Wednesday, 31 December 2008

Do we need nichrome?

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.

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.

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: -
  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.

Sunday, 21 December 2008

Sticking point

sid, who is a regular contributor to the RepRap forums, had an idea to get a soldering iron manufacturer to make a heater barrel assembly for RepRap. He approached a Chinese company with a specification and they sent him some prototypes. He forwarded one to me for testing. It appears that they ignored his specification and just sent a standard de-soldering iron element. Nevertheless it is a nice unit and looks eminently usable.

It has a tube running through the middle with an internal diameter a shade over 3mm. Ideally it needs to be about 3.5mm to cope with the worst filament I have encountered. My green ABS, being a little undersized, fits down it easily.

The heater has a cold resistance of 1.3Ω but, unlike nichrome, it has a big temperature coefficient, so its resistance increases significantly at it gets hot. It appears that it is a 12V 50W heater. We can drive this with PWM using a MOSFET provided the PSU can handle 9A peaks on the 12V rail in addition to what the steppers take, a tall ask. An inductor and diode could be used to reduce the peak current.

The other two wires are a type-E thermocouple. Unfortunately the thermocouple sensor board that Zach designed using the AD595 is for the more common type-K thermocouples. It can be recalibrated for type-E by adding extra resistors. However, the AD595 is an expensive chip because it is factory trimmed for accuracy. By the time you add external components the convenience and accuracy is lost so you might as well just use a cheap op-amp and a micro with an internal temperature sensor for the ice point compensation. E.g. the MSP430F2012 that I use for my extruder controller is a lot cheaper solution than the AD595 and can control the heater and motor as well.

To test the heater I clamped it by the mounting flange in a vice and hooked it up to a bench power supply. I measured the internal thermocouple's output with a millivoltmeter and also inserted a 3mm rod type-K thermcouple down the barrel. Here are the results: -

Voltage Current Power Resistance Temperature Thermocouple Calculated Temp
1 V 0.75 A 0.8 W 1.3 R 43 C 1.5 mV 42 C
2 V 1.20 A 2.4 W 1.7 R 106 C 5.6 mV 102 C
3 V 1.55 A 4.7 W 1.9 R 182 C 9.5 mV 160 C
4 V 1.80 A 7.2 W 2.2 R 275 C 14.5 mV 233 C
5 V 2.00 A 10.0 W 2.5 R 357 C 20.0 mV 314 C

The temperature column is as measured with my type-K thermocouple towards the nozzle end of the barrel. The calculated temp column is assuming 68μV/°C from the type-E thermocouple and a cold junction temperature of 20°C. There is a big temperature gradient along the barrel so the thermocouple reading depends on where it is placed.

As you can see we only need about 5V to drive the heater. The current would start at 3.8A and fall to 2A as it warmed up. This would be kinder to the PSU and safer than using 12V, but 12V would give a much faster warm up time. I expect something better than bang-bang control would then be needed to avoid massive overshoot.

When running horizontally the inlet tube stays cold and the mounting flange is just too hot to hold so it would be ideal for mounting to ABS or HDPE. This is because the barrel appears to be stainless steel, which is a very poor conductor of heat. The element must be towards the bottom so there is a continuous thermal gradient along the barrel.

The nozzle that came with it is made from copper with some type of plating. It had a hole to mate with the tube that sticks out of the end of the heater but it did not go all the way through. In fact it could not, as the tip comes to a fine point. I suspect this is a soldering iron bit that has been drilled out to fit.

I attempted to drill a 0.5mm hole through it but it just snapped the drill. Even drilling a 1mm hole snapped the drill. In the end I drilled a 2mm hole, but the drill bent and came out the side. I think it needed to be sharper for copper. Finally, I cut the point off and filled the 2mm hole with high temperature solder. That is soft enough to easily drill a 0.5mm hole through. It melts at 300°C so should hold up.

The heat damage is where I heated it up with a blow torch in an attempt to remove the broken drill bits. Copper expands a lot more than steel. That did not work so I tried to get it red hot to soften the drill bits so I could drill them away. I failed to get it red hot but I did melt the plating.

The shape is not ideal for making objects but it is good enough to see if I can extrude. In fact it extrudes well. I was able to push a piece of ABS through it easily by hand and it extruded at a very good rate.

The bit/nozzle is clamped on to the end of the barrel by an outer stainless steel sleeve tightened up by a threaded ring at the cold end. I was worried it would leak under extrusion pressure without some sealing. When I stripped it down I found it did leak a little but didn't get far. I suspect it freezes when it meets the outer sleeve.

So apart from the bore being a little too small this seems like a perfect solution: -
  • It needs no construction apart from drilling the nozzle.
  • It is mechanically sturdy.
  • It should be very durable; soldering irons last a lifetime and they run at higher temperatures.
  • It is cold enough to mount with plastic without any insulation. It does after all in a soldering iron although that is probably a thermoset plastic.
  • The nozzle can be easily removed and replaced.
BUT, it has one fatal flaw, exactly the same flaw as my attempt to use stainless steel as a heat barrier in my experimental high temperature extruder. There is a slow thermal gradient along the length of the barrel. That means there will be a point about half way along where it is just hot enough to soften the plastic but not hot enough to make it flow. When you push a piece of virgin filament in it slides past that point easily because it does not have time to soften until it gets to a much hotter part of the tube. If however you stop extruding then the stationary filament has enough time to soften further up. When you push it again it expands to plug the tube but is not fluid enough to move. No matter how much force you apply you cannot move it. To get it going again you have to pull it out backwards, cut off the swollen bit and start again.

The reason the original extruder design does not have this problem is that the thermal gradient is in the PTFE. It is much shorter so the problem region that is soft but not molten is a lot shorter and the walls are very slippery so it can still be shifted.

I can't think of a solution to this problem. You could make the internal tube out of copper but then the top end would be hot so you would need a PTFE thermal break again. Also it would not be an off the shelf product, it would be custom to RepRap. Perhaps a taper at the problem region could stop it sticking.

The next extruder I am building has an aluminium barrel and nozzle and a PEEK thermal break. It won't suffer from this problem at least.