Meaning
Thermodynamic variables outside of temperature influence the speed and temperature at which a polymer melt transitions to a solid state. Pressure dependent kinetics explains why a polymer might solidify earlier than expected when high packing pressures are applied. Increased pressure reduces the free volume between molecular chains, which shifts the crystallization temperature upward.
Understanding this relationship helps in predicting the final dimensions of parts moulded under extreme conditions.
Compression Effect
Molecular mobility decreases as the chains are forced closer together under hydrostatic load. This reduction in volume restricts the ability of the polymer to flow into thin sections of the tool.
State Correlation
Relationships between pressure, volume and temperature define the specific volume of the resin at any point in the cycle. High pressure effectively increases the glass transition temperature, making the material behave as if it were cooler than its actual thermal reading. Engineers use these correlations to calculate the correct hold time required to freeze the gate.
Without this data, a process might be set with a cycle time that is either too long for efficiency or too short for stability. Modern simulation software incorporates these effects to improve the accuracy of shrinkage predictions.
Flash Prevention
Rapid increases in cavity pressure can overcome the clamp force of the machine and force the mould open slightly. Monitoring the pressure dependency helps in setting limits that prevent the resin from seeping into the parting line.
