Determining Polymer Wall Conductance on Smooth Tool Steel
Polymer wall conductance on smooth tool steel peaks above 3500 W/m²K under packing pressure and drops below 600 W/m²K upon thermal gap detachment.
Volumetric Thermal Contraction
Meaning
Density changes that occur as a molten polymer cools and solidifies within the mold cavity determine the overall shrinkage and dimensional accuracy of the finished component. This volumetric thermal contraction must be compensated for during the packing and holding stages of the injection molding process to prevent the formation of internal voids and surface sink marks. The degree of contraction varies significantly between amorphous and semi-crystalline resins due to their different molecular structures.
Cavity Packing
Moulders must inject additional material into the mold cavity as the polymer cools to make up for the reduction in volume. If the volumetric thermal contraction is not countered by adequate packing pressure before the gate freezes, the part will exhibit sink marks or vacuum voids in its thickest sections. This step is critical for maintaining part weight and density.
Warpage Control
Uneven contraction across different sections of a part leads to internal stresses that can cause the part to twist or warp upon ejection. Minimizing the effects of volumetric thermal contraction requires uniform wall thicknesses and balanced mold cooling to ensure that all regions of the part solidify at the same rate. This balanced solidification minimizes dimensional deformation and post-molding distortion.
Material Properties
Semi-crystalline polymers undergo a much higher degree of contraction than amorphous polymers because their molecules arrange themselves into highly dense, ordered crystal lattices. This high level of volumetric thermal contraction requires mold designers to use larger shrinkage allowances when sizing the mold cavity steel for materials like nylon or acetal. Amorphous polymers like polycarbonate shrink much less, allowing for tighter dimensional control.
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.