Thermal thoughts

I attached the wires of my new heater with some small crimps that I cut from connector pins like this:-



to just leave the crimp part :-



I also soldered them with lead free solder which melts at about 220°C rather than the tin lead solder that I used last time, which melts at 183°C. It could do with being higher still so I might see if I can buy some high temperature solder (301°C) for the next one I make.



Rather than insulate with the recommended heat shrink sleeving, which is only rated for 125°C, I used some high temperature woven sleeving. Even that seems to discolour at 240°C. PTFE sleeving might be better.



I pushed the wires from the thermistor into a two pin connector so that I can easily remove the nozzle.

I had to make a new mounting bracket because when I added the pipe around the PTFE insulator to stop it expanding it made it too big to go through the hole in my previous bracket, which also doubles as my spindle mount when milling.

The bit of aluminium box section I cut it from has been waiting for a new life since it appeared in a prototype electronic photo booth on Tomorrow's World in the early nineties.

I calibrated the thermistor by putting a thermocouple into a second hole in the nozzle with some silicon grease. I also put a second thermocouple inside the barrel. I then put a range of voltages across the heater and waited for it to reach thermal equilibrium before reading the temperatures and the resistance of the thermistor.



Notice how dark the J-B Weld has gone when briefly heated to 240°C.

I made the new heater resistance 7.1Ω compared to 8.6Ω last time because I guessed the acorn nut nozzle would have a larger surface area and hence greater heat loss. In fact it has a remarkably similar temperature power curve to my last nozzle.


Note that the difference between the internal and external temperatures is as much as 25°C. The PTFE tape I used to seal the nozzle may not be helping here. I also measured the outside temperature of the J-B Weld using an IR thermometer which read 242°C when the inside was 245°C and the nozzle was 220°C.

After calibrating the thermistor it seems to track the thermocouple within about 5°C.

A riveting read

A recent modification to the RepRap extruder is the addition of two 3mm pins through the clamp and the PTFE insulator to prevent the PTFE slipping out. My PTFE tube is only 12mm diameter compared to the current design which is 16mm so 3mm pins are a bit too big for it. Instead I used some shafts from pop head rivets which are about 2.3 mm.



I drilled the holes 2.2mm to make them a snug fit.



Here are the pins installed through the clamp after being cut to length and rounded of with a grinder:-

Nutty Nozzle

My old extruder nozzle was made from a solid brass rod with a 0.5mm hole in the end.



It was drilled 3.2mm from the other end to accept the filament. The problem was gauging how far to drill from the back. Ideally the hole in the end should be as short as possible to provide less resistance to the filament flow. Drilling too far would write it off so I erred on the side of caution.

I suspected it was a bit on the long side as I got more die swell than I was expecting. Since it is now scrap I sectioned it to find out exactly how long the hole was. It turns out it was about 0.6mm.

The latest RepRap design uses an acorn nut which gives much better access to the back of the hole. I used the smallest drill I have, which is 0.3mm, to start with, I might open it up later.



The dome of a brass acorn nut is quite thick so I opened the hole out from the back using a conical milling bit that I bought for PCB track isolation milling.



The point is actually about 0.3mm so I was able to countersink the hole from the back until the point came through. That means the rim of the hole is very thin indeed and there is a double taper leading to it. The first is created by the drill that made the thread hole in the acorn nut and then a shallower taper made by my mill bit. It will be interesting to see what flow rate and die swell I achieve with this close to ideal shaped aperture.



I turned down the front of the nozzle to a point to give some clearance to the work piece as suggested by Vik Olliver somewhere I can't find now.



The extra hole on the face next to the thermistor is to allow me to introduce a thermocouple during calibration.

J-B Weld Heater

I ordered some Cerastil H-115 high temperature ceramic glue today but it will not arrive for a few weeks so I made another heater with J-B Weld. It is much quicker and easier than using BBQ paint.

I spread a thin layer on the barrel to insulate it and left it for 6 hours to set. Last time I used a thicker layer and turned it down but this time I just spread it very thinly with a spatula while hand rotating the lathe.



Next I wound the nichrome using a nut with a small hole drilled in it to anchor the start. I anchored the end using a piece of copper wire tied to it and pulled over a support round the back of the chuck. I expect this could be done with a drill chuck if you don't have a lathe.



I then added a second, thicker, layer of J-B Weld and left it over night to set.



Finally I slowly heated it up to 200°C in an oven to cure it.



I haven't tested it yet, apart from checking it for opens and shorts but it is pretty much the same method I used first time around: hydraraptor.blogspot.com/2007/07/hotting-up

Back to square one

I moved on from J-B Weld as a means of making a heater because it does not handle the high temperatures I have been using for HDPE. I completely failed with Thermosteel so I decided to have another go with BBQ paint. It seems that I must have the wrong sort of paint because despite helpful advice from Vik Olliver and Forrest Higgs, after a week of trying I can not get it to work.

