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EC-515 Die Casting Defects

Dr. Steve Midson


You cant correct and control defects without first measuring and reporting them. The scrap reporting system must be set up for those who have to make improvements, not just for the front office. The scrap report should be available to everyone in the plant. In fact, it should be posted on the bulletin board so everyone can see it.


The daily scrap report must have the following features as a minimum: 1. It must be available first thing in the morning for the previous day 2. It must categorize scrap (as a minimum): By defect type, By part number, By die, By shift, By operator, By machine.


The scrap reporting system should show long term trends and be able to predict the customer rejects based on the current scrap activity Pareto charts are good ways to show the problems The report should include defects that are not detected until the parts are downstream - and a system developed so these defects can be tracked to the shift and machine that produced them All shots should be reported, even warm-up and scrap that is returned to the furnace at the machine (they cost in die life)


The process is complex, and a continuous reporting system must be set up to provide real time feedback and effective process control if defects are to be controlled. The two major defects in die casting are surface quality and porosity. Both of these require judgment decisions about severity. This means a method of measuring the severity of defects is a requirement and must be devised for many situations.


A rating system is intended to tell you if the defect problems are getting better or worse, or whether changes made in the process are making a difference. What you are looking for is the ability to track any changes or trends, and to know when corrections are needed. This system also allows corrections to be made before the defect level becomes a crisis. The standards used for the rating system may not coincide with the customer standards or the quality dept. Ratings; they are for a different purpose and do not need to coincide.


For example, you may rank a porosity defect from the worst to the best with rankings from 1 to 5. A capability study could be done as follows: Take 6 sets of samples of 5 sequential castings at intervals of 1/2 hr to 2 hr. Rate each casting and average the total. This gives the average quality level; this should be checked against similar studies to determine if the process is improving or degenerating


One of the most difficult problems in developing a rating system is finding a method of reporting and rating porosity The most typical methods are x-ray, machining, or sawing. A cheap and effective method is to use an old lathe to approximately duplicate the customers machining. Always select examples for the rating system and save them. They must not be used for any other purpose.


Thus defect corrections must start with a good scrap reporting system Developing this system may start with defining the names of defects, which means a document or board with samples and names of defects Bottom line: YOU CANT IMPROVE IT IF YOU CANT MEASURE IT!

Die Casting Defects

1. Surface defects 2. Laminations 3. Gas porosity 4. Blisters 5. Shrink porosity 6. Sinks 7. Leakers 8. Cracks 9. Inclusions 10.Solder 11.Carbon 12.Erosion/cavitation 13.Outgassing 14.Bending/warping 15.Flash 16.Stained castings 17.Waves/lakes 18.Drags 19.Ejector pin defects 20.Cold flake 21.Excessive flux

Surface Defects

Causes Of Surface Defects

This part of the class is about those types of defects that appear on the surface of a die casting These defects are called by one of the following names: Cold Flow Cold Shut Flow Lines Cold Chill Non-fill No-fill Poor-fill Laps Lines Run Marks Misruns Others:

Pictures - Surface Defects

Factors Controlling Surface Defects

A general list of the factors that control this kind of defect is shown below. These are controlled by different people, maybe even different companies
The The The The The The The wall thickness casting shape fill time flow pattern (gate design) die temperature metal temperature type of alloy

Who Controls Surface Defects?

The first 2 factors on the list are controlled by the part design The wall thickness is the most critical,
Controls many casting parameters
Required fill time Required die temperature Distance the metal will flow Length of defect free surface that can be made

Wall thickness and casting shape Very important Regarding surface defect problems

Good surface quality requires consistent wall thickness,

The designer and the die caster should focus on reducing heavy sections Try to make the wall thickness constant

Wall Thickness And Surface Defects

The operating parameters for thin walls are very different than for thick walls A thin wall causes the flow to freeze and develop cold flow quickly, For aluminum and magnesium, a typical minimum wall thickness will be about 1.5mm
For a flow distance of about 150 mm or more

For zinc, the minimum wall thickness would be about 1.0 mm

Wall Thickness And Surface Defects

For example, see the change in fill time required by change in wall thickness:
Change in Wall Thickness 2 mm to 1.5 mm 3.5 to 3.0 mm Fill Time Change Required 25% reduction in fill time 12% reduction in fill tim2

Thus a small variation in plunger speed (fill time) on a thin wall (say 2.0 mm ) casting is very noticeable for surface defects, while there would not be much noticeable for the same fill time variation in a thicker wall (i.e. 5.0 mm) aluminum casting

Wall Thickness And Surface Defects

The die temperature will also become much more critical and, in addition to becoming much more sensitive, the reduced mass of the part will not provide enough heat for the die The basic requirement for making a thin wall casting is a very fast fill time in a hot die with a high gate velocity

Wall Thickness And Surface Defects

Summary of wall thickness issues for thin wall parts:
Talk to designer early Get wall thickness the same (consistent) Keep wall thickness variation down with narrow range to toolmaker Expect much a smaller process window, and set up much tighter process controls Use low or very low fill times Use direct feed from gate Use high gate velocities (but within normal range) Use high die temperatures

Fill Time And Surface Defects

Fill time has a major impact on surface quality The fill time is defined as the time beginning when the metal first arrives at the gate and ending when the cavity is full of metal (including the overflows and vacuum runners)


Fill Time And Surface Defects

The fill time can be calculated from the formula given in the NADCA gating course, which is:

ti tf ( S * Z ) Max fill time = K * T f td t Where:

K = A constant T = Average casting wall thickness ti = Metal injection temperature tf = Metal flow temp (solidus) td = Die temperature S = Percent solids at cavity full Z = Conversion for latent heat

