ISO 25178 Surface Parameters for Injection Moulding Optical Components
ISO 25178 areal parameters define 3D optical surface texture to control stray light and guide injection moulding process windows for precision lenses.
Surface Height Parameter Sa
Meaning
Arithmetic mean height deviation represents the absolute vertical distance from the mean plane to all points across a defined measurement area. Surface height parameter sa quantifies the average deviation of the topographic surface from the reference plane, providing a comprehensive assessment of overall texture height regardless of peak or valley distribution. This metric provides a scalar value that averages the absolute deviations of all heights within the evaluated domain.
Mathematical calculation of this parameter treats the surface as a three dimensional grid where the height at each coordinate point contributes to the total sum of absolute deviations. Absolute values prevent positive and negative deviations from cancelling each other out during the averaging process.
Moulding Variance
Polymer surface finish depends heavily on the heat transfer rate between the molten resin and the cavity wall. Higher values of surface height parameter sa often indicate incomplete contact between the polymer melt and the tool steel during the cooling phase. Moulders frequently adjust injection pressure or holding force to force the resin into the microscopic features of the mould, reducing this average deviation value.
Excessive reliance on machine settings to overcome tool deficiencies leads to increased cycle times and higher internal stress in the finished part. Resin rheology affects the outcome since materials with high viscosity require more pressure to achieve the same surface finish as lower viscosity counterparts.
Economic Consequence
Operational costs rise when a moulder must maintain a specific surface height parameter sa that exceeds the baseline capabilities of the existing tooling. Verification requires optical or contact profilometry which adds time to the quality inspection routine for each batch. Virgin resin typically provides predictable shrinkage rates that stabilize the surface texture, while the introduction of regrind material introduces impurities and varying flow rates that increase the variability of this parameter.
Parts failing to meet surface requirements often end up as scrap or require secondary finishing steps to achieve the desired texture uniformity. Moulders calculate the cost of maintaining tight surface tolerances against the risk of aesthetic failure or functional leakage in seals and gaskets.
Material Specification
Engineering datasheets define a surface height parameter sa range to ensure parts fit properly within an assembly. Designers treat this value as a hard constraint when the part must form a vacuum seal or house a bearing surface. Material suppliers provide reference data based on standardized plaques, but the actual part geometry creates different cooling conditions that shift the realized value away from the laboratory figure.
Producers distinguish between the surface quality of a tool-polished steel cavity and the replicated surface on a polymer component. Deviation between the designed specification and the actual moulded result marks the boundary of successful process control.