News: Making sand moulds and cores for iron casting
Following previous blogs about specifying cast iron and pattern making, the purpose of this blog is to throw some light on another highly skilled aspect of the casting process - mould and core making. Our aim is to provide potential clients, be they artists or architects, some understanding of the processes involved and how these might affect their projects.
As with the earlier blogs we will follow the manufacture of cast iron columns for the Gridiron building at One Pancras Square to help explain the process.
What is a sand mould?
Sand casting as a process has been around for thousands of years. It involves creating a shape, encasing it in sand and then removing the original to leave a cavity. This is the sand mould into which molten metal is poured, left to cool and then broken out to reveal a casting. The industrial revolution, technology, new materials, knowledge and skill levels may have made things more sophisticated, but essentially the principle remains the same.
Many castings i.e. bollards and columns are not solid but have hollow centres or cavities. To create a cavity the internal shape of the casting is formed by making a sand core, which is then placed in the mould cavity. The molten iron runs between the mould and the core which, when the sand from the core is removed, leaves a hollow casting.
How are moulds and cores made?
For the purpose of this blog we will be considering mould making in boxes. The alternative is core assembly in the foundry floor which we will deal with in a separate blog in the future.
The process starts with a pattern (see our blog). The pattern is placed in a metal box and sand is then packed tightly around it. Traditionally the sand was mixed with clay (green sand) so that it would remain firm in the mould. Today the sand at Hargreaves Foundry is mixed with a resin and catalyst that allows it to set hard slowly. Once set the pattern can be removed, leaving the mould.
However, to be able to remove the pattern from the mould there can be no undercutting. Undercutting occurs if, when lifting the pattern out, sand gets trapped and breaks or damages the mould. This process is ameliorated by adding taper to the pattern. Taper allows the pattern to be withdrawn from the mould more easily, however, when this is not enough the mould is made in separate pieces and put together like a negative 3D jigsaw puzzle. Complex designs often involve multiple pieces to complete the mould, most notably Sir Antony Gormley’s Angel required 257 separate pieces to complete. Similarly, when casting Kenny Hunter’s ‘The Unknown’, each hand required 6 separate sand cores to allow the pattern to be removed from the mould.
A complex shape doesn’t mean we can’t cast it – it just means that it takes greater skill and more time than a simple shape. We welcome clients discussing their projects with us so that we can determine the best and most cost effective ways of manufacturing their castings.
Moulds are generally made in two halves and joined together one on top of the other creating the cavity into which the molten iron is poured. As mentioned above, if a hollow casting is required a core will be placed into the bottom half of the mould before the top half is put in place. The core sits on ‘prints’ at each end of the casting maintaining the gap between the mould and the core for the flow of metal, and ensuring the correct metal width is achieved. Making the moulds and cores for the Gridiron building at One St Pancras Square exemplifies this entire process.
Ordinarily a half round pattern would lift straight out of the sand mould without any undercutting, However, due to the weave design on the outside of the columns this would not be possible. The pattern was therefore made in three sections allowing each section to be removed at an angle that would prevent undercutting. Once the bottom half of the mould was completed the core was put in place and the top half of the mould placed on top. The moulding box with core is then secured and weighted down ready for the molten iron to be poured in.
Half round column pattern and mould
Weave pattern half round column pattern and mould
Pattern in moulding box
Sand is packed around the pattern within the box to create the mould
Pattern sections being removed from mould (note the angle)
Core being made in the same way as the mould
Core is placed in the bottom half of the mould (note the gap between core and mould)
Top half of mould is put in place
Molten iron is poured in
Finished castings prior to fettling
Columns in place
- The process starts by packing sand around a pattern.
- Sand Moulds have to be made so that there is no undercutting when the pattern is removed. Sand cores are used to create a cavity within the casting.
- We can cast complex shapes or designs but this requires more time and can affect the cost. In terms of heritage or restoration projects, if somebody cast a shape in the past, we will be able to reproduce it.
- It always pays to discuss your project with us and we welcome enquiries from customers.
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 email@example.com Tel: 01422 399111