|Monthly Tech-Tip |
This was about producing the simplest of shapes, open and flared-out. But it was also about achieving a fairly precise fired size so that a standard plastic nursery pot would drop down inside in such a way that, after the plant had reached a specific size, it would be easy to grab it by the rim and extract it. Another aspect of this process was the idea of repeatedly printing a shell mold, pouring a working mold, casting a piece, firing it and trying the plastic pot.
I used my standard jigger shell mold. That enables dropping the freshly-cast pieces in and finishing the lips on my wheel.
I made the bottom and top radiuses and the height parametric. I made the diameter of the pouring ring = top diameter of the pot + 2 mm. The walls are 2mm thick because my 3D printer prints 0.4mm wide (that means two passes for each layer).
Top left: I have 3D printed a shell mold, and filled it will plaster to produce the model. Bottom left: I have smoothed the model on the wheel (using a metal rib and sandpaper). Then I stuck that down to the batt using a sticky clay slip. Then I soaped it. Bottom center: I glued the jigger head shell mold down using the same clay slip. Bottom right: I filled that with plaster. Top right: The mold is ready. Top center: Within an hour I filled that mold with a red stoneware slip to produce this piece (amazingly plaster is absorbent enough after setting to be able to cast). Notice how the plastic nursery pot fits: It's rim is too far down and the top diameter of the clay pot is a little too much.
These molds do not need to fit into my jigger wheel, but it is handy if they do because I can do finishing on them before extracting the piece.
This was part of my nursery plant-pot mold-making project. I held previous shell-molds down with a brick on top. But that was not working well, putting the mold out of shape, making it difficult to pour the slip inside and agitate the mold after pouring to surface the bubbles. I found that the 3D printed mold can be held down by simply using a sticky clay slip (Plainsman Polar Ice)! I printed this one with a flange at the bottom for this purpose. I am pouring the mold on an arborite bat on the wheelhead of a potters wheel. This enables turning the wheel slowly and pouring the plaster into the perimeter slowly through a funnel, that produces a much better surface. If you don't have a super sticky clay you can make one by adding 4% Veegum to a Grolleg porcelain. Actually, Grolleg and New Zealand kaolins are both very sticky, so they could also work.
The cup-head was lathed from a block of aluminum and it attaches to the shaft the same as a regular wheel-head. Plaster molds simply drop in and sit on their shoulder. The shoulder is the only point-of-contact, this prevents chattering while the mold spins when under pressure. I am using these molds for a casting-jiggering process (or just casting). For example, I can cast a mug in the mold, then pour out the slip, wait a few minutes and then, as the wheel spins, finish the rim and inside sure using a 3D-printed template/rib. I do not actually use the jigger arm, it is easier just to hold the template in hand. I can finish the rims on any round pieces made in these molds.
|By Tony Hansen|
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