RepRap: Builders

Several months ago I posted an extruder tip design that used a transistor in a TO-220 package (http://builders.reprap.org/2009/04/inspired-by-nopheads-observation-about.html). It worked, but not for long, probably since I was running the transistor at a temperature(>230C) much higher than its rated maximum of 150C. After I had gone through a few transistors I switched to using a 10 ohm load resistor in a TO-220 package. The first resistor mechanically broke during an overnight print run. It looked like the resistor had hit the printed object. I replaced the resistor, and the second one had been working continuously ever since. I have printed more than 5 pounds of ABS with it so far. I must credit Nophead with the idea of having the barrel temperature drop over as short a distance as possible.

Because of this success, I would like to present the details of the extruder tip design and some instructions on how to make it

1. Heatsink: This holds down the temperature at the end of the extruder barrel. I have used a 6.3mm (1/4”) thick aluminum L bracket with a total surface area of about 15100 square mm (23 square inches). Since heat flow from the barrel is important, the barrel screws into a threaded hole to so that the thread-to-thread contact can conduct heat. I have also made a second experimental extruder with an aluminum heatsink of about half the area. I think that as long as the heatsink is not too hot to touch after several hours of operation, then it would be good enough. This heatsink also plays a structural role, since it is part of the force path involved in pushing the filament into the barrel. In my extruder, I have the pinch wheel motor attached directly to the aluminum L bracket (which means that the motor also heats the bracket).

2. 10-32 nut: This is in the heat flow path between the barrel and the heatsink, so do not use a lockwasher. I suggest brass as a material, though I have also used an aluminum nut made by cutting up a threaded aluminum standoff from my junk box.

3. Extruder barrel: This is a tube made by putting a 10-32 thread on a 4.76mm (3/16") OD, 0.71mm (0.028") thickness stainless steel tube (K&S Engineering stock No. 7113 Stainless Tube 3/16 x 0.028 (4.76mm x 0.71mm) UPC code 14121 17113). In order to put on thread successfully, put a 3.17 mm (1/8") diameter rod in the tube. 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.

4. 10 ohm resistor in TO-220 package. Stackpole P/N TR35T110 5% B , Digikey P/N TR35T110J-ND. Other manufacturers could be used. If you use something different, I suggest picking as large a wattage as possible, based on the idea that larger wattages would have a better thermal connection between the actual resistive element and the heatsink tab, and probably be more reliable. The mounting hole must be drilled and tapped for 10-32 thread. Given how little material is removed by the drill bit, it might be possible to tap it without drilling first, but I have not tried this. The material is copper, and not thick, so be careful to keep the tap straight as it goes in.

5. Thermistor holder: I have done this two ways. The original heater has a small bracket made from copper sheet. More recently I have used a crimp lug with a #10 hole and suitable for 22-18 AWG wire. I take off the plastic insulator and pry open the crimp barrel slightly along the seam. I open the gap enough that the thermistor can be slid in and is held by the spring of the metal.

6. Thermistor: Allied Electronics P/N 254-0019. Use your favorite.

7. 10-32 acorn nut, with 0.5 mm hole drilled in the end (done with a PCB carbide drill).
   

Filament jamming: Others in the reprap community have noted that filament can jam in an extruder barrel if it is allowed to soften back up the filament and form a plug pressed against the inside of the extruder barrel. I have noticed this to some extent with my extruder under experimental conditions, but under normal operating conditions it is not a problem. By normal operating conditions, I mean starting the extrusion process right after the thermistor shows that the tip is up to temperature. It takes a few seconds for the filament to start coming out, but this is on the edge of the print raft so that it does not matter. I use skeinforge, and it does not turn off filament extrusion long enough to allow the filament to jam. I also set the file that skeinforge appends after the print to turn off the heater. This allows good startup the next time.

Frank Davies

Labels: extruder heater

# posted by Frank Davies @ 8/29/2009 07:59:00 AM