Simple experiment

Inspired by Demented Chihuahua's extruder work, I repeated his experiment using what was left of my old heater. I mounted it in a 30mm M6 stainless steel washer and clamped that in a vice. I used my 0.3mm aluminium nozzle, which I counter bored with a 0.7mm drill to reduce the depth of the 0.3mm hole to about 1.5mm.



I powered the heater from a bench power supply and adjusted it manually to about the right temperature. Green ABS is handy for this because it changes colour at 260°C so you can tell when it is too hot.

I can extrude filament by pushing it by hand with moderate pressure. It comes out at 0.4mm but I should be able to stretch it back down to 0.3mm without any problems. Even with a 0.5mm nozzle I can stretch it down to 0.3mm, but I lose positional accuracy because the orifice no longer defines exactly where the plastic goes.

Originally the heater was 5mm longer, with the excess protruding beyond the half nut. I found that cutting that piece off made it easier to extrude. It was probably a relatively cool section so the plastic remained very viscous there.

When a new piece of filament is inserted into the heater it extrudes very easily. After a while some plastic flows backwards and builds at the entrance to the heater. That causes considerable extra resistance. I plan to tackle that by having a short section of PTFE at the entrance with a heatsink the other side of it. The steep gradient across the PTFE should freeze the back flow over a short distance and, being super slippery, should allow it to slide back into the heater.

Another thing I tried was forcing out the plastic using the shank of a 1/8" drill bit as a piston. The further the drill got to the end of the heater the less force was needed to push it. That confirms what I had suspected. The force to push the plastic though the long 3.5mm section of the barrel is very significant compared to the force to squeeze it through the short small hole in the nozzle. So the heater needs to be kept as short as possible. Obviously there will be a point where the extrusion rate becomes limited by the rate the plastic melts if it is too short, but I expect that is much shorter than the current set-up.

New Materials

HydraRaptor's extruder suddenly stopped working in the middle of a build a few weeks ago. I tried upping the temperature and pushing the filament with pliers but it would not budge. All that happened was the heater barrel slipped a few threads in the PTFE insulator.

It was a bit difficult to find out what was wrong because it was full of solidified plastic when cold. I unscrewed the nozzle and placed it in some acetone to dissolve the ABS. It appears that the hole in the nozzle was blocked by burnt plastic. It probably formed when I had some high temperature accidents and experiments recently.

I should have realised the nozzle was blocked, but it has never happened before. If I had then I could have just unscrewed it, cleaned it out with acetone and put it back on again. In the event pushing the heater out of the PTFE pretty much wrote it off.

Not for the first time, I decided to rob parts from the extruder I was making for my Darwin. These are all made from different materials in order to see if small improvements could be made.

The barrel is made from aluminium. It is a better thermal conductor than brass, is easier to machine being a lot softer, and is cheaper.



To make the thermistor more easily removable I mounted it in ring of aluminium with a tapped hole.



The thermistor was glued in with Cerastil H-115 and the ring was screwed onto the barrel with some heatsink compound in the thread. By adjusting the beta I was able to get the reading to agree with a thermocouple inside the barrel to within a couple of degrees. I don't know if that means the ring was at the same temperature as the middle of the barrel or if it was lower and I compensated with a beta value that is not actually the beta of the thermistor. Either way it produces the desired result.

I also made an aluminium nozzle with a 0.3mm aperture. I broke the drill bit as it went through. I am not sure if that was due to the aluminium snatching more than brass does, or me being careless. I have broken loads of small drills recently and blunted some bigger ones by accidentally drilling with my lathe in reverse!



The picture also shows where the thermistor ring mounts.

Another modification I made was to put a PTFE cap over the nozzle.



This has two benefits: -

1. It is a good insulator so it helps to keep the nozzle warm.
2. Being non-stick, and also cooler than the nozzle surface, it stops filament from sticking to it. I use a brush to wipe the nozzle. This works well with HDPE but ABS tends to curl upwards and stick. Since I added this cap the nozzle wipe has worked 100%. It remains to be seen if it works with PCL and PLA.

It is a snug fit but when it gets hot PTFE expands a lot so it slips off. I held it in place with a tiny screw and an indentation in the nozzle made with a drill point.



