Rex Plastics Product Development

Plastic Part Design Fundamentals

Sink and Warp

Plastic Part Design – What is Sink?

Part manufacturers should always give careful consideration to the materials used in plastic part design, as this decision will have significant consequence in the sink & warp of plastic injection molded parts. Why is this? During the cooling stage of a plastic injection molded part, plastic first solidifies at the mold surface and moves inward toward the center. If the plastic is too thick, the center will stay molten for an increased period of time. This causes an inward pulling stress to develop which leads to sink marks on the outer surfaces of the part.

In the image above, notice the sink marks on the part shown to the right.

Preventing Sink in Plastic Parts

Ribs may provide stiffness for plastic injection mold parts, but also can result in sink marks on the outside of the wall. To prevent sink in plastic molded parts, the thickness of the rib should be about 60% of the thickness of the wall. This rule-of-thumb guideline should help keep sink from occurring as the part cools.

Plastic Part Design – What is Warp?

If uneven wall thicknesses exist in any plastic injection molded part, thinner sections will freeze faster than thicker sections, which will introduce stresses in the part between the thick and thin areas. If the stresses become excessive, the part will warp, illustrated below:

The part above on the right-hand side has a warped thinner section.

Preventing Warp in Plastic Parts

Plastic injection molded parts may experience ‘warp’ due to stresses in step transitions between wall thicknesses.  To combat sink, plastic part design can be improved through the use of a ramp. Additionally, the use of gussets can also provide support in corners of plastic parts to help avoid warping.

Learn More

Do you have additional questions about what it takes to create top-quality plastic injection mold parts?  Please feel free to contact us by calling or emailing at any time, and be sure to take a look at our Frequently Asked Questions.  Are you an established OEM, or start-up organization with plastic part manufacturing needs?  Learn Why Rex is the right choice, and why choosing American manufactured parts should be a top priority.