Cavity Surface Texture Effects on Thermoplastic Heat Transfer and Shrinkage
Cavity surface texture increases interfacial thermal contact resistance, delaying resin skin freeze and increasing semi-crystalline thermoplastic shrinkage.
Bead Blasting
Meaning
Mechanical surface finishing propels abrasive spherical media against mould cavity surfaces to create uniform non-directional matte micro-textures. In tooling preparation for polymer processing, bead blasting establishes target surface roughness on hardened steel or aluminium cores and cavities to modify part gloss, mask cosmetic sink marks and adjust ejection friction. The process alters surface morphology without removing bulk parent metal.
Its utility ceases on optical-grade mould surfaces requiring specular reflection or mirror polish.
Media Dynamic
Pressurised pneumatic streams deliver glass, ceramic or stainless steel beads against prepared tool steel. Media velocity, nozzle distance and blast angle govern the resulting surface profile, typically measured as arithmetic mean roughness Ra. Glass spheres produce smooth, rounded micro-dimples, whereas crushed media generate sharp angular peaks that cause severe mould sticking. Toolmakers replace spent media regularly to maintain uniform particle size distributions across large cavity impressions.
Process Friction
Microscopic indentations on cavity walls influence polymer release dynamics and part aesthetics during the ejection cycle. Textured surfaces diffuse reflected light, hiding cosmetic flow lines and minor sink marks across thick-to-thin wall transitions. Excessive surface roughness on vertical ribs creates mechanical undercuts that increase demoulding resistance.
Moulders apply additional draft angle on blasted faces to avoid micro-scuffing during mechanical ejection.
Production Maintenance
Abrasive mineral fillers and high-shear glass compounds erode micro-textures over extended production runs. Polyoxymethylene and reinforced polyamides wear down blasted finishes faster than unfilled polyethylene, causing matte components to become glossy over time. Toolroom maintenance schedules incorporate periodic re-blasting using masked boundaries to preserve critical shut-off edges.
Tooling engineers verify surface roughness profiles with contact profilometers before returning tools to production presses.