Fill Time And Surface Defects

For some rough guidelines, the following are maximum fill times based on calculations and experience and will be reasonable for most castings: Thin wall Average wall <2 mm >2 mm Al, approx. 2 kg .09 sec .1 sec Zn, approx. 1.4 kg .03 sec .05 sec Mg, approx. 1 kg .02 sec .03 sec Note: for high quality surface finish, reduce the fill times shown by about 50%

Fill Time And Surface Defects

Predicting the fill time is best done with the PQ2 calculation The PQ2 calculation predicts the change in fill time and gate velocity from changing any of the following: The gate area The plunger size The machine hydraulic pressure The plunger speed setting

Fill Time And Surface Defects

The PQ2 calculation provides the only way that the fill time can be predicted accurately
Without it you must guess, and this is expensive

Fill Time And Surface Defects

This list shows some of the things that can affect the plunger speed, which in turn changes the fill time and the casting surface finish. Some of these are:
Dragging tip Plunger lubrication Poor sleeve condition Poor plunger condition Poor cooling water flow to the plunger Sleeve deflection Gooseneck and plunger ring conditions Hydraulic pressure Low (or high) nitrogen charge Gate size

Fill Time And Surface Defects

Summary of fill time adjustments:
Set fill time maximum values with calculations and experience, then use a disciplined process Use PQ2 to predict adjustments to get the right values and eliminate costly trial and error Measure and control process variables with a monitor system Maintain control of sleeve and gooseneck conditions to keep the fill time within limits

Flow Pattern And Surface Defects

The metal flow pattern is the criteria in gate design The gate design is a function of design rules as taught in the NADCA gating classes
Flow the short way across the casting Avoid mixing flows if possible

Flow pattern can be simulated with computer programs and reviewed with short shots


Flow Patterns And Surface Defects

These rules include:
Use PQ2 to size the gate and the plunger, using the appropriate gate velocity, fill time, and cavity pressure criteria Divide the casting into zones Proportion gates so as to fill each zone at the same time Flow the short way across the casting Avoid mixing flows if possible (unless close to gate) Gate into heavy section porosity and/or important surface finish areas

Flow Patterns And Surface Defects

Flow pattern rules (continued):
Avoid jet flows at all times and distribute flow as much as possible If possible, set gate location so venting can be used opposite the gate Avoid flow directly on cores if possible (but do it if needed for best flow pattern Keep runner lengths equal (avoid tree type runners) Eddies in the flow path (from cores or openings) will cause swirls, gate to avoid this Gate to allow for high momentum (cores can be bypassed)

Die Temperature And Surface Defects

The die temperatures effect on surface defects and the die temperature operating window will be discussed next A low die temperature affects surface defects by cooling the fluid metal stream and increasing the percent of solidified metal in the metal stream If the percent of solidified metal is high, then it becomes stiff and solid and does not knit together well; And the flow forms "wrinkles", or cold flow A cold die can be compensated for by having a shorter fill time - this means a higher plunger speed In other words, we can exchange plunger speed for die temperature.

Die Temperature And Surface Defects

Measuring die temperature should be done on every job - most dont do it enough, but it is required to really minimize surface defects In general, measuring can be done three ways:
Hand held probe Thermocouple in the die Infra red device

Each has advantages and disadvantages:

Hand held probe: accurate, but must stop the machine Thermocouple in die: continuous, but does not measure surface temperature Infra red: easy, but not as accurate

Die Temperature And Surface Defects

Temperature ranges for good surface finish: (Measured with a hand held surface probe just after the casting ejects) Metal Al Zn Mg Good Finish 250 315oC 230 - 290oC 220 - 290oC Average Finish 190 - 315oC 190 - 290oC 200 - 290oC

Die Temperature And Surface Defects

The operator can control die temperature with the following controls: Die spray Water/oil flow rates Cycle time The engineer can change the design and affect how much difference some of these make, but the operator often controls the actual use of all of them. Thus the operating temperature of the die is often controlled directly by the operator and this die temperature control is probably the most important activity of the operator on the floor. We will review these in sequence

Die Temperature and Surface Defects

Die spray is lubricant mixed with water It is mostly water The water controls the temperature, not the die spray itself The spray cools the die quickly because of the large amount of heat quickly pulled out of the die as the water in the spray boils away


Die Temperature And Surface Defects

Some techniques include: spray in 2-3 second increments with careful adjustment of the spray pattern - spray the areas that need cooling, not just where the casting might stick - automatic sprayers with a set manifold for each job are a good way to keep spray to a minimum Keep spray equipment in good shape, the most important factor is consistency Document pressures, nozzles sizes, flow adjustments, and spray times in detail undocumented changes should not be allowed (changes should not be stopped, just documented)

Die Temperature And Surface Defects

Factors in the spray actions that are very important in controlling die temperature:
Length of time of spray Spray nozzle adjustment, or spray pattern Distance from nozzle to the die Balance between air pressure and lube pressure (drop size and velocity) Minimize over spraying Document everything - spray time, pressures, spray location, mixtures, etc... Be consistent

Die Temperature and Surface Defects

The second die temperature control factor is the adjustments made to the water or the hot oil systems for internal die cooling- this can be done by the operator or technician


Die Temperature And Surface Defects

With water, flow rate is usually more important than the temperature of the fluid. A small difference in flow rate will do more than a large difference in temperature Measuring flow rate is a very desirable way to improve process control
Using flow meters is more and more common Keeping the pressure constant is another way

A constant flow rate can help to eliminate the cold flow that seems to come and go without explanation

Die Temperature And Surface Defects

The flow rate is determined by the smallest opening in the supply line - this is usually the quick connect fitting Water (or oil) manifolds affect the flow rate if they are not carefully designed - there often is more output area than input area
Then which line gets the most flow?

