*Approval Stage (50)* may be automatically skipped if the DIS has been approved. However, if the draft has been significantly revised following comments at the DIS stage (even if the DIS has been approved), committees can decide to carry out this stage. If this stage is used, the Final Draft International Standard (FDIS) is submitted to the ISO/Central Secretariat (ISO/CS) by the committee secretary. The FDIS is then circulated to all ISO members for a two-month vote. The standard is approved if a two-thirds majority of the P-members of the committee is in favour and not more than one-quarter of the total number of votes cast are negative.

*Publication Stage (60)* is where the secretary submits the final document for publication. Only editorial corrections are made to the final text. It is published by the ISO Central Secretariat as an International Standard. Committee secretaries and project leaders get a two-week sign off period before the standard is published.

## **3.4 Conclusion**

In a manufacturing context, Standards are documents that comprise principles, rules and guidelines for the specifications of built parts. They are developed by a group of experts from a relevant field based on consensus building. From the industrial revolution to mass production to the evolution of information technology, Standards play an important role to enhance efficiency, consumer confidence and trust in products, enabling shorter time to market and reduced cost from re-testing or re-certification. This is even more important, especially in Additive Manufacturing, where new technologies, materials, processes and applications are continually evolving.

## **3.5 External Resources**

The following links provide further detail about ISO standards and other resources:

## **References**

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# **Chapter 4 Design for AM**

**Olaf Diegel**

## **4.1 The General Thought Process of DfAM**

Many industries approach AM as a direct replacement for conventional manufacturing technologies. This approach, however, does not fully utilize the unique possibilities that additive processes offer. For example, a production component designed for 3-axis computer numerically controlled (CNC) machining that is manufactured through additive processes will, generally, be more expensive to produce with AM. Additionally, parts generated through 3D printing may still require conventional machining-based post-processing in order to meet surface quality and engineering tolerance specifications. Thus, AM should be approached as a complimentary form of manufacturing through which novel products/forms of products can be generated. In general, it should only be considered as a viable production technology if the value it adds to the product is high enough to overcome the high costs of its relatively slow speed.

Much like the design requirements of established manufacturing technologies, the optimal fabrication capability of AM is dependent on the implemented design. It is through this design for additive manufacturing (DfAM) that parts of increasing geometrical complexity can be generated without the financial constraints found within conventional manufacturing. This enables the use of complex re-entrant geometries, lattice structures and otherwise difficult to produce geometries resulting from topology optimization.

AM also enables several design features that would otherwise be impossible or very expensive to achieve with conventional manufacturing. Through the layer-uponlayer process of AM, it is possible to produce internal voids without complex tooling or dividing the component into several parts. This enables, for example, internal conformal cooling channels to be added to molds to increase productivity and part

O. Diegel (B)

Faculty of Engineering (LTH), Lund University, Lund, Sweden e-mail<EMAIL_ADDRESS>
<sup>©</sup> The Author(s) 2022

D. Godec et al. (eds.), *A Guide to Additive Manufacturing*, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-05863-9\_4

quality. Additionally, AM has allowed for the generation of objects having multiple parts/components without the requirements of post-fabrication assembly.

Additive manufacturing, because of its layer-upon-layer process is, by nature, a slow manufacturing process. This relatively slow speed, especially when compared to conventional manufacturing technologies such as injection molding and casting, means it is also an expensive technology. For this reason, it is important to only use AM for production when it adds enough value to overcome the high cost of producing the part.

The overall thought process that can be useful in redesigning parts for AM includes:

#### 1. **Reduce part to only those features that serve functionality.**

With a conventional technology, such as CNC machining, we try to minimize the amount of cutting required for the part and, therefore, try to remove as little material as possible from the billet we start with. AM is precisely the opposite, because any material that does not serve a real engineering function is just material that has to be processed by the AM system and therefore costs time and money. Anything that breaks the 'even-thickness rule' is just unnecessary material that increases cost, causes more residual stress and therefore more supports and heat treatment.

	- **Consider replacing temporary supports with permanent walls.** Support material can be thought of as a temporary wall that will be removed after the part is printed. So why not consider replacing the temporary wall with a permanent one that then becomes a feature of the part.
	- **Consider changing the angles of features requiring support.** If a feature is horizontal, it will require support material beneath it. But, if you can change its angle, chamfer or gusset the bottom horizontal face at 45 degrees then the need for support material can be avoided.

As an example, let us look at the step-by-step design thought process that can be employed in the redesign of a 100 mm × 100 mm × 100 mm steel manifold.

#### *Eliminate evrything that does not serve a purpose*

The first step of the design process starts with eliminating everything in the manifold that does not serve a real function. In the case of a manifold, the only thing that performs a real function are the pipes, so the goal of the first step is to eliminate all the material that is not a direct part of the pipe network. The goal is the simplest possible representation of the 'block' manifold (Fig. 4.1) with just the actual channels for the transport of hydraulic fluid.

Once the block design has been simplified, the next step is to remove all the excess material of the cube to leave just the pipes that form the manifold channels. The majority of CAD software packages have some built-in functionality, often called a 'shell' function that makes this step relatively easy. In this example, we select all six outer faces of the cube to be removed, and apply the 'shell' function (Fig. 4.2), leaving only the internal channel structure with, in this case, a thickness of 2 mm.

**Fig. 4.1** Simplified 'block' design manifold with only the required in and out channels (Courtesy of Olaf Diegel [1])

**Fig. 4.2** Manifold design after shell operation on block design (Courtesy of Olaf Diegel [1])

#### *Decide on print orientation*

One of the early decisions that needs to be made when designing for AM is the print orientation, as this will affect all other design decisions thereafter. This is because print orientation will have an impact on almost every aspect of the part. **When designing for additive manufacturing, one should always design around the specific orientation in which the part will be printed because part orientation will determine the direction of anisotropy, mechanical properties, surface finish, roundness of holes, support material, etc**.

When we run the manifold example through software used to generate support structures we see that support is generated between all the horizontal pipes (Fig. 4.3). In one of the orientations below, where the large diameter pipe is horizontal, we can see that support has also been generated inside the large diameter pipe.

It is important to remember that, in DfAM, there is no absolute that says one orientation is better, or worse, than the other. Instead, it must be a conscious exercise to examine what the effects of a particular print orientation are on a part. In our example, both print orientations will necessitate the removal of this support material after printing, as well as some surface treatment to improve the surface finish of the areas where the support material makes contact with the real part. This postprocessing increases the amount of labour required to finish the part, extends the delivery time on the part, and increases cost.

One could possibly make the argument that, in the print orientation where the large diameter pipe is horizontal, it will be harder to remove the support from inside the pipe than from all the outside surfaces. So, unless there were some other advantage to having the large diameter pipe printed horizontally, the better print orientation might be the one where it is in the vertical position. However, without fully understanding the context in which the manifold will be used, one cannot guarantee that this is, in fact, the best orientation.

**Fig. 4.4** Support material required by optimised for metal AM design (Courtesy of Olaf Diegel [1])

#### *Eliminating support material*

Support material is, essentially, a sacrificial temporary material, that serves a certain purpose during the print process, but is then removed after the print is finished. In many cases, a design option that is worth considering is to replace the support material beneath each of the horizontal channels with a permanent wall to eliminate the need for support material altogether. **The idea is that the added wall becomes the support material and becomes a permanent feature of the part**.

In the case of the manifold example, we can replace all of the support material with permanent walls, thus eliminating the need for support material entirely. The bottom walls are chamfered at 45°, as that is the angle we have set beyond which to use support material, and we can add elliptical, diamond, or teardrop-shaped holes in the walls to further reduce weight, yet without requiring support material in the holes. As can be seen below (Fig. 4.4), the only needed support material is a very small amount required to weld the part to the build platform.

In the case of the above manifold, the weight of the original 100 mm × 100 mm × 100 mm block design, in steel, would be 7.4 kg. In contrast, the optimised for metal AM design weighs only 600 g. That represents a greater than 94% weight saving, and a proportional reduction in print time and cost.

The above example demonstrates how some simple design decisions can greatly reduce the amount of support material required, and therefore, the post-processing, which means a better product at a lower cost. Remember, however, that in many cases of printing metal AM parts, some support material is unavoidable, but that the designers thought process should constantly be looking for ways to minimise it.

## **4.2 The Economics of DfAM**

This section investigates economic arguments related to AM, namely how designcontrollable factors of this manufacturing and the results of DfAM affected costs relate to operations, material, and processing.

Though the strategies implemented within this chapter apply equally to all AM technologies, a focus is made with respect to metal PBF AM because it is one of the technologies in which cost and time factors are the most significant. This section will evaluate the current economic relationships relative to operations within AM. An analysis regarding the current fabrication related costs is conducted to act as a better framework for discussions relating to design-based 3D printed part optimization. To further illustrate these ideologies, a case study is conducted through which the described DfAM techniques are demonstrated in the generation of previously discussed manifold part. The preferred design option for this example was fabricated with an EOS M290 selective laser melting system, in 316L stainless steel, to demonstrate that it could, indeed, be printed with minimal support material and post-processing.

## *4.2.1 Machine Costs*

Printing parts in which DfAM has not been considered are susceptible to high costs. This is due to the expensive nature of industrial AM systems paired with comparatively slow part production rates. A metal production sized AM system, typically, costs between US\$500,000 to over US\$1,500,000.

A typical operational time (80%) for a metal AM machine results in approximately 7000 h of annual use. A desirable return on investment used by industry recoup the cost of a rapidly evolving technology infrastructure investment is, for example, 2 years. Whilst this may vary relative to individual companies, it will suffice as a benchmark for this work. Additional associated costs might include potential loanbased interest rates (e.g. 5%), installation labour and power consumption (both being relatively minor expenses in comparison to machine time). This simplified cost model is summarized in the Equation below and some examples for the hourly running costs of differently priced AM systems are shown below in Table 4.1.

Machine hourly running costs = *(*Machine purchase cost + interest*)/ (*payback period ∗ % running time ∗ yearly hours*)*

The above equation illustrates a simplified costing model from which hourly operational machine costs between US\$500,000 to US\$1,200,000 have been generated. From this range of potential costs (US\$37–90/h) an average of US\$65/h was used for the examples in this chapter. This is, in some cases, a potentially optimistic value as certain user specific expenses (e.g. overheads) have not been accounted for. Currently, metal AM technologies are associated with long operational times (e.g. dozens of hours to produce a part to, not uncommonly, greater than 100 h) demonstrating an operational cost vs. time vulnerability within the technology. These operational costs are, however, not dissimilar to other conventional high-end manufacturing technologies such as industrial CNC and injection molding machines. However, geometrically simple parts can be produced much faster with these conventional technologies than they can with AM.

## *4.2.2 Material Costs*

Metal AM powders can range from US\$70/kg (e.g. steel or aluminium) to US\$300/kg (e.g. cobalt chrome or titanium) which is significantly more costly than the same material alloys used for traditional manufacturing. Costs for AM polymer materials are also substantially more expensive, between 10 times to up to well over 100 times, than their conventional manufacturing counterparts.

Powder-based metal AM also requires the use of additional support material for overhanging part geometries, to anchor the part to the build platform and to help with heat dissipation. This support material, together with losses from partially sintered powder, yields an estimated 8–10% loss/wastage of material. Thus, an argument can be made relating to the reduction in wasted material (relative to subtractive manufacturing) and therefore cost reduction in production. Currently these material costs are often negligible relative to the machine-time costs. However, the likely future increases in AM production speeds will result in an increased role of material costs in metal AM cost-based viability in the future.

## *4.2.3 Post-processing Costs*

AM typically requires pre- and post-processing to yield a desirable part. This includes operations relating to the heat treatment of parts, support material removal and surface modification. The table below shows the estimated cost proportions of these tasks with values derived from a 2017 Wohlers Report service provider survey [2] (Table 4.2).

