Tuesday 31 July 2007

Hotting up

The latest RepRap heater design consists of insulated nichrome wire wound around a threaded barrel and then stuck down with a coating of J-B Weld high temperature epoxy. I think that is a good way to do it but the insulated nichrome is expensive and I happen to have some nichrome from a heating element. It came from an old hair dryer I think.

Luckily it seems to be the right gauge to give me a reasonable number of turns. The spec was for 8 ohms which gives a maximum wattage of 18W at 12V. That gave me a length of about 340 mm which made 17 turns. I attached some tinned copper tails to make the connections easier to handle. I tied them to the nichrome and then soldered it. On reflection that was probably a bad idea as the solder does not stick to nichrome so if it oxidizes it may go open circuit. Small crimps would be a better I think.

I started by laying down a layer of J-B Weld to insulate the barrel.

After letting this dry for 24 hours I put it in the lathe and turned it down to as thin a layer as I could get before it started flaking off. That was at about 0.2mm.

I added some more J-B Weld to repair the gaps and also used it to attach one end of the nichrome.

After another 24 hours I put it back in the lathe to make the winding.

Finally I added another thick layer of J-B Weld and left it another 24 hours to set. A very slow way of doing it compared to using insulated nichrome and a single coat of J-B Weld.

I tested it by putting a thermocouple probe down the barrel and running it from a variable bench power supply. I heated it up to 200 °C at which point it smoked a bit and the J-B Weld started to discolour. I dropped it back down to 160 °C which only required about 5W of power and pushed some HDPE filament down it. Pressing as hard as I could I got it to extrude some 0.75mm diameter filament though the 0.5mm hole in the nozzle. You can just see a little bit poking out in the picture of the finished article below :-

I don't know how long it will last, the J-B Weld may crack as there is nowhere for the nichrome to expand to. Insulated nichrome would be better in this respect.

There should also be a glass bead thermistor attached to the nozzle to monitor the temperature so that it can be regulated in software. Having seen the resistance of copper stepper motor coils increase noticeably when they get hot, and tungsten light bulb filaments change resistance by a factor of ten, I thought I should try using the resistance of the heater to measure its temperature. The resistance didn't seem to change much so I looked up the temperature coefficient and found it was much lower than other metals, so that is a non starter. I have ordered a couple of thermistors but they are out of stock at the moment so I will have to run it open loop to start with.

The next thing to do is put the pump back together and see if it can extrude.

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