Influence of surface roughness of die sinking EDM on mold releasability in compression molding of thermosetting phenol resin

Improvement in metal mold characteristics such as wear resistance, corrosion resistance and mold releasability is essential for plastic molding. In this study, factors of the mold releasability were investigated by quantitative evaluations of adhesion forces between the processed metal mold surfaces and the molded resins in compression molding of thermosetting phenol resin. Die sinking EDM was employed as the processed surfaces of the metal molds. As the mold releasability tests, the molded thermosetting phenol resin onto the EDMed surface was pulled up in the perpendicular direction from the surface. Consequently, the mold releasability could be quantitatively evaluated by measuring a maximum tensile load which means a mold release force when the molded resin is separated from the EDMed surface. Several workpieces with different surface roughness as the EDMed surfaces were prepared by varying discharge currents and pulse durations. As a result of investigations on relationships between surface roughness parameters and measured mold release forces, the mold release forces tended to increase as maximum height roughness RZ and average length RSm of the surface roughness parameters increase. On the other hand, the mold release forces tended to decrease as peak count RPc increases. Accordingly, it is considered that anchor effects become higher with increase of undulation on the EDMed surfaces. Therefore, the undulation is one of the factors affecting the mold releasability, and it would be possible to improve the mold releasability by reducing the undulations on the EDMed surfaces with EDM conditions.