How to Cast

What is Casting?

Casting is the process by which a liquid material is shaped by pouring it into a mould to set. There are many different materials that can be used for casting, some of which are concrete, plaster, plastics, silicone, rubber,alginates and metals. The process for casting each material is based on their specific properties.
Concrete Casting
Plaster Casting
Slip Casting
There are many different methods for casting concrete, depending on the desired outcome. Concrete is a versatile material as it can be cast at many scales, has great compressive strength and can be cast in many different forms.
Click here for a basic tutorial on casting a concrete plinth.
Plaster is an inexpensive casting material with a quick curing time. It can be used for casting a mould for many materials, in particular metal and slip casting. It is effective for casting objects in high detail, due to its low viscosity.
Click here for a tutorial on how to make a plaster cast.
Slip casting is a ceramic forming technique for the mass-production of pottery and other ceramics, especially for shapes not easily made on a wheel. A slip is a liquid mixture or slurry of clay and/or other materials suspended in water. When poured into a bone-dry plaster mould, the plaster absorbs the excess water from the slip, forming a layer of thick clay on the inside of the mould.
Click here for steps on how to make a plaster mould for slip casting.

How to Make a Mould

It is important to understand the material in which you wish to cast before designing a mould. For example, plaster is extremely fluid before it cures, so it is important that the mould can be made water-tight. Most casting materials require a rigid mould, though some can be cast with soft moulds, such as casting resin in a silicone mould.
Silicone Mould
Alginate Mould
Adjustable Mould
Silicone is a rubber-like material that can be used for safe and efficient soft mould-making. It is effective for casting moulds for detailed forms and is water and heat resistant. Due to its extreme flexibility, the mould can be removed from complex forms without breaking, making it great for making multiple identical casts.
Click here for a tutorial on how to make a basic silicone mould for concrete.
Alginate, also known as Alginic acid, is a compound found within the cell walls of brown algae. It is considered extremely safe to use and is often combined with water to create a viscous gum paste, perfect for creating moulds of dental impressions, hands, feet or other small scale items. The alginate mould is broken apart as to remove the interior cast, and is therefore not reusable.
Click here for a tutorial on casting an alginate mould.
Multi-media moulds can be effective for casting specific forms, especially in making versatile and reusable moulds.
Click here for a reusable and adjustable MDF and fabric mould.

Weight Considerations

If the cast is large, it is important that the mould is sturdy, as to withstand the weight of the material until it has set. Tongue and groove joints, which have been screwed together, are extremely strong and reliable for large moulds.
It is also important to think about the weight of your object before casting as it may be too heavy to lift and move around once poured. If this is the case, talk to the Guru about options. You may want to pour it with dollys underneath the mould so you can move it around and store properly afterwards.
Click below for a Grasshopper tongue and groove box script.

Curing vs. Drying

Curing is a process during which a chemical reaction takes place, resulting in a harder, tougher or more stable substance. Materials such as resin, concrete and plaster go through a curing process once ingredients are mixed and poured.
This process can be quite quick or slow, depending on the material. Each material process has a different curing time depending on their chemical reaction.
Drying is a process consisting of the removal of water or another solvent by evaporation from a solid, semi-solid or liquid. This process usually takes a lot longer than curing but can be sped up with increasing heat and airflow to aid in the evaporation of water.
We have two drying cabinets to aid in the drying process. See 6.8 to find out how to use them.

Working Time

The period of time between the addition of the catalyst and the gel stage is called the 'working time' of a material. Working time ranges depending on material type.
Keep in mind temperature has an effect on curing and effectively working time. The hotter it is in the room or outside will generally speed up the curing process.
Material
Est. working time
Est. curing time
Considerations
Plaster
10 minutes
2-4 hours
(best to leave overnight to ensure it is fully set)
Plasters can cure extremely suddenly, therefore the time between mixing and pouring should be quick.
The volume of plaster when mixing should also be carefully considered, as it is difficult to dispose of effectively. Minimal waste is ideal.
Concrete
90 minutes - 2 hours
2- 4 days to set (enough to walk on)
28 days to fully cure & reach full strength
It is critical that air bubbles have the ability to be removed by tapping or massaging the concrete straight after pouring. If the cast is large you can pour it onto a vibrating plate to help remove the air.

Preparation and pouring

Preparation

Make sure the area is clean and you have enough space to make your pour without interfering with others’ work. Make sure all the equipment you need to mix and pour is ready (e.g. buckets, weighing scales, mixing tools). You will also need to be wearing the correct PPE (e.g. P2 mask, glasses) and make sure everyone in the space is wearing equivalent PPE.

Pouring

Make sure there is enough space for you to pour and that your cast is movable once you have poured. Your cast should be easily moved once you have poured the material in. If it is going to be extremely heavy, considerations need to be made before pouring; such as putting you mould onto dolly's.

Cleaning up

It is important that you clean the space before you leave the Wetworks area for the day.
Dispose of any cement and other wet casting solutions properly. Any liquids of large particles or water with residue in it, please use a sieve to filter out any larger particles before pouring into the sink.
Any large waste products need to be disposed of in the large skip bins at the rear of the Maker Spaces storage area. Small pieces of concrete up to 5kg can be disposed of in the construction waste bin, anything bigger will require a skip. Speak to the Guru if you are not sure of how to dispose of anything.
Make sure any borrowed equipment is cleaned and returned.

Using the drying cabinets

The Maker Spaces has two TS560 Electrolux drying cabinets available for student use. They are located in the materials storage area adjacent to the Wetworks area. These are available for student use upon request from the Guru.

How to use

  • Place cast on a cabinet shelf
  • Set temperature to 40 or 60 degrees (some materials need to be dried slowly, and may crack if set to 60 degrees initially).
  • Cabinets will automatically turn off when they sense that the cast is dry. Sometimes only the outer layer of the cast has dried by this time, however, so it is important to check the cast before removing. If it is still damp, set the cabinet for another round.