Wednesday, October 07, 2009


Working with Cotronics 907

I let the first application of Cotronics 907 on the Rapman extruder barrel air dry overnight. The material was quite hard. The next morning I used some fine grit sandpaper to smooth it down so that the mounting flange could slide over it. It took some time to smooth it out sufficiently, but you can see that it dressed dowquite nicely.

I was also able to dress the PEEK thermal break so that it fit into the extruder barrel socket firmly without jamming.

Unfortunately, I read the oven curing instructions incorrectly and encountered a problem that the Cotronics people were very open about with this product. If you don't cure it properly and heat it too hot and too fast, it tends to form blisters which separate the ceramic slip from what you are putting it on. Thus my careful sanding and dressing of the flange joint was for naught. It is worth noting that once I saw the bubbles I was able to remove the 907 with the wire wheel on my bench grinder relatively easily while it was still warm.

From there I applied a second coat, this time with the flange in place so that I wouldn't have to do any sanding. Doing it that way I was able to put the basic underlayer for the nichrome heating coil, the flange fillet joint and the flange pad all on at one go.

After four hours of air drying, two hours curing in my fan oven at 90 C, another hour of post curing at 120 C and a final post curing hour at 200 C, I was able to secure the nichrome heating coil as per BitsFromBytes instructions on yet again.

On that I put a slip coat of 907 and after it had air dried was able to put a coat 907 to secure the leads for the heater as per BitsFromBytes instructions. I was also able to place the thermistor in its socket as you can see from the following picture.

Using the "helping hands" electronics jig, I was able to secure the thermistor leads and the extruder assembly. This was very handy because it let me monitor the assembly for shorts in both the heater coil and the thermistor while I was applying the 907 ceramic adhesive.

I will be letting it air dry till morning after which I will do the 8 hour curing and post-curing in the fan oven. Hopefully, I didn't overdo the ceramic application and the curing will be successful.

Looking good, the cement too.
Looks better than it is.

The ceramic adhesive is pretty good for sticking the extruder barrel to the alignment flange. It tends, however, to bubble up off of the extruder barrel during the curing process even when the barrel is mechanically roughened and very slowly cured as per Coltronix's instructions.

I won't say that the material is a complete failure, but it is rather difficult to use on the Rapman aluminum heather barrel.

I've ordered a couple of rolls of Kapton tape from McMaster-Carr and am going that way now.
Do you think you could send me a sample of this stuff so I can try it ?

I'll be happy to send you some. Drop me a line at and leave me an address and how much you want.
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