Monday, March 06, 2006


Monday tests on the 6.35 mm filament extruder

I decided to run a few preliminary tests on the 6.35 mm filament extruder before I did further work on it.

I used semolina as a proxy for polymer resin to check to see if the assembly would pump. There was no problem with that. I then inserted the PTFE thermal barrier into the assembly to see if I could pump semolina through that.

The semolina immediately jammed in the thermal barrier passage. It formed a 15 mm plug almost exactly like that I encountered when using the same auger bit in 1/4 inch steel pipe except the steel pipe only made a 5 mm plug before jamming.

The plug was easy enough to clear once I detached the thermal barrier from the polymer pump assembly.

Think that the problem might have been specific to the semolina I then used a few grams of the CAPA resin in powder form that I recently acquired as samples. The same thing happened.

Any ideas?

Sounds like it doesn't jam in the steel bit when it's under pressure, but does in the PTFE.

Does this apply even with a short piece of PTFE? Or copper? Or aluminium?

If short PTFE spacers can be used, perhaps a short water-cooled section could be used to keep the heat in the pointy end?

Could you try putting the feed in through the PTFE block? This might reduce the pressure in the PTFE section, with maximum pressure being born by the steel section.

Vik :v)
It would appear that once the granular polymer media leaves the screw pump it forms plugs at ambient temperature.

It doesn't seem to matter what material forms the pipe it finds itself in. I did notice that plugs in steel pipes tend to form much more quickly than they do in the PTFE pipe of the same diameter. I am assuming that that has something to do with the difference in frictio coefficients between steel and PTFE.

I did the experiment several time and widened the throat of the PTFE transport thinking that part of the problem might have been misalignment between the steel and PTFE sections. That got me a 25 mm plug instead of a 15 mm, my previous experience.
So can the pump go all the way to the nozzle? If the stuff blocks up as soon as it leaves the pump, make the thing one long pump :)

Vik :v)
In standard extruders, the resin is molten well before the screw terminates. I'd guess this isn't just to do shear heating, but also to prevent this jamming you're seeing.

I think Vik has it: The screw has to extend into the heated zone, in some form or other. Might be enough to just weld on a bit of threaded rod...
***The screw has to extend into the heated zone, in some form or other.***

I guess that I need to look for a longer 1/4 inch auger bit.

From what I'd read, though, a screw pump for granular material depends on the friction coefficient for the barrel wall being larger than the friction coefficient for the auger. If I run a long auger through a length of PTFE that would say that the granular wouldn't pump in that section. I should be able to test that idea easily enough, however.

I have a bigger practical problem. Right now I can just about drill a 6 inch hole through steel with the equipment. I've drilled a 4 inch heater block, a 7 inch polymer pump block and a 3 inch PTFE cylinder (badly). I've had to cobble the alignments of those together. If I have to use a longer auger bit, it's going to be significantly longer than the 1/4 inch (or any diameter for that matter) hole that I can drill. :-(
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