Sunday, March 21, 2010
i really need to find a better way of extruding plastic.
our first extruder worked fantastically for a long while, until the heater element got a little too hot and broke. we must have built 5 or 6 different extruder heads in the past month or two with parts from makerbot, and it just hasn't gone well. this is the latest, and i thought we really had it -- until i looked over, and noticed a two inch long blob of plastic steadily coming out from between a swollen barrier, and the heater barrel. a first for me. and, on only the second print.
please, someone, come up with a better method... maybe a ceramic barrier, or something? these PTFE barriers, brass barrels, and wait times for shipping are starting to get expensive after the n_th iteration. o.O;;
I think a tight fit, good temperature control, and a well mounted temp sensor go a long way.
I replaced it with a washerless design from MakerGear, and it is doing much better. The thermal barrier is PEEK, which has a much lower coefficient of thermal expansion than PTFE. The entire system seems to be much more robust.
The temperature being set to 240C seems to give a pretty good extrusion, but like Thomas mentioned, maybe things are just getting far too hot higher up.
If you make your own thermal barrier, drill a 5 mm hole and use an M6 tap.
I would also like to try a stainless steel heater barrel without any thermal barrier. Stainless steel has low thermal conductivity, so the heat climbs from nozzle to base slowly. What that is happening the barrel cools as it looses energy to the atmosphere. There is some stainless steel heater barrel length at which the nozzle can remain at 220C and the base be at an acceptable temperature. A fan blowing over the heater barrel would reduce the necessary length.
I tried a stainless steel barrel like you describe but it did not work. In fact two, one I made myself and the other was a solder sucker heating element.
The problem is you end up with a transition zone about 50mm long. Somewhere along that transition the plastic is past its glass transition but not molten, i.e. like rubber. When you push on it it swells outwards and plugs the tube. It becomes impossible to move forward.
The way to solve that is to make the transition very short, a few mm, by using very thin walled pipe and a heatsink. It works then works very well.
Another solution looks to be Brian's hybrid insulator with PEEK on the outside for strength and PTFE on the inside for slipperiness. I have been testing one for a few days only, but it seems good so far.
I re-used the barrel and nozzle from that, but moved the thermocouple into the junction between the barrel and the nozzle (it's just jammed into the threads between the two), and added some code to detect a loose thermocouple. It hasn't happened yet.
I also use furnace cement instead of Kapton - there's more thermal mass, but a better thermal connection, and less overshoot. I think PTFE starts having trouble at 260 deg C, so ABS at 240 is awfully close, especially if you don't have a stable temperature control scheme.
Finally, I usually use PLA for printing these days, at 210 deg C, so that gets me much further away from the dangerous temps for PTFE.
The most important trick is to use forced cooling on the cold side. Here's a pic:
What you see is a heater/nozzle made out of 6mm diameter brass rod. I drilled a 3.5mm fillament hole through it and lathed down a 10mm stretch to a wall thickness of 0.25mm.
At first I used a simple passive cooler on this nozzle -- that was a disaster, it would *only* jam.
So I (soft)solderd the heater into a length of brass slab (6mm x 25mm) and mounted a chip cooler fan onto the slab (top left in the photo).
The heater is built from an old toaster wire but has the same resistance (and hence the same power) as nichrome heaters. With this power rating it can reach 240 Celsius albeit a bit slowly.
With this metal insulator I have had *zero* jams after I started to use forced cooling...
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