Calculating Multi Cavity Dimension Drift from Steel Expansion Profiles
Calculate multi-cavity dimension drift by subtracting thermal steel plate expansion profiles from localized polymer shrinkage values across operating temperature zones.
EDM Offset
Meaning
Machining allowance represents the physical gap maintained between an electrode and the steel surface during spark erosion. The value of edm offset determines the final dimensions of the mold cavity by accounting for the volume of metal the electrical discharge removes. Precision in this calculation is required to ensure that the finished tool produces parts within the specified tolerance band.
Designers specify different values based on the required surface finish, as roughing passes require larger gaps than finishing passes.
Overcut Logic
Electrical sparks jump across a dielectric fluid to erode the workpiece. The resulting edm offset is essentially the distance of this spark gap added to the physical size of the electrode. If the calculation is incorrect, the cavity will be too large or too small for the intended resin shrinkage.
Most modern software packages automate this by subtracting the offset from the cad model before milling the graphite or copper electrodes. Precise control over current and pulse duration keeps the gap consistent throughout the burn.
Surface Texture
Roughness levels on the mold wall are a direct result of the energy used during the erosion process. A high energy spark creates a larger crater, requiring a substantial edm offset to prevent the tool from becoming oversized. Conversely, a fine spark used for a polished finish allows for a much smaller gap.
Moulders often request specific finishes to manage part release or aesthetic requirements. Each finish grade corresponds to a known overcut value that the toolmaker must apply.
Electrode Wear
Metal loss on the sparking tool affects the accuracy of the geometry over time. Because the electrode also loses material, the effective edm offset can change during a long burn or across multiple cavities. Compensation for this wear involves using multiple electrodes or adjusting the machine settings.
High quality graphite reduces the rate of wear and maintains the integrity of the spark gap. Consistent material properties in the electrode lead to more predictable results in the hardened tool steel. Final inspections usually confirm that the resulting dimensions align with the master design before the tool enters the molding press.
Without this verification, a molder risks producing parts that fail to meet critical assembly dimensions.