This data indicated that approximately 45% of metal AM expenditure is related to the pre- and post-processing required for part production. This value demonstrates the significant economic impact of parts which have not been designed to minimize these processes.

Time-consuming operations within AM are not solely limited to part geometry and post-processing. Such operations relate to the time taken to, for example, the spreading of material for a layer (recoater time) and the implementation of controlled environmental conditions. The requirements for purging (typically the removal of oxygen through inert gas addition) and the heating of the printing region are machine and material dependant. The recoater time is user dependant given the implemented part print orientation which will typically affect the required number of layers/recoating operations. However, if two parts, one well designed and one poorly designed, have the same overall height, then the recoater time will be the same for both, and is not affected by how good or poor the design is.

## *4.2.4 Time Factors That Are Affected by Design*

Similar to the golden rule of injection moulding, where even wall thickness is always recommended, large masses of material, typically, offer little functional advantage. In fact, large material masses can be detrimental as those are the areas that are likely to contain residual stress which can cause distortion of the part. Moreover, in AM, large areas with solid material also greatly extend print times. There are many techniques for removing large masses of material, including the 'shell' technique described below. Others include filling solid parts with honey-comb, lattices, or even porous material. From a time-based perspective, it is best to print a part in the orientation in which it has the lowest vertical height as this will generally have the fewest layers, and therefore the fastest print time. However, because print orientation also plays a major role on part mechanical properties, geometric accuracy, surface finish, and support material, this decision can often be a compromise between print time and these other aspects.

#### *Design factors that reduce laser operating time*

Within most AM, printing time is directly related to the quantity of material which needs to be solidified. This is particularly true of powder-based techniques which employ a processing (sintering/melting) beam (laser or electron) to scan across

**Fig. 4.5** Example form highlighting the typical implementation of laser scanning for *contourlines and hatch patterns* (Courtesy of Olaf Diegel [1])

a region of power to generate a fused layer/slice of a model/part. This scanning operation can be characterised as either a contour or hatching operation (Fig. 4.5).

The contour operation relates to the processing of the slice edge (which will become the part outer surface) and the hatching operation relates to the pattern employed to process the internal material of the part and can occur in various forms (e.g. meander, stripe or chess-board). A good analogue for the laser scanning process would be to draw a shape outline with a pencil (this being the contour operation) and then proceed to 'colour in' the internal area of the shape by running the pencil back and forth (hatching) many times.

As such it is desirable to implement techniques capable of reducing the quantity of material requiring laser scanning. Techniques of solid material substitution with shells, honeycombs or lattices result in faster manufacturing as they avoid unnecessary processing of material.

#### *Design factors that affect print time and cost*

The Table 4.3 further highlights the various stages within an AM process which are affected by part design and in which part design can, therefore, influence the cost of the part.

## *4.2.5 Economics Case Study: Metal AM Manufactured Hydraulic Manifold*

Within AM, metal-based techniques yield the greatest opportunity to optimise the design for products otherwise designed for traditionally manufacturing. One example of which are manifold like objects, as per the example discussed in the previous section. These parts are typically generated by the drilling of channels within a large

**Table 4.3** Factors within AM processes through which DfAM can yield economic improvements

metal block to form a network of interconnecting pipes. Of note is the requirement to drill separate holes through the block to act as internal connecting channels, some of which are then required to be appropriately blocked (plugged) post machining. The Fig. 4.6 below illustrates the previously discussed block manifold example for which a print layer is depicted to highlight the inferred AM material/material processing requirements.

The scanning hatch pattern for the example layer slice will require a relatively long beam scanning distance. For a manifold having dimensions 100 mm × 100 mm, and a hatch spacing set to 0.1 mm, **a scanning hatch distance of approximately 100 m is required per square layer**. For the purposes of this example, a beam travel speed of 330 mm/s and the previously derived machine operating cost of US\$65/h will be utilized. This results in a layer hatching time of 100/0.33 = 303.03'' s = 5.05'' min. At \$65/h this equates to US\$5.47 in operational time.

The first fundamental step related to DfAM encourages a reduction in material requiring processing. The use of a hollowing ("shell") based technique upon the generated model allows for the retention of the established surface morphology as

**Fig. 4.6** A block manifold and an illustration of the laser scanning process to melt a single layer of the part (Courtesy of Olaf Diegel [1])

**Fig. 4.7** A shelled block manifold and an illustration of the laser scanning process to melt a single layer of the part (Courtesy of Olaf Diegel [1])

well as the specification of a desirable uniform wall thickness. The results of this modification are depicted in the Fig. 4.7 below.

This simple alteration to the model results in a greatly reduced material processing (laser scanning) requirement, therefore reducing overall print time and cost. Utilising a 'shell thickness' of 2 mm and the previously described hatch settings **yields a total hatch distance of 4.5 m** (a > 95% reduction over the solid block). This will take approximately 13.6 s to hatch and result in a layer cost of US\$0.24 (saving US\$5.23 per layer). It is worth noting that within powder-based AM the generated cavity will be filled with un-melted powder. For applications in which this is not desirable, this powder can be removed via the inclusion of an opening/hole through which this excess material can be drained. It is worth noting, also, that support material is still likely to occur for the horizontal geometries within the shelled structure. The removal of these would be relatively complex or impossible.

Given the parts requirements, the outer surface (external box form) can be further removed yielding a part having solely the desirable functional form of the manifold thereby further reducing print time and the associated costs. Within this example, limited functional reduction in support material will be achieved via the variation of printing orientation for the manifold. Whilst the support material required for the part can be reduced by implementing an angular alteration (relative to the desired materials operational range) to the various extrusions of the manifold, this would not yield a functionally desirable result. Additionally, within metal AM support material is fundamental in anchoring the part to the build platform as well as aid in the controlled dissipation of thermal energy. As such it is required to minimise the potential thermally generated distortions which could occur to the part during fabrication. As such this example made use of designed permanent supporting structures which when added to the part both avoid automatically generated support as well as acted to reinforce the channels.

Great care must also be taken to ensure no/limited support material occurs within the manifold channels as this will result in flow distortion. The maximum diameter from which AM will no longer require support material will vary relative to the material and machine utilised. Within this example, the utilised material (Stainless Steel 316L) and EOS M290 Printer allowed for a minimum inner diameter of 8 mm for the manifold channels without requiring any additional support structures. The resultant manifold design including the implemented permanent support structures is demonstrated in the Fig. 4.8 alongside a version which depicts the limited automatically generated support material.

With regards to the aforementioned steps required in DfAM; step 1 related to the implemented hollowing/shell technique and removal of all the non-essential material, step 2 was not required as the design geometries were already connected, step 3 identified the potential regions and requirements for support material relative to printing orientation, and step 4 included the reduction in support material via inclusion of permanent support structures and software-based design validation. The resultant manifold developed through this process is shown below (Fig. 4.9). This required

**Fig. 4.8** Resultant manifold design having permanent support structures and model depicting software derived support material (blue) (Courtesy of Olaf Diegel [1])

**Fig. 4.9** A designed for AM block manifold with reduced print time and support removal time (Courtesy of Olaf Diegel [1])

no further surface machining and the channels were manufactured free of support material, the part only needing removal from the printer build plate for use.

A costing comparison of the various stages of the part redesign (solid block, shelled block and optimized DfAM) was conducted through simulating these parts within Autodesk's Netfabb Ultimate 2018 software for an EOS M290 machine. The print material was set to 316L stainless steel and a layer thickness of 50 µm utilised. The simulations identified major reductions in time and cost namely; an approximate ~90% reduction in time, ~90% reduction in machine cost, ~92.5% reduction in weight and material cost. Additionally, external AM bureau quotes were requested and an 87% reduction in purchasing cost was identified. The results of these costing comparisons are shown below (Table 4.4). This case study acts as an example of how AM, and more specifically the DfAM can dramatically improve production cost and part size/weight.

This chapter demonstrates that machine cost and print time is one of the main factors that affects AM part production and the most significant controllable factor is the avoidance of large masses of material. These require extended hatching times, and the avoidance of support material aids in minimizing post-processing. It quantifiably demonstrates that a simple strategy of replacing large masses of material with even wall thickness shells can have a substantial impact on hatching times and therefore machine time and cost.

This chapter establishes the importance of DfAM from an economic point of view. The difference in cost between a part that has been designed for AM compared to that of a conventional part made with AM is so significant illustrating DfAM to be an essential part of successfully implementing AM. Moreover, if one includes the

**Table 4.4** Manifold design print times and cost

less quantifiable added value elements such as reduced weight, improved functionality, reduced time to market, reduced waste material etc., then the argument for the absolute necessity of DfAM is not hard to make.

## **4.3 Polymer Design Guidelines**

A previous assessment of part geometry is recommended so as to evaluate whether additive manufacturing will bring benefits to our part or not, based on a first quick assessment we will be able to decide if AM, conventional manufacturing methods or both of them are required.

Some of the keypoints that must be analysed are:

Although costs involved in the Additive manufacturing production chain vary depending on the technology, average values can be stablished as: (i) material (15%), (ii) human resources (30%), (iii) machine (55%), and (iv) post processes (if required).

Therefore, many variables take part in the process that will affect part production costs.

#### **Part weight/volume**

Given the material optimization that additive manufacturing processes permits due to the design freedom, this is not only an opportunity for the part to be optimized in terms of weight but also an opportunity to reduce the production costs in the AM processes. The more parts we manage to put inside the build envelopment the cheaper the production costs will be.

Hence the design for additive manufacturing is a key factor for the part and process optimization. Normally a part which is conceived for traditional methods will not be economically feasible for AM if the design is not adapted.

#### **Production costs analysis**

As in all the additive manufacturing technologies happens, there is a fix cost which comes from the machine preparation and build preparation, the cost will not vary if only one part is produced or plenty of parts are nested inside the build envelopment, therefore the unitary cost will vary depending on the amount of parts that are allocated in the build envelopment. Some calculations can be done so as to assess the feasibility of additively manufacture a certain geometry. A relationship between unitary costs and number of parts per build is found in Fig. 4.10.

As a summary, although there might be different approaches depending on the part complexity and each situation, there are a few questions that we must formulate so as to decide whether re-desing for additive manufacturing will be required or not:

**Fig. 4.10** Unitary costs versus number of parts in build envelopment (*Source* Skin Project— AIDIMME)

interaction with other parts in the assembly and the working conditions (loads and restrictions). Some of these restrictions might limit our design.

## *4.3.1 Designing for Material Extrusion (MEX)*

Material extrusion is the AM process with the most anisotropy. The bond between the layers can substantially weaken parts in the vertical direction, so this is the process in which choosing the most appropriate print orientation can have the biggest impact.

#### *Material extrusion accuracy and tolerances*

There is a great difference in accuracy and tolerance between different material extrusion systems. This is particularly the case when comparing desktop systems to industrial systems. Accuracy and tolerance can also vary depending on geometric features and print orientation. The only sure way to know the accuracy and tolerance of any particular system is to print a test reference part and to measure it.

The numbers given in Table 4.5 are for industrial quality material extrusion systems. They represent a general tolerance and accuracy for material extrusion technologies.

**Table 4.5** General tolerance and accuracy for material extrusion technologies

#### *Layer thickness*

Layer thickness is one of the first decisions that has to be made when using material extrusion. In general, the thinner the layer, the better the surface quality, particularly on rounded parts, as the stair-step effect will be much less visible. Remember, however, that the thinner the layer, the longer the part will take to print.

If a part is composed mainly of planar geometric features in the vertical direction, then printing it with thicker layers will not necessarily produce a substantially worse surface finishes that if it is printed with thin layers, but it will print much faster. If the part is made up of mainly curved surfaces, then thinner layers may be preferable in order to achieve the smoothest possible curved surfaces.

#### *Support material*

Some material extrusion systems print with soluble support material while others do not. If the supports need to be removed manually, rather than being dissolved away, then access should be allowed so the supports can be broken away. Also, with small and delicate features, care should be taken that they are not accidentally broken when the support is removed.

