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.
Transparent Resin Back Reflection
Meaning
An optical measurement artifact occurs when light returns from the second surface of a clear molded part to interfere with surface metrology. The transparent resin back reflection complicates the measurement of surface roughness on clear polymer lenses and light guides, as the sensor receives signals from both the top and bottom surfaces. In plastic injection molding metrology, this phenomenon must be managed to obtain accurate roughness measurements of transparent components.
Optical Interference
Generation of incorrect surface data occurs when the light reflected from the rear surface of the part interferes with the primary reflection from the top surface. This transparent resin back reflection can create false surface profiles, showing non-existent peaks or valleys. The severity of this artifact depends on the thickness of the molded part and the refractive index of the resin.
Metrology Optimization
Overcoming this measurement challenge requires the use of specialized techniques during surface evaluation. Applying a non-reflective coating or a temporary refractive-index-matching fluid to the back surface of the molded part eliminates the transparent resin back reflection by absorbing or scattering the transmitted light. Alternatively, using a coherence scanning interferometer with a short coherence length allows the operator to isolate the signal from the top surface.
This ensures that the measured roughness values are accurate and reflect the true surface quality of the part.
Signal Enhancement
Proper management of the back reflection ensures that the metrology system can accurately verify the surface finish of the molded part. This allows moulders to maintain quality control without having to coat or damage the finished parts. This non-destructive testing is crucial for high-throughput production lines.