Constipated Extruder

My "New Year" extruder, which is the one on HydraRaptor that I use to build things, stopped working while building the first layer of an object. That is the lowest temperature layer, so the plastic is at its most viscous.



I couldn't get it to work again, so I removed the drive and tried pushing the filament by hand. I couldn't shift it. I measured the temperature of the molten plastic with a thermocouple and it was correct, so I deduced that the nozzle must be blocked. I removed the nozzle and when I pushed the filament this came out: -



It is dark and glassy looking. No idea what caused it, but it seemed to have blocked the nozzle. I cleared it out with a drill and reassembled it. I took the opportunity to measure its performance with my "lead kebab" test jig.

Even though this extruder has a 0.3mm nozzle and no taper in the PEEK insulator, it works better than the tapered PEEK extruder with a 0.5mm nozzle.



The most notable difference is that this one has a much bigger heater chamber, so perhaps a smaller heater bore melts the plastic quicker.

I got this interesting graph of flow against force, averaging over five runs of 20mm : -


I think the steep part of the curve is where the flow through the nozzle dominates the force required and the first part is where the plug friction dominates. The point where I operate it is right on the knee of the curve. I suspect adding a taper would straighten it out, but I don't want to strip down my only working extruder to prove that.

So I don't know what caused the blockage, but it is the second time I have had an extruder block, so it goes to show that a detachable nozzle is advisable.

Taper relief

As tapering the stainless steel insulator made so much difference I went back to my PEEK extruder to try the same thing.



I used the tapered reamer to open it up to 5mm at the bottom end.



I had to remove and replace this with the heater hot. You can see where ABS has run up the thread and then burnt when it met the air. This seems to seal the thread as long as the initial leak is slow enough. I don't think HDPE would seal in the same way, so I run ABS first when I assemble an extruder.

The taper made a big difference. HDPE pushed with 4.6Kg went from 1.1 mm³ to 5.3 mm³ and the times got more consistent. I think it is beneficial in four ways: -

• It removes the friction of sliding the plug along the wall.
• It increases the bore where the very viscous, just-melted plastic is, reducing the viscous drag by a fourth power.
• It thins the hot end of the insulator making the thermal gradient steeper.
• The wall being thinner and having a bigger surface area will allow more heat flow into the melting plastic.

Foolishly I didn't measure any ABS flow rates before I made this mod. ABS extrudes at 4.5 mm³ when pushed with 4.6Kg and only 1.3 mm³ when pushed with 2.3 Kg. This is odd in that the differential between ABS and HDPE is less with this variant.

The performance with HDPE is a bit better than the stainless steel extruder when it was fitted with the same nozzle, but the ABS performance is considerably worse. I can't explain why that would be.

A third variant would be to use a longer PEEK tube with a taper to dispense with the heatsink and hopefully be strong enough without the washer and bolts. I think I will have a look at drive mechanisms for some light relief before coming back to that.

It looks like about 5 Kg force should cover the plastics I have tried so far. I don't think anybody has tried pinch wheel with the slippery plastics (HDPE and PCL) so I will have a go at that.

Simply better

I find it very satisfying when making something simpler also makes it better. I tested the simplified heater / nozzle design using the same stainless steel insulator and heatsink arrangement, so I could get a direct comparison of the results.



The heater warms up a lot faster than the one made with two AL clad resistors. It also extrudes faster and the times are more consistent. ABS pushed with 2.32Kg went from 3.7 mm³ to 4.6 mm³, an increase of 24%. HDPE pushed with 4.6Kg went from 3.8 mm³ to 9.3 mm³!

The nozzle is 0.6mm rather than 0.5mm, which reduces its contribution to the pressure by a factor of 2, but all my other tests have shown that what happens at the other end of the heater dominates the force requirement. As I improve things though, the nozzle hole becomes more significant.

Here are the drawings :-



Although it looks complex it isn't difficult to make with a drill press, drill vice, and some taps and dies.

I glued the thermistor in with Cerastil, but I expect it could just be wrapped in tin foil and jammed in like the ceramic resistor, taking care to insulate the wires of course. I use PTFE sleeving.

I didn't need to seal the threads with PTFE tape. I just screwed them up tight and there was no sign of any leakage.

The next thing to try is putting a taper in my PEEK version to see if that can be made to perform as well as this one.

Of course I haven't built anything yet with any of these designs, so caveat emptor.