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The plastic clamp and cylindrical finned heatsink have been replaced by a single horizontal 6mm thick aluminium plate that combines both roles.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

HydraRaptor's New Year's Resolution

My new extruder has a 0.3mm nozzle compared to 0.5mm that I have used before. The actual filament diameter is controlled by the flow rate versus the head feed rate, so a single nozzle can give a range of filament diameters.

The maximum diameter is governed by the hole size and the die swell. The head movement has to be about the same as the rate that the filament leaves the nozzle, or faster, otherwise the filament squirms about and makes a zigzag instead of a straight line. Fortunately the faster the flow rate, the more die swell there is, which works in our favour when trying to extrude the maximum diameter filament. With the 0.5mm nozzle I could extrude up to about 1mm with ABS and I used that to good effect when making the first layer of the raft. With a 0.3mm hole die swell is more but even so I can only get 0.8mm filament. That makes the first raft layer thinner, so it is less tolerant to the bed being uneven.

I normally extrude at a rate that produces filament the same diameter as the nozzle but it can be stretched further making it smaller than the nozzle. The limiting factor is when the filament starts to snap. I did make some 0.3mm filament with the 0.5mm nozzle but I don't think I got the full benefit of the extra resolution because the filament was less constrained as the nozzle changed direction.



These two gears are both made from the same gcode with 0.3mm filament giving a layer height of 0.24mm. The one on the left was made with a 0.5mm nozzle and the one on the right with the new extruder with 0.3mm nozzle. The latter is slightly better defined. The benefit is more apparent on the underside.



The bottom of the one on the left feels perfectly smooth due to being made on a raft with a very fine surface. It is actually smoother than a sample I have from a commercial machine.

I was disappointed that it did not improve the clockwise slant of the teeth. This must be due to the same effect that makes holes come out too small. The filament likes to cut corners, so when the head moves on a curved path the filament takes a smaller radius path. I noticed that the teeth are straight at the base but slanted at the top, so the effect is somewhat cumulative.

I made another one with the outlines anti-clockwise on every second layer. Here is a video of it being made: -

HydraRaptor RepRapping a gear from Nop Head on Vimeo.

The teeth came out straighter but the edges are slightly more ridged because each layer alternates a little.



The surface is not quite as good as the previous one. I put that down to variations in the feed stock diameter. You need exactly the right amount of plastic to get a good surface.

I also need to up the resolution of my z-axis. 0.05mm is significant with 0.24 mm layers, so I will have to add microstepping like my other axes.

So in summary 0.3mm nozzle gives noticeably better results and can still make 0.5mm filament due to die swell. It is harder to get the raft heights and temperatures correct. To get the same build rate with 0.3mm filament I would have to extrude at 44mm/s, but HydraRaptor is currently limited to 32mm/s. I could probably tune it up to 44 but the vibration gets a bit ridiculous as the moving mass of the table is 9Kg.

Yet another quick heater hack

The ideal off the shelf heater would be a cartridge heater but they tend to be at least 1" long, need mains voltage and are very expensive. Here is a cheap 12V alternative: -



It is a vitreous enamel wire wound resistor that can handle surface temperatures up to 450°C. It is a 6.8Ω RWM 6 x 22 rated at 10W, but I am overloading it somewhat to get 240°C.

I bought a pack of five from RS. Farnell and Newark also stock them I believe.

I drilled a hole to accept it in a 19 x 19 x 8mm block of aluminium with an M6 tapped hole for the heater barrel and a small hole for a thermistor.



The tapped hole is at right angles so that the hot zone is as short as possible. It could be made parallel to get more contact area.

The outside diameter of the resistor measured 6.3mm so I drilled a 1/4" hole for it. That was too tight so I drilled it out to 6.5mm. I then wrapped aluminium kitchen foil around the resistor to make it a tight fit and rammed it in.

Here it is under test with a random bit of tube to simulate a heater barrel.



It needs about 11W (8.7V) to get to 240°C. 14.7W (10V) gives 300 °C. I haven't run it for very long so no guarantees it will last, but I can't see why not.

Compared to the aluminium clad resistors I tried before, these are cheaper and you get a more compact heater with a smaller surface area to lose heat from. Also making connections should be no problem with normal solder because the wires are long enough to cool down.