Porcelain Part Geometry and the Lost Wax Method

Not sure if I have light to shed, but here goes.

I have never made lost wax castings, but I have read about the process. As I understand it, the sprued wax model is encased in plaster of paris: the wax is then displaced by hot molten metal.

For this to work with porcelain, it would have to be (a) hot enough to melt and displace the wax and (b) pretty much a liquid.

There's one thing that might work for you: the rubber molds used to make the wax models for casting would probably give you an easier release than plaster. The rubber takes as much detail as the wax, this is why it's used to replicate the waxes for lost wax casting.

Bruce Hoheb was a gifted sculptor and comfortable making artwork in many mediums: marble, bronze, wood.

I spent many hours working with him and was fortunate to visit the foundry that cast his statutes so I have some experience in how lost wax is performed.

Unfortunately Bruce died many years ago and I lost not only a dear friend but a gifted artist. Regrettably, I have no idea if the lost wax method will work with porcelain. I agree with you, however, that a rubber mold is perhaps the best way to go.

Thanks for responding and your effort to help.

L.J.

One thing to bear in mind is that the lost-wax mold will be broken to obtain the piece. If the ceramic is as fragile as the mold material, then it could be a problem. Perhaps a softer mold material than we usually use for metal castings? Maybe even dissolvable? [p]

another advantage of investment casting is no seam to the finished piece. Is this important for you?

For the time being, I'd be content simply knowing that the material works for the application. The methods used for prototyping will likely differ from those used in production.

I've concluded that a thin, flexible rubber mold, supported in a plaster of Paris mother mold will likely give me the result I want.

I was going to machine a rigid mold and use that to form the green porcelain.

Part of my concern is my lack of experience with the material. Does it shrink while drying? The use of a soft rubber mold, as you suggested, makes that a non issue.

Thanks for responding. I'm going to go study this some more.

L.J.

When I first envisioned the molding and firing process, I figured on using two molds to produce the green part for firing.

I was told that the best way to produce the porcelain part was to use a plaster mold to form it. The primary reason being that the part must be free entirely of moisture (green) prior to firing.

The proven method for doing that, I'm told, is to use a mold made from a material with relatively stable geometry and which absorbs moisture. . . . . . plaster being the popular material of choice.

However, the plaster mold degrades with each use and in time must be discarded. Hence the need for the second mold to cast the plaster one. That is a production issue however and not likely to be a factor when prototyping proof of concept parts.

It's become apparent that I should forget about using wax, bite the bullet and create the two step method. Having used painted latex rubber successfully in the past for casting clear acrylic I suspect that I will wind up painting the prototype with acrylic then backing the latex with a plaster and burlap to build up a "mother mold" to hold the latex geometry. When that is all cured, I'll remove the prototype and pour plaster into the latex.

When that is cured, use that as a mold for the porcelain.

What complicates matters is that I am trying to resolve a multitude of issues, not the least being how thick porcelain can be and still be cast and fired.

Thanks

L.J.

Porcelain can be very thin. I know of no other clay product than can be cast as thin as porcelain. I don't think you can be close to too thick at the 1/4 inch you posted earlier

Thinness depends on the composition of the porcelain.

"Bone" china added ground bone to the porcelain. The phosphorus and some sodium(? alkali metal anyway) reduced firing shrinkage and increased strength of the fired product (probably by more complete vitrification) allowing very thin wall china to be made.

Look at ,say, "Wedgewood" cups to see how thin it can be. They get well below 1/4", down to 1/16", but the shape is a simple one. As shape gets more complex, firing stresses increase and wall thickness often needs to increase.

A fairly high alkali porcelain will generally enable a thin wall, but it can cause problems if alkali is too high.

The rubber mold to make plaster masters seems a good approach. It can also be used to make wax masters which can be melted out of the plaster and predominantly reclaimed.

As someone else pointed out, the plaster disintegrates on firing. provided the water can escape freely from the porcelain before this and it can be fully dried before exceeding 100C this should work well. (If it doesn't internal steam causes micro or macro cracks in your part, with either excessive shrinkage, or growth or a damaged final product.)

Whatever method is used, porosity of the material is not acceptable as it will be polished.

It must have a glass like finish without the need for a glaze coating.

Thanks

L.J.

If that's the case, a bone china type composition would be your best bet

I think it is worth a try as the wax is usually melted out in an oven prior to the metal being poured as as hot metal and burnable wax might cuase a "BURP" and hot metal be sprayed around.....at least in the jewelry area......

Wait till its cold and pour your porcelain in, let it dry out over a good period, maybe with a low heat source.......then either bake in the mould or remove very carefully.....

In the steel/iron casting, where the operator is farther away, this might be acceptable to leave the wax in place, but I believe nowadays they use special hard foams, formed in a CNC machine, buried in plaster or similar, that melt away to nothing when hot metal attacks/is poured on them.

You should contact a company that makes ceramics for various industries as to how they manage it - impellers for car Turbos etc....

Best of luck.....