Design Principles of Plastic Injection Molding

Looking to design a mold? Make sure you’ve checked every box on this plastic injection molding design checklist.

Draft Angle

Make sure the mold includes a draft angle in its design.

This is essential for allowing air to enter alongside the plastic part and preventing it from scraping against the core part of the mold during ejection. Make sure there are no 90 degree angles in your mold. 

Corner Radius

Decide how sharp to make the angles on your part. 

The closer to a 90 degree angle, the harder it is to inject molten plastic into the corner of your part. Consider the proper radius to optimize material flow and part integrity.  All inside corners should have fillets to avoid stress points in the molded parts. 

Parting Line

Decide where you’d like the parting line located on your product. 

The parting line marks where the cavity side and the core side of the mold meet. The location of this line may affect the structure and functionality of your part. 

Ejector Pin Placement

Decide where to effectively place ejector pins. 

Ejector pins are needed to push the product out of the mold. Consider where they are placed to ensure the part is fully and effectively ejected.

Gate Location

Decide where your plastic part is attached to the mold via gates. 

Molten plastic is injected into the part through the gate. The number of gates and where the gates are placed on the product affects how effectively the plastic is injected. 

Wall Thickness/Ribbing

Decide how to strengthen the walls of your part through thickness and ribbing. 

Wall thickness is an important consideration affecting the strength and functionality of your part. It is best practice to have a uniform wall thickness.  Non-uniform wall sections can cause warpage and dimensional control problems. When this can’t be achieved, there should be a gradual transition between the thick and thin sections.  It is also best practice to gate into the thicker section and then flow into the thinner section. Overly thick areas should be avoided, as they can lead to sink marks. Consider coring out thick sections of parts.  Ribbing can be used to strengthen your part without increasing wall thickness. Ribs thickness should be 50-75% of the wall thickness to minimize the sink caused by the thick section where the rib and wall meet.