Plaster - Casting

The following eight points serve as a guide to illustrate the most critical steps of casting plaster and present an overview of all the necessary considerations for the process. Individual points can be used as a reference during the making process or to refresh your memory before you start your model-building project.

  1. Building the mold
  2. Calculate the volume
  3. Workplace setup
  4. Mixing 
  5. Pouring
  6. Cleaning
  7. Demolding
  8. Post-processing

1 — Building the mold

The significance of mold-making can not be overstated. Typically, a substantial amount of time and work is spent to prepare the molds for casting plaster. Making the mold is the stage that, besides properly mixing the plaster itself, determines how successful your outcome will be.

Ensure your molds are constructed to be watertight, sealed, and easy to disassemble. There are various methods and tricks to good mold making; some of them are outlined in the following post:
Simple molds for plaster and concrete

2 — Calculating the volume

Carefully calculate the volume of your mold before you prepare everything. Most CAD software does this step for you if you have a 3d model of your part — check the internet on how to do so for your drawing program.

If you have to calculate the volume by yourself, use the simple length x width x height formula to do so and convert the result to liters. For safety, add 10% to the volume.

Mixing ratio by weight: water/plaster = 2 : 3

Use the following guidelines to calculate how much water and plaster you need:

1L is approximately: 600g water + 900g plaster

3 — Workplace setup

It makes sense to set up your workplace with all the tools you need to avoid surprises during casting and take the time to go through the process step-by-step.

  • Check your formwork to see if there is anything that needs changing
  • Apply mold release, if necessary
  • Prepare the raw materials in separate buckets (water/plaster)
  • Make sure you have all the tools ready:
    • Buckets
    • Spatulas
    • Clay for sealing the mold
    • …

4 — Mixing

The process outlined below is known as the island method and is an intuitive and proven way to prepare plaster on the fly.

  1. Fill the mixing container up to ⅔ with cold, clear water
  2. Sprinkle the plaster evenly over the entire water surface until dry islands form on approximately ⅓ of the surface. Do not agitate the mix at this stage!
  3. Leave the plaster to slack for 1-3 minutes, and wait until the plaster is saturated
  4. Stir for no more than 2-3min. Keep the stirring action below the surface and avoid introducing additional air bubbles
  5. Tap the mixing container on the table to release any air bubbles
  6. Pour the liquid plaster into your mold

5 — Pouring

Fill your mold from the lowest point to the top for the best results. Try to stay at this point and pour evenly. This technique helps to push away any air from the formwork naturally.

If, at some point, plaster starts to leak out, either seal it with clay or sprinkle it with plaster powder.

6 — Cleaning

After casting, residual plaster must be cleaned right away. Plaster is known for rusting, ruining tools, and clogging drains and pipes, leading to costly repairs.

How to clean:

  1. Scrape your buckets and tools to remove any residual plaster and put it into the mud box
  2. Rinse all your buckets and tools carefully in our sink and store them in our drying rack
  3. Clean the sink and remove any hardened bits
  4. Clean your table, and do not forget to check the floor
  5. Put any hardened plaster directly in the skip

Check out our cleaning tools and workspace post for details.

7 — Demolding

An exothermic reaction takes place during the plaster setting process. The heat development, up to 60C, indicates that the plaster is setting correctly. The generated heat of the setting process indicates when the time to demold is reached.

As a rule of thumb, plaster can be demolded ~40-60 min after the pour or when it cools down again.

Initially, the plaster will look gray and feel wet when touched; this is the ideal moment to do any post-processing that might be necessary. The casting will gradually dry over the period of a couple of days and develop its characteristic white color.

8 — Post-processing

The final casting can be processed efficiently with simple hand tools before the part is dry. This way of working has two functions. Firstly, it avoids unnecessary dust exposure; secondly, the plaster is still soft and has better workability than a dried piece. Hand tools like saws, chisels, rasps, scrapers, scalpels, and a surform can finalize any shape or form.