After many attempts the final method I used was to put down three coats of paint using a paint brush in my lathe. Each layer has to be allowed to dry for many hours and then baked with a heat gun. If I apply heat too soon then it blisters. If I don't apply heat then the next coat simply dissolves the coat underneath. The paint has a lot of very volatile solvent in it.



Once I had three coats, I baked it in the oven on full blast (gas mark 9+) to make it hard enough to take the wire. I anchored one end of the wire with a nut that has a small hole drilled through it and attached a weight to the other end to keep it taught while winding.



To keep the wire in place while I painted over it I tied it to a piece of copper wire wrapped round the back of the chuck. To keep the tension in the right direction I used a piece of PTFE left over from a previous experiment to support it. It was an ideal shape and could stand the heat from the heat gun.



The picture above is after one coat of paint. When the paint was applied it was thick enough to completely cover the wire but when it dries it is very thin. I put five more coats on and baked it in the oven. I was somewhat disappointed when it came out like this :-



The paint resembles soot and has no strength to it at all. I can scrape it away with my finger, like I could with the Thermosteel.

Time to step back from this and think again. It is crazy trying to use high temperature paint as a high temperature adhesive. Some makes of paint may work, but you can't complain when other makes don't. I think it makes a lot more sense to use something designed to do the job such as Cerastil H-115. I will order some on Monday. In the meantime I will go back to J-B Weld because it is easy to use and will last for months if I keep the temperature down.

Tough as steel, my a...

I had originally intended to rebuild my extruder with the barrel I bought from BitsFromBytes but I ran into a compatibility problem with the nichrome wire I am using. My wire, as well as being un insulated, is a bit lower resistance than the recommended stuff, which I can't buy in the UK. I need about 300 mm rather than 200 mm to get the required resistance.

Going from a 6 mm barrel to an M6 threaded barrel means that I can only wind it with the pitch of the thread (1 mm) and the diameter is reduced, so there isn't enough room to accommodate 300 mm. A friend suggested using a finer thread which seemed like a plan. I have an M6 x 0.75 tap and die so I thought I would use that. Unfortunately finer pitch means shallower, so I would need a 5.25 mm drill bit which I don't have and I thought it was probably a bad idea to use a shallower thread in the PTFE. So in the end I made a new barrel with a thread on each end and an un-threaded section for the heater :-



I used J-B Weld for my last heater but it did not stand up to the heat very well. The bit near the thermistor, which I know was at 240C, and the ends of the heater remained strong, but the rest turned to a light brown dust with a harder darker skin over it. It was still functioning as a heater until I touched it at which point it started to flake away.



On the packet it states "J-B Weld maximum temperature is 600°F" which is 315°C. I know the heating element is going to be a bit hotter than the barrel, particularly when I was extruding with a fan on, because of the temperature drop across the thermal resistance of the J-B Weld. I doubt that it got to 315°C though. I emailed J-B Weld about this but I didn't get any reply other than an auto acknowledgment. Looking at their website I see the following "withstands temperatures up to 500°F" which is only 260°C so no wonder it failed.

The RepRap instructions suggest using Dulux Spraykote BBQ paint as a substitute for J-B Weld under the heading "But They Don't Sell JB Weld Here". Ironically they only seem to sell that in New Zealand so I got a local BBQ paint.



It turned out to be quite nasty stuff. Probably not a good idea using it in doors but it is too cold to do it outside at the moment. It went on easy enough but the RepRap instructions suggest three coats under the wire and four over it.



There are no instructions on the paint about drying times and re-coating. Impatiently I dried it with a heat gun, as it is rated to 450°C, but it blistered. I should probably have used a hair dryer and / or been a bit more patient. I apologise for the rubbish photo but you can see the size of the blisters.



I tried turning them down with the lathe but the paint just flaked off so I was back to square one.

I then thought I would give ThermoSteel a try. Supposedly it is a steel filled water based epoxy paste similar to J-B Weld but rated to 1318°C, although I don't know how you can have water based epoxy. I read somewhere else it was a ceramic paste which makes a bit more sense to me although I am not a chemist.



My plan was to put down a thin layer and then machine it flat with the lathe, wind the heater and cover it with a thicker layer, a technique I used successfully with the J-B Weld. It does say it is machinable.

When mixed up it resembles wall paper paste with iron filings in it. It was impossible to spread thinly, I had to dab it on to get it to stick.



I let is set over night and then as it says it gets stronger when heated (although it does not say to what temperature), I heated it up to gas mark 9 which is 260°C at the rate our oven warms up and then let it cool down at the rate the oven cools down when switched off. When it came out it looked like this :-



It looked a bit fragile so I scraped it with my fingernail and it came off in much the same way as the J-B Weld did!



