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.
Mold Density
Meaning
Mass per unit volume of a polymer after it has cooled and solidified inside the mold cavity determines the final weight and dimensions of the part. Achieving the correct mold density depends heavily on the holding pressure and the cooling rate applied during the molding cycle. If the mold density is too low, the part will suffer from excessive shrinkage and internal voids.
Process technicians monitor this property by weighing molded parts at regular intervals during a run.
Cooling Influence
The rate of heat transfer from the polymer to the mold steel governs how the polymer chains organize themselves as they transition from liquid to solid. Fast cooling restricts the rearrangement of molecules, which leads to a lower mold density in semi-crystalline materials. Slower cooling allows for greater crystalline growth, which increases both density and hardness.
Adjusting the coolant flow rate is therefore a direct way to fine-tune the density of the finished product.
Pressure Control
Holding pressure forces additional melt into the cavity to offset the thermal contraction that occurs as the plastic cools. Without this active packing, the mold density drops and causes sink marks on thick wall sections. This packing must continue until the gate has frozen completely.
Mechanical Property
Higher material density improves the structural stiffness and chemical resistance of the molded component. However, an excessively high mold density can increase internal molded-in stress and make the part susceptible to stress cracking over time. Moulders must balance these mechanical trade-offs by setting appropriate injection and cooling parameters.
Using regrind instead of virgin resin can also alter the final density due to molecular chain shortening.