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.
Spatial Auto Correlation Length Sal
Meaning
A horizontal surface parameter identifies the shortest distance over which a texture becomes statistically independent. The spatial auto correlation length Sal is used to distinguish between highly directional, periodic textures and random, isotropic surfaces. In polymer molding, this parameter assesses the consistency and orientation of textures on the molded part.
Texture Uniformity
Evaluation of surface texture consistency utilizes the spatial auto correlation length Sal to detect pattern variations. Isotropic textures, like those on acid-etched mold surfaces, produce low Sal values because the texture varies rapidly in all directions. In contrast, surfaces with linear toolmarks or directional graining exhibit high Sal values along the direction of the marks.
Flow Alignment
Injection molding introduces polymer flow patterns that can align the polymer molecules and any filler fibers along the flow direction. This flow alignment affects the replicated surface texture, which can be measured using the spatial auto correlation length Sal. When the melt temperature is too low, the polymer freezes too quickly, locking in highly directional flow textures that increase the Sal value.
Adjusting the mold temperature helps restore the desired isotropic finish of the mold cavity.
Optical Scattering
Excessive directional texture on clear molded parts causes unwanted optical effects. Measuring the spatial auto correlation length Sal provides a metric for assessing the scattering behavior of the part surface. This ensures the parts meet optical clarity standards.