Most of the problems that jewelry mold makers experience with vulcanizing natural and silicone rubbers are the result of faulty or improperly set vulcanizing presses.
If the temperature is too low the mold will not cure properly and will remain soft, gummy and easily deformed.
If the temperature is too high the rubber will become stiff, show increased shrinkage, and may even turn liquid permanently.
If one heating plate is higher or lower than the other, the mold will curl.
Vulcanizer problems are the result of:
• Improperly set thermostats
• Faulty thermostats
• Faulty or burned-out heating elements
Most jewelry industry vulcanizers, whatever their price, are made with unreliable and cheaply made controls, in part because most jewelry mold rubbers are made to be very forgiving of errors. This works most of the time but sometimes things can go badly wrong.
The wise mold maker will check the temperature of his vulcanizer with a good thermometer regularly, just as one might check the oil level in one’s car.
If your vulcanizer has a knob and a round dial, it is wise to mark one or more required temperatures with a pen or other device. It will be very apparent if the dial has been moved accidently.
If your workspace is shared with other people or if children are often in your work, it might be wise to remove and save the adjusting knob from the dial to prevent unauthorized changes.
Do not trust the temperatures that are marked on these dials –they are often wildly inaccurate.
If your vulcanizer has one or more digital temperature controllers you are much less likely to have a problem and not be aware of it. But things can go wrong even with the best-made machine.
Some vulcanizers have a hole drilled in the top or bottom plate for the insertion of a thermometer. This is better than nothing, but will not give an accurate reading. If the top plate, for example, is too cold or too hot, the thermometer in the bottom plate will not show that.
Placing the thermometer between the two plates is better, but will still not give an accurate reading. If the plates are different temperatures, the thermometer will only show the average of the two temperatures. If one plate is 50 degrees too high and the other is 50 degrees too low, the reading will show as correct. Castaldo’s recommended technique is illustrated in the attached photos. Place a small block of wood ½” or 3/8” / 3-6 mm thick between the plates as a cheap and disposable insulator.
Place your thermometer probe first on top of the wood and then later underneath it. If your vulcanizer is large, also measure left and right sides of each plate. Close the plates of your press GENTLY, taking care not to damage the thermometer probe.
Vulcanizing presses, like all electrical heating devices, are designed to reach and hold a set temperature by switching on and off as required by the thermostat. In vulcanizers with a temperature indicating light, you can see this process as the heating lamp goes on and off regularly. When the lamp goes out the press has reached the top of its cycle, when the lamp comes on, it has reached the bottom of the cycle.
Take your temperature measurements at the top of the cycle consistently in each case.
Vulcanizers with digital temperature controllers can maintain a steady temperature within 1 or 2 degrees. For older types, 5 to 10 degrees would be an excellent range although we’ve seen many cases of 25-degree and even 50-degree differences.
Allow the vulcanizer to heat stabilize for an hour or more before beginning your tests. Them measure the temperature in one spot, watch it through several cycles and note the temperatures shown. Then repeat the process for the other plate or, in the case of larger presses, in the left and right areas as well.
There is no need to sit and watch the machine through this procedure. It is enough to go about other tasks and merely check the vulcanizer once in a while.
Both digital thermometers and dial type thermometers work well. Glass thermometers will also work but might break in this process.
If your vulcanizer has dual controls, one for each heating plate, it is a simple matter to adjust the setting up and down as required.
If, however, you have one control for both plates, it may or may not be possible to get close to the right temperature. As a short-term solution you can remove the mold from the press at the half-way point, turn it over and put it back in the press for the remainder of the time. This may even out the temperatures.
The longer term solution is to repair the machine or replace it.