Monday, 11 February 2008

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.


  1. It looks like you are really battling with the extruder barrel.

    You can get spray cans of BBQ paint in the UK.

    I generally spray a coat and let it dry outside for an hour before putting on another coat. After putting six coats on that way I wrap the insulated nicrome wire onto the barrel and then put on another six coats with one hour intervals between them. It takes at least a day to make a barrel that way. Probably better to make several at a go.

    I found that you just can't get in a hurry with BBQ paint. If you do hurry it on you wind up with the heater coil coming loose much quicker.

  2. We have had no problems using Holts "Gun Gum", exhaust repair putty available from Halfords. I don't think we've gone much past 100degC but the stuff is rated for exhausts.
    The medium is waterglass or sodium silicate


  3. Thanks Toby I will look into that.

  4. JB Weld is available form these guys in NZ

  5. try Holts Fire Gum. They say it is good for 1000degC