Help with resistance wires


I’m trying to heat ABS plastic inside a metal tube and use resistance wires around the tube to distribute the heat. I will eventually use this to inject it into a mold. I need to be able to control the temperature so that it is steady at 120 to 150 Celsius. Please help me with a few questions…

  1. Which resistance wire/s would you guys recommend I use?
  2. What’s the best way to insulate them?
  3. Can I use regular relay switches to turn them on/off?
  4. Are there any temperature modules I could use with the arduino to measure temperature accurately
  5. Any other tips?

Thanks in advance!

Hey, A..

Interesting and non-trivial project. More details?

  • Size of metal tube?
  • How much dynamic power may be needed if there is a continuous flow of plastic, compared to a 1-shot heat-and-dispense situation?
  • Approximate time constant of heater to plastic?
  • How close temp control needed?

Too high temp for semiconductor sensors (125C max), so need RTD or Thermocouple, probably. Maybe thermistor?

Solid State Relay would be good. Like

DISCLAIMER: Mentioned stuff from my own shop...

All thes questions are answered on the reprap site. The problem with nichrome wire is that you can't solder to it. Most modern extruder designes use virtuous enamel resistors to heat a block. You can control it with a simple power FET, get a logic level one. A thermistor into the analogue input of the arduino with a 10K pull up can measure the temperature.

The problem with nichrome wire is that you can't solder to it.

I get an "Aluminum soldering kit" and the flux that comes with that works OK on NiChrome...

I have also crimped it in a heavy copper terminal along with a copper lead to power. Seems to survive a long time..

Original Poster: You need to put information or links to your problem to get help here...

I get an "Aluminum soldering kit"

That solder is expensive though, at a recent engineering show they were selling it for £35 for half a meter. You are better off crimping the connections with ferrules.

That solder is expensive though

I think what I got was different. It was a small coil of solder and a separate small tube of flux.. Less than $5 US. I only used the flux and regular lead-tin solder, not the solder in the kit..

Thanks for your replies guys! I really appreciate it.

The tube would be about 2 inches in diameter, and I plan to feed pellets/chips into it. It won't be an extruder, but a home made plastic injection molding device.

I just thought of this: if I wrap the wire around the tube which contains the plastic, wont it short? What can I use in between the wire and the tube so that it insulates current, but not heat? Some sort of insulating paper/sheet. I have no idea if there is such a thing.


Hi, Yes, you need some electrical (not thermal) nsulation and also to try to keep adjacent turns from shorting.

Take a look at the heating elements in toasters and hair dryers and try to adapt some of those materials??

150C isn't THAT high..

Maybe you could surround the tube with tubular ceramic power resistors parallel to the tube, and add outer insulation??

Kapton tape is good up to 400C.

The problem is that most electrical insulators tend to be good thermal insulators. Mica is a good exception but it only comes in flat sheets. Fire cement is a good compromise and might do. There is also an experimental section on pellet feeders on the reprap forum.

The suggestion to use power resistors is a good one: they're robust, and solve your electrical insulation and thermal contact problems. Look for the kind with a metal jacket, which can be bolted onto your heating tube. You'd need to machine flat surfaces onto the tube for good thermal contact, or use a square-section tube.

Wow seems more complicated than I thought.. Can't I just put thermal paste around the pipe and make the resistance wires spiral around it? or maybe just buy the mica paper to wrap the pipe with, then spiral the resistance wires over it?

was trying to do research on mica.. and it seems there are different kinds. is the flexible paper like mica not a heat conductor?

or maybe just buy the mica paper

It is not paper. Mica is a mineral, it doesn't bend

Have you a link on mica paper?

then spiral the resistance wires over it?

How do you stop the spiral shorting out?

Can't I just put thermal paste around the pipe

What is thermal paste?

Look for the kind with a metal jacket,

Those are good but when you blow them up the ends shoot out of the tube and embed themselves in the wall opposite. I know, fortunately I was not standing in the way.

The jackets used in the injection molding industry are real standard stuff. I did code for Atlantis Plastics 20 years ago. Most of the plastics they used which included ABS and ABS with glass fibers were heated to 400-450F going in. You might melt the beads at less but getting the flow needed to mold takes that much on the pipe… maybe the plastic itself doesn’t get the full heat running a shot every so many seconds too. They didn’t use more heat than they needed as the next step is running water through the cooling lines and extra heat would only increase cycle time. When the load screw stops turning the piston rams forward, there’s 2000 PSI going in. You either have a massive press or you make small things.

Doing small hobby plastics, check out Makerbot Industries. You could drop $700-$1500 pretty quick there but the capabilities you get far exceed injection molding though much, much, etc, slower.

Grumpy Mike,

Here's the link to the sheet mica I was talking about. Not sure if it's any good? Will find out this weekend :slight_smile:

There is industrial heat tracing wiring products avalible.