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Design Considerations for Injection Molding

Making the mold for injection molding is similar to making a mold for casting. The major concern is ease of separation; there should be no material preventing the two halves of the mold from splitting. The major restrictions against separation are undercuts and overhangs. Though possible to make, these complications should be avoided if possible. Creating and using a mold with a movable part for undercuts and overhangs is expensive and time-consuming. Also, all corners should be rounded to improve the quality of the part. Below are examples of undercuts and overhangs:

	Basic Part. Easy to make.
	Part with undercuts. Difficult to make: Requires moving part on mold.
	Part with overhangs. Very difficult to make: Avoid if possible.

While it is possible to to make a mold with three or more pieces that assemble to create the shape of the part, these molds are MUCH more expensive than the standard two part mold.

Wall Thickness

Maintain uniform wall thickness insofar as possible, and if changes in wall thickness are unavoidable, make them gradual rather than abrupt.

Holes

Holes are feasible in injection molded parts. However, they are a complicating factor in mold construction and in part quality.

The minimum spacing between two holes or between a hole and sidewall should be one diameter.

Holes should be located three diameter or more from the edge of the part to avoid excessive stress.

Blind holes should not be more than two diameters deep.

Information source: Bralla, Design for Manufacturability Handbook.