Starting with the basics, slip casting involves pouring a ceramic slurry into a porous plaster mold. The plaster will immediately absorb water from the slurry, depositing a layer of particles on the surface of the mold. After a short period the excess slurry is poured from the mold and can be reused. The layer on the surface of the mold is then allowed to dry. As the layer dries, it will shrink away from the plaster. No release is necessary. Shrinkage can be dramatic however, so high precision casting will involve some iterative methods. Expect 5-10% shrinkage, but this will vary with composition of the slip and the wall thickness/geometry of the part.
The amount of drying time of the shell will affect it's workability. Once the part has been demolded and dried completely, it can be fired. The firing process vitrifies the ceramic, reducing the porosity and making it more glass-like.
Here are a few examples:
http://www.commonwealth.nu/projects/9/tropism
Here is a pic of some ongoing research in the lab; this is a pendant lamp, in the unfired state.
There are many variations on the process, but they all share some key steps. The goal for making the mold is to produce a perfect plaster negative of the part. For some forms the mold could be made in one piece, such as an open tile mold, but in general there will be at least two parts to the mold. The line on the part where the mold splits is the "parting line". This line can curve in three dimensions, and therefore be a compound parting line. This line will follow the point on the form where the draft angle changes.The draft angle in any part of the mold can generally never be negative. So for a spherical part, the parting line is any diameter of the sphere, and at that point the draft angle is zero. Since Slip cast parts shrink so much, a zero degree draft angle is OK. In many other casting methods draft angles need to be greater than 5 degrees or the parts will not release easily from the mold.
When a mold consists of two or more parts, these parts must be "keyed" together in order to properly align the parting lines before casting the parts. Well designed keys are tapered or curved so that they eliminate all degrees of freedom exactly as at the point when the mold is fully closed. This means that cylindrical dowels or pins are not a good method. Tapered pins are a better choice, but in plaster the keys are usually cast into the mating surface of the mold.
One of the quickest methods to creating a plaster mold is directly milling the mold from a solid block of plaster. In this case the alignment keys are also machined into the mating surface of the mold. Another workflow for generating the molds is to produce a model or pattern of the final part. This pattern can be CNC machined, 3d printed, or made by hand. The mold is then cast directly from this pattern. In the case of a multi-part mold each piece is cast sequentially, insuring they all fit and key together.
The plaster molds are then thoroughly dried. The mold is assembled, and held with straps or rubber bands. The slip is then poured into the mold. For control, the time that the slip is in the mold should be recorded. Immediately the level of the slip will begin to drop, so fast that it is almost as if it were leaking, but this is normal. The wall thickness builds up rapidly to start. To get a consistent wall thickness it is important to top off the mold and keep it full of slip.once the wall thickness reaches the desired amount, the slip should be poured from the mold. If the mold doesn't naturally drain from the hole, the mold can be rotated to help get all of the "pools" of slip out from inside. The mold and shell are then left to dry. Longer dry times will give a harder "green" state, but will make secondary operations more difficult. In the image above the dry time was 16 hours, and it is completely firm, though it can be carved with a blade. Once the green state dries completely outside the mold, it can be sanded or worked using traditional clay finishing techniques. Once dry it can be fired to the correct temperature based on the formulation of the clay body.
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