Optical Profilometry Basics for Hardened Steel Tool Cavity Verification
Optical profilometry non-destructively maps 3D tool cavity topography (Sa, Sz) to verify polished steel finishes and predict injection part ejection forces.
Surface Roughness Parameters
Meaning
Quantitative mathematical descriptors evaluate profile height variations and surface texture attributes on machined metal or moulded plastic parts. Engineers calculate surface roughness parameters such as Ra and Rz to characterize cavity steel finishes and evaluate polymer replication fidelity. Profilometers measure fine vertical surface deviations from a mean line across defined evaluation lengths.
The boundary of profile roughness parameters stops at micro-scale surface texture and excludes macro-scale dimensional waviness or form errors.
Profile Characterization
Arithmetic mean height Ra provides a single numerical average of profile deviations but hides individual height extreme peaks and valleys. Combining multiple surface roughness parameters gives toolmakers a complete profile description, distinguishing sharp peaks from deep retention valleys. Rz measures maximum peak-to-valley height, identifying deep scratches that increase demoulding friction.
Skewness Rsk indicates whether a surface profile has rounded peaks or sharp valleys, predicting oil retention and wear resistance.
Moulding Correlation
Cavity surface texture dictates the tactile gloss and visual appearance of injected plastic products. Measuring surface roughness parameters on both mould steel inserts and final plastic parts quantifies replication accuracy across processing parameters. High cavity pressure and warm tool steel produce high replication, yielding part roughness values matching the mould.
Low injection pressure or cold tools yield poor replication, resulting in higher part roughness than the cavity steel.
Quality Specification
Engineering drawings specify acceptable roughness ranges to ensure functional seal performance and cosmetic consistency across production lots.