Saturday, January 17, 2009

 

I kow I'm posting too much but.... Heres my collection of others "extruder rules of thumb".

Ok, so everyone seems to be struggling/working to find a better extruder ( whether that's the feeder mechanism, or the hot zone ( and transition zone).

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:
http://geo01005-ideas.blogspot.com/2009/01/exturder-thermal-model.html

and then larry's hot zone working here too:
http://repstrap-cerberus.blogspot.com/2009/01/hot-stuff-first-extrusion-with-new.html


So what do we learn from all this?
1) thermal gradient zone from hot to cold MUST be critically short, to avoid jams.
how?
- 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?

Buzz.

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. :-)

Comments:
One comment on the nozzles. I don't know that anyone has ever tried this back in the early days, but how about mechanical pencil tips? I have been dismantling some of them and I think that *some* could work pretty well. Why drill these tiny holes if we can but a part off the shelf. I have been looking at both threaded types and non-threaded types, sizes 0.7mm, 0.5mm and 0.3mm. I think this should really get it's own topic. Sorry for posting here, but not many people read my blog(for good reason) and this is an important topic.
 
"how about mechanical pencil tips?"

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.
 
Yet too short a distance will cause your extrudate to swell more once it exits the orifice. A 5 mm extrusion (5 mm orifice) becomes 6-7 mm again. This is probably less if it's kept squeezed for a bit longer. So there's an optimum width and depth to keep pressure (from friction) reasonable and the filament small. The new feeder mechanism might cause us to worry less about pressure and extrude smaller filament faster (more resolution and retaining speed).
 
I am not sure that is true Erik. Higher pressure causes more die swell so a shorted exit should need less pressure and so cause less swell.

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.
 
BTW, I have another easy to make heater:
hydraraptor.blogspot.com/2009/01/yet-another-quick-heater-hack.html
 
Thanks for explaining.

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.
 
Yes, my ALCLAD version had two 12R in parallel. I will try one to start with and may add another if I find it is needed.
 
I am using welding nozzle tips which you can buy from screwfix and halfords. They are made from copper(I think) and come with predrilled 0.6 and 0.8 holes and have M5 threads on the top.(5 for £4) They have worked Ok but cause jams due to their length, I have now shortened one and it is running better. As soon as I get some decent runtime I will let you know more.
 
Post a Comment

Links to this post:

Create a Link



<< Home

This page is powered by Blogger. Isn't yours?

Subscribe to
Posts [Atom]