Tooling Surface Polish Impact on First Article Inspection Accuracy
Uncalibrated optical scans on mirror-polished plastic introduce up to fifteen micrometers of false elevations, ruining First Article Inspection accuracy.
EDM Surface Scale
Meaning
Thermal alteration occurring during thermal erosion operations leaves a brittle, recast metallurgical layer on tool steel cavity walls. Mold makers encounter edm surface scale after spark erosion processes, requiring mechanical polishing or chemical etching to prevent premature part ejection failure. Rapid melting and quenching during spark discharges form untempered martensite and micro-cracks along the steel surface.
The boundary of this condition extends only to the heat-affected zone beneath the eroded pocket.
Metallurgy Alteration
Pulse duration and peak current determine the depth and hardness of the thermally modified surface structure. High-energy roughing passes generate thick edm surface scale containing tensile stresses and embedded carbon particles from hydrocarbon dielectric fluids. Micro-cracking within this hardened skin propagates under cyclic injection moulding pressure, resulting in premature mold fatigue.
Fine finishing burns diminish layer thickness, producing a refined surface structure that requires minimal secondary benchwork.
Polishing Requirement
Removing the compromised recast layer demands manual stoning or chemical etching prior to production sampling. Leaving edm surface scale intact inside deep textured ribs causes plastic melt to key into micro-voids during packing. Hand polishing must remove the full depth of the white layer without altering core dimensions or rounding sharp shut-off edges.
Ejection Friction
Unpolished recast layers increase ejection force by trapping cooling resin inside microscopic surface undercuts. Polymer parts molded in un-etched cavities exhibit scuff marks and drag lines along vertical sidewalls.