Another decision that has to be made is what type of support material structure options to use. Almost every different maker of material extrusion printer offers different options for this. But some of the common types of support strategies include supports shown in Fig. 4.11.

**Fig. 4.11** Types of support strategies (Courtesy of Olaf Diegel [1])

**Fig. 4.12** Examples of different interior infill percentage options (Courtesy of Olaf Diegel [1])

#### *Fill style*

Most material extrusion systems allow the user to choose if the part should be printed as a solid part, or as a 'sparse' part in which the interior void is filled with a scaffold structure. Some systems may also give the user the option to specify how thick the outer shell wall should be. Some systems also allow the user to select the infill percentage, or how dense the internal scaffold structure should be (Fig. 4.12).

#### *Other considerations*

A characteristic of the material extrusion process is a "stair-stepping" effect on gently inclined or curved surfaces of parts. This can be reduced by various post-processing techniques (such as acetone vapour smoothing for ABS parts), but this will have an impact on part accuracy, geometrical stability and sometimes material properties.

Material extrusion can, depending on the particular AM system being used, leave a small air gap between the laid down filament for certain wall thicknesses. This is because the software has to make a decision on whether to deposit an extra strand of material in the wall. For example, if the strand of polymer exiting the printer nozzle is 0.4 mm wide, but the wall thickness is 0.9 mm, the software must make a decision as to whether or not to add an extra strand of polymer between the two first tracks, or to leave a 0.1 mm gap. This very much depends on the machine brand and model,

so it is best to do some tests and to find which wall thicknesses are not ideal for each brand of machine.

Holes in a Material Extrusion part are usually printed undersized. If tight tolerances are required, it is best to drill the holes to the required diameter.

Because of the sometimes weaker bond between contour lines and infill (or hatch) lines (Fig. 4.13), self-tapping screws can occasionally strip away the contour material in the screw bosses. A drop of cyanoacrylate (super glue), allowed to wick between the contour and fill material can improve this.

The following pages contain general design guidelines for the material extrusion process.

Comments

Avoid unsupported large flat surface areas. Warping may occur with extended lengths of unsupported walls (i.e., no ribs or intersecting walls). In this case, avoid using the minimum wall thickness

Avoid sharp transitions. Fillets at the points where walls join are recommended In general, an even wall thickness is recommended on all walls, both vertical and horizontal

Horizontal walls

With material extrusion technologies, horizontal walls can, theoretically be as thin as a single layer of material. In practice, however, to produce a horizontal wall with some strength and consistency, at least 4 layers of material are recommended

Again, it is best practice to keep all walls of your product at the same thickness

#### Support material overhang angles

Maximum overhang angle (a)

#### 45°

This is a safe default number. But the angle can vary greatly from printer brand to printer brand, and depends on the material used and desired surface quality

#### Comments

Overhang angles less than 45° (measured from horizontal) require support material, which is normally added automatically by the system software. Be aware that some systems measure the support angle from the horizontal, while others measure from the vertical

Excessive supports that need to be broken away manually will increase post-processing time. Soluble support structures require much less manual labour, but they still waste material and time Horizontal holes (e.g., cooling channel profiles) can often be modified into teardrop or ovals shapes to minimize the need for internal supports that are hard to remove

#### Comments

Large areas of close proximity will slow down the removal of support material. Clearance between parts built separately and assembled later must be at least equal to the general build tolerance of the system

(continued)

#### (continued)

#### Comments

The main challenge with printing moving parts on a printer without soluble support is in the difficulty of removing the support material from between the moving parts

Large areas of close proximity will slow down the removal of support material. Clearance between parts built separately and assembled later must be at least equal to the general build tolerance of the system

#### Comments

Holes are generally built undersized, typically by around 0.2 mm (0.0079 in.) across the diameter (*Note* this value needs to be verified for each machine/material combination being used). This can be remedied approximately by adjusting the CAD model using the values above, or more precisely by drilling out the hole after the part has been built If using self-tapping screws, the column of contour material that surrounds the hole can sometimes be stripped out by the screw. A drop of super-glue, allowed to wick in between the contour and fill material, can alleviate this problem

#### Comments

Pins with small diameters, vertical ones in particular, are prone to breaking off if only supported at one end

Always fillet the pin where it joins the wall. Even 0.5 mm is enough to substantially strengthen the pin

## *4.3.2 Designing for Polymer Powder Bed Fusion (PBF-LB/P)*

What differentiates polymer powder bed fusion processes, be they laser-based or multijet fusion-based, from the majority of other AM processes is that they do not require support material. The part being constructed does not require supports because the unfused powder surrounding the part provides sufficient support. It can therefore be argued that this gives the designer greater freedom than most other AM system.

Parts created using polymer powder bed fusion usually have some degree of vertical anisotropy, particularly for small features that are less than about 25 mm2 in surface area in the vertical direction. Engineers must design around this by, for example, ensuring that highly stressed features are built horizontally rather than vertically.

#### *Powder bed fusion accuracy and tolerances*

There is a difference in the accuracy and tolerance between different manufacturers systems. They also vary depending on geometric features and print orientation. The only sure way to determine the accuracy and tolerance of any particular system is to print a test reference part and measure it. The numbers given in Table 4.6 are for industrial quality powder bed fusion systems.

#### *Layer thickness*

A common layer thickness for powder bed fusion is 0.1 mm, but some systems allow for thinner layer thicknesses. Compared to other AM technologies, however, the stair-stepping effect is less visible on polymer powder bed fusion technologies. It is only on very gently curving surfaces of a relatively large surface area that it is visible.

#### *Avoiding large masses of material*

Designers need to be careful in trying to avoid uneven thicknesses of plastic in their parts and to avoid large masses of material. These can cause distortion in the part and will add substantially to the time and cost it takes to make the part. The same applies with multijet fusion because, for large masses of material, much more fusing agent needs to be deposited by the print head, which can substantially increase the cost of the part.

The following pages contain guidelines on how to design features to be built using polymer powder bed fusion.

**Table 4.6** Tolerance and accuracy for powder bed fusion

#### Comments

Though it is, on occasion, possible to print walls thinner than 0.6 mm, their success is highly dependent on the rest of the part geometry, print orientation, etc

Thin walls with a large surface area are likely to warp during the cooling process. If large surface area thin walls are required, consider adding ribs to stiffen the walls

Thicker walls and any large volumes of material will result in excess heat retention in the part and hence shrinkage, resulting in geometric deformation. Therefore, a maximum wall thickness of 1.5~3 mm is also recommended. If walls must be thicker than this, consider shelling them. This will both help to reduce distortion and greatly speed up print times

In general, an even wall thickness is recommended on all walls, both vertical and horizontal

#### Comments

The required gap between moving parts is highly dependent on the surface area of the faces that are in close proximity. If faces in close proximity have surface areas of only a few mm2, then gaps as small as 0.2 mm between the faces are possible. The 0.5 mm gap stated above is one that will work in most situations and on most different manufacturers systems

Large areas of close proximity will slow down the removal of excess powder. Clearance between parts built separately and assembled later should be at least equal to the general build tolerance of the system

Circular profile through holes

#### Comments

Small round holes, typically below 1.5 mm are related closely to wall thickness. As the wall thickness increases, powder becomes increasingly difficult to clear from small holes. As the wall thickness decreases, smaller through holes become feasible

Comments

Small square holes typically below 1.5 mm are related closely to wall thickness. As the wall thickness increases, powder becomes increasingly difficult to clear from small holes. As the wall thickness decreases, smaller through holes become feasible

(continued)

#### (continued)

Comments Pins with small diameters are prone to breaking off if only supported at one end Always fillet the edge where a pin joins a face

Comments

Larger holes require slightly greater distances to the edges of walls

## *4.3.3 Designing for Vat Photopolymerisation (VPP)*

Although many of the previously covered polymer design rules apply equally to vat photopolymerisation, there are certain guidelines that are specific to resin-based processes.

#### *Resolution*

Vat photopolymerisation resolution in the XY-direction is dependent on the laser spot size and can range from 50 to 200 µm. The smallest feature size can therefore not be smaller than the laser spot size.

Resolution in the Z-direction varies from 10 to 200 µm depending on the choices of layer thickness allowed by the machine. As with other AM technologies, choosing a very fine vertical resolution is a trade-off between speed and quality. For a part that has few curves or fine details, there will be little visual difference between a print at 25 µm and one at 100 µm.

#### *Print orientation*

When orienting a part for vat photopolymerisation, particularly on a bottom-up SLA machine that cures the part from the bottom and pulls it out of the vat of resin, the biggest concern is vertical cross-sectional area. The forces involved with a print sticking to the bottom of the tank are directly proportional to the 2D cross-sectional area of the print. Because of this, a part with large cross-sectional areas, is often best printed at an angle to the plate. Minimizing the cross-sectional area along the Z-axis is the best way to orient parts for vat photopolymerisation prints. Reducing the number of horizontal areas relative to the print orientation, hollowing out components and reducing the cross-sectional area are all steps that can be taken to optimize a design for vat photopolymerisation.

#### *Support material*

Vat photopolymerisation does require support material for overhanging features. This is because the uncured resin is not viscous enough to support features on its own. This support material must be removed in post-processing. On most vat photopolymerisation systems the process of adding support material to the part is largely automated but, with experience, the user can manually edit the supports to avoid having supports in areas where surface finish is critical.

#### *Overhangs*

Overhangs generally pose very little issue with vat photopolymerisation, unless the model is being printed without adequate support structures. Printing without supports can lead to warping of the print, but if printing without supports is necessary, any unsupported overhangs should be kept to less than 1.0 mm in length and at least 20° from horizontal.

#### *Isotropy*

Vat photopolymerisation is one of the AM processes where the parts are relatively low isotropic. This is because the layers chemically bond to one another as they print, resulting in near identical physical properties in the X, Y, and Z-direction.

#### *Hollowing parts and resin removal*

Vat photopolymerisation machines can print solid, dense models but, if the print is not intended to be a functional part, shelling the model to be hollow can significantly reduce the amount of material needed as well as reduce the print time. It is recommended that the walls of the hollowed part be at least 2 mm thick to reduce the risk of failure during printing. If printing a hollow part, drainage holes must be added to allow the uncured resin to be removed from the part. Drain holes should be at least 3.5 mm in diameter, and at least one hole must be included per hollow section, although two holes can make the resin much easier to remove.

#### *Details*

Embossed details (including text) include any features on the model that are raised slightly above the surfaces around them. These must be at least 0.1 mm in height above the surface of the print to ensure the details will be visible.

Engraved details (including text) include any features which are recessed into the model. These details are at risk of fusing with the rest of the model while printing if they are too small, so these details must be at least 0.4 mm wide and at least 0.4 mm deep.

## *Horizontal bridges*

Bridges between two points on a model can be successfully printed, but one must keep in mind that wider bridges must be kept shorter (usually under 20 mm) than thin bridges. Wider bridges have a greater cross-sectional area of contact which increases the chance of print failure during delamination from the bottom window.

## *Clearances for connecting parts*

If parts are being made that need to connect together, it is always best have a certain tolerance between the parts that fit together. For vat photopolymerisation, these tolerances are typically:

If interlocked moving parts are being printed, then the tolerance should be 0.5 mm between the moving parts. The following pages contain guidelines on how to design features to be built using vat photopolymerization.

Comments

Supported walls are walls that are connected to other structures on at least two sides, so they have very little chance of warping. These should be designed at a minimum of 0.4 mm thick. Note that if the supported wall has a large surface area, a larger thickness may be required Unsupported walls are walls that are connected to the rest of the print on less than two sides, and are at a very high chance for warping or detaching from the print. These walls must be at least 0.6 mm thick

Always fillet the corners where one wall meets another wall to reduce stress concentrations along the joint

In general, an even wall thickness is recommended on all walls, both vertical and horizontal

Comments

Holes with a diameter less than 0.5 mm in the X, Y, and Z axes may close off during printing

## **4.4 Metal Design Guidelines**

## *4.4.1 General Design for Metal PBF*

Metal AM technologies can be broken down into the technology categories in the Fig. 4.14 below.

