Optical Profilometry Metrology Protocols for Indirect Tool Cavity Topography Verification
Calibrated silicone replicas combined with optical profilometry yield sub-micron tool cavity topography metrics without destructive sectioning.
Linear Shrinkage
Meaning
Dimensional reduction occurs in molded plastic parts as they cool from the melt state to ambient room temperature. The measure of linear shrinkage represents the percentage difference between the physical dimensions of the mold cavity and those of the cold, solidified part. This value is essential for predicting part sizes, since polymers naturally experience a substantial density increase when transitioning from a liquid to a solid state.
Material Behavior
Polymer chemistry and molecular structure dictate the baseline contraction of the material. Unfilled amorphous resins typically exhibit low, isotropic linear shrinkage, while semi-crystalline polymers shrink to a much greater degree and can be highly anisotropic. This directional variation means that the material contracts differently along the direction of polymer flow compared to the transverse direction, which often leads to warpage.
Incorporating glass fibers or mineral fillers can reduce this contraction but further increases anisotropy, making flow-analysis software essential for predicting the final part shape.
Process Influence
Processing parameters can alter the actual shrinkage of a part compared to the value listed on the resin datasheet. High injection and holding pressures pack more polymer chains into the cavity, which decreases the total linear shrinkage. Mold temperature also plays a major role, as warmer tools allow crystals to grow more fully in semi-crystalline materials, increasing the final contraction.
Tooling Adjustment
Tooling designers use the datasheet shrinkage value to scale the mold cavity dimensions up during the design phase. If linear shrinkage is not properly accounted for, the finished parts will be too small and will not fit within their sub-assemblies. This adjustment must be tailored to the specific polymer grade, as even minor changes in filler content can alter the contraction behaviour.
Dynamic Cavity Pressure Integral Correlation with Part Dimensions in Injection Moulding
Dynamic cavity pressure curve integration over hold time governs volumetric polymer density and predicts final part dimensions before tool ejection occurs.