Optical Profilometry and ISO 25178 Parameters for Cavity Wear
ISO 25178 areal parameters like Ssk, Sku, and Vvc measured via optical profilometry quantify cavity wear, predicting ejection failures before part dimensions drift.
Areal Roughness
Meaning
Three-dimensional surface topography measurement quantifies micro-irregularities across a defined sample area rather than along a single linear profile. Areal roughness evaluates the micro-geometry of injection-moulded polymer components, governing how well a surface releases from a metal mould cavity during ejection. Toolmakers control this parameter during electrical discharge machining and photo-etching operations to establish the required texture on tool steel inserts.
Polymer melt flows across these microscopic peaks and valleys during high-pressure injection, copying the metallic topography into the solidifying skin layer of the part. This geometric transfer stops applying once the polymer freezes below its glass transition temperature against the chilled tool wall.
Tooling Surface Texture
Thermal distribution across the mould cavity dictates how uniformly the polymer fills the microscopic valleys defined by areal roughness. Cavity walls held at elevated temperatures allow the polymer chains to relax fully into every topographic crevice, whereas rapid chilling freezes the melt prematurely and creates short shots of the intended surface texture. Tooling fabricators measure this parameter on both the raw steel insert and the final moulded component to verify that the replication ratio meets production standards.
High shear rates during the filling phase push viscous polymer against the tool boundaries, pressing material into the valleys of the areal roughness profile.
Polymer Shrinkage Stress
Volumetric contraction during cooling pulls the solid skin layer away from the tool steel, altering the apparent areal roughness on the finished part relative to the cavity surface. Semi-crystalline thermoplastics shrink significantly more than amorphous plastics during this phase, creating a wider gap between the original tool finish and the final component topography. Excessive packing pressure forces the molten polymer deeper into the microscopic valleys, locking in high residual stresses that warp thin-walled mouldings upon ejection.
Processing technicians adjust barrel temperatures and injection speeds to counteract the shrinkage forces that distort the micro-geometry of the moulded component.
Release Resistance Force
Interlocking friction between the polymer skin and the tool steel increases exponentially when the areal roughness exceeds the optimal threshold for a specific resin grade. Virgin polyethylene tolerates aggressive surface textures without tearing because the material retains high elongation at break, whereas recycled regrind batches exhibit brittle failure under the same mechanical drag. Mould designers calculate draft angles based on the peak height distribution of the areal roughness to prevent parts from sticking during the opening stroke of the injection press.
Ejection pins punch through the component wall when the release resistance created by excessive micro-texture surpasses the structural integrity of the cooling polymer.