Machining ESD plastic looks like ordinary machining until a part fails a resistivity check after it has been cut. The grade carries a conductive filler, usually carbon black or carbon fibre, that forms a network through the polymer, and that network is what moves charge away from the surface. Cutting, sanding and heat can disturb it.
This guide explains where the conductive network sits, how normal shop practice can damage it, and what to measure before a part is accepted. It is written for engineers who have a resistivity band on the drawing and need the process to meet it, not just the feedstock.
How dissipative grades get their conductivity
Static-dissipative and conductive plastics are made by compounding a base polymer with a filler. Carbon black gives a low-cost dissipative grade in the 10^6 to 10^9 ohm-per-square band; carbon fibre gives a stiffer, more conductive material that can reach the conductive band. The filler does not have to touch everywhere: it forms a percolation network, and conductivity depends on how continuous that network is.
That is the key point for machining. The network is a three-dimensional structure through the whole section, so as long as the filler loading is comfortably above the percolation threshold, cutting a surface exposes fresh filler and the part stays conductive. Below the threshold, or in a poorly dispersed batch, the margins are thin and small changes can tip the part into the insulative band.
A common mistake is to assume a dissipative sheet behaves the same on every face. Extrusion and compression routes can orient the filler differently, and a cut face can read a little higher than the moulded skin it came from. That is normal, and it is one reason a specification should define exactly where the reading is taken.
We stock dissipative and conductive grades as sheet, rod and tube across our ESD materials range, and the first question on any enquiry is always which band the part has to hold.
Where machining damages the conductive network
Three habits cause most resistivity drift: heat, abrasion and contamination. Heat is the most common. A dull tool, a fast feed or a dry cut raises the surface temperature, and carbon-black-loaded polymers can re-anneal or smear at the skin, changing the local filler concentration.
Abrasion from sanding and polishing is next. A belt or flap wheel removes the resin-rich layer and can pull carbon black out of the top surface or smear it into a film that reads differently from the bulk. Grit media also embed foreign particles between the probes and the surface.
Contamination is quieter but real. Coolant, hand oils and release agents leave a resistive film, so a part that measured well an hour after cutting can fail after it has been handled and wiped. Choosing a grade that holds its band after machining ESD plastic is the subject of our ESD material selection guide.
Coolant, heat and the safe process window
For most filled grades a sharp carbide cutter, moderate speed and a steady feed keep the surface cool enough that the network is untouched. Flood coolant or a strong air blast carries heat away and clears the swarf before it can be re-cut. The aim is a free-cutting action, not a burnished one.
Where the material is heat-sensitive, such as carbon-filled PEEK or a glass-filled grade, a slower surface speed and a positive rake reduce friction. In our experience the parts that fail are almost always the ones cut hot, and a polished-looking surface with a burnt smell is a warning, not a finish. The whole aim when machining ESD plastic is to remove material before the surface gets hot enough to reflow, not to leave it bright.
Surface resistivity versus volume resistivity
The two measurements answer different questions. Surface resistivity describes how easily charge moves across the surface, and it is the number that matters when the part bleeds charge away at the point of contact. Volume resistivity describes conduction through the body of the material and governs whether the part can be grounded through a fixing.
A part can pass one and fail the other. A thin, resin-rich skin can read insulative on the surface while the bulk is still conductive, and a contaminated surface film does the same. For a lightly filled POM dissipative grade the gap is wider than for a well-loaded PEEK, but it still has to be checked on every part.
Grounding is the other half of the picture. A dissipative part only protects a device if the charge it collects has somewhere to go, so the fixings, the mating metal and the earth path matter as much as the polymer. Resistivity sets the pace of the bleed; the ground path decides whether it works at all.
| Grade type | Resistividad superficial | What it does | Typical example |
|---|---|---|---|
| Insulative | Above 10^12 ohm/sq | Lets charge build up | Unfilled POM, PEEK, PEI |
| Dissipative | 10^6 to 10^9 ohm/sq | Bleeds charge slowly | Carbon-black POM, PEEK ESD |
| Conductive | 10^3 to 10^6 ohm/sq | Discharges fast, grounds well | Carbon-fibre filled grades |
| Volume conductive | 10^3 to 10^6 ohm-cm | Carries current through the section | CF-filled PEEK, PPS |
Questions buyers ask about machining ESD plastics
Can I sand or polish an ESD part?
Only with care, and never as a routine finish. Sanding removes the resin-rich skin and can pull filler out of the surface, which is exactly the layer the probes read. If a fine finish is needed, keep the cut cool and take the last pass with a sharp tool rather than abrasive paper.
Does coolant change the resistivity?
Wet cutting is fine, but residue is not. Flood coolant keeps the surface cool, which protects the network, yet the film it leaves behind is resistive and will fail a surface test if it is not removed. Wash and dry the part to an agreed method before measuring.
What should the drawing say about resistivity?
State the band, the test method and the condition of the surface at the time of test. A bare figure of 10^6 ohm per square means little unless it says whether surface or volume is meant, and whether the part is measured as-machined, cleaned or after ageing.
How soon after machining should a part be tested?
As soon as the part is clean and dry, and again after a defined settling period. Surface condition can drift over the first hours, so a reading taken on a freshly wiped part is not the same as one taken the next day. Where the band is narrow, fix the test point and keep to it for every delivery, and pull a reference block from our stock shapes so the incoming and outbound readings are comparable.
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