Optical Profilometry Metrology Protocols for Indirect Tool Cavity Topography Verification
Calibrated silicone replicas combined with optical profilometry yield sub-micron tool cavity topography metrics without destructive sectioning.
EDM Micro-Pitting
Meaning
Small craters formed during electrical discharge machining occur when the spark energy creates excessive thermal disruption on the tool surface. These flaws known as edm micro-pitting appear as localized porosity or irregular depressions in the hardened steel of a mould cavity. Such features are transferred directly to the moulded plastic part, appearing as tiny raised bumps or a sandy texture.
The presence of these defects often indicates a poor balance between metal removal rate and dielectric fluid stability.
Resin Interaction
Molten polymer flows into these microscopic voids and creates a mechanical bond that resists ejection. High edm micro-pitting density increases the total surface area of the cavity, which generates higher frictional resistance during the part removal cycle. When processing engineering resins like polyetheretherketone, these pits act as stress concentrators that can cause surface tearing.
Parts intended for optical or fluid sealing applications usually fail inspection if these craters are present.
Tool Maintenance
Polishing or lapping the steel remains the standard method for removing the damaged layer. Because edm micro-pitting reaches into the heat affected zone, superficial buffing might not eliminate the underlying metallurgical weakness. Removing too much material to fix the pits might push the cavity dimensions beyond the specified tolerance limits.
Energy Control
Careful selection of the spark duration and peak current prevents the formation of these craters. Low amperage finishing passes with graphite electrodes minimize the heat input and result in a smoother finish. Monitoring the dielectric fluid for contamination helps ensure that the electrical arc remains focused and consistent throughout the machining process.