Designing plastic parts for strength

 Overview:  Interior trim parts must withstand substantial pressure and stress specifically parts such as door panels and handles. This document explains various design options to help strengthen the part.

Plastic Filler Materials: Some plastics have fillers added for additional strength. These plastic types tend to be strong however, they can wear out the mold. These fillers have a tendency to wear out mold components like gates, runners, and manifold systems. They can also have effects inside the mold when having to flow around sharp corners or into small features. Some mold vendors prefer to place inserts into high wear areas making the area in question easy to replace. This tends to add more cost to the mold, but can save in the end by reducing the need to completely rebuild an entire mold.

Using Ribs for Strength: Using ribs is probably the most common way to strengthen a part. Ribs give strength to a part and prevent a part from warping. Ribs can be a challenge if they are too tall. You have to be aware of how thick a rib is at the base of the part. Ribs that get too wide at the base can produce a sink mark in the opposite side of the part. This is not good for cosmetic parts because these flaws can be apparent. One consideration is to reduce your rib versus wall thickness percentage. The general rule is to make the rib thickness 60% the thickness of the part. You can reduce this to 50% of the part thickness to help keep the thickness at the base to a smaller size.

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Coring versus Ribs:Sometimes ribs are not the appropriate tool to use. In the following example, a door handle is created and shown with its outer shape. 

In this scenario, the part needs to look and feel as if it is a solid part. The part has a thickness that is too large to mold with a multitude of flaws including sink marks and warping. If you try to shell the part and add ribs for strength, you might create a sharper edge on the backside making it uncomfortable to grasp.

In this situation, coring can be a viable solution. By removing a series of pockets on the backside, you can simulate the feel of a complete part, but have the necessary walls in it to avoid the problems.

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