Effect of Peak Cavity Pressure on Unreinforced Polyamide Linear Shrinkage Rates
Elevating peak cavity pressure compresses unreinforced polyamide melt, suppressing specific volume and reducing post-moulding linear shrinkage rates.
Sink Mark Risk
Meaning
The likelihood of localized depressions forming on the surface of a molded part as it cools is determined by the wall thickness, gate location, and packing phase settings. A high sink mark risk is usually associated with thick rib intersections or heavy bosses where the volume of material remains hot for a longer duration than the outer skin. As this inner core cools and contracts, it pulls the already solidified outer skin inward, creating a surface depression.
Process adjustments like extending the hold time can help mitigate this cosmetic issue.
Geometric Influence
Ribs and bosses must be designed to be thinner than the main wall to prevent localized heat retention. When a rib is too thick, the increased volume of plastic creates a region of slow cooling. This slow cooling rate raises the sink mark risk because the material cannot be packed sufficiently through the thin adjacent walls.
Keeping the rib thickness to sixty percent of the main wall thickness helps maintain a uniform cooling rate.
Process Variable
Low packing pressure or short hold times prevent the machine from adding enough resin to offset thermal contraction. If the gate freezes before the packing is complete, the sink mark risk rises dramatically. This risk is managed by optimizing the holding phase parameters on the press.
Material Property
Semi-crystalline polymers present a higher level of cosmetic vulnerability because they undergo a much greater volume reduction during crystallization than amorphous resins. High melt temperatures also increase the amount of thermal shrinkage that must be compensated for during the cycle. Moulders can reduce this risk by choosing resins with lower shrinkage rates or by using chemical foaming agents to generate internal gas pressure.
This internal pressure pushes the polymer skin against the mold walls as it cools, eliminating the depressions.