How to Make Plaster Mould

Traditionally, original models used in the ceramic industry have been turned on a potter’s wheel or hand-carved from set-up plaster, metal, wood or clay. Mould making in ceramic plants has been simplified and thousand of patterns and original models have been formed with plaster of paris.

Materials:

  • plaster of paris
  • modeling clay
  • separating agent (ordinary soap or petroleum jelly)
  • original model

Tools:

  • weighing scale
  • sponge
  • knife
  • basin
  • pail
  • rubber band
  • dipper
  • hacksaw blade
  • ruler
  • compass
  • corrugated box or plywood
  • pencil
  • sandpaper (water-proof)

Procedure:

1. Apply a separating agent to the entire original model. Repeat the application for two or more times. Wipe off the excess separator from the model.

2. Locate lines to divide the original model exactly through the axis with the aid of a compass or ruler and pentel pen.

3. Place the original model on the table and lay flat. Mount the sides of the original model with a plastic clay mass (lower half buried and the other half exposed). Smoothen the top surface of the clay mass by leaving 0.32 cm (1/8-inch) allowance lower than the dividing line. Trim the sides of the clay mass leaving 2.54 cm (1-inch) allowance from the sides of the original model.

4. Place a walling around the entire assembly with corrugated box. Put a rubber band around the walling for additional reinforcement. The height of the walling must be 1 higher from the original model.

5. Prepare the plaster:

5.1. Mix the plaster of paris with water to form a slurry. The guide to plaster to water ratio for different types of molds is as follows:

Mixture Consistency:  (Materials = Plaster of Paris (grams) = Water (ml)

  • Slip cast mold for intricate design = 100 = 75
  • Mold for Jiggering = 100 = 67-70
  • Mold for Pressing = 100 = 67-70

5.2. Stir the mixture for 3 to 4 minutes until it is about to set or until its consistency is like that of a condensed milk. A vacuum agitator may be used for stirring to minimize the formation of bubbles and to attain a homogenous mixture.

6. Immediately pour the mixture of plaster of paris carefully inside the clay walling. Set the plaster of paris for about 5 to 15 minutes. Complete setting can be observed by trial scratching of the surface of the plaster or by noting the increased temperature.

7. Remove the walling and turn upside down while the plaster is still soft. Remove the clay bedding and trim the excess .38 cm (1/8-inch) allowance mentioned in no. 3.

8. Scrape and smoothen the edges with knife to assure good fitting. Cut locks into the edges to serve as interlocking knob with the second half.

9. Repeat the same procedure (numbers 4, 5, and 6) to complete the other half and also the base/bottom portion of the model. This time, apply separator on the plaster cast mold before pouring the mixture of plaster of paris.

10. Make locks at the bottom. Locks on the other side of the mould should be different in shape for easier assembly of plaster mold cast. Apply separator and put a walling around the bottom, then pour plaster mixture.

11. Clean and smoothen all outer parts with sandpaper to remove the unnecessary parts of the mold. Make the outer part of the mold flat and rectangular for easy handling, and for better appearance without occupying much space. All parts of the mould must have the same thickness.

For more information contact:

Industrial Technology Development Institute
DOST Compound, Gen. Santos Ave.,
Bicutan, Taguig
Tel. Nos.: (632) 837-2071 to 82 locs. 2182, 2218, 2180;
TeleFax Nos.: (632) 837-3167, 837-6150, 837-6156
Web: www.dost.gov.ph

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