Tuesday, October 06, 2009


Getting away from fire cement...

I'm currently finishing building a Rapman variation on Darwin. The construction has gone remarkably smoothly until recently when I began to build up the extruder's heater barrel. I had the whole thing finished with the nichrome and thermistor properly embedded in fire cement which encased the aluminum barrel.

When I tried to insert the PEEK thermal break, however, things went wrong. The PEEK cylinder jammed in the extruder barrel's sleeve and in trying to get it back out again I managed to apply a bit of torque to the barrel. This shattered the fire cement.

I was able to recover both the thermistor and the nichrome heater wire, largely because the fire cement, even oven cured, has the consistency and tensile strength of dry silt mud. I cleaned the barrel with a small screwdriver and electrical pliers with zero trouble. You can see the results here.

I had used ceramic coatings before with nichrome and called Danny at Cotronics in Brooklyn. He recommended their 907 formulation which is, if I've read the data sheet correctly, a mica-based ceramic adhesive formulation with considerable compressive and tensile strength. Better still, I was able to order it through a retailer, McMaster-Carr, here in California in Long Beach. I was able to get it ordered, shipped and delivered overnight using a local courier service for about $5.

You apply 907 with a brush. The adhesive in liquid form is a bit like very wet clay mud getting an even coat on the aluminum barrel was quite difficult. Once dried, however, it developed a very hard, glassy surface as you can see here.

Right now I am trying to decide whether I should sand it down before or after I heat cure it. If I'd been smart I would have painted it on after I installed the heater barrel flange, then having to slide the flange over the ceramic adhesive wouldn't have been an issue.

It adheres very tightly to the aluminum.

Hmmm... that could work very well. Given the temperature capabilities, I might have to look into that for a high temp extruder.
If you're going to build one in the next six months let me know and I'll send you some of this 709. It's far more than I'll reasonably be able to use and my Scots-Irish DNA hates to see things go to waste.
What is the McMAster-Carr part number ?
You can see it here.

BTW, don't run out and buy this stuff immediately. I haven't got it to work successfully yet.
Good luck. I'm thinking of building a hotplate bed to test a heated build surface idea to get around ABS warping. Lots and lots to finish first, though.
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