How to cast solid porcelain?

I haven't worked with porcelain, but I assume it would work like other clays in a press mold. I've used 2-part plaster press molds to make drawer pulls, beverage coasters, and the like. The biggest problem is that the clay must totally fill the mold, but it's pretty hard to determine the *exact* amount of clay to do this. So you have to make the clay wad a little larger, and have some place in your mold for the excess to squeeze into, and trim it off later.

Or, depending on what you need the solid porcelain for, you might be able to get away with normal slip-cast hollow porcelain, and fill the finished fired piece with plaster or concrete, etc. if you just need a heavier weight.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Thanks, Bob. What I am wanting to make is a "pass-through" bushing, like the type you see on top of the transformers mounted on utility poles.

I want to make some smaller, though. Each piece with be about two to three inches in length and an inch or two in diameter, with a hole through the middle. They will need to be fairly sturdy, so perhaps the press mold technique, making them out of one lump of porcelain would be the best idea.

Thanks,

Jon

This sounds like a good candidate for an extrusion. Or are you shooting for the typical insulator design, with a series of ring ribs on the outside? Might make for a tricky mold, since you'd need at least 2 parts for the outside so you could get the ribs out, plus some kind of center core for the hole. I wonder if you could extrude a hollow tube, then somehow squish it slightly between two mold halves to make the ribs.

Or maybe press-mold a solid piece, then add the hole later when its set up a bit.

The other thougth that comes to mind is to make a 2-piece insulator, an outer ribbed piece by slip casting and an inner cylinder that is a simple extrusion. When assembled, the insulator would be hollow. If you really need this to be a single piece, you could consider joining the two parts before they are dry, but I don't know what's involved in getting an extrusion to join to a slip casting, since the casting slip be full of sodium silicate and will probably have a different shrinkage rate.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

HI all, This sounds very much like what I have made for my kiln. We built an electric kiln using reclaimed kiln bricks. The elements needed to pass through to the wiring outside the kiln and I had to make porcelain bushes for them to run through. These bushes were approx 3/4" diameter and approx 3" in length. The top of each bush was wider approx 1" for about 1/2" down. Of course they were hollow. How to make? Well I made a slightly larger solid master out of regular clay and, from that, made a two piece mold (open at either end). When using the mold I plugged the bottom with clay and poured the porcelain slip in. I waited approx 20 mins (usual time for slip casting) and then unplugged the bottom and let the remaining slip drain. This created the central hole - making it into a tube. When dry I then fired with the next glaze firing to 1250C. Hope this helps. JM

Had another thought about how to make these if you want to have the circular ribs/fins that typical HV insulators have. Assuming you are only making a few prototypes or replacements (not a big production run) you could assemble them as a stack of clay "washers". This requires almost no special tooling, and is really adaptable. You can make paper or tarpaper templates, or use some "found" objects as cookie cutters. Roll out a slab of clay the thickness of one fin ring, and cut a batch of large washers out of that. Roll out another slab the thickness of the spacing between rings and cut out smaller washers. Let the washers set up a bit between pieces of drywall to keep them flat, then stack them over a dowel with slip between them. Remove trom the dowel before they shrink too much.

The advantage (besides not having to mess around with making a model or plaster molds and worrying about relief and parting lines), is that you can change washer diameters at whim, or the hole diameter, or the thickness of either part. You can even make a "cone" series of fins that get larger toward the bottom. If you want to make a larger bushing, you can stack more washers together.

Hope this helps!

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

In message , Bob Masta writes

What an elegant solution! I only wish I needed some insulators, so that I could make them this way!

you can cast in a similar way as slip casting. You just have to make sure that there is a resevoir available for the slip to feed the bulk of the cast. It will get solid provided you mold is not saturated. Use a fairly dry mold and a releasing agent such as talc. You can sollid cast any ceramic in this manner provided that the particle packing is not so dense that water cannot escape through the bulk. Porcelains are fine, but not so much so I believe. (You should be able to adjust that with you particle suspension / deflocculation shceme.)