Areal Surface Texture Parameter Measurement in Micro Injection Mould Cavities
Areal surface metrology in micro-cavities requires non-contact optical instruments with matched bandwidth filters to verify steel texture and polymer replication.
Demoulding Shear Force
Meaning
Mechanical resistance encountered during the ejection of a cured polymer part from the mould steel arises from frictional forces along the core and cavity walls. In injection moulding, demoulding shear force depends on the coefficient of friction between the polymer and the steel, alongside the normal force generated by the shrinking polymer. As the resin cools, it contracts around the mould core, creating a tight grip that must be overcome by the ejector pins.
Proper tool surface finish and the use of internal release agents reduce this drag during ejection.
Friction Factor
Resin selection directly influences the sliding resistance between the solidified part and the steel surface of the tool. Crystalline resins like polyoxymethylene exert a higher clamping force on the core because of their high volumetric shrinkage, whereas amorphous resins like polycarbonate present a higher coefficient of friction. Moulders often apply textured finishes to the steel to reduce the contact area, which lowers the ejection force required.
This prevents the ejector pins from puncturing the warm plastic, a defect that renders the part unusable and increases scrap rates during high-speed runs.
Ejection Stress
Excessive mechanical load during the ejection stroke can cause permanent deformation on the finished part. When this resistance exceeds the strength of the warm polymer, the part can warp or crack. This extends the cycle time.
Mould Draft
Tapered walls on the core and cavity elements are designed to release the contact immediately after the ejector system starts its stroke. Even a small draft angle of one degree reduces the demoulding shear force significantly after the first millimetre of travel. This layout ensures that the mechanical load on the ejector pins remains low, which extends the working life of the ejector mechanisms.