News: Why is Pattern Making so important?

07 November 2013

One area where customers for iron castings often need the most advice is Pattern Making. The purpose of this blog is to help customers understand the critical nature of pattern making in relation to Design, Manufacture and Cost.

To achieve this we will look at what patterns do, how they are made and how including the foundry early in the planning process can achieve the best results, and save time and money.

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.

pattern making 1

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.

pattern making 2

Left: Polystyrene maquette by Antony Gormley, with blue refractory coating and in a supporting frame, being packed in sand in the foundry floor
Right: The finished casting, still with support frame in place, ready for fettling

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.

A good example of this was on the Gridiron building at One St Pancras Square. From an early stage we established a close working relationship with David Chipperfield Architects and BAM Construction. The challenge was to make 396 columns of varying sizes and profiles, all with precisely the same complex weave design, on time and on budget.

A master pattern was engineered out of model board using a CNC machine and based on CAD drawings provided by the architects. The master pattern itself was a 1.5 metre half round section, from which individual resin copies were taken to make up the different patterns required for the different sizes and profiles of columns. Each time the pattern required alteration or maintenance new resin copies were taken from the master pattern ensuring that quality and conformity were maintained. The patterns created were robust enough to withstand repeated use throughout the manufacture of all 396 columns, and of a quality that ensured the desired effect of the weave design was achieved. By working closely with the clients and investing in the pattern we were able to manage down the manufacturing costs.

pattern making 3

Left: Master pattern based on CAD drawings supplied by the client
Centre: Resin copies building up the pattern
Right: Full column pattern

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.
To talk about your project, contact Andy Knight, Foundry Manager at aknight@hargreavesfoundry.co.uk Tel: 01422 399111
Contact us via Linkedin or Twitter or speak to Andy on Twitter.