As the plastic solidifies in the injection 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.

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 molded, it is a good idea to maintain consistent wall thickness and avoid thick areas whenever possible. 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. 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.

See the Protomold Design Guide for other helpful design information.

This month's survey is in two parts. First, let's find out what 3D CAD software everyone is currently using. Then let's see which package designers would like to use if given the opportunity:

What 3D CAD software... ...do you currently use? AutoCAD Inventor AutoCAD Mechanical Desktop Pro/E SDRC Solid Edge SolidWorks Unigraphics Other None - 2D Only ...would you prefer to use? AutoCAD Inventor AutoCAD Mechanical Desktop Pro/E SDRC Solid Edge SolidWorks Unigraphics Other None - 2D Only

Let us know and we'll include the results in next month's Rapid Injection Molding Design Tips.

Suggestions for future survey questions related to the field of Rapid Injection Molding are welcome - just email them to us at marketing@protomold.com.