A high temperature extruder?

The standard RepRap extruder can't quite handle the temperatures for HDPE for very long. I have found a high temperature replacement for J-B Weld. The main weak point remaining is the PTFE thermal barrier. PTFE is an excellent thermal insulator but it is not very strong mechanically. It also expands by about 0.5mm at 225°C. Worse than that it seems to slowly creep the more I use it, which makes a mockery of my z axis calibration. Since I got it working again I have re-calibrated it four times and each time it has grown: 0.3mm, 0.2mm, 0.15mm and 0.3mm. I.e. it is now 0.95mm longer than when I built it and a further 0.5mm when it is hot.

I have come to realise that stainless steel is quite a poor conductor of heat compared to other metals:-

Stainless Steel	Brass	Aluminium	Copper
17 W/mK	109	250	400

I bought some stainless steel pipes on eBay that have an outside diameter of 6.4mm and an inside diameter of about 3.5mm. I cut a 50mm length, tapped it and screwed in into a medium sized heatsink. I tapped the other end and screwed in my experimental high temperature heater. I applied heatsink compound to both threads.



I put a thermocouple in the heater and adjusted the power to get 240°C inside the brass part of the barrel. That only required 7.3W. I put another thermocouple at the top of the stainless steel barrel and that only reached 50°C.



Although this is just a lash up, it looks really promising. I can get the temperature even lower by using a CPU heatsink or a small fan. I will make a nozzle out of aluminium or copper with a built in heater and thermistor.

Not only will this stand temperatures up to the limit of the thermistor, which is 300C, but it is also much more rigid and does not change in length significantly with temperature. It should also reduce the amount of molten plastic because of the thermal gradient down the SS barrel. That should give less extruder overrun.

Filling in

I have been experimenting with various infill patterns. Here is a 40 x 10mm block made with 0.5mm filament at 50% fill: -



For simplicity I used alternating horizontal and vertical lines rather than diagonal. The layer height is 0.4mm so the width is about 0.6mm and so are the gaps. A couple of things that weren't obvious to me at the beginning were: -

The first and last lines of the fill must be adjacent to the outline so that the U turns on the alternate layer above have something to rest on, otherwise they curl upwards or downwards and don't bond to the outer skin. That means adjusting the gaps slightly to make the overall width correct. When the fill is 100% I adjust the filament width slightly to exactly fill the interior. Easy enough with a rectangular object but probably not with an irregular polygon.

The fill lines probably should line up with the those two layers below so that the intersections form a solid column of filament from top to bottom, otherwise some sag may be expected. Again trivial for rectangles but could get tricky to generalise.

Here is 33% fill, i.e. the gaps are about twice the filament width: -



This is 25%. Notice how, although the filament is laid down in a perfect square wave, when it shrinks it pulls itself to the first harmonic. A physical low pass filter!



And here is 20%: -



I found that when putting a lid over the top it struggled with an infill this sparse, so I settled on 25% as the limit for making closed boxes.

All the above are done with filament stretched to 0.5mm. When extruding through a 0.5mm orifice, left to its own devices the filament would be about 1mm due to die swell. I decided to try the same pattern with 1mm filament, i.e. with no stretching: -



As you can see the filament holds the square wave better but what is not obvious is that without stretching it sags a bit in the gaps where it is not supported from below. So some stretching is beneficial, when it comes to spanning voids, but it does increase corner cutting.

As I mentioned before, with my old nozzle, I could extrude 0.5mm filament at 16mm/s. This is what happens with the new one which has an exit hole which is too shallow: -



One unfortunate characteristic of FDM is that errors tend to be cumulative. What I mean by that is if, for example, the U turn of the zig zag fails to bond to the outer wall then that causes the next layer to have nothing to rest on, so that fails as well. The defect then propagates all the way up the object. With 100% fill, any errors tend to have less effect on the layers above.

Rather than slow down my experiments I decided to go to 0.75mm filament at 7mm/s until I make a new nozzle. Here is a 50% fill: -



I also added a bit of overlap between the fill and the outline at the u-turns to get a better bond.

So does the infill density affect warping? I made several test blocks and it looks like the answer is not much. However, I have come to realise that the warping takes hours to fully develop after the object is removed from the base so I will leave them overnight before attempting to make measurements.

The pros and cons of nozzles

HydraRaptor seems to be running reliably again, touch wood. I did have one scare when it started making noises like a machine gun when I had left it running unattended. It turned out that the shaft encoder code wheel on the extruder motor had fallen off. That caused the firmware to think it was far behind and so it applied maximum power in an attempt to catch up, which caused the GM3 gearmotor's torque liming clutch to slip. I added it to the list of sanity checks to put in my extruder firmware :-

• If the shaft position gets more than, say, half a turn behind then give up.
• If the thermistor resistance is too high then the thermistor is open circuit so turn the heater off.
• If the thermistor resistance is too low then the thermistor is short circuit.
• If the heater has been on for more than 5 seconds and the temperature has not risen then the heater is open circuit.
• If the heater has been off for 5 seconds and the temperature has not dropped then panic, the transistor is short circuit.

All these checks are necessary for safe unattended operation in my opinion.

The solution to the code wheel problem was to extend the shaft of the GM3 with a piece of brass rod :-



I have managed to perform quite a lot of tests with HDPE and it is clear that the new acorn nut nozzle behaves quite differently to the previous one piece design.

The original nozzle looked like this and had a 0.5mm hole that was about 0.6mm deep: -



The new nozzle is made from an acorn nut turned to a point. I also has a 0.5mm hole, but it is tapered at about 45° so the the part of the hole that is 0.5mm diameter is very thin :-



The differences this seems to make are: -

1. The die swell, i.e. the amount the filament expands from the hole diameter, is a little less.
   
2. The amount of filament that extrudes after the motor is switched off has increased quite a lot. The excess is wiped from the nozzle, but by the time the head has moved from the brush back to the workpiece, a few more mm have leaked out making for a messy line start. I think this is because the shorter exit hole makes it easier for the plastic to escape.
3. If I move the head quickly with the extruder off, then the filament snaps. It quite often leaves a blob that sticks to the workpiece. With the longer hole it stretched to a long thin string rather than snapping.
4. I used to be able to lay down 0.5mm filament at 16mm/s by stretching, but now I can only do this reliably at 8mm/s as the filament has a tendency to snap. I think it is too easy to pull it from the new shaped hole.
   

When stretching the filament it has a greater tendency to cut corners. I think this is mainly due to not running the fan, but may also be because the nozzle is too pointy. A wider nose will help to push the corners down.

I can't run the fan because the heat loss from the bigger nozzle causes the heater to work harder, raising the temperature of the barrel above the point where the PTFE distorts. I need to insulate the nozzle so I will try making a new one similar to this one with a PTFE cover over it.

Here is about where I am at with extrusion quality: -



This is a rectangular block about the size of the extruder pump (60 x 20 x 15mm), with a 50% fill. I forgot to put a top surface on it but that is perfectly possible. It was extruded at 220°C (measured at the nozzle) with filament stretched to 0.75mm at 7mm/s. The layer height is 0.6mm and the pitch is 0.9mm. Some warping still evident but it has come a long way from my first attempts.