Monday, August 16, 2010

Plaster Moldmaking 01

Today I worked on the feet of the BJD for awhile, then I began organizing my checklist for plaster mold making. It has been a long time since I've made a multiple piece plaster mold for slip casting, so I needed to do some research.

A one piece plaster push mold is very simple. It doesn't need to fit with another mold piece, so no registration keys are needed. Nor is any mold separator needed, because there is no second mold half. The important thing to remember about making a plaster push mold is that the model should have no undercuts that would lock it into the mold. Here is the one piece plaster push mold for the base faces I'm working on.

Things get more complex when more than one mold piece is being made. First of all, the model needs to be embedded halfway in a clay bed. The halfway point is called the parting line. There can be no undercuts. Remember that undercuts will lock the model into the mold, so likewise, a casting would also be locked into the mold, if there are undercuts. In the following photo of a two-piece plaster mold, note that the wax leg is embedded halfway into one half of the mold. If the first piece of the mold were being made, this half of the mold would be made of clay. Note the three little hemispheres on the mold halves. These are registration keys. Registration keys are needed to align the multiple pieces of the mold. In this mold, the slip or wax is poured through the bottom of the foot. A third piece of the mold, called a spare is missing. The spare provides a reservoir of extra slip to feed the casting when it is thickening on the walls of the mold. Without a spare, I had to continually keep the casting topped-off by pouring more slip or wax into the bottom of the foot. A mold spare is a good thing to have.

In this photo, a six piece plaster mold is shown. Note the two plaster plugs for the shoulder sockets. There are also two plaster plugs for the hip sockets.

Here you can see the plaster pieces for the shoulder and hip sockets a little better. Is there anything missing in this mold? Look and see if you can find the missing thing.

Here, the other hip socket plaster plug has been moved away from the model. Did you find the missing thing yet? Where are the registration keys? Well, there aren't any carved registration keys in this mold. It is difficult to tell from the photo, but there is enough curvature of the mold to act as a registration key.

Don't forget to put registration keys in multiple piece molds! It is probably a good idea to always do it, even if it isn't absolutely necessary. There are many different ways to put registration keys in the mold. I may have used a coin to carve the registration keys here.

Besides plaster and clean water, it is very handy to have a sealer, such as Orange Shellac and some Denatured Alcohol for cleaning the shellac brushes on hand when making plaster molds. The brand name does not matter. Use whatever brand is available to you. Once a surface has been sealed, it can be coated with several layers of mold separator. A good mold separator for a plaster mold for slip casting is a 50/50 solution of water/soap. I'll give more details about sealers and separators when I make the plaster molds.

Never pour plaster into the drain pipes. Never. Always have a big bucket with some water in it for cleaning the mixing bowls, brushes, spatulas, stirring sticks, and other tools. Wash your hands off in the bucket as well. Plaster in a drain pipe is a clogged drain.

Some of the tools you will need for making plaster molds are a scale, some clean mixing bowls, a spatula, brushes, registration carving tools, and a nice clean table to work on! I use this Formica covered table to make molds on. The table should be level.

This is a plaster mold making coddle. It is made from 1x4 inch pine that has been sealed with Orange Shellac. An angle bracket has been screwed onto one end, which makes it adjustable.

This is a better picture of the coddles, showing the angle bracket on the end of each piece of pine.

I am planning to make a multiple piece plaster mold of the leg that I have been working on. I will use the mold to cast some CompoBell doll composition slip into to check for shrinkage, and other things.

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