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Quick Heaters

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My objective is to insert a PVC tube in a the (cold) mold. The heater should heat the mold to an ideal temperature to melt the tip of the PVC tube inside the mold (for a few seconds) and then the heater must switch off and the mold must be cooled down immediately by circulating water through the outer sleeve of the mold.

The mold is made with steel for experiment only. I inserted the tail of the mold in a 45w soldering iron as shown in the picture. My test was successful.

I plan to build a set-up in which I can use molds of various working sizes. I mean, the outer dimensions of the mold will remain the same only the hole diameters would vary to match various sizes of PVC tubes. The actual molds will be coated with chrome or nickle. And, if required, I can change the material of the mold to aluminum, copper or brass for better conductivity. The mold would be installed inside a cooling sleeve, which cover only the body of the mold (excluding the tail).

After testing the trial mold on a soldering iron, I planned to use a soldering gun which heats the tip more quickly.

Here is my proposed idea to quickly heat-up the mold. The transformer's secondary winding is 2/3 turns of 4 SWG solid copper wire to give 2-4volts but high amperes like 400 - 500A. The rest circuit is self-explanatory. The pictures of the mold is also attached to give you the idea of the size of the mold.

Lastly, I want to use 4 such heaters connected parallel and work at the same time.


What kind of cables/wires should I use to connect the Transformer-Relay-Heater ?
What should be the amperage of the relay to switch on the Transformer?




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  • 4 weeks later...

I wouldn't try to switch the secondary current.  Switch the primary on and off, as is done in a soldering gun.  You will pay dearly for a relay that will work in your secondary circuit.  Also, you will be adding contact resistance to the secondary loop where it will affect performance.  Remember that you have 12 volts supply in the car, but only a few tenths of a volt in your rig.  The voltage drop across any contacts in the secondary loop would seriously affect performance.

You are not going to see anything approaching 1 volt across the tip of a soldering gun.  After writing a rambling speculative comment here I decided to just measure the voltage across the tip of my 140 watt soldering gun.  It was 0.2 VAC across the tip conductors at the point of connection to the gun (after a few seconds warmup - voltage increases with temperature of the tip) and about 0.3 VAC across the conductor bars emerging from the gun body (indicating a low but finite resistance at the tip clamping nuts which I recently cleaned and tightened).

Remember that the conductors inside the gun are a single loop of heavy copper alloy bar and the tip is a short, heavy copper conductor.  You are not going to develop much voltage there.  I wouldn't be surprised to find that the tip itself contains a section of some alloy with a higher resistance in order to concentrate the heat where it is useful, but I don'know if that's true.

The required conductor size has a lot to do with your duty cycle.  My gun is rated at 120 volts, 1.2 amps primary, or 144 watts and I measured 0.3 volts across the gun conductor bars.  That implies about 500 amps in the secondary loop and this is only a medium size gun.  It would be quite impractical to design your secondary conductors for that continuous current.  Look at the size recommendations for 500 amp welding cable that carries current for much longer periods than your rig will.  Welding cable conductors for 500 amps is about the size of your thumb.

I'd take a clue from the soldering gun manufacturers, although part of their conductor selection may have to do with physical strength.  Time for some experimentation on your part.  Consider using copper tubing.  Since you are going to be dirculating water anyway for cooling your mold, you could circulate water through your secondary conductors, allowing much higher current to be carried for the amount of copper in the conductors.

Use a switch on the transformer primary with the rating of at least that of the switch on the microwave.  That's all the transformer can handle.

This looks like a good application for a SSR (Solid State Relay) with a current rating of, say, double your calculated primary current since you will be controlling it with a timer. You won't have as large an inrush current to deal with as you would with a motor or incandescent lamp load but you might have a brief surge and an inductive kick to deal with.  Use a suppressor.  The SSR manufacturers have app notes to guide your selection for various types of loads.


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...and then the heater must switch off and the mold must be cooled down immediately by circulating water through the outer sleeve of the mold.

If you need/want the mold to cool quickly, you want a material like "ceramic" which has good thermal conductivity, but a low thermal capacity. Basically somethng the conducts heat but won't retain any.
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Your numbers are wrong.
4V at 500A is 2,000W. But a big soldering gun uses only about 100W.

Agreed but I think he may have measured the open circuit voltage and closed circuit current which doesn't necessarily mean it's dissipating 2kW.

Anyway, this tread is over three weeks old. I don't think the original poster will return so why bump it?
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