In this chapter we will give recommendation for powder bed fusion technologies based on lasers and electron beams, which are the most widely spread technologies for producing metal parts.

Metal powder bed fusion is an AM process in which thermal energy selectively fuses regions of a powder bed. Materials used include stainless steel, tool steel, aluminium, titanium alloys, nickel-based alloys, cobalt chrome, and precious metals

**Fig. 4.14** Hierarchy of metal AM technologies (Courtesy of Olaf Diegel [1])

such as gold. The part being constructed normally requires supports (sometimes called anchors) to be added, built from the same material as the part. These supports are removed manually after the build process.

If we compare metal PBF AM parts to conventionally produced metal parts, AM parts, with no post-processing other than support material removal and shot-peening, would perform as follows (Table 4.7):

With suitable post-processing, however, AM parts can, in some cases, approach the mechanical properties of forged or wrought parts.

#### *The PBF-LB/M AM process*

The powder bed fusion process begins by spreading a thin layer of powder onto the build plate, and an energy source is then used to scan the powder and fuse it together where required. The process is then repeated for subsequent layers (Fig. 4.15).

As metal AM is an expensive process (see the chapter on the economics of AM for details), and parts can require substantial post-processing, you need to have a good reason to make a metal AM part. In general, parts that are not specifically designed for metal AM are not worth doing with AM. There are exceptions to this, like spare

**Fig. 4.15** The overall metal AM build process (Courtesy of Olaf Diegel [1])

parts for example, but, in general, the geometry of the part should be complex enough that it cannot easily be made through traditional manufacturing.

#### *Topology optimisation*

Because of the high costs associated with metal AM, topology optimisation is an excellent technique to apply to metal parts. Because one of the benefits of AM is its ability to make very complex geometries, topology optimisation, which can often create cell-like biologically inspired structures, offers the ability to make much lighter components than with conventional manufacturing. Please refer to the chapter on topology optimisation for further information on how to apply this technique to additive manufacturing.

#### *Lattice structures*

Lattices are another excellent way of producing light-weight parts, but extremely strong, parts that can also reduce the time and cost it takes to make metal AM parts. A lattice is a cellular structure made of repeated unit cells to form a larger volume. There are many options for the shape and size of such lattice cells, and for the pattern in which they are is repeated. Lattices can be uniform, where the same cell size is repeated in all directions of the part, or variable, where the size and spacing of the cells is different in different directions. There are four main techniques for applying lattice structures to additively manufactured parts:

#### **Convert the entire part into a lattice**

This technique transforms the entire volume of the part into a lattice structure (Fig. 4.16). It is commonly done for medical implants, and parts where the exterior surface of the part is not critical.

#### **Fill the inner body of the part with a lattice structure, leaving an outer shell of a specified thickness**:

In general, this method requires salt-shaker holes so that the unsintered powder can be removed from inside the part. If designed correctly, the internal lattice structure (Fig. 4.17) also acts as the support material for heat transfer within the part.

#### **The part is subdivided into solid and lattice areas**

Here a conscious decision is made as to which features of the part remain solid, and which get converted to lattices. The easiest way to achieve this is usually to split the part up into its different regions in the native CAD software the part was created in, and to then import the separate parts into the lattice conversion software to convert the required parts into a lattices while leaving those that must remain solid untouched (Fig. 4.18). Once this is done, a Boolean operation can be performed to join the lattice parts and solid parts to form a single part ready for AM.

#### **Variable lattice structure based on FEA results**

This uses any of the above techniques but, instead of a constant cell-sized lattice, uses a lattice structure where the cell size and spacing varies based on a finite element

#### 4 Design for AM 105

**Fig. 4.16** Complete lattice structure (Courtesy of Olaf Diegel [1])

**Fig. 4.17** Interior lattice structure (Courtesy of Olaf

Diegel [1])

analysis to produce a dynamically-sized lattice structure. The more highly stressed areas of the part wither use thicker lattice members, or a denser spacing of the lattices.

The strut diameters used in lattice structures must be of a diameter such that they can both be manufactured as well as provide the required mechanical properties to the part. Theoretical minimum strut diameter for metal AM is around 0.15 mm. Common sense, however, tells us that a 0.15 mm strut will have relatively little mechanical strength or resistance to fatigue. A more sensible minimum strut diameter to use is, therefore, between 0.5 mm to 1 mm.

When designing lattices, it is important to use lattices that are self-supporting and can be printed without requiring support material. It is possible to have horizontal struts, but they must be short enough to have a surface area below that which requires support material. In the lattice cell shown below (Fig. 4.19), if forces are applied in the directions shown, then design B will resist the force better than design A, but

**Fig. 4.19** Lattice cell A is weak against the forces, Lattice B is stronger but hard to print because of the horizontal strut. Lattice C both resists the forces and is easier to print (Courtesy of Olaf Diegel [1])

may not be printable because of the horizontal strut. If, however, the cell is printed after being rotated by 90 degrees as shown in C, it will resist the forces and it will print better.

#### *Overhangs and support material*

Though it may sound like repetition, it is important to emphasize that supports are absolutely critical in the production of metal AM parts. Supports should not only be considered during part design but can become one of the factors that influence AM part design. The angle and surface area of any overhanging feature of the part determine whether the part will require supports. It is almost always a trade-off to orient the part for minimum build time, easy to remove supports (particularly from inside the part), surface quality, and part warpage. Some aspects will improve, while others deteriorate, depending on the support material being used.

In metal AM, support structures have several functions:

Most metal AM pre-processing software allows the selection of a number of different support types, each of which has different heat transfer and mechanical strength characteristics. Some of the support types offered by most AM preparation software include solid, walls, trees, cones, lattices, blocks, points, lines, webs, and gussets (Fig. 4.20). Which type of support to use very much depends on the part geometry, and how much residual stress it will contain, and how hard the support material will be to remove. The best advice to understand the effect of different types of support offered by your system is to design a part made up of a series of bridges, under which each bridge is printed with a different type of support. The effect of each type of support can then be observed, both in its impact on surface finish and on removal difficulty.

**Fig. 4.20** Examples of different support types. Note that these are just used to show examples of different types of support material, and may not be ideal for this particular part (Courtesy of Olaf Diegel [1])

## *Printing parts with large horizontal surfaces*

Parts with large horizontal areas of material will require much stronger supports than the rest of the part. This is because the sudden change in cross section to a large molten sheet of material will cause substantial stress and will, in all likelihood, cause cracks in the part if the support is not very strong and dense. In such situations, one must sometimes use 'solid' supports. If at all possible, parts should be oriented so as to avoid large horizontal flat areas, or sudden changes in surface area.

#### *Angle for support material*

A general guideline for angles that do not require support material are angles greater than 45° from horizontal. This does, however, vary from material to material and machine to machine. Specific angles relevant to particular materials are given in the table on "Feature type: overhang angle" later in this chapter. Some manufacturers specify angles from horizontal, while others specify them from the vertical. Remember also, that these angles represent the minimum angle at which the part can be produced without supports. In general, using angles that are steeper than those minimum, will yield parts with better surface qualities.

#### *Unsupported angles, overhangs, and bridges*

The area melted at the focal point of the energy beam cools very quickly and the stress generated tries to curl the material upwards. Supports act as an anchor to the build plate to avoid such upward curl.

## **Angles**

Poor surface roughness is the result of building directly on loose powder instead of using the support structure as a building scaffold. It occurs because the laser penetrates the powder bed and agglomerates loose powder around the focal point instead of dissipating the heat through the support structure. At a certain point, the unsupported angle becomes such that the part either has an unacceptable surface quality or crashes the recoater mechanism.

#### **Overhangs**

Overhangs differ from self-supporting angles in that they are abrupt changes in a part's geometry, such as a small feature that protrudes horizontally at 90 degrees. Powder bed fusion is fairly limited in its support of overhangs when compared to other 3D printing technologies. In general, any design with an overhang greater than 0.5 mm (0.020 in.) will require additional support to prevent damage to the part. As an overhang extends past about 0.5 mm, the surface quality either becomes unacceptable, or the upwards curl can become such that it causes the recoater mechanism to crash.

#### **Bridges**

A bridge is any flat down-facing surface that is supported by 2 or more features. The minimum allowable unsupported distance for the powder bed fusion process is around 2 mm (0.080 in.). Parts that exceed this recommended limit will have poor quality on the downward facing surfaces and may not be structurally sound. They can also cause the recoater mechanism to crash.

## *Residual stress*

One of the most challenging aspects of producing good metal AM parts is residual stress. Like any welding process, metal AM induces a substantial amount of stress on the parts. This is one of the reasons why support material is often needed on metal parts. This residual stress and stress concentrations must be relieved through heat-treatment before the parts are removed from the build plate. Residual stress can, in some cases, be so large that it causes the entire build plate to bend, or the part to detach from the build plate, or crack the part itself. Residual stresses are stresses that remain in a solid material after whatever caused of the stresses has been removed.

Residual stress can occur from a variety of mechanisms including:

## *The very best solution to combatting residual stress is to try and eliminate as much of the residual stress out of the part as possible through its design*

## *Designing to reduce residual stress*

There are a number of relatively simple design techniques that can be employed to minimize residual stress. These include:

In addition, many of the design rules for conventional casting apply equally to metal AM.

If large masses of material are completely unavoidable (which is rare), use different laser hatch parameter settings to minimize the build-up of residual stress.

• Smaller chess-board hatch patterns will, for example, create less residual stress than bigger ones, or than large scan areas. But they will slow down the build process a bit.

**Fig. 4.21** Stress concentrations are the areas where cracks will most naturally form. Eliminating such stress concentrations can be critical to part quality (Courtesy of Olaf Diegel [1])

• Rotate each hatch scan, usually by 67°, for each layer.

Ultimately, some residual stress is unavoidable. The real question, however, is whether it will affect the function of the part. A well designed for AM part, however, can often require minimal, or no, heat treatment compared to a part that has not been designed for AM.

#### *Stress concentrations*

A stress concentration is a location in a part where stress is concentrated. These stresses occur both within the AM fabrication process, and in the heat treatment of AM parts. With metal AM, this is a design opportunity where a well-designed part can minimize the areas of stress concentration. Fatigue cracks almost always start at areas of stress concentration, so removing the areas in which such defects can occur can minimize such defects and greatly increases the fatigue strength of the part (Fig. 4.21).

The best way of minimizing the amount of heat treatment required is to design your parts to have as little stress induced or concentrated in them as possible. Simple strategies, like filleting all sharp corners (reduces stress concentrations), even wall thicknesses, and avoiding large masses of material (reduces residual stress), can help a lot. In the simple part below (Fig. 4.22), for example, the sharp internal corner has a good chance of causing a stress crack. In addition, the sharp corner has a larger mass of material than the horizontal and vertical walls and will, therefore, contain some residual stress which could cause the wall to distort. In contrast the filleted corner has eliminated the possibility of a stress crack, and it is even wall thickness has minimised the potential of residual stress.

#### *Horizontal holes*

In metal AM, horizontal holes (or holes angled below the minimum support angle) over a certain diameter will require support material inside the hole. For pipes that

**Fig. 4.22** Example of simple filleting to eliminate both stress concentrations and residual stress (Courtesy of Olaf Diegel [1])

are not straight, in particular, the support can be hard to remove from inside the pipe. As a general guideline, holes below a diameter of 8 mm (0.314 in.) can be printed without supports. If larger diameter holes are required, the most common technique is to change the hole from circular to a shape that can be printed without the need for support material. These shapes commonly include ellipses, teardrops, and diamonds (Fig. 4.23).

