Manufacturers in almost every industry are looking to design lighter parts, whether for quicker cars, cheaper air travel, or providing that extra edge on the competition in a sporting event. But it's often difficult to know where to begin, with the endless combinations of materials, processes, and design tools available today. Whether you are interested in putting existing parts on a diet, or starting from a blank sheet, the following five approaches to light-weighting may help identify strategies that can work for you.

First, let's clarify our objective in light-weighting: the word optimize is sometimes used loosely and can lead us astray. It's better to think of these approaches as steps towards an optimum and recognize that achieving total optimality is a climb up an asymptotic mountain. How far you choose to climb depends on how much a pound costs to your design. Extra baggage on the way to orbit costs a lot more than on your average road trip.

1 Materials Selection

To get started, you need to select the right material. Why use steel if you can get away with aluminum? There's a weight reduction of approximately two thirds on the table, albeit with compromises: reduced stiffness, higher cost, and more difficulty in welding. A great way of visualizing these tradeoffs and rankings of mechanical properties is with Ashby Charts, which essentially represent the menu of materials that an engineer can select from (more on that later).

Figure 1. Ashby Chart for evaluating stiffness-to-weight ratio of conventional materials. This essentially represents the menu of (conventional) materials that engineers have to choose from. (Chart created using CES EduPack 2018, Granta Design Ltd.)

These charts can compare any combination of properties; the one shown in Figure 1 being useful for evaluating stiffness-to-weight ratio. Note the logarithmic scale. Serious lightweighting can occur just through proper material selection. Our menu of (conventional) materials varies over factors of millions in modulus, and thousands in density. Surely, some opportunity for light-weighting lies somewhere in between.

2 Part or Function Consolidation

Material on its own is just feedstock. Only with shape comes a structure or a part. But even using the best material can result in an overweight part if the form doesn't fit the function. Many assemblies are designed as such due to manufacturing rules, fastened together with nuts and bolts. These alone can add up significantly in weight, not to mention assembly time.

Figure 2. A pipe assembly lightweighted through part consolidation and conformal ribbing. (Image Credit: nTopology)

Approaching such problems from a function-first mindset, without the shackles of traditional manufacturing rules, can trim a lot of the excess that is not directly contributing to the overall function. The bulky flanges and bolts in the pipe system in Figure 2, for example, don't help us at all with controlling fluid flow (i.e. why we're even building a part). Why try optimizing something that shouldn't even exist in the first place?

3 Conformal Ribbing

Figure 3. Conformal ribbing on a turbopump casing, produced with electron beam melting (EBM) by Zenith Tecnica. (Image Credit: nTopology)

Conformal ribbing is one of the more popular methods on this list and is very prevalent in aerospace. Whether built from polymers, metals, or composites, conformal ribbing aims to enable thinner walls while improving buckling resistance (otherwise you're left with a crumpled soda can).

For polymers, ribbing can be very affordable and easily mass-produced. Look underneath a plastic shipping palette or fold-out table, for example, or even the side of a milk crate. You will find conformal ribbing (Figure 3), which engineers use to lightweight (and reduce material usage) in these relatively simple structures.

Figure 4. Machined metal isogrid on a spacecraft capsule. The high cost to get vehicles to orbit motivates advanced lightweighting techniques. (Image Credit: NASA)

But what can we do with stiffer and stronger materials? Isogrid, shown in Figure 4, has long been popular in the space industry, with design handbooks dating back to the 1970s. This is very expensive to produce via conventional machining — the cost-per-pound for machined metal isogrid is likely only justifiable for high-end aerospace applications — but additive manufacturing is now making this technique much more accessible.

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