The oven is the whole problem
Powder coating cures typically at 180 to 200 °C for 10 to 20 minutes. Plating and e-coat lines add chemical exposure on top. Most elastomers simply do not survive that repeatedly. Nitrile hardens and shrinks. EPDM does better but takes a set. Natural rubber degrades almost immediately.
Silicone is the correct answer for hot masking, and for a specific reason: it retains its elastic properties across the full oven range and returns to size afterwards. That recovery is what makes it re-usable rather than disposable.
Getting the fit right
A plug that is too loose lets powder creep under the edge and leaves a ragged mask line. A plug that is too tight distorts, becomes difficult to remove hot, and can pull coating off the surrounding surface as it comes out.
- Tapered plugs suit a range of hole sizes and are the most forgiving choice for a mixed workload.
- Pull-through plugs give the cleanest mask line and the fastest removal on repetitive work, but need the hole size specified accurately.
- Caps suit external features — studs, threads, spigots — and should grip on the cylindrical surface, not the end face.
- For threaded holes, size on the minor diameter, not the nominal thread size.
Removal temperature matters
Silicone plugs come out most easily while the part is still warm but handleable. Left to cool completely, cured powder at the mask edge can grip the plug and tear it. Build removal into the line at the right point and plug life rises noticeably.
Hardness
Around 50 to 60 Shore A suits most masking. Softer plugs seal better on rough or out-of-round holes but tear more easily during removal. Harder plugs last longer mechanically but seal less well and need tighter hole tolerance. If your holes are as-cast or as-drilled rather than machined, go softer.
What to send us
Hole diameter and depth, whether it is threaded, the cure temperature and time, and roughly how many parts per batch. If you have a plug that nearly works, send it — adjusting an existing geometry is quicker than starting from a drawing. We mould masking plugs from 6 mm up to 110 mm diameter.
Have a part that fits this description?
Send the dimensions and the operating conditions. We will tell you what compound it should be in and quote it — including saying so if it is not a job for us.