So not getting very far with making a new heater. I have no idea why my ThermoSteel is a weak crumbly substance instead of something resembling steel. Should I have heated it a lot more? Have I been sold a small pot of wallpaper paste with Iron fillings in for £12.75?

I am not sure what to do now, perhaps try the BBQ paint again, use J-B Weld as I know it at least works for several months or make an induction heater.

Bottoming

As well as repairing my extruder I aimed to bring it closer to the latest RepRap design. The machined heater barrel has been replaced by a drilled out brass bolt. Partly out of laziness, and partly out of a lack of confidence in my skill with a lathe, I bought a ready made barrel from BitsFromBytes.



I was surprised to find it came with a modification that I had blogged when I made my first nozzle. That was to turn down the end that fits into the PTFE holder to get round the fact that I didn't have a bottoming tap to make the thread go right to the end of the hole in the PTFE. Ironically, I had bought a set of taps which included a bottoming one in the meantime. Well at least I thought I had :-



The tap on the left is the one from my cheap set of taps which only has one for each thread. The boxed set are the new ones that I bought with a taper, second and bottom tap. The single tap seems to correspond to the second tap, which makes sense for a compromise. The thing I don't understand is why the bottom tap still has a point and some taper. I was expecting it to be straight with a flat end, so I made it thus with a grinder :-



To try it out, I made a test thread in a scrap of PTFE and cross sectioned it :-



As you can see the flat end of the heater barrel butts up nicely to the end of the thread in the PTFE. I think it is important not to have a void here as it will fill with molten plastic which will freeze when the heater is switched off. It might then be difficult to melt it again as it is insulated from the heater.

The end of the barrel that is turned down is then a pretty good fit for the acorn nut, although a simpler solution is probably to bottom the thread in the nut and the PTFE and then go back to a plain heater barrel.

Nuts

Since my first attempt at making the RepRap extruder the design has moved on to use a brass acorn nut as the nozzle. It has the advantages of making the extruder easier to fabricate, allows the aperture size to be changed by swapping nuts and allows blockages to be cleared. I have to say that I never experienced a blockage with my single piece nozzle, but I can see how it could easily happen if a bit of dirt bigger than the aperture gets into the barrel.

Unfortunately brass acorn nuts, otherwise known as dome nuts and cap nuts, are expensive and hard to get hold of. I got a couple of un-drilled ones from BitsFromBytes.



My plan was to start with the smallest hole I could drill and expand upwards to see what effect it had and then drill the other to the size I found to be the best. Stupidly, I overestimated how thick the dome was and put a centre drill right through the first one. So now I have one with a 0.3mm hole and the other is about 1.1mm.

This is the 0.3mm bit I used :-



If you use a drill or a drill press it is easy to snap drill bits this small but it is actually very easy to drill 0.3mm holes with a lathe. I spin the nut in the chuck and hold the drill in my fingers. I drill from the inside of the dome. The drill finds its own centre and then I apply light pressure. I expect the same could be done by spinning the nut in a drill chuck.

The RepRap design for the heater barrel is just a flat ended threaded brass tube made from an M6 bolt. This is easy to make but not the ideal shape. Brass acorn nuts seem to be machined from a solid piece of brass. The internal thread is made by drilling and tapping. Because it is a blind hole that means that the thread does not go all the way to the end. If you screw a flat ended barrel into it then it stops short of the end, leaving a void that will fill with molten plastic as can be seen here. Molten thermoplastics compress under pressure, so ideally the amount of molten plastic in the extruder should be kept as small as possible to make the start stop response as fast as possible.

I decided to sacrifice my over drilled nut to find out the inside profile by cross sectioning it :-



Not surprisingly, the inside profile matches a 5mm drill as that is the correct size for tapping an M5x1 thread.



This is how far a flat ended barrel can enter :-



This is my attempt to match the profile :-



And this is the improved fit :-



The chamfer at the end is not quite right. My DeWalt bits have a 110° angle but the standard appears to be 118°.

I decided to take a look at steel acorn nuts :-



These are a completely different animal. Rather than being machined out of one piece they consist of a nut with a dome pressed into it.



They have some advantages and disadvantages : -

• They are cheaper and more commonly available.
• They are smaller so less thermal mass.
• The dome is much thinner, about 0.4mm rather than 1mm, so it is easier to get a short hole.
• Steel has a much lower thermal conductivity than brass so plastic may cool down in the nozzle.
• Steel has a different thermal expansion rate than brass. Fortunately it is less so it should get tighter as the extruder warms up.
• The steel dome might spring out under the pressure of extrusion.
• The flat ended heater barrel goes in further but leaves voids at the side.

These are just the nuts I have managed to buy. I have no idea how much they vary from one supplier to another.

