We have another riddle for you to introduce this month's Design Tip:
"When is circumference less than 2πr and when is it greater than 2πr?"
The answer is when you're talking about a cone or a potato chip shape, respectively (see illustrations). What's the point? Well, if your injection moulded part is filled from the center and the resin shrinks less in the flow direction than in the transverse (as with any glass or carbon filled resin), the part will want to warp into the shape of a cone. But if the material has the opposite shrink characteristic, as with unfilled nylon, then it will tend to warp into a potato chip shape. Either way, you may not get the final geometry you're expecting.
Which brings us to this month's discussion about warp - something hard to predict with precision but usually manageable with the knowledge of a few fundamentals. We've covered part geometry-based techniques for managing warp in our October 2003 Design Tip, but two additional considerations are the characteristics of the resin and the nature of the gate(s) to be used.
As illustrated in the Protomold Design Guide resin properties table, the tendency for resins to warp varies significantly. Good or excellent dimensional behavior can be expected from a polycarbonate/ABS alloy (e.g. Cycoloy), but only fair or poor results should be expected from materials like thermoplastic elastomer (e.g. Isoplast) or glass filled nylon (e.g. Zytel). Of course, there are other considerations to take into account when selecting a resin such as additional mechanical properties and cost.
In addition to looking at alternative materials, sometimes warp can be reduced by changing the nature and/or location of the gate(s). For example, in the case of the disk-shaped part illustrated above, rather than locating a single gate in the center of the part it may be advantageous to have several equally spaced gates around the circumference of the part as illustrated in the figure to the right. Although this may result in knit lines (covered in next month's Design Tip) where the resin flows meet, the multi-gate approach may cause the overall stresses to balance and help to avoid the cone or potato chip effect.
Very seldom can you get everything you want in one part, which is what engineering tradeoffs are all about. But as a well informed designer, you can make a big difference in our ability to make your parts, and more importantly the success of your project.
You can visit the Protomold Design Guide for other helpful Rapid Injection Moulding design information.