Meaning
Formation of an oxidized layer or crust on the surface of injection mold cores and cavities when exposed to high temperatures and corrosive gases. In the molding of engineering resins, tool steel scaling degrades the heat transfer capability and the surface finish of the tool. This oxidation reduces the efficiency of the molding cycle.
Choosing the correct steel alloy prevents this premature degradation.
Thermal Resistance
High-temperature operation during the molding of polymers like polyphenylene sulfide accelerates the oxidation process. When tool steel scaling occurs, the resulting oxide layer acts as an insulative barrier that slows down heat transfer from the plastic melt to the cooling channels. This insulation extends the cooling phase of the injection molding cycle and increases production costs.
Steels with higher chromium content resist this oxidation by forming a protective passivating layer.
Surface Finish
Microscopic roughness from oxidation transfers directly to the surface of the molded parts, causing cosmetic and functional defects. With tool steel scaling, the smooth, polished cavity surface becomes pitted and uneven over thousands of cycles. This roughness causes the molded plastic parts to stick in the mold during ejection, leading to scuffing or part distortion.
Demolding forces increase, which puts additional stress on the ejection mechanism.
Maintenance Cycle
Periodic polishing and chemical treatment of the mold cavities are necessary to restore the tool to its original condition. To manage tool steel scaling, molders must pull the tool from production for regular cleaning and bench work. This maintenance involves removing the oxidized layer with fine abrasives without altering the critical dimensions of the cavity.
Applying a specialized protective coating such as chromium or titanium nitride can significantly extend the time between these maintenance intervals, allowing the molding press to run longer before experiencing dimensional drift or ejector failures.
