Friday, January 16, 2009
ceramic resistors as a low-tech heater element
Take 1or 2 x 10W ceramic resistors, strap them to the barrel with wire, twist wire tight twice, and hey-presto, an instant heater element. No nichrome or waiting for glue to set, no special engineered parts/blocks ( except the usual tip and shaft ), no bolts, or washers, and voila!
Initially, with just one 5.6ohm resistor in air, I got 280dec C (measured on the outside surface of the unit with a thermocouple) with my powersupply, and without any insulation around it. That told me I had a go-er... so .....
Now with two resistors in parallel, and the increased thermal losses from the brass and increased surface areas, my "heater" reaches 200degC+ without insulation, and with a breeze blowing :-)
I expect it'll extrude just fine when I get the other parts of my extruder built, and insulate it, and if-necessary give it a bit more juice than the ~10.5V I have across it now, but YMMV.
I used a bit of JBweld-like stuff to fill the air-gaps as air is a poor thermal transfer agent. Actually I used Loctite gasket sealant, but it's irrelevent as it's not structural, just for improved thermal transfer. you could just as easily use CPU grease in this case, if you like.
Downside: longer "hotzone" shaft, resulting in theoretically more potential for ooze of hot material.
anyway, here tis: In these pics, it's running, and at-temp!
I looked into using these ceramic resistors - the data sheets suggest they will run reliably at 250 degrees C. I bought some and ran them briefly.
The only problem, as you point out, is they are long in the 7 & 10 watt packages but i wonder if we could 'push' a 4W one?
My idea is to put the resistor inside the heater - co-axial with the filament. The heater barrel would be made from a machinable ceramic with a 3mm and 10mm overlapping coaxial hole. The filament melts, running alongside the resistor casing - direct heating.
Most of the heat is kept inside the barrel and in the plastic. My main concern is the melt reansition zone will be oddly shaped and/or high friction and I am watching Nophead's work on this hot/cold area with much interest.
Actually, I should actually put 'work' on one side and do some tests myself :)
The resistor could intersect the filament feed at a shallow angle so as the filament is pushed along the resistor's flat side, it melts and flows around to the other faces of the resistor to even out heat-flow.
I ordered a variety of power resistors from Digikey. I'll be experimenting with these and I'll report back with what I found.
could have an extruder barrel go right through them. This one is good up to 350 C
I currently have 2x 5.6-Ohm 10W resistors in parallel, giving me masses of available heat ( it peaked at 290dec INSIDE the shank, once I got it to temp, and removed the breeze), and I have them strapped to either side of the shank. why not use 4x5W resistors at (say) 3ohms each, and wire them into seris pairs, and then wire that into paralell. the result is the same as I have now, only using 4x smaller packages that will "surround" the shorter shank at 0,90,180,270deg, still held on by some random stiff wire ( in my case 1.2mm galvanised fencing wire ,but that's irrelevant )
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