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.
Rz
Meaning
Arithmetic mean height of the five tallest profile peaks minus the five deepest valleys calculated over a single sampling length defines rz. This surface topography parameter quantifies the vertical roughness of a moulded part or film texture. Measurements occur along a line perpendicular to the mean plane of the feature.
Surface finishing processes rely on this metric to ensure functional contact areas for gaskets or friction fits.
Profile Sensitivity
Variations in resin viscosity during injection moulding alter the fidelity of tool texture transfer. High rz values frequently indicate insufficient packing pressure or incorrect cavity temperatures where the plastic fails to replicate the grain of the steel. Regrind integration changes flow characteristics and shifts these roughness results outside of design tolerances.
Proper calibration of the stylus instrument remains mandatory for reliable readings across diverse polymer grades.
Dimensional Influence
Material shrinkage across a cooling cycle impacts the final roughness profile. Tooling wear on textured inserts leads to a gradual reduction in peak height as abrasive fillers erode the metal geometry. Specifications for engineering components differentiate between the rz requirement of the raw material datasheet and the actual performance of the finished injection moulded object.
Verification requires selecting a sampling length that aligns with the structural intent of the part surface.
Production Boundary
Measurement of rz provides a consistent indicator of tool condition but ignores the horizontal spacing between peaks. Technical limits for this parameter depend on the intended mechanical function, such as lubrication retention or optical clarity requirements. Absolute control over the roughness value proves impossible when raw material batches exhibit high variability in melt flow index.
Consistent process parameters remain the sole method for stabilizing the surface finish of mass produced plastic components.