Optimizing Core Layouts for Polymer Flow and Sink Prevention
Balancing standing steel thickness below sixty percent of nominal wall and maintaining turbulent cooling prevents cosmetic sink marks.
Electrical Discharge Machining
Meaning
Subtractive manufacturing operations utilising high-frequency electrical spark discharges erode conductive workpiece metals submerged in a dielectric fluid medium to create complex tooling cavities. Toolmakers producing injection moulds, extrusion dies, and compression tooling for plastics depend on electrical discharge machining to produce deep ribs, sharp internal radii, and intricate surface textures impossible to cut using conventional rotational milling cutters. The process applies controlled voltage pulses between a shaped electrode or continuous wire and the tool steel workpiece, melting and vaporising minuscule metal volumes with every transient spark.
Because physical cutting forces remain zero throughout the erosion process, hard-to-machine hardened steels such as H13, S7, or stainless 420 can be processed post-heat treatment without mechanical distortion or tool deflection.
Subtractive Mechanism
Spark erosion takes place across a microscopic spark gap maintained continuously between the electrode and the tool surface by an automated servo drive system. Dielectric oil or deionised water acts as an electrical insulator until the applied gap voltage exceeds the dielectric breakdown threshold, initiating an instantaneous plasma channel. Temperatures within this plasma zone reach between 8,000 and 12,000 degrees Celsius, melting a micro-crater of metal on the mould steel while the dielectric fluid flushes away solidified debris particles.
Sinker configurations employ machined copper or graphite electrodes to plunge three-dimensional geometries into cavity blocks, whereas wire variants guide a brass wire to cut through-holes, core slide tracks, and ejector pin channels.
Mould Surface
Intense local thermal cycles generate a recast layer, commonly known as the white layer, over the eroded steel surface alongside an underlying heat-affected transition zone. This thin recast layer contains brittle martensite, micro-cracks, and residual tensile stresses that impair tool fatigue life under cyclic injection moulding pressures. If left untreated, the brittle surface induces premature tool failure, pitting under corrosive resin off-gassing from PVC or flame-retarded polyamides, or causes moulded parts to stick during ejection.
Removing this layer requires hand stoning, chemical polishing, or dry grit blasting before placing the injection mould into production.
Texturing Control
Controlling electrical discharge machining discharge parameters, such as peak amperage, pulse-on time, and pulse-off duration, directly governs final mould cavity roughness. Toolmakers select specific spark energies to produce standardized, isotropic matte textures directly on cavity walls, eliminating downstream bench finishing steps for consumer housings. This electro-spark finish masks minor visual defects on injection moulded parts, including sink marks over nominal ribs, flow hesitations, and weld line witnesses.
Excessive surface roughness on steep cavity sidewalls, however, increases demoulding friction, demanding larger draft angles to prevent part scuffing or drag marks during core retraction. Precise spark energy regulation remains fundamental to keeping mould cavity dimensions within tight micron-level part tolerances.