|Monthly Tech-Tip |
The challenge: Make large stackable calciners for roasting clay powders in a gas kiln. By "roast" we mean to about 1000F, a high enough temperature to kill plasticity but low enough that no particles get sintered together. We need these to process kaolin, Alberta Slip and Ravenscrag Slip.
At first, it seemed this would be simple. But it was not. The problem is that the mold, which I wanted to work for casting or pressing, must have flared walls (straight up would hamper mold release). How is it even possible to stack flared vessels? The shape is shown below. The piece is a little longer than wide, when these are stacked with 90-degree rotation the bottom corners sit on the in-facing corners. This design affords strength and helps prevent warping during drying and firing. The shape has proven so strong that we can dry pieces upside down in the sun, even when the vessels are made using non-grogged pottery clay bodies.
Using three templates we are able to cut the sides from 3/8" thick rolled slabs. With 45-degree angles in the right places, these can be press molded using standard stiffness pottery clay. They can be hand-joined on the corners relatively quickly.
This was done in Fusion 360. We originally planned for a 6mm wall, but later had to thicken them to about 8mm. All of the walls have an outward draft. This proved especially important when making the original mold - had they been more upright it would have been very difficult to get the wood box out of the plaster poured mold.
This box forms the inside shape of the plaster mold to be made. The finger joint is the most popular technique in laser cut joining at 90-degree angles. But it also works with near 90-degree joins (although sanding is required to smooth the overlaps). The key to making finger joints is creating interlocking tabs with a cut depth equal to the thickness of the material stock being joined. Fusion 360 has features for woodworking that simplify the design and joining geometry. This box is made from 13 pieces that fit flawlessly together, it was amazing. Glue held it all together very firmly. After sanding we varnished the whole surface and glued the box down to a varnished board and added retaining boards to hold the plaster-containing form for the next step.
We ordered fourteen custom cut pieces from an online service provider (to construct a model of a calciner). It was just a matter of uploading the CAD file (which of course had been drawn to match the exact thickness of wood to be used). One would think that the laser would burn the wood but it does not, it just darkens the color of the cut surface.
This will produce a vessel able to hold 4 US gallons. We cast this around a wooden box model placed upside down within a waxed cardboard form. We poured in three stages. The first pour put minimum pressure on the cardboard and secured it and the wooden form in place. The second and third went with no issues. The mold, after drying, weighs about 50 lbs. We tried several times to slip cast the vessels but so many issues arose that it turned out to be far easier to slab-build them (by pressing the slabs into it and joining the corners using traditional methods).
Since slightly longer and wide, when rotated 90 degrees the outside corners rest over the inside corners. Those inside corners are thus dual purpose, giving the calciners strength and stackability.
First I glued the cut plywood sections, varnished the box, stuck it down to a varnished board, encased the form in a strong cardboard enclosure, waxed the inside of that, poured the plaster in three sections, and finally, lifted the mold off (it expanded slightly on set and extracted easily).
This is made from Plainsman M340, it is a pottery clay used mainly for throwing on the wheel. Yet it proved to be no problem to dry this in the sun and fire it, no cracks appeared. This shape is amazingly stable, although this piece is fired to near vitrification (about 2% porosity) it has stayed square. This was a test piece to prove the concept could be first-fired without failure, subsequent pieces were made using special clays and the in-facing load-bearing corners were thickened and strengthened.
This is a standard pottery stoneware. It was easily able to survive a first firing to cone 6 without issues. But, it contains plenty of quartz particles, they pretty well guarantee that the first time it subsequently passes too quickly up or down through quartz inversion it will crack. And it did!
|By Tony Hansen|
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