Injection Mould Cavity Surface Roughness Measurement Using Silicon Replicas
Indirect silicone replica metrology resolves deep mould cavity roughness parameters non-destructively within five percent variance of direct steel scans.
SPI Finish
Meaning
An industry standard defines the specific surface texture and cosmetic appearance of a mould steel cavity and the resulting plastic part. Developed by the Society of the Plastics Industry, the SPI finish classification ranges from highly polished diamond finishes to rough grit-blasted textures. This system provides a common language for designers, toolmakers, and moulders to specify surface quality.
Standard Classification
The standard divides surface finishes into four primary categories, each with three distinct sub-levels. Grade A designates high-gloss finishes produced using diamond powders, ideal for optical lenses. Grade B represents semi-gloss finishes achieved through fine paper polishing, while Grade C uses stone polishing to eliminate tool marks.
Finally, Grade D uses dry blasting with glass beads or sand to create a dull, textured surface. Each grade has specific roughness values that can be measured and verified using profilometry, ensuring consistency across different toolmakers.
Moulding Performance
Surface texture directly influences how easily a part can be ejected from the mould cavity. Highly polished surfaces can create a vacuum, causing the part to stick to the metal. Adding a slight texture or draft angle resolves this, while textured finishes can hide sink marks and scratches.
However, rougher finishes require larger draft angles to prevent scuffing during ejection.
Production Cost
Achieving and maintaining high-quality finishes significantly impacts tooling budgets. Polishing a cavity to a Grade A finish is a labor-intensive process that requires skilled manual work. Over time, corrosive resins like PVC can degrade the polished surface, necessitating regular repolishing.
Moulders must balance cosmetic needs against these maintenance costs.