A cavity-pour mould uses a model and an outer form to create a controlled void that is filled with liquid rubber. A matrix mould uses a rigid support shell to define that void around a model, allowing consistent rubber thickness with less material on larger or irregular shapes.
Method selection comes before product selection. Consider the model’s shape, undercuts, surface sensitivity, orientation, number of castings and the flexibility needed for demoulding.
Quick answer
Choose the mould type from the geometry, establish parting lines that release cleanly, seal and release-test the model, design vents and pour paths, control rubber thickness, then support the cured rubber so it returns to shape for every casting.
What you need to know
- A mould can lock mechanically even when release agent is used.
- The support shell carries shape; the rubber captures detail and allows release.
- Porous models normally require sealing before release agent.
- Always run a small compatibility and cure test on valuable models.
Tools and materials
- Original model and stable base
- Compatible sealer and release agent
- Mould-box materials or support-shell material
- Plasticine or non-sulphur clay as required
- Selected silicone or urethane mould rubber
- Accurate scale or volume measures
- Mixing pails, spatulas and brushes
- Registration-key and vent materials
- Required PPE
Step-by-step method
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Study the model
Identify undercuts, fragile areas, highest points and likely seam locations. Decide how the finished casting will be removed and where any seam can be cleaned.
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Choose cavity-pour or matrix
Use a simple cavity pour where a box can contain a reasonable rubber volume. Consider a matrix method for larger, irregular or vertical models where controlled wall thickness saves rubber.
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Prepare and test the surface
Secure the model, seal porous materials and test the release system in an inconspicuous area. Confirm that clay, sealer and rubber chemistry are compatible.
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Build the form and parting system
Make leak-free walls or a rigid support shell. Add registration keys so rubber and shell realign. Plan pour holes, air vents and shell fasteners before mixing.
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Measure and mix accurately
Pre-mix components if instructed, then measure at the stated ratio. Scrape the container thoroughly and use a double-mix method when recommended.
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Apply or pour
For brush work, capture detail with a thin first layer before building thickness. For a cavity pour, introduce rubber at the lowest practical point in a narrow stream and allow air to escape through planned vents.
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Demould, inspect and store
Wait for full cure, remove the shell in sequence and flex the rubber rather than forcing the model. Inspect thin zones and trapped bubbles. Store the clean mould supported in its shell.
Watch the demonstration
The video is supporting material. Follow the current product instructions and safety data sheet for the products in your shop.
Common mistakes
- Choosing a method after the mould box is built
- Ignoring mechanical lock
- Using sulphur clay with a cure-sensitive silicone
- Skipping a release test
- Making a rubber skin with uneven thin areas
- Forgetting keys, vents or a shell opening sequence
Safety
Read the mould rubber, sealer, release agent and shell-material SDS. Some products require ventilation and specific glove or respiratory protection.
Do not use a general mould-making process directly on a person. Life casting requires a product intended for skin contact and a dedicated procedure.
Frequently asked questions
What is the advantage of a matrix mould?
It controls the space between model and support shell, producing a consistent rubber thickness and reducing material use on large forms.
Do silicone moulds always need release agent?
Not always for the model, but silicone-to-silicone applications usually require a suitable release. The model material and surface finish decide the system.
Why did my silicone stay sticky?
Possible causes include ratio or mixing errors, incompatible clay or sealer, contamination or temperature outside the cure range.
How thick should mould rubber be?
Enough to capture detail and resist tearing while remaining flexible for release. Follow the product and method guidance rather than one universal thickness.
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This guide provides general educational information. Products, substrates and working conditions vary. Follow the current technical data sheet, package directions and safety data sheet for every material you use. Structural, code-regulated or safety-critical work may require a qualified professional.