Thursday, April 02, 2009
Inspired by Nophead's observation about short thermal transitions being necessary as well as by a desire to try a different way of heating the extruder, I have made the following extruder which is working well so far.
1. 6.3mm (1/4") thick aluminum L shaped extrusion as heatsink. Total surface area about 15100 square mm (23 square inch). Hole is tapped for 10-32.
2. 10-32 brass nut. This is in the heat flow path between tube and heatsink, so no lockwashers, etc.
3. Tube made by putting a 10-32 thread on a 4.76mm (3/16") OD, 0.71mm (0.028") thickness stainless steel tube. In order to put on thread successfully, put a 3.17 mm (1/8") diameter rod in the tube.
4. TO-220 PNP transistor (P/N NTE-153) with mounting hole drilled and tapped for 10-32 thread.
5. Thermistor holder made from copper sheet. Hole is thru hole for 10-32, not tapped.
6. 10-32 acorn nut, with 0.5 mm hole drilled in the end (done with a PCB carbide drill).
Putting the thread on the tube with a 10-32 tap is tricky, but putting a rod inside the tube to hold it straight helps. The tubing gets so thin that you can see ridges on the inside of the tube pushed up by the teeth of the die.
The PNP transistor has a rating of about 150C. I am taking a chance at running it at 240C for extruding ABS, but it has worked for about 10 hours run time so far without failure. I chose a PNP device so that I could connect the collector to the frame of the unit, since the tab is the collector. Here is a schematic of my drive circuit. I adjusted the pot until there was 1V across the 1 ohm resistor. With a 12V power supply, this means that the transistor is dissipating 11W. One reason that I used the PNP transistor is that it is possible to use the transistor characteristics to measure its temperature. I have not tried this yet, since it would be somewhat complicated, and I wanted to get some extrusion hours under my belt.
It is possible to get power resistors mounted in the TO-220 package. They might be a higher temperature rating and would be less complicated to drive with the standard circuit.
I have not found it necessary to have any more heatsinking than the L bracket (item 1), and I do not have a fan on it.
I have successfully used this with a pinch wheel consisting of a knurled nut.
Here are some pictures of the unit mounted on my repstrap.
I'll be interested to see the long-term behavior, whether there will be sufficient migration to cause failure, etc.
It would also be more efficient than having the TIP-110 dissipate waste heat on a PCB while it could heat the target (the tip) instead!
I'd be interested in knowing about a longer run-time... see how it fares above the rated temperature.
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