Finite Element Thermal Stress Analysis on Large Format Angled Tooling Shut off Steels
FEA-driven cold clearance offsets on angled shut-off steels eliminate thermal expansion interference and galling failures in large injection moulds.
Plastic Strain
Meaning
Irreversible, non-recoverable dimensional deformation occurs within materials once applied mechanical stresses exceed their native elastic yield limit. In injection moulding, this mechanism applies equally to structural tool steels undergoing permanent micro-cracking under extreme local clamp stresses and to solidified polymer parts retaining permanent orientation and warpage following mould ejection. Tracking plastic strain allows mold builders to predict tooling fatigue life and enables part designers to prevent permanent deflection, stress cracking and structural failure in service.
Material Response
Hookean elasticity ceases when external loading forces molecular chains in polymers or atomic slip planes in metals past their reversible yield points. At this stage, plastic strain initiates through dislocation migration inside polycrystalline tool steels or irreversible molecular uncoiling, disentanglement and crazing within polymeric substrates. Unlike elastic deformation, which fully rebounds when processing pressure drops to zero, permanent strain remains locked inside the material structure.
Moulded components produced with virgin amorphous polymers like polycarbonate retain permanent local strain when packed excessively into frozen gates, making them susceptible to environmental stress cracking under chemical exposure.
Mould Tooling Degradation
Tool steels experience localized permanent deformation when injection and clamping forces exceed safe fatigue thresholds. Slender core pins, sharp shut-off corners and thin cavity ribs are frequent sites of localized plastic strain caused by peak cavity pressures reaching 200 megapascals. Repeated production cycles turn microscopic plastic deformations into low-cycle fatigue cracks, eventually chipping parting line edges and producing part flash.
Toolmakers prevent permanent steel deformation by selecting premium through-hardened alloy steels like 1.2344 or vacuum-melted H13, tempering components to hardness ranges that balance high yield strength against brittle fracture vulnerability.
Processing Manifestation
Processing conditions dictate the intensity and distribution of permanent deformation across both tooling and moulded parts. Elevated melt temperatures, uneven cooling water paths and unbalanced holding pressures generate steep internal stress gradients that resolve into unwanted plastic strain as parts cool outside the tool. In semi-crystalline polymers like polyamides, non-uniform shrinkage produces asymmetrical internal stresses that permanently twist thin-walled profiles out of drawing specifications.
Molders reduce post-mold deformation by optimising mold cooling uniformity, prolonging packing times to balance volumetric contraction, and occasionally annealing parts to relieve internal stresses before assembly operations begin.