Because rapid injection molding is a true injection molding process, it shares many of the characteristics of traditional injection molding. Much has been written regarding design guidelines for injection molding. Yet, the design guidelines can be summed up in just a few design rules.
1 Injection molding design better uses uniform wall thicknesses throughout the part. This will minimize sinking, warping, residual stresses, and improve mold fill and cycle times.
For the perfect uniform wall thicknesses design of injection molding, there are some tips for each part.
Avoiding Warp:
Sink Marks—As the plastic solidifies in the mold it freezes from the outside (near the mold 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.
Avoiding warp by minimizing---Warpage due to stresses in step transitions between wall thicknesses can be improved through the use of a ramp
Avoiding Sink:
Thicker and non-uniform wall thicknesses can often result in sinks in the material due to the same solidification physics described above.
2 Injection molding design better uses generous radius at all corners. The inside corner radius should be a minimum of one material thickness.
3 Injection molding design better uses the least thickness compliant with the process, material, or product design requirements. Using the least wall thickness for the process ensures rapid cooling, short cycle times, and minimum shot weight. All these result in the least possible part cost.
4 Design parts of the injection molding to facilitate easy withdrawal from the mold by providing draft (taper) in the direction of mold opening or closing.
5 Injection molding design better uses ribs or gussets to improve part stiffness in bending. This avoids the use of thick section to achieve the same, thereby saving on part weight, material costs, and cycle time costs.
Rib-to-wall thickness ratios:
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.
Here are some general injection molding design guidelines to help you for learning the injection molding design, but for design perfect products, you have to learn more and practice more in your work.
