This step-by-step mould making tutorial shows you how to make a simple, one-piece silicone block mould of a small, decorative object.
The Basics:
- Block moulds are generally the simplest type of mould.
- They usually require more rubber than other methods (like skin moulds).
- They are made by pouring liquid rubber into a containment area around the model (or “master”).
- These containment areas can be made from a variety of objects (melamine-laminated particle board, PVC pipe, plasticine clay sticks, plywood, foam core, Perspex,corex, and much more).
Supplies & Tools Needed to Complete this Project:
- A Model
- Silicone Rubber
- Brush
- Plasticine Clay or Hot Melt Glue
- Mould support
- Drill (to create vent hole)
- Measuring Tape
- Mixing Container & Mixing Tool (e.g., Pails & Stainless-Steel Spatula)
- Digital Scale
- Spirit Level
- Personal Protective Equipment
Step One: Select Containment Area
In this case, melamine-laminated particle board was used as the “mould box” material. The mould box walls were screwed together and placed on top of a base board. Sometimes, a mould strap is used to hold all these pieces together.
If the mould box is not going to be used to support the mould during the casting process, make sure that the rubber is thick enough between the model and mould walls so that it doesn’t deform when filled with casting material.
Generally, it should be at least 10-12mm” on all sides. [Figure 1]. Also, make sure that the mould box is tall enough to accommodate the appropriate mould height – at least12mm of rubber above the model.
Figure 1: Minimum of 6mm of space on all sides of model
If the mould is going to be self-supporting when casting, increase the wall thickness to between 10 and 12 mm
Step Two: Secure Model to Base
Securely fasten (e.g., Screw or glue) the model to the base board.
If the model is not secured, the poured rubber may leak beneath it, and it may begin to float.
NOTE: If the model is porous (e.g., plaster, wood, stone, masonry, pottery), drill a vent hole in the base board underneath the model to allow air to escape [Figure 2].
Step 3 Seal the Mould Box
To help prevent rubber from leaking out of the mould box, seal the interior edges (between walls and between walls and base board). Warmed plasticine clay was used for this process [Figure 3]. Hot melt glue is another option.
Step Four: If Necessary, Apply Sealer
Sealing Porous Models
Generally, porous models need to be sealed before a release agent is applied and before rubber is poured. The sealing agent should be determined based on the model material and rubber mould material.
For many porous models, several coats of paste wax, petroleum jelly, shellac, paint, melted paraffin, lacquer, or PVA (polyvinyl alcohol) solution work well.
Step Five: Determine How Much Liquid Rubber Is Needed
How to Calculate:
For a Box
- Calculate the volume of the mould box (L x W x H= Volume) [Figure 6].
- Estimate the volume of the model.
- Subtract the estimated volume of the model from the volume of the mould box.
- Divide the result (the volume of the mould area) by the specific gravity (Density) of the moulding rubber. (The Specific Gravity is on average 1,12 but check the data sheet of the product you are using)
The resulting number is the amount of liquid rubber needed.
Step Six: Weigh & Mix Rubber
Once the amount of rubber necessary for the project is determined, weigh, and mix the rubber.
Silicone RTV is catalysed at between 2-5% depending on the manufacturer. Check the data sheets for the exact percentage.
Before mixing, rubber should be at room temperature and all tools should be ready
Some “working times”, the time you have before the rubber begins to cure, can be very short.
If tools are not ready, the product may cure before entirely poured.
The working time for Silicone Rubber is usually between 15 – 20 minutes at 25˚ C.
Great care must be taken to ensure proper mix ratio. We recommend measuring all rubbers by weight on a digital scale, not by volume.
To measure out the catalyst use a syringe.
Mix the rubber and catalyst thoroughly [Figure 7].
Before pouring the mixed rubber into the mould box, some may choose to use a vacuum chamber or pressure pot to remove air bubbles from the mixture. This is not always necessary and should be determined on a case-by-case basis.
We suggest that once you have mixed the rubber in a container, It is a good idea to transfer it to a new container and mix again. This will avoid having unmixed rubber from the sides and bottom of the first mixing container, thus avoiding soft spots in your final mix This is called a double mix.
Step Seven: Pour Rubber
Make sure the mould box is level before pouring the rubber. Uneven moulds will eventually lead to uneven castings.
Start by pouring rubber into one corner of the mould box. This allows the rubber to flow across the model. Avoid pouring directly onto the model [Figure 8]. This helps to prevent trapped air bubbles.
Figure 8: Pour rubber into rubber, allowing it to flow across the model.
Ensure that the rubber meets an appropriate height level and that all parts of the model are sufficiently covered.
Step Eight: Allow Rubber to Cure
Cure time varies from rubber to rubber. Some rubbers can be de-moulded in 3-4 hours, while others should not be removed from their mould boxes for at least 24 hours. Cure time will decrease with higher temperatures.
Please be patient. De moulding too soon can cause deformities in the rubber.
Step Nine: De-mould
Once the rubber has cured, carefully de-mould. The mould box walls may need to be removed in order to remove the mould.
The mould is now complete [Figure 9] and the casting process can begin [Figure 10 & 11]!
