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 molded 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 Molding design information.

Last month we published the first issue of the quarterly Protomold Rapid Injection Molding Journal. The intent of the Journal is to provide useful information about the business and technology involved in the process of bringing new products to market. Let us know what you think, and we'll include the results in next month's Rapid Injection Molding Design Tips.

What did you think of the Summer Rapid Injection Molding Journal? Excellent Can't wait for the Fall issue. Good A decent start. Could be better. Fair Saw it. Read it. Tossed it. Poor Why did you send this to me again? Not Applicable. What Journal? I never saw it.

If you didn't receive a copy of the Journal in the mail and would like to check it out, you can either download a PDF or send an email to marketing@protomold.com and we'll be happy to mail one to you. Alternatively, if you did get one in the mail but don't want to see it again, just let us know via an email and we'll take you off the list.

Please suggest any ideas you may have for future survey questions. Just e-mail them to us at marketing@protomold.com. We asked "How many mechanical CAD engineers or designers has your company hired to date in 2004?"

Of the approximately 200 people that responded, when given the choices of a staffing change of none, minimal (< 10%), moderate (< 50%), heavy (> 50%) or not applicable - it looks like the "economic recovery" hasn't yet had a very big impact overall. Let's hope business picks up for everyone over the remaining months of 2004.