Another new material I used was Polyetheretherketone (PEEK) instead of PTFE for the thermal break. This has similar insulating properties to PTFE and a slightly better working temperature range. It machines well but forms burs very readily.

I found it much sturdier at working temperature, I don't need a pipe clip to stop the barrel popping out now, but I think it may be a bit harder to push molten plastic through, being less slippery.



The other thing I changed was I used insulated nichrome. When using bare nichrome I have to put down a thin layer of Cerastil to insulate the barrel, leave it to set, then wind the heater and cover it with more Cerastil. That makes it a two day job. By using insulated nichrome I can just wind it straight on the barrel and then cover. But what I didn't think about was that I normally make the soldered connections under the Cerastil, which I could not do this way. All in all I think bare nichrome is best as it makes a much neater job. Here is the previous heater that I made way back in March :-



So after all these "improvements" how did the new extruder perform?



Not very well! I tried it with green ABS first but could not get it to extrude reliably. I swapped the nozzle for my previous 0.5mm brass one and that got it working.

I then switched to some plain ABS that I bought a while ago but have not been able to use because it is very oval. It was too wide for my previous extruder. This extruder has a 3.5mm bore so it should easily fit but I could not get it to work reliably. It takes an enormous force to push it into the extruder. I am not entirely sure why. If I pull it out and push some green in I can extrude the plain that is left in the barrel easily so it isn't any harder to push it through the nozzle but it is to push it into the heater.

Since I foolishly changed every material at the same time it is hard to evaluate which things are better and which are worse. I have recently formed the opinion that the extruder design is far from optimum. I think we need a much sharper thermal gradient and a shorter heater barrel. I think a lot of force is wasted pushing slightly softened plastic down the thermal break.

My next attempt will have a very short thermal break with a heatsink at the cold side. I will also make it easier to strip down and reassemble. A problem with the current design is that once the heater barrel is screwed in and full of plastic it is hard to remove it.

Experimental extruder

I want to see how much of the Darwin design I can make out of HDPE as that is the plastic I have the most of and is the easiest to get hold of. It should also be the cheapest but I think I got a very bad deal with mine.

To extrude HDPE quickly, without losing accuracy, requires a fan blowing on the work piece while extruding at around 240°C. The PTFE insulator in the extruder starts to lose its strength under these conditions and it also extends about 0.5mm due to thermal expansion. The JB-Weld heater insulation also degrades rapidly. To address these problems I am working on a design using stainless steel as the insulator, which I first blogged here. Here is a second lash up I made to progress the idea :-



At the bottom is a brass nozzle made by the man himself, Adrian Bowyer, and is described here. It has already been superseded with the anti-ooze design shown here.

Above that is a brass barrel that came from BitsFromBytes, with my experimental Cerastil heater on it. I attached a thermistor to the barrel with JB-Weld.

The brass barrel is screwed into the end of a 1/4" stainless steel tube. The other end has been tapped with a 1/4" UNF thread and screwed into a small north bridge heatsink from a PC motherboard (40 x 40 x 15mm). I drilled through the centre and tapped it. To lock it in place and give a good thermal connection I made a square nut from a piece of 10mm aluminium bar. I spread heatsink compound on the threads.

The top of the stainless steel tube is screwed into an old PTFE barrel to join it to the pump. The barrel had swollen so that it wouldn't hold an M6 thread anymore, but fortuitously it seems to have swollen just enough to match 1/4" UNF.

This is by no means the final design, it is far too long and flimsy, it's just to test the concept using existing parts.

I also wanted to try insulating the barrel and nozzle with PTFE. I made an end cap that fits over the nozzle by plunging an 8mm end mill into a 12mm PTFE rod :-



The idea of this is to keep the fan wind off the nozzle and also give it a non-stick surface so that when filament curls upwards and will not stick to it. I also insulated the stainless steel tube with a piece of 12mm PTFE rod with a 7mm hole drilled through it. Here is the completed assembly :-



The gap in the PTFE where the heater and thermistor are and where the wires emerge is covered with fiber class wool. I hate the stuff, I only have to think about it for it to make me itch all over. It is a much better insulator than PTFE though, but I wanted something smooth and slender to not disrupt the airflow from the fan too much.

