Meaning
Surface degradation on mating metallic tool components arises when microscopic particles weld together during high pressure sliding contact. Micro galling creates localized transfer of material from the moving part to the stationary mold element. This process occurs during the rapid actuation of slide cores or ejector pins in an injection environment.
The phenomenon generates surface roughness that exceeds the initial polish specifications of the steel cavity. When sliding surfaces lack sufficient lubrication or thermal expansion clearance, the microscopic welding becomes widespread. Engineers quantify the severity of damage through optical profilometry to measure the depth of craters left by torn metal.
The boundary for this condition starts when material transfer causes a visible change in the finish of the molded part or impedes the mechanical movement of the tool.
Tooling Geometry
Precise clearance settings determine the likelihood of material accumulation on guide pins and sliding shutoffs. Designers specify clearances that allow for the thermal expansion of mold steel during the heat of a production cycle. If a design provides insufficient space for oil film retention, the surfaces contact without a protective barrier.
Contact between naked steel substrates under high tonnage loads triggers the localized diffusion that characterizes micro galling. Designers adjust the surface hardness of opposing components to minimize the tendency for adhesive wear between identical alloys. Different hardening treatments create a hardness gradient that discourages the welding of sliding surfaces.
Resin Interaction
High fill loadings in thermoplastic compounds exacerbate the abrasive wear caused by sliding metal contact. Fibrous additives like glass or mineral particles scrape the surface of the mold and strip away protective oxide layers. Once the bare metal becomes exposed to the high pressure of the injection shot, micro galling develops rapidly.
Moulders observe that resins with high viscosity require higher injection pressure and faster velocities to fill the cavity. These settings increase the frictional heat generated at the tool surface which softens the steel alloy. Resin regrind often contains contaminants or high levels of fiberglass fines that accelerate the degradation of the mold finish.
Virgin resin grades with high crystallinity exhibit lower adhesion to the metal surface compared to soft amorphous materials.
Production Outcome
Financial losses originate from the unscheduled maintenance required to refurbish damaged mold surfaces. Technicians replace or weld and re-machine the affected areas when the surface texture transfers to the molded product. Scrapped parts represent the primary economic burden when the surface degradation becomes severe enough to reject the molded goods.
Production schedules suffer when tool components require removal for polishing to restore the original finish. Consistent monitoring of ejection force provides an early warning of surface degradation before the damage propagates to the cavity. High ejection force values indicate increased friction from microscopic adhesion points on the sliding elements.
The total volume of defective output scales linearly with the degree of surface adhesion occurring inside the tool.
