As the plastic in the mold cools from the outside it can cause pulling on outer walls resulting in sink marks. Thinner wall thickness will help to prevent this. Where possible always try to design a part with thinner and consistent wall sections to minimize warp and sink marks.
Wall Thickness by Material Guideline
The proper material selection for your desired part thickness can also help. See table below for injection moldable material selection and recommended wall sections for your molding project.
|ABS||0.045 - 0.140|
|Acetal||0.030 - 0.120|
|Acrylic||0.025 - 0.500|
|Liquid crystal polymer||0.030 - 0.120|
|Long-fiber reinforced plastics||0.075 - 1.000|
|Nylon||0.030 - 0.115|
|Polycarbonate||0.040 - 0.150|
|Polyester||0.025 - 0.125|
|Polyethylene||0.030 - 0.200|
|Polyphenylene sulfide||0.020 - 0.180|
|Polypropylene||0.025 - 0.150|
|Polystyrene||0.035 - 0.150|
|Polyurethane||0.080 - 0.750|
Fiber reinforced plastic allows for larger thicker parts
Part Warp 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 warp.
Tip: Reduce the Boss diameter and strengthen with ribs
Part Draft on Plastic Injection Molds
Plastic parts should be designed with draft to prevent sticking and ejector pin push marks on the show surface during the molding process. Also, an adequate amount of draft is needed for surfaces that will be textured. Don't forget ribs should not exceed 60% of wallthickness.