Matching Tool Steel Selection to Initial Production Commitments
Match tool steel hardness and chemistry directly to committed shot counts and resin abrasiveness to prevent premature cavity failure and tooling cost overruns.
Polyoxymethylene Corrosive Degradation
Meaning
Thermal decomposition of acetal homopolymers and copolymers releases volatile acidic off-gases that chemically attack tool steel tooling surfaces. Severe exposure to polyoxymethylene corrosive degradation pits mould cores and strips protective plating across cavity vents. Overheating acetal resin above its thermal threshold triggers depolymerization into gaseous formaldehyde, which oxidizes into formic acid inside unvented cavity pockets.
The degradation mechanism stops when melt temperatures remain within safe processing limits and cavity venting permits rapid off-gas clearance.
Acid Attack
Formic acid condensate generated during resin overheating directly reacts with iron content in un-plated cavity steel. Chemical pitting roughens polished mould surfaces, degrading component surface finish and creating micro-undercuts that hinder ejection. Unvented dead ends in runner systems accumulate corrosive condensate, causing accelerated localized pin pitting.
Melt Temperature Boundary
Residence times exceeding ten minutes inside heated barrel zones cause rapid polymer chain scission. Operating near upper temperature limits exacerbates gas liberation and accelerates tool surface damage.
Corrosion Mitigation
Preventing chemical damage requires applying surface coatings like electroless nickel or titanium nitride to steel cores. Standard stainless tool steels like 420 or 1.2083 offer superior resistance to acidic off-gassing compared to standard P20 steels. Purging processing machinery with non-corrosive polymers prior to thermal shutdown removes residual acetal melt from the barrel.
Mould maintenance protocols for acetal tooling include immediate solvent washing of parting lines to strip acidic residue after production runs.