Polyamide Cavity Pressure Influence on Moulded Shrinkage Rates
Increasing peak cavity pressure compresses polyamide melt during solidification, directly counteracting phase-change volumetric shrinkage and tightening component tolerances.
Specific Volume Gradient
Meaning
Density variation across the thickness or length of a molded part. A specific volume gradient is caused by uneven cooling and pressure distribution during the injection and packing phases. This variation leads to internal stresses that can cause the part to warp or fail prematurely.
Thermal Influence
Material near the mold wall cools faster and becomes denser than the material in the center. This specific volume gradient is influenced by the thermal conductivity of the polymer and the cooling circuit design. High gradients often result in visible sink marks on the surface.
Packing Pressure
Maintaining high pressure throughout the cooling phase helps to minimize the specific volume gradient. If the pressure drops too soon, the core of the part will have a markedly lower density than the skin. This difference creates a pull on the outer surfaces as the core shrinks.
Proper pressure management ensures that the material is distributed evenly across the geometry.
Integrity Limit
Parts with a uniform density are more resistant to impact and environmental stress cracking. Minimizing the specific volume gradient requires precise control of the injection speed and mold temperature. This ensures that the part has consistent mechanical properties throughout its volume.