Coffee Mug Slip Casting Mold via 3D Printing

For traditional potters, all of a sudden it looks like it could be possible to slip-cast heavy stoneware mugs, and do it in such a way that they look thrown. 3D design and 3D printing along with new clay body options have suddenly made this possible and practical.

The precision possible with just an ordinary consumer 3D printer has been among the most astounding observations in our recent successes in making slip-casting molds. And, what seemed like the best success in the last project, the pouring of a rubber case mold into a 3D-printed block mold, has turned out to have issues. This time I am going to pour the case mold using traditional plaster (into a 3D-printed block mold of course). The advantage will be the ability repair and improve surface issues created by the printer (which is impossible with the rubber).

3D printing is also going to make it possible to create a multi-piece mold much more easily. Even better, later it will be practical to swap out the bottom piece, this will enable custom embossed logos and designs. 3D printing is also going to make it possible to create a top piece, with a matching pouring spout that will enable the production of a rim that looks like it has been thrown on a potters wheel.

Another unexpected option that has surfaced relates casting rate. I plan to make these mugs thick-walled, like thrown ones. This is not normally possible in slip casting since it requires long casting times. These produce uneven wall thicknesses. However, we have recently found body recipes that cast much faster, actually too fast for normal thicknesses. That means it should be possible to cast these mugs in 10-15 minutes, hopefully to a 5mm wall thickness.

Another development has been black and dark red burning casting bodies. These interact with glazes to produce beautiful stoneware-type surfaces. The beauty of slip casting is that you can mix your own clay body recipes and fine-tune their degree of vitrification, color and glaze fit.

Ceramic transfers are best applied at the leather hard stage, we are planning to develop a process to do that at the time the mugs are removed from the molds. Thus only bisque firing, clear glazing and final firing will be needed. We will also experiment with green glazing, which would mean single firing.

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AI-Imagined mug I chose for a new slip-casting mold project

This shape is one of many suggestions I got from MidJourney.com when I described what I needed. Consider the advantages of this for slip-casting:
-The piece has thick walls, that will help prevent warping during firing (enabling using a clay body that is highly vitreous).
-The shape, wide at the bottom to narrower at the top - and then a flared rim will also resist warping.
-The flat sides will be practical for applying ceramic transfers (this decoration can be done exactly that way). Of course, the transfer will have to be cut in a semi-circle to lay properly.
-The heavy handle should make it possible to cast the mug with the handle attached.
-The smaller foot ring sets it up off the table and should enable stacking.
This surface character will be possible using an opacified light-colored glossy oatmeal glaze - the pigment in an iron-bearing body should bleed through wherever the glaze is thinner (e.g. on contour edges like the rim).

My drawing to enable printing a mock-up

To make this I offset the profile sketch by 0.88 mm outward, this enables printing a thin shell of the shape. The total shrinkage from forming to the final firing product will be about 10%, thus the 0.88 will shrink down to 0.8 mm, that is exactly double the 0.4mm width of my printhead. I rotated the .88 mm profile to create this shell. In the slicer, I shrunk each of the models by 10% to emulate clay shrinkage.

Printing the final size mock-up underway

Notice I printed the handle in two halves. This made it unnecessary for the printed support to hold the handle up if the mug was printed whole (and it avoided the poor surface that results on down-faces on top of support). These measures cut print time in half. The pieces were easy to glue together and glue onto the mug. The final printed model turned out to be too large, so I scaled it down to prepare for the next step of printing a block mold.

First half of the case mold is drawn and printing

Notice the rib stabilizers, these will keep it rigid. I will fill these cavities with plaster before casting the case mold from this. I am going to print a mount onto which the leather hard mug will fit. The flat bottom will thus be better for embossing the base. Foot rings can also be adhered to that if needed.

