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.
Autocorrelation Length
Meaning
Spatial decay mapping defines the statistical distance over which polymer chain orientations lose correlation within an extruded film sheet. This mathematical metric evaluates molecular relaxation patterns across extruded polyethylene film webs, governing optical clarity and mechanical tear resistance. The parameter ceases to apply when molecular weight distribution narrows beyond the threshold required for meaningful orientation analysis.
Spatial Resolution
Polymer scientists compute autocorrelation length from laser light scattering profiles captured along the machine direction of blown film lines. Extrusion line speeds and chill roll temperatures dictate the final value during processing runs. Molecular memory retention creates severe gauge bands and uneven shrinkage when the physical distance shifts outside strict manufacturing tolerances.
Virgin resin lots maintain predictable chain relaxation curves, whereas regrind fractions introduce compositional variance that disrupts spatial continuity. Laboratory technicians measure film samples under standardized tension to eliminate thermal noise before running calculations. Datasheet values provided by compound suppliers rarely match the actual performance observed on shop floor production machinery.
Thermal Decay
Temperature profiles across the annular die determine how polymer chains relax before quenching freezes the amorphous domains. Barrel heaters and internal bubble cooling air volumes control molecular stretch ratios immediately above the extrusion die lip. Excessive heat input elongates the spatial decay distance, leaving finished parts vulnerable to curling and dimensional distortion during subsequent thermoforming steps.
Part specifications demand tighter control over molecular orientation than generic resin datasheets require for raw pellets. Cooling rate fluctuations generate localized variations in structural uniformity across wide sheet extrusions.
Structural Variance
Melt fracture defects appear on final products whenever local shear rates exceed the relaxation capacity of high density polyethylene compounds. Die gap geometry and screw compression ratios influence the spatial persistence of polymer chain alignment. Tooling wear alters melt flow velocity profiles inside the extrusion head, changing the statistical distance without any adjustment to resin grade or barrel temperatures.
Downstream winders compensate for minor fluctuations by regulating tension profiles across the web surface. Molecular relaxation behavior dictates the ultimate mechanical integrity of extruded packaging materials.