15 August 2016

In previous blogs we have explained making sand moulds using wooden patterns. Core Assembly is a different way of making sand moulds for iron casting. Simply put a mould made in this way is like a 3D jigsaw puzzle in negative - the technique requires great skill and takes years to master.

Hargreaves Foundry has made thousands of castings in this way, so why do we use this technique rather than traditional patterns? Well in our case it is to do with casting size, crane limits and storage. Some of the very large engineering castings would not be possible in our foundry without using this technique, and we certainly don't have the storage space for large numbers of eight metre long wooden patterns.

In terms of cost generally Core Assembly saves money on the pattern making but costs more for the moulding. So for a one off job there may be a saving with Core Assembly, however, if multiple castings are needed this could defray the saving.

How does it work? - the bluffers guide to Core Assembly

It starts with an engineering drawing - a detailed plan showing the overall shape, internal and external dimensions, radii etc. Next we prepare a pit in the sand floor of the foundry, the base of which is levelled out carefully ready to start the assembly process. As this is Core Assembly we won't be making patterns to create the outside shape, we will be making sand cores that create the internal spaces. The gaps between these cores are where the metal goes and forms the casting shape. So, we do need core boxes made from wood, just like a pattern, to make all the different sizes and shapes of the sand cores that will be required. The individual cores are prepared in the core shop by 'ramming up' the core boxes with sand. Once the sand has set the cores are taken out of the boxes ready for assembly. The next step is to piece these sand cores together inside the pit and build up the mould, more or less from the inside out. This part of the process requires skill, attention to detail and accuracy on the part of the moulders, as each individual core has a precise relationship with every other core. If one is slightly out of place they will all be out of place.

Once the mould is built up it needs to be securely and tightly packed in sand, and weighted down ready for the molten iron to be poured in. It is critical that none of the cores move even the slightest amount during the pouring process as this would render the entire casting scrap. When it has eventually cooled, (a process which takes days) the casting is broken out of the mould and moved to the fettling shop where the sand cores are removed and the fettling and shot blasting then completes the process. As with all our bespoke castings we only use recycled scrap iron and all the sand cores can go through our reclamation unit to be used again.

The nature of the process, working in negative as it were can be confusing. Fortunately our moulders, thanks to training and experience, know exactly what they are doing and are extremely skilled at this particular technique. In fact when they look at the drawings they see the spaces and think of it in terms of the sand needed to fill those spaces.

To help demonstrate Core assembly we produced this time lapse video of a grinding bed being made. This particular casting has a finished weight of 9.5 tonnes, required 60 different cores, 11 tonnes of sand, 50tonnes of weight plus clamps to hold it down and 12 tonnes of molten iron to pour it. The film lasts 11 minutes and follows a process that took place over a six week period.


Foundry contact Andy Knight, Foundry Manager at Tel: 01422 399111 Contact us via Linkedin or Twitter or speak to Andy on Twitter.