Cavity Surface Texture Effects on Thermoplastic Heat Transfer and Shrinkage
Cavity surface texture increases interfacial thermal contact resistance, delaying resin skin freeze and increasing semi-crystalline thermoplastic shrinkage.
Mechanical Interlocking
Meaning
Physical adhesion mechanism joins two distinct materials through the flow and solidification of one component into the microscopic pores, crevices or undercuts of another. In multi-component injection moulding and insert overmoulding, mechanical interlocking creates structural bond strength without requiring chemical molecular bonding or adhesive primers. The molten polymer penetrates surface roughness features on metal inserts, porous substrates or pre-moulded plastic cores before solidifying into a rigid mechanical joint.
The phenomenon stops providing joint integrity if shear forces exceed the shear strength of the interlocking polymer protrusions.
Surface Engagement
Microscopic and macroscopic surface irregularities create physical anchor points for advancing melt fronts. Shot blasting, chemical etching, laser texturing or knurling creates undercut geometries on metal inserts prior to placement in the mould. Injected polymer enters these cavities under high packing pressure, filling microscopic voids.
Solidification captures the frozen polymer fingers within the substrate matrix, generating strong resistance against tensile separation and shear displacement.
Processing Dependency
Injection speed, melt temperature and tool temperature determine how effectively resin penetrates fine substrate pores. High melt viscosity and cold tool steel cause the advancing melt skin to freeze prematurely, preventing complete penetration into micro-undercuts. Semi-crystalline polymers like polyamides flow well into textured substrates under elevated packing pressures, while high-viscosity amorphous resins require larger macroscopic grooves.
Insufficient pack pressure leaves interfacial voids that compromise joint stiffness and create leak paths.
Assembly Performance
Hybrid metal-plastic components and multi-shot moulded parts rely on physical interlocks to maintain structural integrity across thermal cycling. Differential thermal expansion between metal inserts and polymer bodies induces internal stresses at interlocking junctions. Incompatible polymer combinations in two-shot moulding rely entirely on mechanical dovetails and undercut holes when chemical compatibility is absent.
Quality control engineers perform pull-off and torque tests to verify that mechanical interlock designs withstand required service loads without debonding.