Adjustment valves should be easy to see, easy to use, and have large openings for good flow The water pressure at each machine should be consistent, and not vary
Many plants should have new piping installed

Die Temperature And Surface Defects

Hot oil systems will often make a significant difference in the surface defect rate for two reasons:
It keeps the die hot during stoppages - the start up scrap is often a high percentage of the total scrap, (this is where hot oil can easily pay for itself) It can add heat to the die as needed to get better surface conditions

Hot oil units cool about half as effectively as water, so the thermal design must account for this to get the cycle times desired
Use higher flow rates, move lines closer, make larger, etc..

Die Temperature and Surface Defects

Adding overflows are another method of increasing die heat


Die Temperature And Surface Defects

Some things to consider:
Do not place water lines around the outside edge of the cavities (cool the cold areas) Give priority to cooling/heating lines, even if this means moving ejector pins or other changes Do not use the same line to control temperatures in both a hot area and a cold area Depth of the line is critical, set depths carefully Size of line must match the flow rate Treat water because deposits of only .005 cut heat transfer by about 40%

Die Temperature And Surface Defects

Another important control for the die temperature is the cycle time. The die temperature at any given time is the direct result of the number of pounds of metal that went through the die in the last one to two hours. A consistent cycle time is one of the most important factors in good defect control The cycle time should be measured and displayed to the operator or technician in some way to get good control

Die Temperature And Surface Defects

Die temperature changes slowly, which can cause some delayed effects, and perhaps some confusion Example: A change that adds nozzles to the spray manifold, and shortens the spray time with even more cooling than before, and also shortens the cycle time There will be two effects, short and long term. The short term from the increase in spray cooling, the long term from the change in cycle time Increasing the cooling from spray will reduce the die temperature quickly (short term) Reducing the cycle time will increase the die temperature (long term)

Die Temperature And Surface Defects

Summary of correcting surface defects with die temperature:

Measure die temperature to know where to change and how much Establish temperature goals for minimum defects Increase die temperature in the defect area by: Reducing spray Reducing water flow rate Adding overflows Increase overall die temperature by: Reducing cycle time Increasing hot oil temperature and flow rate

Die Temperature And Surface Defects

Summary (cont.):
Use good spray practices and keep consistent Use good computer aided thermal analysis for cooling/heating line design Measure flow rates and control Use good engineering to develop good quality water at consistent pressures Minimize start up scrap and marginal production situations with die pre-heating; Keep die hot during short stops

Metal Temperature And Surface Defects

The metal temperature can make a significant difference in the surface finish In general, the most desirable situation is to keep the metal temperature at a high range, but not high enough to cause a lot of other problems
Keeping the zinc at 430oC max, and the aluminum at about 690oC magnesium should be kept at about 677oC

Control metal temperature on furnaces to within +/- 5oC

Use consistent times between ladling and shot If possible, control temperatures in shot sleeve Use consistent set point, do not use metal temperature as a variable unless absolutely necessary


Laminations have several sources Usually they are the result of metal flow conditions where one flow lays on the top of another, and the flows were too cold to mix as they came to rest This is shown in the next overhead

Flow paths 1 and 2 meet, and are relatively thin layers at this point - they can be peeled away from the layer underneath (flow 3) by grit blasting, machining or similar activity




The correction of these kinds of laminations is to correct the process conditions, which include:
Flow pattern (gating design) Gate location Gate velocity Fill time Die temperature Metal temperature

These layers can come from the way the metal flows inside the casting, which may be due to the geometry of the casting In this case, the flow pattern can be difficult to change with gate modifications, so the important corrections would be:
Decrease fill time Increase die temperature Increase metal temperature if possible Changing gate velocity (either up or down) may also affect these kinds of laminations

Generally changing fill time is the best It is very common that laminations are due to some metal being splashed into the cavity while the plunger is at slow speed; In this case the correction would be increase the length of fast shot (move the switch towards the pour hole) Laminations are also possible from a flexing die - when the intensifier comes in, the die may flex and another layer of metal could be added outside the initial casting skin - the correction is add support to the die

Laminations can also come from oxide skins. These skins come from the holding furnace, or may be formed in the cold chamber during injection These will be random in location, and usually are fairly small, perhaps .08 (2mm) in size. When dislodged by machining or sanding, they can be mistaken for porosity The corrections are good metal handling, including:
Skimming the holding pot properly Keep the time in the cold chamber to a minimum Filtering Fluxing and degassing properly

Another cause of laminations is flash captured in the casting This happens when the die is not cleaned properly, and the flash left on the die drops into the cavity as the die closes The incoming metal will not remelt the flash and cause it to mix with the rest of the casting; In fact, the molten metal may barely adhere to the flash This flash can make a very weak spot in the casting, causing cracks in addition to layers on or near the surface (laminations)

The corrections include those activities that reduce flash, such as:
Cleaning the die between shots Not postponing die repairs Using good process design to select appropriate metal pressures Proper adjustment of intensifier settings Engineering the die cooling to keep die expansion as even as possible


Porosity is the biggest problem in die casting. The two basic types of porosity in die castings are:
Shrinkage Gas

It is critical that those who are responsible for solving defects determine the kind of porosity before trying to correct it Each kind takes a completely different corrective action, but they can look alike

It is important that some time be taken to review porosity before starting to make corrections A quick examination can be misleading Generally, a porosity defect should be examined under 5 to 10 power magnification