3D printed plaster filled case mold ready for pouring block mold

This is part of a project to make a slip-casting mold for a coffee mug. In the slicer, I split the print into two pieces 22mm up from the base. This enabled doing the bottom section right side up and the top one upside down. That drastically cut the amount of printed support generated (and thus printing time). I scotch-taped the two halves together and filled it with plaster to produce a rigid block mold. The two halves fit so precisely it is difficult to tell where they join. The big benefit of printing it upright like this is that the all-important front face is very flat (there is some warpage on other parts but that does not matter).

First mug case mold extracted from 3D-printed block mold

3D printed block mold, plaster case mold

The actual mold (top left) vs 3D drawn one (lower). The case mold (top right) separated from the 3D print easily. The join between top and bottom sections of the 3D print is invisible. An advantage of making plaster case molds, vs flexible rubber, is the extra opportunity to smooth surfaces and fix defects (this case mold has some small surface defects at the top of the handle, these will be easy to remove from working molds). The rigidity of the plaster case mold enables the use of 3D printed rails having a flange that goes under the edge all the way around, this reduces issues with potential plaster leakage.

Flattening the mating mold faces

The 3D printed mug outlines align almost perfectly - but the mating faces of the two halves are not perfectly flat. After the halves are dry they can be sanded flat, greatly improving the fit. The absence of notches is what makes this possible. The fact they are missing is not a problem, it is easy to match the halves perfectly when strapping them together in preparation for pouring.

Side rails for production of working mold

I made these side rails specifically to fit this case mold. To be more rigid I printed a wall thickness of 1.2mm. The flange at the bottom fits under the mold and assures that no plaster will leak under and displace it upward (provided of course that the vertical flanges clamp together with a tight fit).

First mug in my newly created mold

This test mold is thin-walled yet I can cast three thick-walled mugs in three hours. This clay is L2596G, a buff burning cone 10 stoneware - the mug on the lower right has been fired to cone 10 oxidation. Achieving 4-5mm thick walls is not a problem if the casting slip employs a large particle kaolin intended for this purpose (e.g. OptiKast). And it is deflocculated properly.

The flared lip works as expected, keeping the rim nice and round. No cracks have appeared at handle joins, even for pieces left in the mold overnight. The mold halves mate with each other very well and the seam is easy to remove. The seam on the base is an issue - I have to be careful to line up the halves well before clamping the mold strap - this is a warning for accuracy during the mold production stage. And the possible motive for a three-piece mold if I get more serious about this piece.

Closeup of the surface of freshly cast mug

This is L4023F (a test body like our H440 cone 10R body). I did not sand or alter the surface on the 3D printed block mold or on the plaster case mold I cast from it. The polygons are produced when the 3D CAD software converts from its native format to an STL file that slicer software can use. These are not visible in the final glazed piece either.

Heavy stoneware mugs using my new slipcasting mold

These are made using the L4558B red burning cone 6 stoneware. The inside glaze is GA6-B. The outside glaze on the left is G3948A iron red. The one on the right is G3933EF. No rim warping occurred, despite the very heavy handle.

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Glossary	Slip Casting A method of forming ceramics. A deflocculated (low water content) slurry is poured into absorbent plaster molds. As it sits in the mold, usually 10+ minutes, a layer builds against the mold walls. When thick enough the mold is drained.
Glossary	3D Design 3D Design software is used to create dimensionally accurate objects by sketching 2D geometry and transforming it using tools to rotate, extrude, sweep, etc. The software generates the polygon surface.
Projects	Medalta Ball Pitcher Slip Casting Mold via 3D Printing A project to make a reproduction of a Medalta Potteries piece that was done during the 1940s. This is the smallest of the three sizes they made.
Projects	Nursery plant pot mold via 3D printing
Typecodes	Mold making using 3D printing An ordinary consumer 3D printer has many exciting possibilities for making many types of molds, it is a place where people having both artistic and mechanical abilities can get a double the dopamine!
Media	Design a Triangular Pottery Plate Block Mold in Fusion 360 Lilly will take you step-by-step through the process of parametrically drawing a triangular plate with curved sides and rounded corners, for 3D printing to pour a plaster working mold.

By Tony Hansen
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