Standard ISO 25178 Parameters for Optical Profilometry Inspection
ISO 25178 areal optical profilometry replaces subjective 2D stylus traces with quantitative 3D parameters that accurately govern tool wear and part demoulding.
Ssk
Meaning
Areal height distribution metrics describe the symmetry of surface profile variations around a mean plane. Quality technicians measure Ssk, the surface skewness parameter, to distinguish between surfaces dominated by sharp peaks and surfaces characterized by deep valleys. A symmetrical height distribution yields an Ssk value of zero, while plateau surfaces with isolated valleys return negative values.
Datasheet roughness average values cannot differentiate a peak-heavy profile from a valley-heavy profile.
Distribution Symmetry
Third moment statistical calculations quantify profile deviation asymmetry across the spatial sampling grid. Positive skewness indicates sharp structural peaks rising above a flat background plane. Negative skewness signifies flat load-bearing plateaus broken by narrow micro-grooves.
Texturing methods for moulding inserts manipulate skewness to optimize lubricant retention without increasing peak friction.
Functional Impact
Negative skewness on mould core pins reduces part release friction by minimizing peak contact points. Sliding components with negative Ssk retain mold lubricants within micro-valleys during high temperature cycles. Regrind polymer processing introduces hard particulate contaminants that scratch tool surfaces and drive skewness toward positive values.
Mold polishers target negative skewness to improve tool wear life.
Measurement Boundary
Extraneous dust particles or optical outliers artificially skew statistical results toward high positive numbers. Filtering algorithms must remove non-functional extreme height outliers prior to parameter calculation. Verification protocols require averaging multiple sampling areas to ensure representative skewness metrics.