While making a new heater I decided to try using stranded tinned copper tails rather than the solid tinned copper wire I used previously. The idea being to put less stress on the Cerastil covering.
I started with a standard piece of 7 x 0.2 stranded copper wire and removed the insulation. I found all seven strands too bulky so I decided to see how many strands I needed to carry 2A. I found that a single strand was cool to touch at 2A but very hot at 4A. I figured two strands would be sufficient for some margin.
The fact that a strand gets hot at 4A, and in fact red hot at about 5A, got me thinking that we could just use a single strand of copper for the heater. Nichrome is expensive, not that easy to obtain, and difficult to make connections to.
I measured the resistance of a strand 52cm long as about 0.3Ω (my meter only gives one digit). The strand measured 0.17mm diameter. Calculating its resistance from the resistivity of copper I get 1.72 x 10-8 x 0.52 / (π (0.00017/2)2) = 0.39Ω.
At 4A the voltage drop was 2.6V giving a resistance of 0.65Ω and a power of 10W. The thermal coefficient of resistance is 0.0039 for copper so the calculated temperature of the wire is 20 + (0.65/0.39 - 1) / 0.0039 = 191°C. It was certainly hot enough to cut through ABS.
10W and 190°C are not far from the operating conditions of an extruder. I tried winding it on the bobbin I had made for my heater but it was about twice as long as I could accommodate. I am trying to make a very short heater at the moment so I went back to using nichrome. Also 2.6V @ 4A is too much for my current drive circuit but it would be easy to come up with a switch mode converter to drive it, or simply use the 3.3V rail of a PC PSU.
So it has definite possibilities. Making the connections would be trivial. Just start with a piece of 7 strand wire and cut it down to one apart from at the ends. Some high temperature solder would keep it neat but would not be essential. A standard heater barrel with some insulation would be about 7mm diameter so 24 turns would be required. If you keep it taught and wind it in a lathe or drill chuck you can get about 2 turns per mm with some concentration. That would easily fit the space currently allocated for the heater.
Wednesday, 31 December 2008
Saturday, 27 December 2008
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.
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.
Friday, 26 December 2008
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: -
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.
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: -
- It is a good insulator so it helps to keep the nozzle warm.
- 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.
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.
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