The heat insulator in the picture above is an experimental one turned from a bar of soapstone.

Extruder postmortem

So my extruder died at the beginning of the month. I have been busy with other things so I have only just got round to thinking about rebuilding it. Here is a list of things that failed :-

I got my extruder working in mid August. At the end of September it ejected its PTFE barrel from the clamp, breaking the heater wires. This is a common problem and stems from the fact that PTFE has the lowest coefficient of friction of any known solid material. I tightened it further and it didn't slip out again. However, when I came to dismantling it I noticed that the end that was in the clamp has been compressed by about 0.3mm. The 3.2mm drill that I made the hole with is now quite a tight fit so the hole may have shrunk slightly. I will drill it out to 3.3mm because some of my 3mm HDPE filament is slightly more than 3.2mm where it is a bit oval.

One curious thing is that it now looks to have a thread in the entry hole.



I am struggling to explain this. The only way I can think it may have happened is as follows :-

The filament has a thread cut into one side by the drive screw. Before I added my feed spool the filament used to rotate as it went through the extruder and had a thread cut all the way round. Perhaps it transfered its thread to the PTFE, which is quite soft.

The final failure mode was the nozzle jumping a couple of threads and leaking molten HDPE. It rammed the nozzle through the object being made and damaged the bed underneath.



The bottom end of the PTFE barrel has swollen by about 0.3mm. PTFE is known to creep, i.e. when subjected to a prolonged force it very slowly flows. It has no memory so it does not spring back when the pressure is removed. I think this is why the top of the barrel shrank and the bottom expanded. There is a lot of pressure in the barrel and it is close to PTFE's maximum operating temperature. PTFE melts at 327°C but it starts to degrade at 260°C. I have been running at 240°C which is a bit too close for comfort.

My barrel, at 12mm, is smaller than the current recommendation which is 16mm. This may have contributed to the failure.

The recommended solution is to fit a pipe clip, but I didn't have one small enough, so I pressed a short section of 15mm OD pipe over the end, which is a tight fit. It had the desired effect. When I first made the barrel the thread was a snug fit. When I dismantled it I noticed it had become quite sloppy. With the ring of metal in place it is now a tight fit. I will use some plumber's PTFE tape to seal it as well.



The other thing that was on the point of failing was the flexible drive shaft. Strands started breaking and the more that broke the more it flexed, so it was a kind of avalanche effect. I estimate that the shaft had rotated about 100,000 times so the flexing backwards and forwards must have caused metal fatigue. I probably have the most mileage on this part of anybody so far so it could be a sign of a design flaw. There are a couple of problems with my implementation which certainly won't have helped.

The first is that I used some 2.5mm cable I had to hand rather than the 3mm recommended. It doesn't seem like a big difference but I think the rotary strength is at least a cube law which would make my cable roughly half as strong. It was left over from a garage door installation but I don't think I have any worries there as 100,000 flexes is about 75 years use!

The other contributory factor is that the bearing lands on my drive shaft are a little bit eccentric. This stems from the fact that my watchmaker's lathe is not really big enough for this work. Fundamentally the hole through the headstock is not big enough to take the 5mm threaded rod.

Each time the shaft rotates it opens and closes the pump halves a little. This makes a big torque variation over a revolution because of the strong springs holding it closed. That caused the cable to wind up and unwind a little each revolution. It actually modulated the filament width and gave the objects a basket weave appearance. Here is a good example :-



I bought some new parts from www.bitsfrombytes.com, which wasn't an option when I first made the extruder.



That will allow me to give the flexible drive a fair test. If it proves to be unreliable then I will switch to direct drive. I found out from the core team that it is not required for any of the polymers currently used, only things like Field's metal.

Another thing that was starting to fail was the J-B Weld holding the heater wire to the nozzle. It is supposed to be rated to 315°C but it had started to crumble with my extruder running at 240C. The other problem I had with my J-B Weld is that it does not cure in the specified time at room temperature. I have to bake it to make it strong. I emailed J-B Weld but got no response apart from an automated reply.

David commented on my last post suggesting Thermosteel ,which is good for 1300ºC, so I will try that next. A couple of the core team use BBQ paint which handles 600°C so I will try that as well.

If it's not one thing it's another!

The third post in a row about my extruder breaking, not a good start to the year!

Now that I am making solid test shapes rather than hollow ones the extruder is working a lot harder and all it seems to do is break down. The drive cable started to disintegrate this afternoon, I could hear the strands breaking :-



It was still limping on however when this happened :-



The brass nozzle started to come out of the PTFE heat barrier. This was ironic because it was only yesterday that I said I had not had this problem in a forum discussion. Others have had it happen and the collective wisdom is to use a pipe clip round the end of the PTFE to secure it.

And the JB Weld which insulates the heater wire has turned to dust :-




So some rebuilding to do!