Designing parts with consistent wall thickness is a fundamental rule of plastic injection molding, and ignoring it can lead to sink, warp and inaccurate or non-functional parts. Yet the functional requirements of consumer, medical, aerospace and industrial products often leave designers little consideration for the material flow and fill properties of plastic, both of which are at least partially determined by wall thickness.
Pay close attention to rib-to-wall thickness ratios. To prevent sink, the thickness of the rib should be about half of the thickness of the wall.
This month’s tip discusses:
- Guidelines to avoid cosmetic defects associated with thin and thick features
- Material alternatives to improve wall thickness consistency
- Important questions to ask about material properties
- The benefits of design for manufacturability analysis
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Knowing the material your parts are going to be manufactured in early on in your development process can save you time, money and a lot of frustration. You should work closely with your manufacturer during design, so they can help you identify potential material issues before any parts are actually molded. Because some part geometries inherently work better with certain resins, your manufacturer can help guide you toward the appropriate material options.
Issues that can result from selecting an incorrect material:
- High fill pressure
- Poor cosmetic finish
- Shorting or burning
Uniform Wall Thickness
With injection molding, we talk a lot about how uniform wall thickness helps improve mold fill versus thin features that can restrict the material and create a number of the aforementioned issues. Having connected walls that are too thick and too thin can affect how a part cools, thus creating sink and warp. Furthermore, the same issues can arise if your entire part is too thick or thin.
Watch rib-to-wall thickness ratios. To prevent sink, rib thickness should be about half of wall thickness.
This is why the appropriate rib-to-wall thickness ratio must be followed. The appropriate thickness for a rib that is extruded from another surface is approximately half the thickness of the adjacent surface. This is the optimal part design to provide strength while at the same time reducing your chances for significant warp or sink. Learn more on uniform wall thickness in plastic parts on our website.