Skill builder simple molds for plaster and concrete

Simple molds for plaster and concrete

Simple mold boxes are constructed with readily available materials and provide a flexible working method during design.

Many examples and considerations in this post work on a smaller scale suited for model building where concerns like hydrostatic pressure are secondary.

First of all, creating good castings takes time! 

It involves making a positive, a negative, a casting, and, most of the time, some clean-up. The process can be more successful if you use the right combination of mold and casting material and learn how to hide seems in your model to allow for more complex assemblies.

Mold types:

The construction of the mold depends highly on its complexity. Undercuts and enclosures require careful planning of the casting process. The following mold typologies should serve as a guide to identify possible trouble spots when making your own molds.

Mold with simple contours

This is the most common situation for producing small block-like shapes that can be used directly as buildings or, with some post-processing, be adjusted to become more complex. We recommend using foamcore and hot glue to construct simple molds like these.

Mold with complex contours

More complex shapes can be achieved by combining materials like foamcore and polystyrene plates or by using single-walled 3d-prints as molds.

Mold with enclosure

Enclosures or details like windows or recesses can be realized with small blocks of styrofoam glued to the primary mold with double-sided tape or contact cement. The styrofoam is removed with a knife or other tools after the plaster has set. 

Mold with undercuts

More intricate shapes require multipart molds that can be disassembled without destroying the casting. In such cases, looking ahead to devise a strategy for removing the individual parts is vital.


Features like holes, champers, and bevels can be done after the casting is made. Having a clear picture of what features are part of the mold design and what features can be done in a second step by drilling or carving speeds up the production of molds dramatically.

Working without a mold

Castings and reproductions of simple, one-sided parts can be made with clay or other sculptable materials by creating an impression that forms the mold.  


Pouring direction
Thin sections

Undercuts / Enclosures
-Try to avoid undercuts and enclosed parts, and consider splitting your shape into different parts
-Undercuts and enclosures can lead to trapped air and incomplete castings

Disassembly logic
-Build your molds with disassembly in mind.

Wall strength and minimal sections
-Try to avoid lengthy and narrow sections as much as possible

Material combinations
-Use material combinations that make sense. Cardboard and concrete are difficult to combine

The structural integrity of your mold
-Build your mold strong enough and ensure it does not leak. Fill your mold with water before casting to identify possible weak spots.

Release agents
-Separating agents can lead to cleaner castings with fewer surface defects if applied correctly

Mold-making materials

Good mold-making materials are easy to assemble, are non-porous, and provide the necessary flexibility to make changes on the go. Selecting a suitable mold-making material depends highly on the material that you want to cast and the result that you want to achieve.
Mold MaterialPlasterConcreteRelease Agent
White Cardboard👍👍👍👎Release Spray
Grey Cardboard👎👎Wax, Petrol Jelly, Shellac
Foamboard👍👍👍Release Spray
Polystyrene👍👍👍👍👍👍Release Spray
Styrofoam (EPS, XPS)👍👍👍👍👍👍Petrol Jelly
Chipboard👎👎Plastic Tape
Phenolic Plywood👍👍👍👍👍👍Release Spray
Timber👍👍👍👍Release Oil
Silicone👍👍👍👍👍Release Spray
Polyurethane Rubber👍👍👍👍👍👍Release Spray

Materials like MDF and cardboard are porous and unsuited for creating molds without applying separating agents to them.

Skill builder casting plaster

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.

Liliana Siewczyk

Liliana Siewczyk

Workshop Manager Casting Workshop

Initially working as a carpenter for some years, I went on to study Industrial Design in Basel. There I intensively worked with different materials and technical processes. In particular, I am interested in the interface and interaction between analog craftsmanship, modern digital techniques and new materials. Following my previous job as a product developer I am now very happy to be part of the RAPLAB team at ETHZ to support the students in their projects.

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