Gas Porosity

Gas Porosity
Gas porosity is the biggest single problem in die casting The high gas content prevents heat treating or welding and makes the strength unpredictable


Gas Porosity
There are three major sources of gas porosity for die castings:
Trapped air Steam Gas from lubricant

Gas porosity is round and generally smooth, although it can be flattened to some extent by pressure The actions to reduce gas porosity, in general, are not the same as the actions for reducing shrink porosity


Gas Porosity

Gas Porosity Trapped Air

Any turbulence in the metal movement that allows some air bubbles to be trapped in the metal
These bubbles will remain trapped when the casting solidifies

Air can be trapped in:

Shot sleeve Gating system Die cavity

Starting with the shot sleeve, we will review potential sources of trapped air and possible corrections

Gas Porosity
The first step is to maintain the same pour rate and shot delay time
Especially important if the fill % is below about 50%.

When the fill is less than 50%, a wave is generated by the pouring action
This wave travels back and forth from the parting line to the shot tip

The time at which the plunger tip starts to move and its speed and acceleration are designed so air is not trapped A surfing wave traps air


Gas Porosity
If the wave is met by the tip as it moves forward, then extra splashing and sloshing is generated, and this captures some bubbles. However, if the tip is started forward just after the wave has been reflected from it, then the tip chases the wave, and this will give the best chance for minimizing air entrapment The timer that sets the time delay between the end of pour and the start of shot will determine when the tip starts forward in relationship to this wave

Gas Porosity
The next part of the sequence that can add trapped air (bubbles) and porosity is the acceleration of the plunger to the slow shot speed This acceleration rate should be slow enough to keep the metal from tumbling over (surfing), and fast enough to prevent trapped air between the generated wave and waves reflected from the die This acceleration rate will vary with the percent fill and the length of the sleeve, but the usual range will be between 2 and 2.8 inches per second per inch of travel

Gas Porosity
The optimum acceleration profile can be closely approximated by a straight line (linear acceleration) when the sleeve fill is below about 50% (which is where most of the problems occur) Above about 50% fill, the optimum acceleration trace will be more of a curve Using these methods, the acceleration will normally cause the plunger to reach the critical slow shot speed 1 or 2 inches before sleeve full This is very close to the start of fast shot, so there is little time to spend at the slow shot speed The trace shown on the next page is for a fill percentage less than 50%

Gas Porosity
30 25 Plunger Velocity (in/sec) 20 15 10 5 0 0 1 2 3 4 5 P l u6 nger P7 o s i t i o n 8( i n ) 9 10 11 12 13 14


Optimum acceleration profile with a 32% full sleeve, (3 in. sleeve, 20 in. length, 400 ton cold chamber machine) Note that the straight line closely approximates the optimum profile

Gas Porosity

Typical overall shot profile at 32% fill, using linear acceleration. 400 ton cold chamber machine

Gas Porosity
The next phase of the shot profile will be the critical slow shot speed - this will be the speed that minimizes the trapped air during the slow shot phase. This speed is calculated from the formula:
Css = k x [(100 - %fill)/100 ] x tip dia

Where k = 22.8 for ips (inch system) This speed will minimize the air trapped in this portion of the shot

Wave Formation in Sleeve

Slow shot velocity too slow Slow shot velocity too fast

Ideal slow shot velocity

Gas Porosity
The following settings should be considered important when trying to reduce air trapped in the shot sleeve. While one of these settings may not seem to be important by itself, there are interactions and its recommended they be repeated as close as possible once a good setting is found

Pour rate Delay time before shot Pour hole speed Change over point from pour hole speed to slow shot speed Slow shot acceleration Slow shot speed Fast shot start point


Gas Porosity
The next location of trapped air is likely to be in the runners. Any sharp corners or small to large area changes in the metal flow path in the runner system will cause air entrapment The main rule is that the runner has smooth, rounded corners, that it has ever decreasing area from the plunger to the gate



Gas Porosity
Effect of short ejector pins





Gas Porosity
Once the metal starts to enter the cavity, it will normally flow at a high velocity, very turbulent flow condition, and will trap some of the air present as gas porosity The flow pattern design should be such that the metal tends to push the air through the cavity to the vents Much of the air in the shot sleeve and the cavity can be pushed out the vents or the vacuum system Vents must be sized correctly and go to the edge of the die if they are to be effective Vents must be kept clean of flash & lubricant buildup

Gas Porosity
To summarize, the control of trapped air porosity will involve a check list like the following: Plunger control settings
Pour rate Delay before shooting Pour hole speed Start slow shot point Slow shot speed Fast shot start point

Runner area


No square corners No low or high ejector pins Decreasing runner area in the metal flow path Vent location at last place to fill Vents sized right and go to edge of die Vents to be kept clean


Gas Porosity
Steam is the second source of gas porosity Steam comes from water on the cavity surface when the metal arrives This gas is mostly trapped in the metal because there is little chance to push the gas out the vents - the gas is not present until the metal arrives, and so is mixed with the turbulent metal flow as soon as it is generated


Gas Porosity
The water is mostly from the die spray but it can also be from other sources Some of the water will evaporate from a hot die, but you cannot count on this happening Therefore, it is critical that the die be dry when it is closed Other sources of water on the die: Leaking water lines Dripping overhead sprayers Leaking hydraulic cylinders


Gas Porosity
Steam porosity tends to be either a few large large bubbles or a group of smaller bubbles If it is from a water line leak, the bubbles may always congregate in about the same location


