How to Avoid Costly Mistakes in Injection Molding

To cut costs associated with injection molding, prioritizing part moldability during the design phase is critical. Optimizing moldability from the outset can saves thousands of dollars.

Wall thickness is the foremost consideration in designing for moldability, according to one process expert, followed closely by optimizing ejection from the mold.

The Goldilocks principle applied to wall thickness
To avoid walls that are too fat or too thin, designing a wall thickness between 0.040 and 0.140 in. (1.0 to 3.5 mm) is ideal, according to injection molding engineer Jack Rulander of Protolabs in Maple Plain, MN. The company provides an array of digital manufacturing expertise.

“Designs that are too thick will be prone to voids/sink/high shrink, often leading to warp,” Rulander advised. “Too thin of a wall, and you will be prone to hesitation, where the melt flow slows down or stops along a particular path. This leads to cracking, weak knit lines, short shots, and more.”

To achieve optimal part ejection, he continued, engineers should add at least one degree of draft to ensure ejector pins and blades pop parts off a mold easily. Material choice plays a key role in how much draft is needed, because “semi-crystalline is often high shrink, while amorphous materials are generally low shrink. Texture and surface finish also play a role. For part ejection, undercuts and internal features may present problems and may need to be redesigned to allow successful molding.”

Consider gate vestiges
Final part appearance is another consideration for part and process design, Rulander noted. “Injection molded parts are also left with gate vestiges and witness lines from parting/shutoff surfaces. Tab gates, which are most common, offer a lot of flexibility for placement, but also can leave a larger vestige when compared to other options. For cosmetic parts, it’s important to find the ideal location where that mark is less apparent. When it comes to functional parts, these same marks or flash (excess plastic) on parting/shutoff surfaces may impact sealing.”

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