## *4.4.2 Design for Laser Powder Bed Fusion (PBF-LB/M)*

The design guidelines below apply to laser powder bed fusion metal processes. The guidelines will vary from machine manufacturer and model to machine manufacturer and model so, if in doubt, it is recommended to print a test piece to verify each set of design parameters.

**Fig. 4.23** Hole shapes that can be printed without the need for support material (Courtesy of Olaf Diegel [1])

The following pages contain guidelines on how to design features to be built using laser powder bed fusion.

(continued)

#### (continued)

Comments

Problems may occur with extended lengths of unsupported walls (i.e., no ribs or intersecting walls). Without adequate reinforcement, large surface area thin walls are likely to distort. In this case, avoid using the minimum wall thickness, or reinforce the wall with ribs, gussets, or use extra support material to prevent it from distorting

Always fillet the corners where the walls meet another surface. A good rule of thumb is to make the fillet ¼ of the thickness of the walls

#### Comments

Overhang angles less than the numbers shown above (measured from horizontal) will require support material, which may be added automatically by the system software. Excessive supports that need to be removed manually will increase post-processing time

Beware that some manufacturers measure support angles from the horizontal, while others measure it from the vertical

Feature shapes (e.g., cooling channel profiles) can often be modified to minimize support requirements, and horizontal holes less than 8 mm (0.236 in.) can be built without supports. Se the chapter of this book on the design guidelines for horizontal holes

#### Comments

In general, with metal AM, all moving parts will need to be welded to the build platform, or connected to each other, so that they are now swept away by the recoater system. They only become moving parts once they have been cut off of the platform or the joining links have been cut

Large areas of close proximity will make the removal of supports more difficult. Clearance between parts built separately and assembled later must be at least equal to the general build tolerance of the system

Comments

As the thickness of the part increases powder inside the slots or holes may become hard, or impossible, to get out

Values for horizontal features are not available as they are too dependent on each specific machine

If possible, fillet all sharp internal corners to avoid stress concentrations

Comments

It is good practice to fillet the bottom of all pins and bosses. A general guideline is that the radius is ¼ of the thickness

Values for horizontal features are not available as they are too dependent on each specific machine

Built-in external screw threads

(continued)

#### 4 Design for AM 115

#### (continued)

Threads should always be built vertically, if at all possible

Comments

Though threads down to about M4 can, theoretically be printed, their surface roughness means they will need to have a tapping die run over them to clean up the thread

Tapping is recommended for all threads and sufficient space must be left around the post to allow access for the tapping die

Fillet the bottom of the boss where it meets the wall to avoid stress concentrations. A good rule of thumb is to make the fillet ¼ of the thickness of the walls

## *4.4.3 Design for PBF-EB/M Guidelines*

Electron beam melting is a powder bed fusion process that uses an electron beam as the energy source to melt each layer of powder. The electron beam is controlled by electromagnets that move it over the powder in a controlled manner to draw the slices of the part to be produced (more in Sect. 1.7.2).

Comments

It is possible to build vertical walls with a thickness of 0.6 mm in solid material, but this can be difficult to achieve in all orientations and with walls of large surface area. A safe recommended wall thickness is 1 mm. For short lengths, such as in lattice structures, different melt strategies can be used so the part can be as thin as 0.3 mm. However, this is not suitable for structural walls as they can suffer from delamination or layer shift Always fillet the corners where walls meet each other

#### Comments

Holes, slots or tubes built into EBM parts at any angle will be filled with partially sintered powder. This block of powder allows different diameters to be built without the need for supports but can be hard to remove unless access to the hole is easily done with blasting media or with hand tools, so this must be considered during the design stage

A minimum diameter of 1 mm vertically or 0.5 mm horizontally is recommended to ensure that rough surfaces do not cause the holes to close up

In walls thicker than about 2 mm, vertical holes will generally need to be no smaller than 2 mm and, for horizontal holes, no smaller than 1 mm

#### Comments

As there is a partially sintered cake of powder surrounding the parts that are built, access must be given to allow trapped partially sintered powder to be blasted away from small gaps, holes and mechanisms. Larger spaces may have to be included around complex parts to ensure the cake can be removed. In general, 1 mm clearance is usually sufficient to thermally isolate each part on the build platform

Screw and threads

(continued)

#### 4 Design for AM 117

#### (continued)

Threads should always be built vertically, if at all is possible

Comments

Because of the relatively rough surface finish of EBM, all threads will need to be tapped/machined

Fillet the bottom of the boss where it meets the wall to avoid stress concentrations. A good rule of thumb is to make the fillet ¼ of the thickness of the walls

## **References**

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# **Chapter 5 General Process Simulations**

**Axel Nordin, Per Kristav, Julia Ureña Alcázar, José Ramón Blasco, Olga Jordá Ferrando, Mario Martínez, and Luis Portolés**

Simulation of AM products can capture a number of aspects. Apart from the traditional types of simulation of the end product, such as mechanical, thermal and fluid analyses, it is possible to simulate the AM build process. While simulating products intended for AM can sometimes be performed in exactly the same way as with products intended for traditional manufacturing, there are several aspects that may require a specialized workflow. In particular, the great benefit of AM in offering "complexity for free" is also a major challenge in simulating the part performance. In this section, a review of the various tools and methods for simulating AM products will be made.

This chapter will be structured according to the basic workflow of setting up a simulation, that is, a geometry needs to be defined and most often discretized, material properties need to be defined, boundary conditions need to be applied, analysis settings need to be defined and results need to be interpreted, as illustrated in Fig. 5.1.

## **5.1 Simulation**

## *5.1.1 Geometry Definition*

The task of defining the geometry for analysis is obviously a part of the design process, which is dealt with in other chapters of this book, but some aspects are worth noting regarding simulation.

A. Nordin (B) · P. Kristav

Faculty of Engineering (LTH), Lund University, Lund, Sweden e-mail<EMAIL_ADDRESS>
J. U. Alcázar · J. R. Blasco · O. J. Ferrando · M. Martínez · L. Portolés Metalworking, Furniture, Wood, Packaging and Related Technological Institute, AIDIMME, Valencia, Spain

**Fig. 5.2** Left: Original part intended for manufacture. Right: Simplified model intended for optimization (Courtesy of Axel Nordin, original model by Olaf Diegel)

#### *Simplification*

A geometry intended for production is often not ideal for simulation. More often than not, simplification steps need to be taken to make it feasible to analyse the geometry. However, since many use cases of AM relies on exploiting "complexity for free" it is not always straightforward to exclude complex features from the analysis. Traditionally, features such as non-structural rounds, lettering, screw threads, small holes etc. are eliminated from the geometry before analysis (as seen in Fig. 5.2). This can either be done through the original modelling software, or through specialized pre-processing defeaturing software. This process is largely manual and often requires considerable effort if the model is complex. Moreover, it is important to not oversimplify the model by removing features that may initially look irrelevant to the performance of the product.

When dealing with models intended for AM, it may not even be possible to simplify away features such as lattice structures or organic shapes with many rounds. Although computing power is ever increasing, many simulations of highly complex lattice structures may not be currently feasible to run, at least not if several iterations of the design are expected. There are approaches, such as homogenization, where complex small-scale structures are replaced by a solid material that replicates the properties of the structure. These approaches, however, require additional work to characterize the material response and are not easily applicable when the small scalestructure varies in, for instance, lattice member diameter.

#### *Facet-Based and Parametric Models*

Traditionally, the geometry intended for simulation is represented in a parametric CAD format, such as STEP. Parametric formats represent curved surfaces as, for instance NURBS (Non-uniform rational basis spline), which can in turn be discretized into finite elements or volumes with varying density. As parametric models are represented by surfaces, it is straightforward to determine aspects such as curvature, which simplifies discretization and application of boundary conditions. Moreover, parametric models enable non-geometric data to be linked to the model, such as surface-ids and material data, which can help reduce some of the modelling effort when analysing several variants of the same basic concept.

However, within AM, it is common to have geometry represented as facetted polygon meshes in, for instance, the STL-format. As the facetted representation is already discretized and does not easily allow for the identification of surfaces such as planes and cylinders, facet-based models pose several challenges for simulation pre-processing. While there are some tools that simplify the selection of connected areas of triangles, these selections will typically only be valid for that particular mesh. If a slightly modified version of the model is to be analysed, all selections will have to be updated again manually.

There are several tools for transforming a facet-based representation into a parametric model, sometimes called reverse engineering tools. These tools analyse the facetted mesh to identify areas that can be represented as, for instance, cylindrical or planar surfaces and can fit NURBS geometry to the underlying facets. This makes discretizing and applying boundary conditions easier, but the process is computationally expensive, often requires manual adjustment and will need to be redone if the mesh is altered.

## *5.1.2 Discretization*

For discretizing a facetted model without first translating it into a NURBSrepresentation, the main approach is to voxelise it. This process generates a number of volumetric elements, or voxels, approximating the facet-based model. The voxels are usually aligned with the coordinate system and may also be aligned to the face of the mesh, to better approximate the shape of the mesh, as shown in Fig. 5.3.

This is a robust method for making a facet -based model compatible with simulation and is less computationally expensive than re-interpreting the mesh as surfaces. Table 5.1 gives a summary of the advantages and disadvantages of the different methods.

**Table 5.1** Summary of the advantages and disadvantages of the different methods

(continued)

(continued)

If you are doing the analysis for validation and will not be analysing several iterations, re-interpreting the geometry is probably a good option. If you are analysing several concepts or will be iterating on the design, voxel-based meshing is probably a good option.

## *5.1.3 Material Properties*

While many research teams are very actively characterizing the material properties of AM parts, it is still hard to know beforehand what material properties the finished part will have. This is in principle due to the large number of parameters that affect the part properties, such as type of energy source (e.g. electron or laser beam), energy density, scanning strategy, powder properties, heat treatment, part geometry, part orientation, etc. While build process simulation tools can take many of these factors into account on a small scale, it is not currently feasible to simulate an entire product in such detail. Instead, most simulations are based on estimated properties calibrated from printed samples. Moreover, factors such as yield and ultimate strength, fatigue life, surface roughness, and thermal and electrical conductivity can only be estimated and need to be tested to ensure reliable results. As AM is inherently anisotropic, the material properties also vary in the different directions and need to be tested individually. Moreover, many AM methods produce parts with considerable residual stresses, which may need to be accounted for through, for instance, build process simulation. The produced geometry will also differ from the CAD-model, not only due to the tolerances of the machine, but more importantly due to the residual stresses and deformations. This effect will be more or less pronounced depending on the production method, part geometry, support strategy and heat treatment. Part warpage can be compensated for by build process simulation.

## *5.1.4 Boundary Conditions*

Applying boundary conditions to AM parts is, in principle, no different from parts intended for traditional manufacturing. The only new challenge is if the model is facet-based, and thus lacks information about underlying surfaces and volumes, as shown in Fig. 5.4. Depending on the type of analysis, it may be beneficial to convert

**Fig. 5.3** Left: Orthogonal voxel meshing. Right: Face-aligned voxel meshing (Courtesy of Axel Nordin)

the facet-based model into a parametric model using reverse-engineering tools such as SpaceClaim. Otherwise, the boundary conditions will have to be applied to individual elements after discretization. As this type of selection does not typically automatically update if the facet-based model is modified, applying boundary conditions for repeated analysis of similar designs can be time-consuming and is not easily automated.

## *5.1.5 Post-processing Results*

Results need, as always, to be analysed with care. In AM products, that are often complex with many small-scale structures such as lattices, this is particularly true. Stress concentrations are not always immediately obvious and mesh independence should, as always, be checked for. However, due to the limitations of facet-based models in terms of resolving small features, it may not always be possible in practice to simulate the model as precisely as needed. Therefore, predictions about maximum stresses and fatigue life are not always accurate and should be validated through physical testing if the design is intended for critical applications. However, for concept design evaluation and comparison, the results should yield usable numbers.