The wires are sleeved with PTFE insulation and then plugged into a floppy drive connector. So everything at the hot end is good for about 300°C.

How well does it work? Well it took me a long time to be able to get it to extrude HDPE semi reliably. Thermally it works well. With the fan off and the barrel at 250°C the heatsink only gets to about 45°C, easily cool enough to mate with HDPE, ABS and probably PLA and PCL as well. With the fan blowing it cools down to room temperature. The heater power goes from about 60% to 80% so the insulation works well enough. A better idea might be to lag the pipe with a thin layer of fiberglass wool and then wrap it with PTFE baking parchment to give it a smooth outer surface. Or maybe an outer metal pipe with fiberglass in between.

Mechanically it is not that great. It seems to a need lot of force to extrude. I had to open up the hole in the nozzle from Adrian's 0.4mm to my standard 0.5mm. I also had to up the temperature to 250°C. I think this is mainly due to where I am measuring it and how I calibrated the thermistor. Previously I measured the nozzle temperature and calibrated it with a thermocouple inserted into a hole in the nozzle. With this version the thermistor is in a notch on the surface of the heater barrel and I calibrated it with a thermocouple inside the empty barrel. Looking at the value of beta that I got I think that it is considerably hotter inside the barrel than the thermistor is outside. I am not sure how this is. With the heater on the outside of the barrel I can't see how the inside could be hotter. Perhaps the thermal connection of the thermistor to the barrel, via JB-Weld is not as good as it it could be. When sited in the acorn nut nozzle it was half buried in a hole.

Even with the nozzle removed it is quite hard to extrude 3mm filament by hand. Part of this has to do with how long the total barrel is and the fact that it has three joints. The inside of the stainless steel barrel is not as slippery as the PTFE. It might also be the case that the molten section extends further up the barrel causing more viscous friction. I plan to shorten the whole thing considerably: I will combine the clamp with the right angle bracket and take the tube right up to the base of the pump. I will support the heatsink with a cradle structure resembling an upside down table. More importantly, I will shorten the heater barrel by combining it with the nozzle and screwing the tube into it. Making it from aluminium, which is two and a half times a better conductor than brass and easier to machine, should make it easier to get a consistent temperature measurement.

As there is a continuous temperature gradient down the stainless steel, the point at which the plastic melts will be about halfway up so I think the heated nozzle can be quite short indeed. The limiting factor is how long it takes the heat to get to the centre of the filament with the very poor thermal conductivity and high specific heat capacity of the plastic.

Here is an HDPE version of the opto bracket with my best PCL version behind :-



I have no idea why it is so grey. It is not as neat as the PCL one but most of the errors are due to blobs forming when the extruder moves between extruding. These cause the nozzle to be displaced sideways when it gets close because it is so flimsy. Shortening it and supporting it properly will improve matters for sure. I also need to incorporate Adrian's anti-ooze valve somehow.

Basket case

I was using two old component spools to hold my feedstock, see all-wound-up, but I don't have any more so now that I have four polymers I decided to give Vik Olliver's design a try. It has the advantage that you don't have to spool all the filament on, you can just drop in an open reel if that is how your filament comes.

This is my take on it: -



The uprights are 15x15mm aluminium angle. The beam across the top is a piece of 20x10mm channel. The bearing is a standard ball bearing and I reduced its internal diameter with a couple of bushes I had lying around. I then used a bolt with holes through the head as an axle. I found it in the road while I was on a walk wondering what to use. That is the third piece of HydraRaptor that I have picked up in the street.



The baskets are £4.21 in B&Q and have a plate in the bottom with a central hole just right for feeding the filament through. As the machine pulls the filament from the centre of the reel, the basket rotates to prevent it becoming twisted.

It works very well and has the advantage I can buy as many baskets as I have plastics and just take them on an off as needed.

It also allows the filament to rotate in the extruder but ironically, since I tweaked my extruder, PLA no longer feels the need to rotate. Presumably it rotates if the friction between the screw and the plastic is higher than between the plastic and the filament guide. I think that gives a clue to which of my tweaks made all the difference in reducing the extruder torque needed. I think it was adding the washers to space the top of the pump apart so that the screw bites in progressively and sharpening the screw thread.