“What is a sink mark?” “Where will the sink marks be?”
Sink marks on plastic parts are in indentation in the surface, caused by the plastic shrinking and “pulling” the surface inward. Visually this causes a “low spot” or “depression” or “valley” in a part that can be seen and felt. In some cases, these cause an obvious visual defect where the surface is not smooth and consistent, rendering the part unacceptable.
The technical term is differential shrinkage, which means different regions of the plastic shrinks at a different rate than others.
These marks are undesirable and lessen the quality of the part, both visually and dimensionally.
SOLIDWORKS Plastics packing simulation.
Simulation allows you to detect sink marks by accurately predicting the shrink that occurs through the part. As shown on the sample part below, the rib structure is opposite the outside wall, which is very normal for plastic part design. However, the ratio of the thickness of the rib to the thickness of the wall is the leading indicator is the surface will have sink marks. Ironically, every part designer knows that for a given material, there is a part design rule for this thickness ratio. Pushing these boundaries is the norm. Most of the time it is the hidden thickness inside of the part geometry that is not easily seen.
How The Solution Helps
Once you identify where the sink marks are going to be and how extensive, this starts the redesign process the part itself or the evaluation of the molding process. They key is to find these defects before the mold is created. Changing the part design or mold design before cutting chips is far less expensive in the virtual world than after the mold has been created.
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BlueBill Engineering Services