Meaning
Surface fracturing occurs when fatigue or mechanical stress removes microscopic flakes from the hardened exterior of a steel mould component. Micro-spalling creates pits that disrupt the surface finish of moulded parts and alters the flow path of molten resin. Tooling steel undergoes this degradation when contact pressures exceed the yield strength of the substrate.
Processing Fatigue
High injection pressures and rapid cycles accelerate the deterioration of metal surfaces exposed to abrasive fillers in polymer compounds. These forces drive hard particles into the cavity wall during every fill stage. Mould maintenance schedules track these events by monitoring increases in injection pressure requirements as the surface roughness increases.
Surface irregularities catch the material and demand higher ejection force which risks further damage to the part and the tool.
Economic Consequence
Parts produced in worn cavities often fail visual inspections due to undesirable textural changes or flash development at the site of the pits. Rejection rates climb when the mould integrity drops below established quality limits. Purchasing regrind material adds a layer of risk because internal impurities act as localized stress concentrators that hasten the breakdown of the steel finish.
Manufacturers avoid this cost by balancing cycle times against the expected lifespan of the tooling surface.
Surface Specification
Dimensional tolerances define the acceptable depth of material loss on a cavity before the tool requires reconditioning. Engineering documents state these limits in microns to differentiate between manageable wear and total part failure. Operators verify the finish using profilometry equipment during scheduled downtime to determine if the cavity requires polishing or full recoating.
Proper heat treatment of the base metal remains the most effective defense against the initiation of surface pits.
