Design Guidelines

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By Jeremy Losek, Sep 14th, 2021

Injection Molded Plastic Part Design Guidelines

For design engineers who are new to designing for plastic injection molding, there can be a knowledge gap in making sure the CAD model is properly designed for this manufacturing process. There are certain, very specific design features that must be considered in the design stage in order to avoid moldability issues that can produce undesirable results in the molded parts. If they’re not considered from the beginning of the design process, and they’re not discovered until the Design For Manufacturability (DFM) review, design changes can add time and cost to an injection molding project.

Draft angle is one example. If a part does not have draft angle designed into it where needed, it may not eject from the mold cleanly and smoothly, and the part will have to be redesigned with proper draft angle.

To help avoid costly and time-consuming design mistakes, we created a plastic injection molding design guide. It provides information on the most important design requirements so they can be incorporated at the initial design stage. The sample tips below illustrate the type of information provided in the complete guide. Download the complete design guide by filling out the form below.

plastic injection molding basic injection molding design guidelines

Plastic Injection Molding Part Guidelines
Proper Design of Bosses
Plastic Injection Molding Part Guidelines

Bosses find use in many part designs as points for attachment and assembly. The most common variety consists of cylindrical projections with holes designed to receive screws, threaded inserts, or other types of fastening hardware. Generally, the outside diameter of bosses should remain within 2.0 to 2.4 times the outside diameter of the screw or insert

It is suggested avoiding bosses that merge into sidewalls because they can form thick sections that lead to sink. Proper bosses should be positioned away from the sidewall, and if needed, use connecting ribs for support. Try using open boss designs for bosses near a standing wall.

Plastic Injection Molding Part Guidelines
Plastic Injection Molding Part Guidelines

Normally, the boss hole should extend to the base-wall level, even if the full depth is not needed for assembly. Shallower holes can leave thick sections, resulting in sink. Deeper holes reduce the base wall thickness, leading to filling problems, knit lines, or surface blemishes. Because of the required draft, tall bosses (those greater than five times their outside diameter) can create a filling problem at their top or a thick section at their base.

Plastic Injection Molding Part Guidelines

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