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.
Root Mean Square Surface Roughness Sq
Meaning
A statistical amplitude parameter represents the standard deviation of surface heights to evaluate finish quality. The root mean square surface roughness Sq provides a robust measure of the deviation of the surface from the mean plane, making it more sensitive to extreme peaks and valleys than the average roughness Sa. In plastic injection molding, this metric defines the benchmark for both mold cavity finishes and the resulting plastic parts.
Finish Characterization
Quantification of a surface using Sq utilizes a square root calculation of the squared height values. This mathematical approach highlights outlier features that could alter the optical or tactile properties of the molded part. For high-polish optical lenses, the parameter is a standard datasheet value that reflects the potential for light scattering.
Mold Replicability
Perfect replication of the mold surface is a primary goal in high-precision molding. The root mean square surface roughness Sq of the molded plastic is compared against that of the mold steel to determine replication efficiency. Processing variables such as cavity pressure and melt temperature must be optimized to ensure the polymer flows into the micro-textures of the mold.
If replication is incomplete, the parts exhibit a higher roughness than the tool, indicating a need for process adjustment.
Part Performance
Excessive roughness on mating molded parts leads to friction and premature wear during operation. Monitoring the root mean square surface roughness Sq during production runs ensures that the parts remain within the wear limits specified by the design. This prevents premature component failure in the field.