## **5.2 AM Build Process Simulation**

There are now several commercial vendors of build process simulation software. While they differ in several respects in terms of capabilities and material libraries, the basic principles and main features are similar. A build process simulation can take into account the layer-by-layer build process of a part including the machinespecific parameters, support material, heat treatment and the removal of support material (Fig. 5.5).

**Fig. 5.5** Build process simulation workflow (Courtesy of Axel Nordin)

## *5.2.1 Geometry Definition*

As geometry is commonly available in facetted formats, the simulation software supports both parametric and facet-based formats. To fully capture the effects of parameters such as build-plate heat conduction and the effects of surrounding loose powder, the part needs to be positioned in the same position and orientation as it will be built in. Apart from the product geometry, the geometry for the support material also needs to be defined or imported. Some software can automatically create support material based on the part geometry, but it is often more convenient to generate the support material geometry in a dedicated build preparation software.

## *5.2.2 Discretization*

As models are often facet-based, voxel-based meshing methods are common for build-process simulation. This is a robust and convenient way of discretizing complex non-parametric models, but the amount of detailed control over the discretization is limited. Separate discretization settings for support and part geometries is usually

**Fig. 5.6** Left: A voxel filled to 70% by the model geometry. Right: A voxel filled 15% by the model geometry (Courtesy of Axel Nordin)

possible to reduce the computation time. As voxel-based discretization often has the same voxel-size throughout the model, it may not be possible in practice to fully resolve small features such as lattice structures. Instead, a sort of homogenization of the small-scale features is possible in some software. Homogenization in this case means that the properties of each voxel is based on how much of its volume is filled by the actual geometry, as illustrated in Fig. 5.6. It is, however, important to make sure that the voxel-size is chosen with respect to wall-thicknesses and minimum feature sizes in the model.

## *5.2.3 Material Definition*

The material properties are typically provided by the software vendor.

## *5.2.4 Build-Process Parameters*

To fully represent the build-process, the intended AM machine needs to be described. This is typically done by inputting information such as energy densities, scan speeds, layer heights, scan strategies. However, unless a full thermo-mechanical simulation of the beam-path is done, other approaches for approximating the material behaviour during printing is needed. Typically, this is achieved by printing and measuring a set of calibration specimens, designed to capture information about the residual strains in the material in different directions, as shown in Fig. 5.7. This type of calibration data, however, is only valid for a particular set of build-process parameters, and thus needs to be done again if, for instance, the laser power or scan strategy is altered.

**Fig. 5.7** Top: Calibration specimens printed in different orientations. Bottom: Measurement of part warpage as support is removed (Courtesy of Axel Nordin)

Using calibrated data like this allows for several layers to be simulated in bulk, greatly reducing the computation time.

## *5.2.5 Post-processing*

Commonly available results include stresses, strains and deformations, but most software can also display plots of likelihood of part failure, likelihood of recoater impact, missing layers due to deformation of the part during print. Additionally, it is common to be able to export a deformation-compensated version of the original model, where the predicted deformations of the part due to the build process are used to compensate the geometry in the opposite direction. As there is no direct relationship between the deformations and the correct compensation, several iterations of simulation, compensation and re-simulation may be needed to reach a geometry that is sufficiently close to the original model to meet the dimensional specifications.

## *5.2.6 Limitations*

While build-process simulation is becoming more common and the accuracy and computational efficiency is increasing, it is still best used for a rough estimate of problems that may arise during printing and to rule out completely unfeasible buildorientations or designs.

## **5.3 Optimization**

As with any design, optimizing the part performance is crucial. Design optimization can be done in a number of ways, such as size optimization and topology optimization. Size optimization is often done on parametric models to fine-tune dimensions and is a relatively well-established field with many commercial software options. While size optimization is a very general method that can be applied to almost any optimization problem where constraints and objectives can be quantified, it quickly becomes unfeasible as more parameters are added. On the other hand, topology optimization can quickly indicate where material should be added or removed from a design to optimize its performance. However, topology optimization is limited in the types of constraints and objectives that can be handled and is thus not as general as size optimization.

## *5.3.1 Topology Optimization*

Methods for topology optimization (TO) started being developed around the same time as additive manufacturing. Due to the organic and often quite complex shapes produced through topology optimization, the usefulness of the approach for products intended for traditional manufacturing methods has been limited, but with AM the number of feasible applications has grown. The number of commercial software intended for TO has grown quickly over the last years, as has the number of applications. At its core, topology optimization is used to minimize or redistribute material within a design space according to certain constraints and objectives, such as minimizing the compliance or deflection of the part. While TO has mainly been used for structural applications where light weight is of importance, other types of physics are possible (e.g. thermal, fluid flow, eigenfrequencies). Topology optimization for AM is not limited to applications where the end product needs to be as light as possible, for instance within the aerospace industry, but is also beneficial for reducing the print cost and production time.

TO can largely be divided into 8 steps.

## *5.3.2 Define Design Space*

The design space is the volume within which the TO algorithm can redistribute material. In order to give the optimization algorithm, the best possible starting point, the design space should be as large as possible. An overly constrained design space will not allow the algorithm to find the optimal load paths, as illustrated in Fig. 5.8. A good starting point can be to look at the interfaces to other components that need to be respected. It is also beneficial to reduce the complexity of the design space as much as possible, that is, to remove unnecessary details like small rounds, threads, decorative elements or holes. Another factor in determining the design space is build size limitations of the intended machine. As with any optimization decision, iteration may be necessary to find the optimal design space if the results indicate that the optimal load carrying path exists outside of the design space.

## *5.3.3 Define Non-design Space*

As surfaces where the product interfaces with other parts, such as fasteners or bearings, should remain unaltered by the optimization, these should be marked as nondesign space. While it sometimes is possible to simply exclude them entirely from the design space, it is often convenient to be able to apply boundary conditions on the non-design spaces, as shown in Fig. 5.9.

## *5.3.4 Define Boundary Conditions*

As with any analysis, the boundary conditions need to be chosen with care. They should as closely as possible represent the real loading of the part. It is easy to, for instance, add a constraint that will take up forces in all directions, whereas in reality the support might only take up forces in compression. However, many topology optimization software are limited to conducting linear analyses, where, for instance, compression only supports may not be available. In those cases, special care needs to be taken to ensure that the supporting area is sufficient for the loads during validation.

As most structures will be subject to several different loads during operation, it is important to make sure that all major loads that may act on the part are represented. If one is missing, the optimization will typically recommend a design that is weak in that direction. A good strategy may be to add a new load case for each force acting on the structure.

## *5.3.5 Define Constraints and Objectives*

There are a number of possible constraints and objectives that may be set up for TO. The most commonly available objectives include minimizing the compliance (*i.e.*, maximizing the stiffness) and minimizing the mass. These objectives will obviously not generate useful design by themselves and need to be coupled to constraints. For instance, minimizing the compliance is often linked to a constraint on the percentage of the volume that should be retained. Minimizing the mass is often linked to a constraint on the stresses or deformation of the part. As knowing beforehand what the optimal design should weigh is impossible, it is recommended to experiment with several percentages. It is also worth mentioning that stresses calculated during TO are only rough estimates, and do not necessarily correspond to the results calculated during validation. The topology optimization can remove or change the mechanical properties of individual elements which makes it difficult to accurately predict the stresses. Moreover, the mesh density that is suitable for TO is not necessarily suitable for stress calculations. While research is being done into how to get more accurate stress values, it is recommended to add additional constraints to the optimization, such as deflection, to get more robust results.

Apart from structural constraints, many software vendors have added support for taking AM-specific manufacturing constraints into account. One can, for instance, reduce the need for support material by adding an overhang constraint, which limits the number of surfaces with an angle below the defined support angle and build direction. As the build direction may not initially be known, however, it may be beneficial to run optimizations in several build directions to evaluate the difference in build height, amount of support material, and part performance. It common for the overhang constraint to be contradictory to the structural objectives, and thus there is often a need to compromise between the amount of support material and the part performance, depending on whether cost or performance is more important.

## *5.3.6 Define Optimization Settings*

Most software will allow the user to define parameters such as mesh density and minimum feature size. The mesh size affects the level of detail that the resulting geometry will have, and of course the computation time. In theory, the finer the mesh, the more optimal the results (as for instance Michell structures shown in Fig. 5.10), however, computational limits and production constraints will typically mean that the mesh size is fairly coarse, compared to traditional FEA meshes. This also means that traditional TO is not suitable for generating structures with the typical lattice structures, and other approaches are needed for generating them.

**Fig. 5.10** A Michell structure (Courtesy of Arek Mazurek). https://commons. wikimedia.org/wiki/File:Mic hellCantilever.jpg

## *5.3.7 Solve*

Solution time will go up with the number of load cases and the mesh density. While reducing the number of load cases to reduce the computational time may be tempting, doing so will yield results that are not useful. Reducing the number of elements, on the other hand, will not typically affect the overall distribution of material, but will only have local effects. For initial optimization of concepts, it is therefore recommended to adjust the number of elements.

## *5.3.8 Interpret the Results*

The results from a topology optimization is most commonly in the form of a meshbased model, although there are a number of experimental methods that aim to make the process from optimization to manufacturing more streamlined. While it could be possible to directly print this part, doing so is often not preferred, as the mesh from the TO is quite coarse, may have many stress concentrations, and may not fit well with interfacing components. Due to this, there is a need to remodel the mesh-based result to make it suitable for manufacturing and use. There are three main approaches for this, as illustrated in Fig. 5.11:

#### *Mesh-Based Smoothing*

There are several tools for smoothing the facet-based result and many smoothing algorithms. While this may produce satisfactory results for one-off prints, the facetbased model will not have any link to the initial CAD model, and it will be difficult and time consuming to do parametric changes to it, such as increasing dimensions.

**Fig. 5.11** Approaches for post-processing the topology optimization result (Courtesy of Axel Nordin)

#### *NURBS Interpretation*

There are several tools for reverse engineering a facet-based model into a parametric model, as discussed earlier. Some require manual input, and some are more automated, but all will require some amount of manual manipulation to get good results. Although these tools generate parametric models, the complexity of them is often very high, with few planar or cylindrical/spherical surfaces. Moreover, the model is "dead" and there is no information about the features or design intent. This makes it difficult and time-consuming to continue working with the model in standard CADpackages, and the models are not easily parametrically controllable, if changes to the dimensions need to be done or if one wants to conduct a size optimization.

#### *Manual Re-design*

This method is obviously the most time-consuming, but also generates a model with high parametric controllability and design intent. If the model is to be further adapted for manufacturing or optimization, manual re-design is usually the most straight-forward option.

## *5.3.9 Validate*

A design validation always needs to be done on the reinterpreted model as the results reported during the TO are not usually very accurate.

## *5.3.10 Topologic Design with Altair Software*

#### 1. **Software Selection**

Once the initial geometry evaluation for PBF-EB/M technology is completed, it is time to optimize the geometry using the computer tools. The software selected is Altair Inspire and the main reasons for this selection are explained below:

Figure 5.12 exhibits the workflow of Altair Inspire: geometry setting, case loads assignment, analysis and optimization, results evaluation and CAD model translation.

**Fig. 5.12** Workflow of Altair inspire (https://www.altair.com/inspire/) (Courtesy of Altair)

#### 2. **Geometry**

First of all, it is important to learn the available abilities to create and modify the models. It is crucial to develop the initial geometry and widen the software´s optimization options. Generally, all the space available inside the maximum volume of the part should be included in the optimization process unless there are technical reasons not to, such as mounting space or interferences with other parts in its normal function (Fig. 5.13).

Simplifications are also required with these kinds of tools. In order to speed up the calculations, it is recommended that all the small fillets, rounds or chamfers that are not necessary in the structural behaviour should be deleted. This deletion should pay special attention to not include simplifications that can introduce significative errors in the results. Nonetheless, the final optimized shape should be validated afterwards through a new analysis.

