Saturday, January 17, 2009
I kow I'm posting too much but.... Heres my collection of others "extruder rules of thumb".
I've just read nophead's beaut new heater/hot zone here: http://hydraraptor.blogspot.com/2009/01/fanless.html
and then there's geo's thermal modeling here:
and then larry's hot zone working here too:
So what do we learn from all this?
1) thermal gradient zone from hot to cold MUST be critically short, to avoid jams.
- thermally insulating material in gradient zone above heater (typically PTFE, sometimes stainless).
- reduced cross-section of material directly above hot zone reduces heat transfer by effectively using "air" as more of the insulating material. ( eg, remove threads and some metal in stainless components)
- use heat sinks at the top of the gradient, to distribute wasted heat that gets through the zone, and effectively drop temperature more suddenly. either large perpendicular ones, or circular turned ones work. Should be of thermally conductive material, contrary to the first point above.
- other points?
2) hot zone should be short, to avoid slumping and dripping, and keep heat focused and un-wasted.
- complexity of custom-made heater elements is daunting ( either nichrome, or other engineered solution)
- minimise the thermal mass of the hot zone ( make it small), including hte nozzle.
- insulate the key areas ( the hot area, and the tip), either with fibre-glass, high-temp silicon blanket, or with PTFE ( typically just over the tip), to improve efficiency, reduce convection, and stabilise the temp ( keep the tip as host as the rest).
- connecting to non-hot-zone areas via the "thermal gradient" area is non-trivial.
3) nozzle constriction zone (ie section of small hole) should be as short as practical to reduce the internal pressure, and hence load/wear on the motor and parts.
- carefully engineer this part for minimised thermal mass, or buy it from those that can.
- should be removable, as it is known to jam, or need swapping.
- modified acorn-nut, or custom-lathed brass parts are both the norm, and both work well.
Anything else I should have included?
P.S. Has anyone done a water-cooled thermal-gradient zone? It seems like it's the best way to do this. I think I have to try. :-)
Yeah, it's been tried by a number of people. The thing that you have to remember about the extruder orifice is that the distance between the exit of the orifice and the inside of the extruder barrel has to be as short as possible lest you get eaten up by the friction generated by pushing highly viscous molten plastic through a very narrow channel. With my extruder that distance is running 0.1 mm max.
Think about how long that channel will be in a mechanical pencil tip and you will quickly see the problem.
Having said that I haven't noticed much difference in die swell with different nozzles. The hole size affects it. The smaller it is the more swell you get. Different plastics swell more, HDPE much more than ABS. The faster you extrude the more it swells.
However, I just stretch it back down to the nozzle hole size so its pretty irrelevant.
Yes, I had read the topic. I like the idea of just having to connect two endpoints of a resistor. When kept long enough, the thin leads probably won't even get too hot to solder them. I even added the resistor to our ordering list. It would make for a nice and compact heater.
Have you thought about two of these resistors (with twice the resistance) in parallel. This way the heater barrel would be heated from two sides.
Links to this post: