Cavity Surface Texture Effects on Thermoplastic Heat Transfer and Shrinkage
Cavity surface texture increases interfacial thermal contact resistance, delaying resin skin freeze and increasing semi-crystalline thermoplastic shrinkage.
Residual Pressure
Meaning
Fluid pressure remaining within the mold cavity after the gate has frozen and before the mold opens must be minimized to prevent ejection difficulties and part deformation. This residual pressure arises when the packing and holding phases force excess material into the mold without giving it sufficient time to shrink or relax before demolding. Monitoring this variable is critical to ensuring a clean release of the part from the tool steel.
Release Behavior
High clamping and packing forces can result in excessive material being locked within the cavity. If the residual pressure is not allowed to decay before the mold opens, the plastic part will remain pressed hard against the cavity walls, causing scuffing and high ejection forces. This mechanical stress can damage both the part and the ejector mechanism.
Part Stress
Molded components that are ejected while still under significant stress are highly prone to warping and cracking. The presence of high residual pressure during demolding indicates that the polymer chains did not have sufficient time to relax from their stretched configurations. This high level of internal stress can lead to environmental stress cracking when the parts are exposed to chemicals or load.
Measurement Method
Cavity pressure transducers mounted behind ejector pins provide direct real-time feedback on the state of the mold. Operators use this data to adjust the hold time and hold pressure to ensure that the residual pressure has dropped to near zero before mold opening. This optimization minimizes mold wear and increases process repeatability.