Introduction: Casting Plaster

Gather your materials.


  • Set square 45
  • Gluestick
  • Pencil
  • Scissors (strong enough to cut through high impact polystyrene)
  • One sheet (25cm by 46cm) of high impact polystyrene (1mm thick)
  • 2 sheets of A4 white paper
  • At least 1 roll of scotch tape
  • 6 cups of plaster
  • 4 cups of room temperature water
  • 2 rolls of masking tape
  • Exacto knife
  • Sanding stand

Step 1: Step 1: Sketching the Mold

  1. Use the ruler within the set square to draw a straight line 2 inches long on one sheet of your A4 paper.
  2. Put the protractor within the set square against the beginning of the line and mark the angle of 120 degrees.
  3. Use the ruler and place it so that it is connecting the beginning of the line and the mark that you have just made.
  4. Draw along the ruler to make a 2-inch line connecting the base and mark.
  5. Connect the end of the base with the tip of the triangle using your ruler to ensure a straight line.
  6. Repeat points 1-5 for 15 triangles to make the form above.

Step 2: Step 2: Cutting Out the Mold

  1. Align your piece of paper along a corner of your polystyrene (making sure your shape is not coming off any edges)
  2. Glue all edges of your paper on to the polystyrene, making sure your drawing is facing up.
  3. Cut along the lines that you have drawn, cutting out every triangle. (please be careful to cut accurately)

Step 3: Step 3: Putting Your Mold Together

  1. Use the scotch tape along all of the edges, making sure to use at least 2-3 layers on each edge to ensure you have no leaks.
  2. Fold the shape together and continue to tape sides in order to make the shape above. (be sure when you put the shape together, the tape is on the outside of the mold)
  3. Make sure to remove all of the paper from your polystyrene before forming the mold.

Step 4: Step 4: Making the Core (Repeat Steps 1-3 for the Shapes Above)

  1. Use the set square to measure out the rectangles in the image above on your second piece of A4 paper
  2. Use the same method you use to make the triangles for the mold
  3. Glue your drawing to the remaining polystyrene
  4. Cut the shapes out
  5. Stick them together using the same precautions as you did for the mold (except make sure the tape remains on the inside of the core)

Step 5: Step 5: Casting Plaster

  1. Once you have ensured that your mold and core are completely leak-proof, grab your two rolls of masking tape and place your mold on top, making sure the top of your mold is facing up.
  2. Once you are sure that you are ready to pour, mix together 6 cups of plaster 4 cups of water, and stir until combined (55 - 70 seconds)
  3. Slowly pour into the mold (not to the top) checking to see if your core can still fit
    1. You want to make sure that when you add your core, the plaster comes to the brim of the mold
  4. Try your best to align the core to the center of the mold, by making sure that every side of the core is an equal distance to the brim of the mold.
  5. In order to secure the core in place, use tape to hold the top of the core steady.

Step 6: Step 6: Removing the Mold

  1. After 1-2 days, you can remove the mold
  2. Using an Exacto knife, carefully cut the tape in order to release your casting from its mold. (be sure not to cut into the plaster)
  3. Remove the core by squeezing the sides inwards until they loosen up enough to take it out.
  4. Sand as needed