When designing a part to be injection molded, you do not need to be an expert. Most molding issues we encounter could have been avoided by following four general guidelines.
1. Wall Thickness Consistency
One of the most basic design parameters is to keep the wall thickness consistent. Parts with a uniform wall thickness will not warp, will fill in properly, and minimize shrink variability. But how much wall thickness is typically allowed? Ideally, there should be no variation, but wall thickness variations should not exceed 10% in materials that have a high shrinkage.
2. Proper Gate Location
A part must have a gate, which is the opening that allows the plastic to be injected into the mold. Gates that are most effective are ones where they enter the thickest part of the cavity, and then flow to the narrower areas. Since the gate will be slightly visible on the part, it is best to have it on a non-cosmetic surface.
3. Radius Corners
If there is one thing that plastic does not like, it is sharp corners. Sharp corners are stress risers that can cause part failure. The molten plastic needs to be able to navigate around corners with ease. Corners with a radius will allow the plastic to flow more easily. In contrast, corners with sharp corners will result in molded-in stress.
Draft is when the side walls in the mold are tapered in the same direction that the mold opens. Draft facilitates the removal of the part from the mold. It is important to note that different degrees of draft are required based off part geometry and surface texture. A tool should use at least 1 degree of draft for all vertical surfaces (2 degrees works very well for most parts).
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