What is a pattern?
A pattern is simply an exact model of what is to be cast. They
are traditionally made of wood, but can also be made of resins,
fibreglass, plastics and even polystyrene. A sand mould is
formed around the pattern, so that when the pattern is removed
it leaves a cavity into which molten iron is poured forming the
casting. Any casting can only ever be as good as the pattern
from which it is made.
That may sound relatively straight forward, however, there is a
great deal more to it than meets the eye. Good patterns can save
time, money and achieve greater reliability throughout the
casting process. Well made patterns, in wood for example, can be
used and re-used hundreds of times to make new sand moulds.
So without undergoing a 5 year pattern making apprenticeship
what do customers need to know to help them get the best
results?
Design
The form of any casting, engineering, architectural or art, is
entirely the customer’s choice / design. Some designs are
straightforward to cast, others can provide challenges, which
could affect cost but not our ability to cast it. The pattern is
designed to ensure not only a perfect replication of the
original design, but also with the minimum waste or cost in its
manufacture. So with any job, either one off or more especially
multiple castings, involving the foundry early in the design
process will lead to better quality and also savings in
production costs.
Another critical element of pattern design is the method system.
This ensures the metal flows into the mould correctly, neither
too quickly or slowly, too hot or too cold. It’s a bit like
Goldilocks – it needs to be just right. A method engineer at the
foundry will work this out and the pattern will be made
accordingly.
Manufacture
Patterns are traditionally made from wood, although other
materials are used today. Pattern makers need exceptional craft
skills to replicate precisely the object to be cast, whether in
wood, fibreglass or resin. They also need a thorough knowledge
of foundry process.
Ideally pattern makers like to work to engineering drawings,
however, these aren’t always available. Architectural drawings
with measurements can be adapted. For heritage projects we have,
in some circumstances, replicated castings from photographs with
detailed measurement references shown. We have also used
original old castings as patterns, although this can sometimes
have drawbacks in terms of the quality of casting that can be
achieved. Using a flawed or degraded original can only mean a
flawed or degraded casting. If necessary it may be possible for
us to restore or replicate an original casting so that it can be
used as a pattern.
Left and Centre: Replicating a 150 year old rainwater head using
wood with resin mouldings
Right: The finished pattern next to the original
As stated the purpose of the pattern is to create a sand mould
into which metal can be poured. Mould making involves packing
sand tightly around the pattern which is held in a box in a
process called ‘ramming up’. For multiple castings the pattern
can be re-used many times and therefore needs to be robust
enough to cope with this process.
Artists sometimes provide their own ‘patterns’ or maquettes, in
polystyrene. This is a slightly different process in that the
polystyrene pattern is used as the mould, then packed in sand.
When the iron is poured in at 1,350 to 1,500 degrees centigrade
the polystyrene is ‘lost’ (pretty much vapourised actually)
leaving a casting occupying the space it had previously
occupied. This is similar in principle to the ‘lost wax’ process
often used in bronze casting.
Costs
Involving the foundry early in the planning process can help
manage the costs of a project, especially where multiple
castings are required. With a full understanding of customers’
needs the patterns and the ensuing manufacturing processes can
be designed to maximise efficiency and minimise costs.
Summary
So to summarise in brief;
Any casting can only ever be as good as the pattern from which
it is made.
Patterns are designed to ensure the best outcome in terms of
replication, minimising waste and cost of manufacture.
Pattern makers are skilled craftsmen who make patterns in a wide
range of materials
Involve the foundry at an early stage so they can apply their
skilled expertise to save you time and money.
Find out More
In use in construction and art for centuries, modern Cast Iron
can be a sustainable, cost effective, locally sourced product
for structural and aesthetic applications.
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