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.
Chemical Etching
Meaning
Controlled acid etching removes selective areas of tool steel to produce decorative grain textures, geometric patterns or stippled finishes on mould cavity surfaces. In injection moulding tooling, chemical etching transfers photographic film patterns onto cavity walls through acid baths, imparting leather, woodgrain or geometric textures directly into moulded polymer parts. The process alters only the near-surface microstructure to prescribed depths.
It stops applying once corrosion penetrates through protective photoresist barriers or exceeds mechanical draft angle allowances.
Process Chemistry
Photolithographic masks protect targeted cavity regions while acid formulations dissolve exposed steel. Multiple etching passes build multi-depth textures, creating complex visual grains on instrument panels and consumer enclosures. Nitric or hydrochloric acid mixtures must match the exact metallurgy of P20, H13 or stainless mould steels to avoid uneven pit formation.
Toolrooms wash and neutralise acid residues completely to prevent progressive corrosion across tool shut-off lands.
Draft Consequence
Etched cavity walls introduce micro-undercuts that increase mechanical resistance against the moving mould half during opening. Moulders apply a general rule of one additional degree of draft angle for every twenty-five micrometres of texture depth. Inadequate draft leads to scuff marks, white-blush stress marks and part retention on the wrong mould half.
Polypropylene and thermoplastic elastomers replicate fine chemical grains faithfully, while high-viscosity resins require elevated pack pressures to fill micro-cavities completely.
Texture Longevity
Mould steel hardness and resin abrasive content dictate the operational life of chemically etched patterns. Glass-reinforced polymers abrade fine textural edges over several hundred thousand shots, causing gloss variation and loss of visual depth across high-wear gating zones. Mould maintenance requires localised re-etching or laser restoration to match original master grain plaques.
Procurement specifications define allowable gloss tolerance ranges to control acceptable textural degradation before tool refurbishment.