**Fig. 5.13** Altair inspire geometry tools (https://www.altair.com/inspire/) (Courtesy of Altair)

**Fig. 5.14** Altair inspire case setup tools (https://www.altair.com/inspire/) (Courtesy of Altair)

Linked to the optimization process and the software layout, a distinction between design parts and non-design parts is required. Therefore, all parts that will have loads or constrains must be defined as non-design parts. To meet this requirement, the user should adapt the model using the geometry tools.

#### 3. **Load Cases Setup**

After completing the geometry modifications, the load cases that will feed the software must be set in order to obtain the optimization. The typical tools to set loads such as forces, pressures or torques and constrains in any freedom degree can be easily found and applied in this software (Fig. 5.14), as in many other CAE software. Furthermore, in most cases, it is also necessary to set fasteners, joints or connectors to bond the parts.

Other less usual conditions that the selected software (Altair Inspire) allows us to add are imposed displacements, accelerations, temperature conditions and concentrated masses.

Contact definition between parts can also be found in this section and it offers us the possibility to define the faces in contact as bonded, contacting or without contact.

In this section, the material properties to the components also need be set. The more accurate these properties are, the more trustworthy the results will be. However, it is also important to know that this software does not allow anisotropic materials setting and this is a significant deficiency because additive technologies normally require this material behaviour. The way to compensate this deficiency and assure safety is to consider the weaker properties. That is, the structural problem is solved as an isotropic material. Again, it should be mentioned that this optimization is a first stage and the final design will need to be validated.

#### 4. **Analysis/Optimization**

On one hand, an option to perform structural analysis is included. Before the analysis can be launched, the user has to set the analysis parameters. The software chosen includes only a few parameters that can be modified: mesh size, contact's type and accuracy. This is due to the simplicity and navigability of the software concept.

The analysis can be completed before and after the optimization. It is good to analyse it prior to the optimization to know the stress level or the model behaviour, in order to help the user, set the parameters in the optimization set up. The analysis is performed after the optimization to check the proposed shape in the different load cases. However, this second analysis can display high levels of stress concentrated in specific points because the raw model shape has imperfections that will be removed in the final model.

On the other hand, the optimization analysis offers different approaches depending on the objective. It can maximize the stiffness or minimize the mass. For the stiffness maximization, the program needs the mass target. For the mass minimization, the safety factor is required.

All of these analyses require the parameters of mesh size (defined as thickness constraints), level of accuracy (faster/more accurate) and contact's assumption (sliding only/with separation). In case of error, the user can modify the size mesh. Reducing it can help avoid meshing problems due to geometry imperfections. An alternative option is suggested: to check and correct the geometry imperfections, since this reduces the work time increase that would result if a smaller mesh size were used.

Finally, the user can select what load cases are used in the optimization. It is not possible to assign different weights to each load case directly. Different weight assignments need to be made through new load cases modifying the load.

#### 5. **Results/Comparison**

The results should be evaluated visually first. Some cases can give us incomplete shapes with reinforcing linking elements that are not fully developed. If it happens, there are two options: (1) the user can modify the result directly, increasing the mass with a slider tool included in the software, or (2) the optimization can be recalculated, increasing the level of mass or the safety factor.

The junction between design and non-design spaces is usually a week point and also needs to be evaluated to decide if it would meet the structural requirements. These issues can be pointed out as difficulties, even if they are accepted, they should be checked in the next phases (Fig. 5.15).

The next step is to establish the goals and to select the results that allow the evaluation and comparison in the different optimized models. The most used results are the stress level, the safety factor and the displacement. The software has utilities

**Fig. 5.16** Comparison results (https://community.altair.com/community) (Courtesy of Altair)

to assist us in the comparison process, by displaying the selected outcomes in every optimization case (Fig. 5.16).

The user has to be aware that models include the imperfections generated in the optimization process and that the surface roughness is not considered. These two points can lead to an overestimation of the model in fatigue cases. This is a common issue in PBF-EB/M technologies because they have considerable surface roughness. A validation check is strongly suggested to be carried out considering this factor after the optimization.

#### 6. **Conversion From Optimized Result to Buildable Geometry Model**

The conversion of the selected optimized model to a buildable geometry model can be done in the proposed software by three means:

#### 7. Key Observations

The use of the proposed software offers a good approximation of an optimized structure in a reasonable time. However, depending on the use, the final design may need to pass an additional structural analysis.

The user should take into account that AM may leave a considerable surface roughness and that fatigue is not considered in this optimization process, for the applicable cases.

There are multiple options to achieve an optimization and it can be difficult to manage all the results and evaluate them. It is recommended to define the selected measurable factors first.

The tools to adapt the optimized shape to a CAD model are being improved. Nonetheless, the adaptations might still be complex when the user tries to translate the model between the different software.

Lattice structures can help lighten the SLS study models, but they may increase the difficulties in their use on the CAD software and the 3D printer machine.

## *5.3.11 Topologic Design with Altair Software for PBF-EB/M*

As PBF-EB/M is a technology that needs to build supports, the software has a specific tool to set a limit and optimize the geometry adapted to this manufacturing process. This tool is applied if the model exceeds the slope degrees limit from the fusion bed. This tool is called Overhang shape control in Altair Inspire software. It allows users to set a maximum degrees/angle from the baseline prior to the analysis, however, this requires a decision on the building direction and this decision will condition the result (Fig. 5.18).

This tool is very useful for PBF-EB/M because the use of supports can be avoided. The main limitations include: that the user has to define the building direction; once set, the tool will not consider the use of supports and this can entail a heavier result. If one of the main goals is mass reduction, this option will probably not be

the appropriate approach. For mass reduction the material should be used to build supports instead of reinforcing the model.

A suggested workflow to optimize the use of this tool would be to try each piece with and without the tool and subsequently, check if adding supports uses a lesser amount of material.

## • **Example 1. Topological optimization of a demo bracket for Ti6Al4V processed by PBF-EB/M**

This is an example of topological optimization for a structural AM part suitable for the aeronautical sector. Figure 5.19 shows the original bracket, the optimization result and the polynurbs geometry.

Different options of topological optimization are represented in Fig. 5.20.

**Fig. 5.19** Topological optimization. Bracket for PBF-EB/M technology (*Source* Skin Project— AIDIMME)

**Fig. 5.18** Altair inspire overhang tool (https://www. altair.com/inspire/) (Courtesy of Altair)

**Fig. 5.20** Different options of topological optimization. Ti6Al4V bracket processed by PBF-EB/M (*Source* Skin project—AIDIMME)

## *5.3.12 Topologic Design with Altair Software for PBF-LB/P (SLS)*

Since PBF-LB/P technologies do not need supports in the building process they offer flexibility in regard to the model shape. The only shape restriction will be to consider the spaces required to ensure the dust removal. The available utility to create lattice structures can be interesting in this type of manufacturing technologies due to its capacity to lighten the model even more.

The process of optimization with lattice geometries follows the same premises aforementioned, but adds an additional optimization using the model generated by the first result. Apart from the objective and contact parameters, the user has to set the percentage of lattice structure, the target length of the bar elements and the minimum and maximum values of the bar elements' diameter. The software will provide a result substituting the base material by bar elements following the lattice structure percentage given (Fig. 5.21).

On the contrary, the use of lattice structures with too many bars may be a problem for the CAD software or the file management by the 3D printer machine.

As a reminder of the material properties, the user has to be aware that the results do not take in account the surface roughness in case of fatigue loads. Therefore, the model needs to be validated properly.

#### • **Example 1. Topological optimization of a demo bracket for PA12 processed by PBM-LB/P (SLS)**

Same example as PBF-EB/M case. Figure 5.22 shows the original bracket volume together with three optimization results where a volume reduction up to 87.62 % can be found.

Optimized AM parts processed in polyamide by PBF-LB/P technology can be observed in Fig. 5.23. After production, mechanical testing are performed to evaluate the mechanical behaviour.

**Fig. 5.21** Altair inspire lattice optimization (https://community.altair.com/community) (Courtesy of Altair)

**Fig. 5.22** Topological optimization of a bracket for PBF-LB/P technology—volume reduction. (*Source* Skin project—AIDIMME)

## **5.4 Lattice-Based Topology Optimization**

As traditional TO is typically not suitable for generating very fine structures with high complexity, hybrid methods have been developed. There are different approaches,

**Fig. 5.23** SLS optimized parts in polyamide and mechanical testing (*Source* Skin project— AIDIMME)

some create a free lattice structure from the underlying geometry, and some rely on a cell-based approach. The cell-based approaches first determine an optimal pseudodensity field within the design space, and based on this density field, a lattice geometry can be generated. The general workflow is to define the lattice type, define the cell size, define the wall/strut thickness, define the minimum/maximum density, solve, interpret the results and validate.

## *5.4.1 Lattice Type*

The lattice type can range from simple lattices like cubic trusses to more complex shapes like gyroidal or other triply periodic minimal surfaces. The lattice shape should be chosen with respect to the type of loading the part will be subject to, and with respect to printability and build orientation. One should be especially careful to avoid lattice types which require support material in larger cell sizes.

## *5.4.2 Define the Cell Size*

The cell size will determine the dimensions of each lattice cell. While small cell sizes may be useful for creating self-supporting structures, larger cell sizes will be computationally more efficient to generate and validate. Moreover, large cell sizes may be beneficial for getting loose powder out after printing.

## *5.4.3 Define the Shell Thickness*

As many applications require a smooth outer surface, it is usually possible to define a shell thickness of the geometry where the lattice structure is excluded. The choice of thickness depends on the application, but in order for the lattice structure to have any effect, it should be chosen as small as possible.

## *5.4.4 Define the Minimum/Maximum Density*

The density will affect how thick the walls/struts in each lattice cell are. As a very low density may result in non-buildable wall thicknesses, it is important to set a minimum density requirement. Exactly what the lower limit should be depends on the machine and on the software, so some experimentation may be required. Additionally, a very high density will lead to lattice cells with very small openings, which, in turn, make powder removal difficult. Therefore, a maximum density should also be set. A good starting point may be 20% minimum density and 80% maximum density.

## *5.4.5 Interpret the Results*

The results from the optimization is either a geometry or a density field, but there will typically be built-in tools for converting the density field into a facet-based model. As the lattice-structure itself should not need any further manipulation, it does not need to be converted into a parametric format. Depending on the software and output lattice structure, some sort of mesh smoothening may be beneficial to reduce stress concentrations.

## *5.4.6 Validate*

Validation of lattice structures is difficult as the complexity is very high, the models are typically facet-based, and depending on the lattice shape, there could be many stress concentrations (Fig. 5.24). Moreover, as lattice structures do not lend themselves to easy post-processing, the surface finish can be rough and be detrimental to fatigue life. Therefore, lattice structures in critical applications, especially where cyclic loading will occur, is not recommended without physical validation.

**Fig. 5.24** Stress concentrations in lattice-based design (https://community.altair.com/community). (Courtesy of Axel Nordin)

## **5.5 Non-parametric Mesh Modelling**

Non-parametric modelling normally starts from the 3D data obtained from a scanned object. A laser scans an object in three dimensions. The result is points is space, or rather a "point cloud". This point cloud is triangulated into a mesh before the nonparametric modelling starts. Common formats are the obj (object) and the stl (stereo lithography)-formats. The mesh may later be cleaned, repaired and altered according to the operators' demands. The scanners that are the easiest and fastest to operate are normally the scanners that are intended for the consumer market. Some of these are handheld and can for example be attached to an iPad, see Fig. 5.25. Football sized objects can easily be scanned by turning the object by hand or by simply waking around the object. The disadvantage with these scanners is the low scanning precision. The precision is, in practice, at best +− 2 mm. The top end of the handheld scanners operate in a similar fashion, but with higher precision, they are however more complex to operate and the entry level price is about 7000 e.

