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Design Guidelines
Uniform wall thickness
Good injection moulded part design relies on consistent wall thickness to minimize the potential for warped or distorted parts.
Sink Marks—As the plastic solidifies in the mould it freezes from the outside (near the mould surface) toward the inside. In thick sections this results in inward pulling stresses (due to contraction) that can cause sink marks in the outer surfaces of the part.
Part Warpage—In addition, because thinner sections will freeze faster than thicker sections there is also the possibility of stresses building up between thick & thin sections, resulting in part warpage.
So in the design of parts to be injection moulded, it is a good idea to maintain consistent wall thickness and avoid thick areas whenever possible.
Avoiding warp by minimizing
Warpage due to stresses in step transitions between wall thicknesses can be improved through the use of a ramp.
The use of gussets can be helpful to provide support in corners to avoid warping.
A. High stress concentrations
B. Reduced stress concentrations
C. Thinner walls result in shrinkage during cooling
D. Gussets provide additional support to reduce warpage |
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Thicker and non-uniform wall thicknesses can often result in sinks in the material due to the same solidification physics described above.
The use of thinner, uniform wall thicknesses helps to avoid sink.
A. Boss in corner causes sink
B. Thinner walls on boss eliminates sink
C. Thick walls cause sink, warp & excess shrink
D. Thinner walls give accurate part |
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Thin ribs on thicker walls may provide stiffness but also can result in sinking on the outside of the wall. This rule-of-thumb guideline should help keep this from happening.
Watch rib-to-wall thickness ratios—To prevent sink, the thickness of the rib should be about half of the thickness of the wall.
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