Cerastil and soap stone

As a bit of light relief from continually repairing my extruder I decided to have a play with the Cerastil H-115 high temperature cement that I have bought.



The minimum quantity that I could buy was 1Kg, which cost aver £100 including shipping and VAT. You only need a couple of grams to make an extruder heater so it is actually cheaper than things like J-B Weld.

It is labeled as a hazardous substance and comes with a material safety data sheet which says it can't be disposed of in domestic waste and must not enter the sewage system. The hazardous components are identified as potassium silicate and sodium fluorosilicate. When I looked them up on the web I found that the former is added to growing medium and in cosmetics and the latter is one of the chemicals added to water for fluoridation. So they don't seem very hazardous but I suppose it's a matter of concentration.

I am assuming that once it has been cured, by the addition of a little water, that it is then no more hazardous that a ceramic potted resistor like this :-



We are no longer allowed to put electronics in domestic waste in the UK but you can just take it to the local tip.

I masked a brass heater barrel and applied a thin layer.



I left it to set for 24 hours and then wound it with two strands of 0.1mm nichrome twisted together. That gives me just 110mm for 8Ω, to keep the heater short. I attached copper wires with high temperature solder and then put a thicker layer of Cerastil over the top. I then left it another 24 hours to cure.



It looks a bit lumpy because of the solder joints underneath.

I mounted it in an insulator that I turned from soapstone and ran it for a few hours at ~290°C.



The bottom of the soapstone barrel got to about 120°C. After the test the Cerastil looked exactly the same, unlike J-B Weld which goes very dark. The soapstone did discolour though at the hot end.



So where has this experiment taken me on my quest to make a durable extruder that covers the full range of thermoplastics? Well I will definitely be using Cerastil from now on as it seems the perfect adhesive for potting heaters, not surprisingly as that is what it is designed for. It is a high temperature adhesive that is a good electrical insulator and a good thermal conductor. I am not sure I can recommend it for the RepRap project though because it is very specialist and not widely available

I am also not sure about the soapstone. I was surprised it changed colour but I don't know if it matters or not. It looks like it would need to be twice as long, or have a heatsink at the cool end. I am also a bit worried about its strength.

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.