Meaning
Prevention of localized surface depressions on molded parts requires the maintaining of adequate pressure on the solidifying polymer melt to compensate for volumetric shrinkage. Through sink mark suppression, molders adjust injection velocity, pack pressure, and hold times to ensure that the outer skin of the part remains flat against the cavity walls as the core cools. This process variable is critical for cosmetic parts where surface blemishes would cause the component to fail visual inspection.
Tool Design
Proper rib-to-wall thickness ratios are the most effective way to address surface depressions before the steel is cut. In sink mark suppression, the thickness of reinforcing ribs should not exceed sixty percent of the nominal wall thickness of the main part. This ratio prevents the thermal mass of the rib-to-wall intersection from creating a hot spot that shrinks and pulls the outer skin inward.
Process Tuning
Dynamic holding pressure is used to feed extra polymer melt into the shrinking core of the part during the packing phase. Under sink mark suppression, the hold time must extend beyond the gate freeze-off time to prevent the material from back-flowing out of the cavity. Moulders also utilize gas-assisted molding or foaming agents to push the resin outward against the tool, which is a common remedy for thick-walled components.
Economic Tradeoff
Extending the hold and cooling times to address surface blemishes increases the total cycle time of the production run. This balance in sink mark suppression shows that high cosmetic standards directly increase the cost per part.