The number of different types of scanners on the market is immense, they may scan everything from wear and tear on small mechanical parts to the topography of entire landscapes. Another type of scanners are the ones that are not handheld. What they all have in common is that the scanning unit is fixed or mounted on an axis controlled by for example a step engine. These scanners can offer precision up to about + − 0.01 mm depending on the circumstances. Some of these scanners have the appearance of a box with a door and with a rotating plate to place the object to be scanned at the bottom. Naturally they come in different sizes. One drawback is that they have to have rather large dimensions if one, for example, intend to scan a human head or a human limb for later prosthesis manufacture.

The non-parametric manipulation of a mesh let the user change the density of the mesh according to his or her demands. Ideally, the mesh is made as light and with as few vertices and triangles as possible without jeopardizing the description

**Fig. 5.25** An iSense scanner attached to an iPad (Courtesy of Per Kristav)

relevant for the surface at hand. The minimal number of triangles to describe a perfectly straight rectangular surface is two. Double bent surfaces require a far greater number of triangles, and hence a more dense and complex mesh to be described with sufficient accuracy. Non-parametric mesh modelling will however let a user to add and reduce the complexity of the mesh locally according the requirements on the surface accuracy.

Normally when an object has been scanned or a CAD file has been saved in order to be able to print the object one loses some of the information. The scanned object is no longer identical to the fully defined measurements of the product, and its original CAD/Step file. This causes problems if one wants to continue to manipulate a scanned object or want to return a 3D print file to parametric software. A solution to this shortcoming is to deploy non-parametric modelling. Exact measurements may, in some circumstances, be of marginal importance. The purpose may for example be to print a human bone, or an object with a purely aesthetical purpose. In these cases, only the scale, level of detailing and quality of the surfaces are important. Another example is if one wants to print or make a mould from for example a scanned branch from a three, or a seashell.

When one has turned such objects to digital data for further form manipulation one has, as mentioned, converted the original surface into a mesh. This mesh can be kept as a mesh surface or be converted into a solid mesh body. There are however several mesh formats. What they all have in common is that they triangulate the surface of a volume into a triangular mesh. First, when this has been done, one may let 3D printer software slice this mesh into slices. This process is however, an obligatory and last step also for parametric CAD models to be able to print in 3D.

Non-parametric CAD-programs such as, for example, MeshMixer and MeshLab let the user manipulate a mesh as in Fig. 5.26 rather than like parametric CAD software as, for example, Creo and SolidWorks that normally produce the mesh only as a last conversion strep to facilitate 3D printing. One may nowadays however, import a mesh in some traditional parametric CAD software, but it is still a delicate, restricted process that require a mesh of limited complexity.

The actual manipulation of the mesh can in some cases be described as working in real clay, but on a 2-dimentinal computer screen. The main tools in Meshmixer and Meshlab could be divided into three main types. One type is filters. Meshlab have a great selection of different mesh filters. A typical function of a filter is to repair a mesh with undesired holes, reduce mesh complexity with a mathematical formula, or make a "ruff" mesh more complex or smoother. Another type of tools are different sculpt and brush tools that let the operator manipulate the mesh locally. A typical function of such a tool let the operator to drag, inflate, smoothing, flatten or move a selected part of the mesh as in Fig. 5.27. The last type are tools that let the operator offset, duplicate, transform, shell, solidify and merge/subtract different mesh bodies from each other. There are numerous software on the market for the manipulation of meshes. Some focus on repairing and "cleaning up" damaged meshes, some focus on optimizing mesh complexity whilst others let the operator create new interesting forms with for example "un-proportional scale" and "sculpt" tools.

The drawback with the mesh format is that it will always present a soft double bent surface as a number of straight triangles, and the claylike manipulation will not result in fully defined surfaces as in parametric software. The strength is that a user may create amazing organic forms that would be next into impossible or very time consuming to create with parametric modelling by deploying extrusion of two-dimensional sketches, this is exampified in Fig. 5.28.

**Fig. 5.27** The brush tools for non-parametric modelling in MeshMixer (Courtesy of Per Kristav)

5 General Process Simulations 147

## **References**

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# **Chapter 6 Applications of AM**

**Damir Godec, Tomislav Breški, Miodrag Kataleni´c, Axel Nordin, Olaf Diegel, Per Kristav, Damien Motte, and Jože Tavˇcar**

## **6.1 AM in Tool Making Application**

Application of AM in toolmaking can be historically considered as a second wave of AM application (so-called Rapid Tooling). Although AM processes are mainly considered as a process for final production, their potential can be used for making tools for conventional production processes (e.g. injection moulding of polymers). Processes of design and production of complex tools such as moulds for injection moulding of polymers are typically the bottlenecks in development and production process of final products and strongly influence their time-to-market.

One of the trends at the market is shifting from mass to custom production which requires more flexibility in tool production processes. Therefore, the main objectives of AM in toolmaking application is to reduce lead-time required to make the tools, as well as to improve the efficiency of the tools (shortening the production cycle time and improvement of parts produced in the tools).

At the end of 20th and in the beginning of twenty-first century there was a lot of excitement about AM application for toolmaking, but requirements for the tools for serial or mass production (e.g. for injection moulding) and AM possibilities are not in accordance. AM in toolmaking applications is exposed to very strict demands because of common processing parameters for such tools (e.g. pressures, temperatures, impact loads, abrasion, etc.) which must be wear resistant, with very narrow dimensional tolerances and with high surface quality.

Injection mould inserts have to fulfil combination of three main requirements:

• strength ≥ 500 N/mm2

A. Nordin · O. Diegel · P. Kristav · D. Motte · J. Tavˇcar Faculty of Engineering (LTH), Lund University, Lund, Sweden

D. Godec (B) · T. Breški · M. Kataleni´c

Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb (UNIZAG FSB), Zagreb, Croatia e-mail<EMAIL_ADDRESS>
D. Godec et al. (eds.), *A Guide to Additive Manufacturing*, Springer Tracts in Additive Manufacturing, https://doi.org/10.1007/978-3-031-05863-9\_6

At the moment, the mentioned requirements (especially the last two ones) have not been achieved on the market of the AM equipment and materials. However, all the companies in the field are continuously trying to meet these requirements. This refers especially to the speed of the manufacturing, precision of dimensions and forms, and the number of materials that can be processed by AM technologies. Also, the properties of usable materials are being continuously improved, contributing to the improved properties of final products. In this way, the differences between traditional procedures of mould production and alternative AM technologies are expected to be reduced in the future.

AM application for toolmaking can be realised in three ways:

In case of short-run tools, only a few moulded parts can be produced within such tools before moulds are worn or damaged. Example of this type of tools is silicone mould (Fig. 6.1). The main advantage of this tooling approach is that such tools can be produced within one day, the process is very simple and the costs are relatively small.

A small-batch production of a hundred to a few thousand parts can be run with so called bridge moulds (Fig. 6.2). Final number of produced moulded parts depends on applied materials (paper, metal, polymers, etc.). Bridge moulds can also be produced in relatively short time (from one day to few days), which also depends on applied materials and additive technologies. Bridge tools for injection moulding are not intended to be replacement for soft or hard tools used in mid- and high volume production. Instead, they are intended to fill the gap between soft and hard moulds as well as a substitute for 3D printed prototypes.

**Fig. 6.1** An example of AM silicone mould: left—original 3D printed part, top—4-piece silicone mould, bottom—3 mouldings in different materials (www.scott-Am. com, Courtesy of Ronald Simmonds, photographer Giulio Coscia)

**Fig. 6.2** An example of AM PolyJet (bridge) mould (Courtesy of Stratasys)

Finally, with *hard moulds* (Fig. 6.3) it is possible to produce up to a few hundred thousand moulded parts. Their durability is similar to the conventional moulds. They require much more time and expenses for production, compared to the soft and bridge moulds.

Figure 6.4 shows relations of moulds produced with *PolyJet* technology and moulds produced with classic technologies from common mould materials and AM technologies for production of metal bridge/hard moulds. It has to be stressed that A and B processed materials (e.g. unreinforced polyolefines) are far less aggressive, and C and D groups are more aggressive materials regarding mould cavity wall wearing (e.g. reinforced polymers).

**Fig. 6.3** An examle of AM hard mould (Courtesy of SIRRIS)

## *6.1.1 AM Silicone Short-Run Moulds*

Silicone short-run moulds can be considered as AM moulds when master models (patterns) are produced with one of the AM technologies. Generally, the process consists of three main steps: master model making (AM), master model preparation for casting (definition of mould parting plane, definition of mould runner system, etc.) and silicone casting around master model for production one- or two-piece moulds. After the silicone casting and curing, silicone block is removed from the frame, master model is removed from the mould cavity and the mould is prepared for casting materials such are ABS, polyurethanes or polyamides into the mould cavity (Fig. 6.5) (Fig. 6.6).

In case of two-piece silicone mould (Fig. 6.7), two-step process is applied. In first step, lower half of the box is filled with modelling clay and silicone is poured into the upper half of the box creating the first half of the mould. After silicone curing, the box is rotated 180°, the clay is removed from the box, parting plain is coated with separation agent and the process is repeated to create the second half of the mould.

In case of the production of more complex mould geometry, within a silicone mould, different metallic or polymeric inserts can be used. Those inserts can be a good replacement for thinner silicone mould parts, and they can prolong mould durability.

#### 6 Applications of AM 153

**Fig. 6.5** Production process of one-piece silicone mould (www.scott-Am.com, Courtesy of Ronald Simmonds, photographer Giulio Coscia)

**Fig. 6.7** Production process of two-piece silicone mould: **a** setting the master model, runner systems and modelling clay system, **b** ouring the silicon into the box (core production), **c** box rotation, pouring silicone into the box (cavity production)—finish (Courtesy of Andreas Gebhardt)

## *6.1.2 AM PolyJet Bridge Moulds*

*PolyJet* and *PolyJet Matrix* processes enable application of more than hundred different materials based on acrylic resins, which can mimic the properties range of the materials from elastic to rigid. For manufacturing bridge moulds, mould materials have to be strong enough (tensile, flexural, compressive and bending strength), tough, and resistant to high temperatures in order to maintain mould cavity dimensions. Four materials can be selected as the most appropriate for the *PolyJet* bridge mould manufacturing: *RGD 525* (white high-temperature material), *RGD 5160-DM*, *RGD 5161-DM* and *Digital ABS plus* (ABS-like green materials). Table 6.1 shows some basic mechanical and thermal properties of those materials.

**Fig. 6.8** Process of ABS-like mould insert hardening (Courtesy of Stratasys)

**Table 6.1** Properties of PolyJet materials suitable for mould production

Material RGD 525 is the strongest material, but its toughness is 5–7 times lower compared to the other materials. RGD-5160-DM material is suitable for production of details with wall thickness down to 1.5 mm, while RGD-5161-DM for wall thickness down to 1.0 mm. If high impact resistance and shock absorption are requested, Digital ABS plus is the most appropriate. By hardening process in furnace (), heat deflection temperature can be increased for all materials up to 30%. Increasing the heat deflection temperature is very important for mould inserts for injection moulding, where mould material is heated in cycles as hot polymer melt fills the mould cavity (Fig. 6.8).

## **6.2 Design Rules for Bridge PolyJet Moulds**

In design of bridge *PolyJet* moulds it is possible to apply basic guidelines for design of classic moulds for injection moulding. Nevertheless, because of specific properties *of PolyJet* materials for production of bridge moulds, it is necessary to make some modifications in design concept. This is necessary because of compensation of mechanical, thermal and dimensional characteristics of such plastic moulds. Some of the basic rules for design of *PolyJet* bridge moulds are:

**Fig. 6.9** Increased draft angles (Courtesy of Stratasys)

**Fig. 6.10** Extension of the back face of the core/cavity (Courtesy of Stratasys)