Meaning
Localised micro-deformation within amorphous and semicrystalline polymer domains creates craze formation during injection moulding whenever tensile stress exceeds local yield limits. Injection pressure profiles govern this phenomenon by dictating molecular orientation during cavity fill and subsequent packing stages. Thermoplastic resins such as polycarbonate and polymethyl methacrylate develop these microscopic fibrillar regions under excessive residual cooling stresses.
The defect boundary sits precisely at the threshold where microscopic voiding transitions into macroscopic brittle fracture across a moulded part.
Thermal Gradient
Barrel temperature settings and mould wall conditions heavily influence craze formation through uneven volumetric shrinkage rates. Cooling line imbalances induce thermal shock across the part cross section during high speed injection cycles. Resins processed below recommended melt temperatures retain frozen stresses that trigger premature surface crazing under minimal applied loads.
Processing engineers monitor coolant flow rates to eliminate local hot spots that exacerbate internal residual stress fields.
Resin Economy
Virgin polymer supplies resist craze formation far better than heavily degraded regrind material containing shortened polymer chains. Molecular weight distribution shifts downward during repeated thermal processing cycles, lowering the critical stress threshold for microscopic void nucleation. Purchasing specifications must cap regrind percentages to prevent unexpected mechanical failure in structural moulded housings.
Economic pressures to reuse runner scrap frequently introduce invisible structural weaknesses unless melt flow index values remain tightly controlled.
Stress Threshold
Tensile loading applied to finished components accelerates craze formation when external forces align with residual orientation vectors. Part geometry features sharp internal corners that concentrate stress and initiate micro-cracking during ejection from rigid steel tooling. Component specifications govern allowable service loads to ensure operational stresses stay well below the critical material limit.
Service failures trace directly back to improper packing pressures that lock high internal stresses into the moulded matrix.
