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.