Meaning
Microscopic surface texture produced on hardened steel mould cavities determines the ejection behavior and cosmetic optical properties of injection moulded parts. Quantified through surface roughness parameters, tool steel surface finish directly influences demoulding friction and resin flow drag. The specification dictates polishing procedures, electro discharge machining parameters or photo etching standards applied to cavity walls.
The scope of this parameter covers forming steel surfaces, excluding unmachined mould plate exteriors and clamping slots.
Cavity Polishing
Progressive hand polishing using diamond pastes achieves mirror finishes on optical tool steel. Coarse abrasive marks left from CNC milling must be systematically replaced with finer grit scratch patterns to prevent localized stress risers. Toolmakers work through diamond paste grades down to one micron to produce SPI A1 optical cavities.
Residual scratches on cavity steel mirror onto moulded transparent parts, causing light scattering and aesthetic haze. High hardness steels like S7 or H13 resist pitting during high gloss polishing, maintaining surface consistency across production runs. Molded parts produced from highly polished cavities release cleaner, provided vacuum relief venting is adequate.
Ejection Friction
Textured cavity finishes increase contact area with polymer melts, altering mechanical ejection force requirements. Coarse EDM surfaces trap resin within micro voids, demanding larger draft angles to prevent part tearing during demoulding. Optimizing surface finish balances aesthetic matte requirements against demoulding force limits.
Cosmetic Defect
Inconsistent tool steel finish causes localized gloss variation across moulded part surfaces. Sink marks opposite thick ribs appear more prominent against mirror polished cavity walls than on textured surfaces. Selecting appropriate steel finishes masks minor moulding defects while maintaining visual quality standards.
