If your mold is not fully vulcanized — more about that in a minute — it will complete it’s final cure as you are cutting it. It will “take a set” as you are pulling back one side or the other, stretching it, etc. It will finish it’s cure in a curled, distorted shape.

This is a common problem with both natural rubber and silicone rubber compounds.

Another clue the mold might not be fully vulcanized is if you are having trouble getting it to release from the aluminum mold frame or mold plates. Uncured silicone rubber is a glue — think window caulking or household glue. Fully cured silicone rubber sticks to nothing – one of it’s major advantages.

Jewelry industry vulcanizers are notorious for being poorly made, poorly calibrated and often out of calibration or just plain wrong. Even the expensive ones. And the thermostats on even the good machines go bad over time.

Make it a practice NEVER to trust the dial settings on your vulcanizer. They are usually wildly off. But the rubber is very forgiving most of the time. It’s only when the inaccuracy is major that the problem occurs.

Buy a good quality thermometer from a jewelry supply, laboratory supply or industrial supply house. The metal probe type with a round dial is good. Digital versions are OK, too. Glass thermometers will work well but are, of course, fragile.

Here is our recommended test technique:

Turn on your vulcanizer and let it heat up and stabilize with the plates closed for 1 hour.

When the indicator light is OFF, showing that the thermostat is not calling for heat and thus has reached its set temperature, open the plates slightly and insert a small block of scrap wood — 1/4″ or 3/8″ (6 to 12 mm) is fine — with the thermometer probe on top of that.

The wood is a cheap and disposable insulator. It’s there so that you can measure first the top plate by itself and then, later, the bottom plate. Without this your thermometer would be are averaging the temps of the two plates, which is not useful.

Leave the thermometer in place for a few heating cycles — when the indicator light goes on and off — and note the temperature of the top plate. Then reverse the thermometer & wood block positions and repeat the process.

If both reading are close, but too low, simply set the dial higher.

I recommend taking the dial off and hiding it so that no one can adjust it further after you’ve reached the right temperature.

If one plate is significantly different than the other, it’s time for repair or replacement.

For more information, see How to Test Your Vulcanizer.