A mould, or matrix, is a hollow form into which a liquid or plastic material is placed so that it keeps the required shape once it has set. In building you meet moulds mainly in precast concrete, in patterned and photo-engraved concrete, and in the manufacture of plastic and metal components.
Where moulds are used
Precast concrete. Hollow-core slabs, wall panels, stairs, beams and columns are cast in steel or timber moulds in a factory. Because the mould is reused many times, dimensions are more accurate than with in-situ casting and hardly any construction moisture reaches the site.
Patterned concrete. A form liner of elastic polyurethane or silicone rubber is laid into the formwork and presses a relief into the concrete: timber grain, stone texture, a pattern, or even an image. Because the rubber flexes, the panel can also be released from undercuts.
Plastics and metal. Gutter brackets, window profiles, clip systems and ironmongery are injection moulded or pressed in steel tools. The high tooling cost is only recovered over long production runs — which is exactly why standard sizes are so much cheaper than bespoke ones.
Points to watch
- Release. A mould has to come apart. Faces are therefore given a slight draft angle and a release agent is used
- Repetition pays. Tooling cost is spread over the number of units. One unique item from a new mould is expensive; twenty identical items are far cheaper each
- Dimensional accuracy. Concrete shrinks as it cures, so the mould is made slightly oversize relative to the finished dimension
- Maintenance. Damage, rust or hardened concrete residue in the mould reproduces itself in every following unit
- Mould versus template. A template is a guide you work against; a mould forms the product itself
Related terms
- Precast concrete
- Formwork
- Patterned concrete
- Template
- Counter mould
- Release agent
- Plastics
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