Optical Profilometry Basics for Hardened Steel Tool Cavity Verification
Optical profilometry non-destructively maps 3D tool cavity topography (Sa, Sz) to verify polished steel finishes and predict injection part ejection forces.
SPI Finish Standards
Meaning
Standardized visual and roughness classifications established by the Society of Plastics Industry categorize surface finishes on injection mould steel cavities. Tool makers utilize SPI finish standards to specify exact cavity surface preparation grades ranging from diamond polish or textured blast surfaces. The standard categorizes finishes into letter grades, dictating the required finishing process and expected surface roughness value Ra. The classification boundary applies to mould cavity steel preparation and does not guarantee identical surface roughness on moulded plastic parts due to resin shrinkage and replication limits.
Polishing Protocol
Progressive abrasive steps using diamond pastes and abrasive stones create specific surface textures on tool inserts. Following SPI finish standards dictates the exact abrasive sequence required to produce Grade A-1 optical specular mirrors down to Grade D-3 textured matte finishes. Diamond polishing produces mirror finishes required for optical lenses and clear covers.
Stoned finishes provide economical release for unexposed structural plastic parts.
Ejection Behavior
Cavity surface roughness directly influences the mechanical sliding friction experienced by cooling polymer parts during ejection. Adherence to SPI finish standards assists mold designers in choosing appropriate draft angles and ejection pin layouts for specific finish grades. Highly polished Grade A finishes can increase vacuum adhesion on flat core surfaces, requiring draft and air release valves.
Rougher Grade C and D finishes require larger draft angles to prevent sidewall scuffing during ejection.
Cost Impact
Achieving high specular polish requires extensive manual labor by skilled mold polishers, increasing initial tooling costs.