Quantifying Micro Structure Transfer Loss in Laser Textured Injection Tooling Cavities
Polymer micro-structure transfer loss in laser-textured tooling stems from melt freezing kinetics and trapped gas counter-pressure, requiring variotherm control.
Micro-Ejection Shear
Meaning
Mechanical resistance forces oppose the release of a moulded polymer part from a cavity during the demoulding phase of the injection cycle. This resistance is quantified as the micro-ejection shear, which occurs at the interface between the solidifying polymer and the metal mold wall. It represents the localized frictional stress generated as the part begins to move along the tool surfaces.
High ejection forces can warp delicate parts or damage micro-featured surfaces. Proper control of this shear force is essential for producing high-precision optical parts and microfluidic channels.
Friction Mechanism
Static friction between the polymer and the mold surface must be overcome at the onset of ejection. The micro-ejection shear is determined by the surface roughness of the mold cavity, the shrinkage of the polymer around core pins, and the coefficient of friction of the material. As the polymer cools, it shrinks onto the steel cores, creating a normal force that increases the frictional resistance.
If the resin has high shrinkage, the resulting shear stress during demolding rises significantly.
Tooling Parameter
Mould cavity finishes and draft angles are engineered to minimize release forces. A low micro-ejection shear allows for smaller ejector pins and thinner part walls, reducing the cycle time and tool wear. Polishing the mold or applying thin-film coatings such as diamond-like carbon can reduce the friction coefficient at the interface.
This allows the moulder to operate at lower ejection pressures, preventing part deformation. The draft angle represents the primary mechanical means of reducing this shear, as even a small taper rapidly separates the part from the tool. When the taper is too small, the pin must push harder, which can punch a hole through the warm polymer.
Defect Mitigation
Part defects like stress marks or ejector pin push-through occur when demoulding forces exceed the structural strength of the warm plastic. Monitoring the micro-ejection shear provides a real-time indicator of part quality and mold wear. When this shear force rises, it indicates that the cavity surfaces may be degrading or that the cooling cycle is too short.
Moulders can lower the shear by adjusting mold temperature, holding pressure, or resin formulation.