13/08/2026
Why Resistance Reading Value Can’t Be Detected on Cut Edges After Drilling Conductive Injection Molded Parts
Core Causes & Detailed Explanations for Missing ESD/Conductivity Readings on Drilled Surfaces
1. Interrupted Conductive Filler Network (Primary Root Cause)
Conductive plastics rely on interconnected conductive fillers (carbon black, carbon fiber, carbon nanotubes) to form continuous conductive pathways.
1.High-speed drilling bits directly sever the integrated conductive filler framework inside molded products.
2.The crisscross conductive network on the inner wall of drilled holes is fully broken. The fresh cut surface is only covered by pure base resin with no uninterrupted conductive channels.
3.If both electrodes of the resistance tester are placed entirely on newly cut surfaces, the conductive circuit will be disconnected, resulting in an out-of-range insulation reading.
2. Insulating Pure Resin Film Formed by Frictional Melting
Heat generated by high-speed drilling slightly melts the plastic at the cutting position. Molten pure resin migrates to the outermost layer of the hole wall and forms an insulating thin film that completely covers internal conductive fillers. The testing probe only contacts this non-conductive resin layer, so valid resistance values cannot be captured. Higher drilling speed or dull drill bits aggravate this molten insulating coating.
3. Improper Test Point Selection (Common Operational Error)
Placing both electrodes solely on the inner cut surface of a single hole: the conductive network is already split with no accessible conductive path between two probes.
One electrode on the cut surface and the other on intact flat surface but spaced too far across the hole, breaking all conductive connections.
Insufficient probe pressure fails to pierce the surface insulating film to reach conductive fillers underneath.
4. Skin-Core Structure Leads to Uneven Filler Distribution
Injection molded conductive products naturally form a skin-core gradient: During rapid cooling after injection, conductive fillers migrate toward the core layer, while the outer skin layer contains high-purity resin with minimal conductive additives. A drilled cut surface creates a brand-new skin layer inherently lacking conductive fillers, compounding the insulation effect alongside broken conductive networks.
5. Performance Differences Among Different Conductive Fillers
1.High-loading carbon black compounds: This insulation phenomenon after drilling is most obvious due to severe resin enrichment on cut surfaces.
2.Carbon nanotube (CNT) modified materials: The issue is less severe, yet deep-hole high-speed drilling still fractures conductive networks and yields no readable resistance.
3.Long carbon fiber reinforced conductive plastics: Fibers are heavily snapped during drilling, making cut surfaces highly insulating.
Practical Improvement Solutions for Customers
1.Avoid testing on fresh cut surfaces: Place one electrode on an undamaged flat original surface of the part.
2.Lightly polish inner hole walls with sandpaper to remove the surface insulating resin film and expose internal conductive fillers.
3.Reduce drilling rotation speed to cut down frictional melting and minimize insulating resin overflow on cut edges.
4.Formula optimization: Improve filler dispersion and add compatibilizers to reduce the thickness of the insulating resin skin layer of finished products.
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