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Injection Molding Parts Design Guide

People who design parts have to work with many materials besides plastics and many processes besides injection molding. They need some understanding of all these but can't be expected to be the complete expert in any one of them. That's why we should not work in closed rooms. The plastic parts designer has to start the thing off but eventually it has to become a collaboration with the rest of the supply chain. In the meantime there is some help for the designer in the shape of design guides, design for manufacturing techniques, and injection molding design rules embedded in some CAD systems.

A common fault that I have found with part designers is a failure to define completely the service conditions and life of the part. Until that is done, you can't make the optimum choice of material.

The commonest causes of failure in injection molding are notch sensitivity, poor weld lines and environmental stress cracking. The notch effect is a mechanical failure under load and is caused by stress concentration at sharp corners. The need for radiussed corners has already been discussed but the designer's intent can sometimes be undone when the injection mold designer not unreasonably removes a radius to simplify the injection mold and cut costs. This again is where you need collaboration. It's the same with weld lines. The design has a bearing on it but weld position and quality is a function of the gate position and injection molding process. Environmental stress cracking causes failures in service and is little understood within the plastics industry let alone the design community. It happens when injection molding is exposed to some medium, frequently a lubricant, that causes mechanical failure at a stress level far below that normally regarded as safe.

There are many injection molded part design guidelines out there. Try Googling for PDF on materials supplier sites. My own design guide was written originally for Asian Plastics News. I make no claim for originality. I drew on published sources as well as my own experience but you will find the same material in many other sources. I have not revised the guide for some time but with that proviso, I don't think they will do anybody any harm and the final chapters cover sources of further information for part designers.

General Part Design Concepts
When designing a part, there are a few general rules to follow:
  • The part wall thickness should be as uniform as possible. Transitions from thick to thin areas should be gradual to prevent flow problems, back fills, and gas traps.
  • Thick sections should be cored out to minimize shrinkage and reduce part weight and cycle time.
  • Radius / fillet all sharp corners to promote flow and minimize no-fill areas.
  • Deep unventable blind pockets or ribs should be avoided.
  • Avoid thin walls that cannot be blown off the cores by air-assist ejection.
  • Long draws with minimum draft may affect ease of ejection.
I see a lot of tooling support problems in hot runner injection molds as this is my focus. Many times the daylight of the injection mold restricts the designers ability to invest the proper surface support into a hot half that is paired up with the cavity. this of course returns in a plastic injection mold that is causing flash that is difficult to overcome as the more the mold maker adjusts on the parting line the more the cavity flexes out of shape and the worst the flash gets. The other major issue is that too often the end molder either plans to run the plastic injection mold in far too large of a press or requires that the injection mold be built for small of platen size not giving the designer the molds to build the necessary support in to the slides and cores that is required. This will always result in a poor output of the plastic injection mold if the processor is able to "process around" the root cause which was just a poorly built mold and improper calculation of the required tonnage and support for the plastic injection mold to maintain the shutoffs needed to accurately develop robust processes.

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