Gas Porosity
Checklist to reduce gas porosity from steam:
To much water based die lubricants on the die (the die must be dry as it closes) Leaking water lines Leaking water pipe connections Crack in the die into a water line Sprayer dripping on the die as it closes Water glycol hydraulic fluid getting on the die


Gas Porosity
The third source of gas porosity is lubricant The lubricant used on the dies or on the plunger can generate gas when heated by the incoming metal This gas (like the steam) is only formed when the metal arrives, and so it is not possible to force most of the gas out the vents ahead of the metal flow All lubricants give off some gas when heated to the temperature of the molten metal - the amount and type of gas will vary from lubricant to lubricant

Gas Porosity
The biggest single lubricant source of gas porosity is the plunger lubricant The usual problem is that the lubricant is applied ahead of the plunger much heavier than is needed This is especially true when a dragging or worn tip is nursed along with extra lubricant


Gas Porosity
Check list of actions for gas porosity from lubricants:
Check the amount of plunger lubricant Reduce the die spray lubricant Look for pockets where the lubricant can accumulate on a cold die



Blisters are another version of gas porosity, the gas just happens to be near the surface of the casting. The trapped gas is under high pressure at the end of fill and the metal may shrink and squeeze it more. When the casting is taken out of the die, and the die surface is no longer there to hold the casting shape, pressure from the trapped gas is able to push up a blister.


The same corrections used for gas porosity apply to blisters:
Reduce trapped air Reduce spray and plunger lubricant Eliminate water on the die Correct venting and vacuum problems


Blisters should be eliminated by correcting the gas porosity problem. However, you can:
Cool the die in the immediate area where the blisters occur
Cool the blister area with die spray Cool the blister area by adjusting water lines Cool the whole die by slowing the cycle time

Cool the casting immediately after ejection by quenching in water (this will keep the skin strong and resist blister formation)

Reduce metal temperature (but watch for other problems)


Shrinkage Porosity

Shrink Porosity
Shrinkage develops because the metal occupies less space when solid than it did when it was liquid. For die casting alloys, the difference in volume will be about 6% to 8% This extra space will be concentrated at the last point to solidify, which is the hottest spot in that section of the casting


Shrink Porosity
Since the location of shrink porosity is also the hottest spot in that section of the casting, it is usually in the center of a heavy section This hot spot location can be controlled to some extent by die temperature The hot spot can often be moved by changing the die temperature, therefore the shrinkage porosity can be moved


Shrink Porosity
An even casting temperature will cause the porosity to spread out, and to be roughly on the center line (the last point to solidify)

centerline porosity



Shrink Porosity
If there is a large temperature difference, then there will be large porosity at the last point to solidify



The spray and the water line keep one end of the casting cold, the gate keeps the other end hot, and the shrink porosity will tend to be at the hot end

Shrink Porosity
If the hot and cold spot can be reversed, then the shrink porosity will follow the hot spot. Shrink porosity will always be in a hot spot in the casting




Shrink Porosity
Note that the temperatures that effect the location of shrink porosity are inside the casting itself The die temperatures will influence the internal casting temperature, but they are not always effective in controlling it completely Shrink porosity in a heavy section will be harder to move, shrink porosity in a thin section will be relatively easy to move


Shrink Porosity
Shrink porosity is rough and irregular in shape and this characteristic is the quickest and easiest way to identify shrink porosity The shape and appearance of the porosity comes from the way the casting solidifies


Shrink Porosity
The first metal to contact the die surface freezes quickly and forms the skin The skin is a very strong, dense, and fine grained surface with very low porosity, due to the rapid solidification/freeze rate Once the skin has formed, the rate slows down and a dendritic structure starts to appear



Shrink Porosity
Dendrites are tree like structures that form in the solidifying liquid The dendrites grow slowly, and by the time the last metal solidifies, there will be a lot of dendrites in this area


Shrink Porosity
It will be difficult to identify shrink porosity in some castings, and without identifying it you may very well take the wrong action for correction thus the identification is a key factor. You should know that the walls of shrink porosity have a characteristically rough structure The shrink porosity is usually crack like in appearance, and is jagged, rough and irregular in general Occasionally it can have a rounded and smooth appearance, when this happens it is sometimes called worm holes

Shrink Porosity

Shrink Porosity
To summarize on the effect of temperature - it will move the porosity or spread it out - not necessarily reduce it - controlling porosity with die temperature is possible only on some castings The shrink porosity location is determined by the temperature difference between areas in the casting - the hot and the cold spots determine the location of the porosity Watch temperature difference between die halves Remember, you can heat up the cold spot as well as cool the hot spot

Shrink Porosity
Shrink porosity can be reduced with metal pressure Die casting is a high pressure process and the only reason die casting machines use high pressure is to reduce shrink porosity The pressure can fill some of the voids as they develop, but timing and temperature are critical and very hard to control


Shrink Porosity
The intensifier systems used with die casting machines are there only to add pressure during solidification and thus to reduce shrink porosity Timing is critical because: the porosity is not there when the metal is liquid, so pressure at that time doesnt help (it will only add to flashing) After the casting is solid, the pressure obviously will not help Therefore, the only time pressure can be used to feed more metal into the developing porosity holes is during solidification


Shrink Porosity
The most common die casting alloys have a freezing range - which means they go through a mushy stage as they solidify It is during this mushy stage that we can add pressure and reduce shrinkage porosity by filling some voids as they are forming Some typical freezing ranges for the most common die casting alloys would be as follows:
380 aluminum 384 aluminum 413 aluminum 45oC 65oC 8oC


Shrink Porosity
Using pressure to fill the emerging shrinkage voids during this mushy stage is a key factor in controlling shrinkage porosity. Several key control elements important in the use of pressure are:

