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.
Ejection Friction Coefficient
Meaning
Demolding forces during the ejection phase of injection molding depend directly on the shear resistance at the interface between the cooled polymer and the mold steel. The ejection friction coefficient determines the force required to push the solidified part out of the mold cavity once the tool opens. This value is influenced by the polymer type, the surface finish of the tool, and the temperature of the mold plates.
Force Requirement
High ejection forces can cause pins to push through or deform the molded part during the release phase. If the ejection friction coefficient is too high, the mechanical stresses applied by the ejector pins can crack or distort the warm polymer. This behavior is common in deep-draw parts where draft angles are inadequate to relieve the friction.
Release Agent
Additives incorporated into the resin or sprayed onto the mold steel lower the resistance of the part to demolding. Adjusting the ejection friction coefficient downward with internal lubricants like zinc stearate or fatty acid esters facilitates smoother release. This reduction in friction limits cosmetic scuffing and cycle-time delays.
Surface Finishes
The texture applied to the mold steel alters the contact area between the polymer and the metal during ejection. While a highly polished surface may seem low-friction, a low ejection friction coefficient is often achieved through textured or bead-blasted mold finishes that prevent vacuum formation. Moulders must select the proper tool texture based on the specific resin’s tendency to grip polished steel surfaces.