Meaning
Localized surface depressions on an injection moulded part occur when the outer skin collapses into the shrinking internal polymer melt. The formation of sink marks occurs in thick sections, such as ribs or bosses, where the material takes longer to cool and shrink. This defect is cosmetic, but it can also indicate internal voids or structural weakness.
The problem is managed by adjusting process parameters or redesigning the part to have more uniform wall thicknesses.
Defect Origin
Thick areas of the part solidify slower than the surrounding thin walls, creating a reservoir of hot melt that shrinks as it cools. If the gate has frozen or the hold pressure is insufficient, this shrinkage pulls the solidifying skin inward, creating sink marks on the surface. This defect is particularly common in resins with high crystalline shrinkage, like polyethylene or polypropylene.
This pull occurs because the internal vacuum created by shrinkage is stronger than the skin’s resistance. This cosmetic issue is highly visible on textured surfaces.
Processing Remedy
Moulders can reduce this defect by increasing the pack pressure or extending the hold time to push more material into the cavity. Lowering the melt temperature or the mould temperature also helps by reducing the total volume of thermal shrinkage. However, these adjustments must be balanced against other defects like flash or high cycle times.
If the process window is too narrow, the tool must be modified. This process adjustment is the first step in troubleshooting.
Design Action
Part designers avoid this issue by keeping the thickness of ribs and bosses below sixty percent of the nominal wall thickness. This design rule ensures that the thick section cools at a rate similar to the rest of the part. If the rib is too thick, a sink mark is almost inevitable.
This design precaution prevents expensive tool changes later.