The casting configuration, especially between the gate and the point of interest (most important factor) The amount of metal pressure available at the end of the plunger stroke - the packing pressure Solid The intensified metal pressure The die temperature Mushy Zone The gate freeze time Liquid The injection temperature


Shrink Porosity
Typical values for minimum pressures would be:
ALUMINUM Static 3,000 psi 20 MPa Intensified 8,000 psi 55 MPa

These should be regarded as minimum for good casting quality internally - lower pressures are sometimes used when the internal quality is not that critical For heavier section castings, some die casters use 10,000 psi (70 MPa) as the minimum intensifier pressure

Shrink Porosity
Another operational factor is biscuit size Cavity pressure decreases very rapidly once the biscuit reaches a certain minimum size This minimum varies with the size of the shot sleeve, the metal temperature, the fill time and other factors Below this minimum, the internal soundness of the casting will deteriorate very rapidly


Shrink Porosity
1 DENSITY 0 .8 0 .6 0 .4 0 .2 0 0.1 0.3 0.5 0.7 0.9 1.1 B IS C U IT T H IC K N ES S 1.3 A P P R O X IM A T E D E N S IT Y , PER C EN T


Shrink Porosity
Corrective actions for shrink porosity
Check the process design for appropriate metal pressure, both static pressure and intensified pressure (plunger size, pressure settings) Check plunger tip and sleeve condition Check biscuit thickness for consistency and the appropriate value


Shrink Porosity
Other considerations
If possible, move the gate close to the problem area to feed metal during solidification phase Use squeeze pins to add pressure on the casting after the gate is frozen
These pins are usually activated from 2 to 12 seconds after the end of fill - the casting conditions must be the same from shot to shot to make them effective, (something many die casters cannot do because they dont control the process adequately) The squeeze pins are also effective on leaker problems, (leakers will be discussed later)

Shrink Porosity
Hot chamber machines have similar problems, mostly they have metal leaking by the plunger - this causes low pressure at the end of the shot and lack of pressure just when you need it Trying to run too close to machine capacity in hot chamber machines can cause low pressure at the end of the stroke Shrink porosity can occur at the gate because this tends to be a local hot spot porosity in this location should respond to better pressure management (temperature control also)


Sinks (surface depressions) are a form of shrink porosity A sink forms when the shrink porosity is close to the surface, and as it cools it pulls the thin skin on the die surface in towards itself The shrink porosity is close to the surface because the surface of the casting is the hottest point in that area of the casting




The shrink porosity is formed in the location shown when the casting is first made. As the casting is run longer and longer, the die temperature gets hotter and hotter in the areas marked hot



It is typical to have a section that is heavy enough so the gate is frozen before the area with the shrink porosity has solidified (so the shrink porosity cannot be fed from the gate as it solidifies) The three hot areas shown in the last diagram cause the shrink porosity to gradually move down towards the flat surface - the flat surface will likely get hotter than any other area


The flat surface gets very hot also, then the hot spot gradually moves very close to the surface As the shrinkage porosity starts to contract, it pulls the skin away from the surface As soon as the skin is pulled away from the surface, heat flow is blocked to the die which makes the conditions worse



Eventually, the casting solidifies with the sink in the surface The clue that the spot is getting hot and that a sink might form is when the surface of the casting starts to get rough (or frosty in appearance) The operator should notice this condition, and start to spray and/or adjust cooling line flow What is needed is a change in temperature balance between the hot and cold areas



Leakers are another form of shrinkage problems There may not be any large voids, in fact there may not be any visible porosity All that is needed is a continuous path and enough space for gas or liquid to get through


This size of the space between dendrites depends on the temperature differences that existed at the time of freezing and the ability to feed new metal in during freezing The center of the casting, or the last point to solidify will have a loose dendritic structure that is porous The skin, however, is not porous - thus most casting would allow at least a little gas to pass through if it were not for the dense non-porous skin


It takes a break in the skin (usually on both sides of the casting) to generate a leak through the wall A typical situation is shown below


One way that a break occurs is when the last point to solidify is on the surface. When this happens, the surface generally has the rougher or frosty appearance, and the dendritic structure is close to the surface Another way is to expose a break by machining off some of the surface


The break in the surface often is from a machining action on one side and a shape condition on the other so that a hot spot is generated on the surface




The first correction should be to try to minimize the temperature problem on the surface This can be done (sometimes) by running this area cooler in the die; changing spray patterns, changing or adding a water line; changing to one of the high heat transfer die materials The temperature difference between die halves is something to look out for (not over 100 deg f max) Remember that heating up a cold section can also help restore thermal balance


Adding radius where possible around the leaker area is a good idea, but can only help so much - adding more than about a .18 to .32 inch (4 to 8 mm) radius does not help much Adding pressure in that area can help squeeze pins can work well; Moving a feeder gate near the leaker area can help if the pressure is managed correctly


Also check the metal temperature - a lower injection temperature may make a little difference, but it may also cause other problems It is best to give every opportunity for a good skin to develop in the area where the leaker appears, be sure the die surface in this area is very smooth and clean.



Cracks, or tears, or hot cracks have many causes, but usually are at least partially caused by shrinkage cracks on the surface Most often, the casting is stretched in the die as it cools because the die doesnt change dimensions while the casting is cooling and contracting. The stretching causes cracks at the weak point (the last point to solidify)


Shrinkage cracks on the surface occur during solidification, and have a dark surface - cool the corners or heat up the adjacent areas, add radii to the corners for this type of crack For castings that crack while cooling in the die, the crack will also be at a hot spot increase radii


Mechanical stress can cause cracks when the die opens or the casting ejects (or during slide operations) Cracks at the base of long cores or fins, dragging or sticking of projections into one die half may indicate die shift when the dies separate The factors in die alignment should be checked, such as: die droop (no die carrier), worn guide pins, worn linkage, worn tie bar bushings, worn shoes under the moving platen, uneven tie bar stress, etc.

