How to fill a negative impression (i.e., cast)

Prepare the cast: 

1. Skirt the mold to increase the height of the cast. You may use masking tape and plastic or plaster.
2. Estimate the amount of water needed to fill the cast (could fill negative cast with water) and pour into mixing bucket.
3. Reinforce your marks on inside of cast with an indelible pencil.
4. Coat the walls of the plaster socket with a parting agent, like 50:50 soap-to-water mixture or baby power.
5. Set the mold upright in the sandbox and ensure is very stable.
6. Select the proper length of pipe (3cm off bottom of cast) and check the orientation of the pipe. Aim for a perpendicular angle between the top of the mold and the pipe so that it fits smoothly on the platten when pulling plastic.

Fill the mold:

1. Using the bucket of water, add a scoop of plaster until the plaster sits on top of the water like an island.
2. Begin stirring to mix thoroughly using a whisk (or piece of water pipe).
   1. For large casts such as a spine model or transfemoral cast you may use an electric stirrer. Cover bucket with splatter guard if using the electric stirrer.
3. While stirring, slowly add plaster and stir until a pourable relatively thick consistency is reached (like pancake batter, but smooth consistency).  Plaster that is too watery will be slow to set.  Too thick and it will not pour from the mixing bucket easily and leave air holes and ridges in the positive model. If adding vermiculite, aim for 70/30 mix.
4. Remove pipe from cast and pour plaster into cast.
5. Replace the pipe in the plaster filled cast with a holder in a vertical position, 3cm off the bottom of the cast.
6. Tap the side of the cast to bring air bubbles to the top.

After filling: 

1. Do NOT pour plaster down the sink. It will clog the pipes.
2. Remove excess plaster from the mixing tool on the end of the drill.
3. Place the bucket on the edge so that it can set up and be easily broken out later.
4. When plaster is set use a utility knife and make a cut down the entire posterior section of the cast.  Be sure the cut is all the way through to the plaster positive mold.
5. Peel away the plaster wrap from the cast.