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.

Fanless

I have not succeeded yet in getting the stainless steel barrel to extrude easily, so I had a go at improving my aluminium extruder to remove the need for a fan.



A lot of heat is lost from the large flange on the top of the heater barrel and transfers by convection to the heatsink above and by conduction through the bolts.



When I stripped it down I noticed the bolts had loosened.



The PTFE had shrunk lengthways and expanded in diameter and was no longer making a good compression seal.



Although no plastic had escaped it had leaked under the PTFE.



I think it was leaking so slowly that it oxidized where it met the air and went hard, stopping further flow. I hadn't run it very long so it may have escaped eventually.

So PTFE is obviously no good in compression at these temperatures. I replaced it with PEEK, which is a shame because it is about ten times more expensive.

I also replaced the aluminium flange with an M8 x 25mm steel washer insulated from the barrel by a PEEK collar.

Here are the parts, the PEEK section is drilled in situ to get perfect alignment: -



And here it is assembled: -



I put some PTFE plumbing tape over the hot end of the PEEK before pushing it into the aluminium in an attempt to improve the seal.

The heatsink now runs at 80°C without the fan. I would have liked it to be lower but as long as it is below the glass transition of the filament and the clamp it should be OK. I tried insulating the bolts with PEEK washers but that only dropped the temperature by 5°C, so not really worth the effort.



After one heat cycle I noticed the bolts were not as tight as they should be so it looks like PEEK creeps a bit as well. Perhaps glass filled would be better.

It is disappointingly complex with lots of machined parts but it does work very well. The heater power has dropped to 50% from about 80-90% with the fan. ABS filament extrudes manually very easily and even HDPE only requires moderate force. I think actually having the long heatsink preheating the filament to just below the glass transition is probably a benefit.

It is too early to say whether this design will be reliable but other than the PEEK section leaking there isn't anything likely to fail. I don't mind making things that are difficult to make provided I only have to do it once.

I also have a much simpler design in mind that should achieve the same short transition zone.