Evolving extruder

The fanless version of my new year extruder works well but is not the easiest thing to make.



I have redesigned the lower half to be a lot simpler. I also wanted to see what would happen if I made a cavity of molten plastic inside the heater. Up till now I have been trying to minimise the amount of molten plastic to reduce ooze, but according to Anon's comments here, professional machines have a relatively large melt chamber. I wondered if plunging the filament into a chamber of already molten plastic would make it any easier to feed.

This is a cross section of my design: -


The plastic clamp and cylindrical finned heatsink have been replaced by a single horizontal 6mm thick aluminium plate that combines both roles.



The easiest and most accurate way to have made this would have been to mill it with HydraRaptor. If I make another, that will be the way I do it, but I made this one with a hack saw, a file and a drill press. I start by gluing a paper template on the aluminium with stencil mount.

I then centre punch through the cross hairs and drill all the holes. The paper can be removed easily by dissolving the spray mount with paraffin. The larger mounting holes fit the Darwin X-carriage and the smaller ones fit HydraRaptor. The group of four holes allow the standard filament guide to be attached. The Darwin extruder clamp has slots but slightly oversized holes are fine.

The 8mm counter bore was a bit tricky. I drilled it with an 8mm drill and then bottomed it with an 8mm end mill. That showed that my drill press / mill is not really stiff enough to mill aluminium with an 8mm bit even though it has a 45mm thick steel pillar. I don't think HydraRaptor would have any problem doing it slowly with a small end mill. It probably doesn't make much difference if the counter bore does not have a flat bottom, so simply drilling would suffice.

Moving down the design is the PEEK insulator that forms the short thermal transition zone.



This is 8mm PEEK rod tapped with an M8 x 1 thread so it can screw into the heater block. I used the metric fine pitch because I didn't have the correct tap drill for M8 x 1.25 (6.75mm). The 3.5mm hole down the middle is drilled in-situ to ensure it lines up with the hole through the heatsink.

Small diameter PEEK rods are far more reasonably priced: 250mm x 8mm is only £3 here and is enough to make about 20.

The heater is a block of aluminium with a 6.5mm hole through it to take a vitreous enamel resistor for the heating element as described before.



As well as the tapped entrance to the melt chamber there is a small hole to take the thermistor.

I made this on my lathe, using a four jaw chuck. It could however be made with a drill press if the nozzle screwed into it instead of it screwing into the nozzle. The lathe gets all the faces perfectly square but there is no reason why it has to be accurate.

The next part down is a PEEK collar to insulate the heater from its retaining washer.



This is the only part I haven't thought of a way to make without a lathe. It might be possible to mill it with the right shaped cutter.

It snaps into the stainless steel washer and is a tight fit to the M6 spout so it anchors the nozzle laterally as well as vertically. It is counter bored at the back to reduce the thermal coupling.



Here is the assembly: -



It leaked a little bit of ABS but it seemed to stop when the leaked ABS oxidised. I should have sealed the joint with PTFE plumbers tape as I normally do. Apart from that it seems to work very well. I was able to manually push ABS through a 0.5mm nozzle very easily, at great speed. HDPE extrudes pretty quickly as well. When I stop pushing, it stops pretty quick. I think with a reversible drive ooze should be OK.

The design is much shorter than the previous one which will increase the build volume on Darwin. It is also very rigid so will not deflect when extruding.

I intend to simplify construction further. Rather than drill the stainless steel washer I can use the technique Ian Adkins uses on the BfB extruder where it is trapped between nuts and washers on studding. The only reason I did it this way was because I had the stainless steel bolts but did not have any M3 stainless studding.

I will also look at screw in nozzles. Andy Hall uses copper welding tips. The exit hole is a bit on the large size but I can reduce it by blocking it with high temperature solder and then drilling that, as I did with the solder sucker bit I tried.

The next task is to make a reliable drive mechanism to go on top.