The cracks at ejection are usually accompanied by drag marks of some sort. Check the ejector plate for worn bushings, be sure the ejector plate operates straight. Look for undercuts from erosion in the die that cause the casting to hang up



What are Inclusions/Nonmetallic Inclusions a) Particles of foreign material in a metal matrix; b) any nonmetal material in the die casting alloy. Usually oxides, refractory particles, and sludge, but can be any material foreign to, and essentially insoluble in, the metal matrix.


Inclusions are mostly a problem in aluminum die casting, but there are issues in zinc and magnesium also Can cause quality issues Strength Hard spots Flow issues The most prevalent type of inclusions are oxides


Oxide Inclusions
The cast alloy is shiny, as evidenced by a casting that has been machined or a furnace that has been just skimmed The gray surface on castings or on the surface of the liquid metal is oxide. This oxide layer on a casting can be very thin from a few microns thick to a few thousands


Oxide Inclusions
Oxides in the furnace can not be totally eliminated Aluminum we use is recycled, has had a lot of exposure to air and has generated oxide Oxygen is picked-up during metal melting and handling Once formed in the furnace, the oxide particles or skins remain


Oxide Inclusions
Aluminum, zinc, and magnesium oxidize to form dross When aluminum oxide is first formed
Fairly soft Less dense than the molten metal Gamma Al2O3, dross


Oxide Inclusions
Exposed to 1800oF (1000oC) or higher in the presence of more oxygen gamma aluminum oxide transforms to very hard more dense phase
Alpha Al2O3, corundum Next to diamond on the Mohs scale Grinding wheel material


Oxide Inclusions
Corundum can form in the melting or holding furnaces in most plants The oxides stick to the wall and are scraped off in the cleaning procedures

1800 TO 2000 DEG




AL ALLOY - 1350


Oxide Inclusions

Casting which contained dross from dip-out well


Oxide Inclusions

Cross-section of a casting containing dross

Oxide Inclusions

Residual corundum particle from improper furnace cleaning


Oxide Inclusions

Dispersion of corundum particles that can look like porosity


Refractory Particles
Furnace refractory particles come off the wall during furnace cleaning


Refractory Particles
Typical forms: brick, mortar, castable, crucible Some common refractory materials


alumina alumino-silicates zircon graphite clay-graphite silica silicon carbide

Refractory Particles

Refractory particles, corundum, and flux


Intermetallic compounds whose formation is composition and temperature dependent Factors contributing to sludging
Metallic impurities Some alloying elements Low holding temperatures Swings in temperature

Once formed almost impossible to dissolve


Fe (iron), Mn (manganese) and Cr (chromium) can form sludge in aluminum alloys if the concentration is high enough


Aluminum sludge factor
Fe + 2Mn + 3Cr 3Fe + 2Cr + 3Mn

Hold to </= 1.8

Note: Drop in Fe can Cause tendency to solder



Sludge particles in aluminum alloy



X-ray of a hard spot (more dense inclusion)


Excess Flux
Fluxes are used for various functions
Cover fluxes protect the melt from oxidation Wall-cleaning fluxes react with wall build-up Degassing fluxes remove hydrogen Drossing or cleaning fluxes assist in partial removal of oxides and reduction/recovery of metal from dross Refining fluxes may modify, grain refine, or remove specific metallic impurities

Too much flux gets entrained in the metal


Excess Flux

Flux inclusion

Excess Flux

Mixture of flux and oxide


Control of Inclusions
What we dont want!


To control dross
Minimize exposure to air and metal temp Use proper drossing procedures Allow enough time after disturbing molten metal bath


To control corundum
Minimize formation of Al dross Dont use higher than necessary metal temperature Allow at least 30 minutes after furnace cleaning for settling of particles


To control refractory particles and sludge
Use proper furnace cleaning procedures Control furnace temperature Stay below sludge factor of 1.8 Allow at least 30 minutes after furnace cleaning for settling of particles


Oxides can be removed by filtering, fluxing, or by de-gassing the liquid metal Refractory particles, sludge, intermetallics can be removed by filtering



Soldering phenomenon occurs when the molten aluminum enters the die and contacts directly on steel die cavity The molten aluminum stream removes the applied surface lubricant film and the iron oxide layer or other coatings then erodes grain boundaries and pits the die surface At a high enough temperature and pressure a reaction takes place that causes the formation of an aluminum-iron intermetallic and a direct fusion between the die and the casting

Keep the gate velocity to a minimum, calculate the gate velocity to stay below about 1600 ips (40 m/s) in aluminum, 2500 ips in zinc (60 m/s), and about 3000 ips (75 m/s) in magnesium - this is to avoid washing the coating off the steel Zinc alloys tend to solder in areas away from the main metal flow The zinc alloys do not form a compound with the die steel on the surface of the die (see build up), but build a layer on top of the steel Die temperature is most important to keep this type of zinc solder from forming

Surface roughness is also important, a polished die surface will reduce the tendency to form the die solder in zinc (it is probably better called a build up) Keeping the die temperature is most important to keep this type of zinc solder from forming Draft angle is also important, especially if the die temperature cannot be controlled



The best solution is keeping the die steel cool solder will not start then Cooling methods include die spray, adding water channels, slowing the cycle speed, and reducing metal temperature. Also consider other die materials
Anvilloy Bi-metallic cores Niobium

Other solutions
Draft angle add to die Gate velocity keep low Die surface roughness keep smooth

Build Up
A die build up usually comes from lubricant that was not evaporated from the die If it is darker in color, it is often referred to as carbon Review the die temperatures with the die spray supplier to get the proper lubricant - keep ratio under control Use of hard water in the die lubricant can sometimes cause build up


Build Up
Review the die temperatures with the die spray supplier to get the proper lubricant Keep die spray dilution ratio under control Dont spray too long Be consistent



Erosion - Burn Out

Erosion and burn out refer to defects in the casting that come from the effects of having some die steel eroded away Erosion generally occurs in aluminum die casting, usually where there are high metal velocity and high steel temperatures The biggest factor in erosion is usually uncontrolled and poorly managed gate velocities


Erosion - Burn Out

The gate velocity should be kept about in the following ranges:

Aluminum, 1000 to 1600 ips (25-40 m/s) Zinc, 1200 to 2000 ips (30-50 m/s) Magnesium, 1200 to 3000 ips (30-75 m/s)
Die temperature is next most important, and should be kept as low as possible Additional factors include the metal temperature, the inclusions in the metal, the type of alloy, proper gate design and flow pattern


Erosion - Burn Out

Burn out is a term usually applied to zinc dies, and is mostly the result of cavitation Cavitation comes from gas bubbles in the metal imploding when the go from a high pressure area in the runner to a low pressure area in the cavity - if they happen to be in contact with the die steel when this happens, then cavitation occurs Sometimes the gate location can be changed so the metal doesnt impact on the die steel just past the gate opening

Erosion - Burn Out

The best correction for this is to minimize the air bubbles - this includes factors such as the following: Using two speed plungers Using runner sprue designs Using proper and carefully designed runners and gates


Gate Porosity - Outgassing

This defect occurs when the casting is heated, usually for a secondary operation like painting, and gas comes out of the casting This is due to the expansion of the trapped gas in the casting that escapes through a porous section of the casting It is not be possible to eliminate all the trapped gas, so most of the work should be focused on reducing the porous-ness of this section of the casting


Gate Porosity - Outgassing

To reduce the problem in the overflows, minimize the number of overflows used and keep the gate to the overflow as thin as possible For outgassing at the gate, keep the gate as thin as possible, keep the gate area cool as much as possible, and increase metal pressure Ensure there is enough pressure to feed the shrinkage For all types of gate entrances to the casting, do not have thin die sections that can build-up heat.

Gate Porosity - Outgassing


This hot spot increases the amount of shrink porosity formed at the gate which contributes to outgassing problems

Bending, Warping

Bending - Warping
Bent and warped castings have many different causes, starting with design, die build, machine conditions, and operating conditions Once the design is set, the major operating factor is that the casting and the die always be at the same temperature when the casting is ejected The use of a thermocouple to eject the casting at the same temperature every time is a valuable aid to precise dimensional control


Bending - Warping
The die and the machine must be maintained in good condition to minimize bending and warping



Flash occurs when liquid metal flows into an area of the die where it is not expected, such as the parting line, under a slide, or along side an ejector pin


Flash can be almost eliminated if proper attention is applied to tool design and operational factors A robust die is required, there can be no die deflection Good thermal balance is required so the die fits well at operating temperature The machine locking conditions must be proper; such as equal load on the tie bars, proper die set up, linkage not worn, platens flat, etc Metal pressures and impact force should be no more than necessary


Flashing can occur for one of three reasons:
1. The die seal-off is poor. 2. The die and machine do not work together to seal off the mold cavity. 3. The impact force at the end of the cavity fill exceeds the clamping force of the machine.

To understand the reason why flashing occurs, you must understand the condition of the machine, the tool, and the forces occurring within the machine at the end of cavity fill.


Determining the root cause of a flashing problem is sometimes quite complex. As an operator, you need to observe the die:
If the casting flashes in exactly the same way every shot, then the tool or machine is likely the problem. If the flash pattern around the cavity changes drastically, especially as a die heats up during startup, then it indicates two possible causes:
The impact force is exceeding the clamping force of the machine. Thermal expansion within the tool is allowing metal to escape the die cavity.

Flash Summary
The two keys to minimizing flashing are:
Die design. Machine maintenance.

Die design.
Die squareness. Thermal design. Die seal-off. Centering the part within the die. Wear plate and lock design.

Stained Castings

Stained Castings
Dark gray or black discoloration Almost always involves die or plunger lube Typically, too much lube has been applied Check: Amount of lube being applied Dilution ratio Application procedure Other sources dirt scrap


Waves and Lakes

Waves & Lakes

Typically in zinc Caused by metal flow problems Two flow fronts meet, one cooler than the other, however, enough heat for re-melting for the two fronts to attach this is the difference between laminations and waves & lakes.

Waves & Lakes

Plunger acceleration needs to be accelerated before metal hits the gate Fast shot transition early enough Gate design Flow pattern Die temperature Metal temperature


Looks similar to solder, but is caused by the die dragging on the part during ejection Check:
Draft angles Ejection problem, such as bending Erosion that may have caused an undercut Distorted, mushroomed cores Flash on slides Die temperature


Cold Flakes

Cold Flakes PSP - ESP

Occurs in only cold chamber machines Causes irregular breakout at the gate Most collects at the gate Can cause flow problems May be difficult to see without microstructural examination


Cold Flakes PSP - ESP

Minimize shot delay time Keep fill % in sleeve as high as possible Keep sleeve temperature as high as possible Keep metal temperature high