In the ever evolving field of filtration for iron casting, groundbreaking technologies are making significant strides, with additive manufacturing, commonly referred to as 3D printing, at the forefront, accompanied by other next generation filter types. However, it is important to recognise that these technologies may not come cheap, at least not for some time.
TRANSFORMATIVE SHIFT IN IRON FILTRATION
For those focusing on sand casting, especially for larger iron castings in ductile and high chrome, typically weighing one tonne and upwards, the primary driver for this transformative shift is the pressing need for increased capacity. Foundries recognise the advantages of filtration, but its implementation becomes progressively challenging as casting sizes grow. Foam filters, for instance, are currently available up to 300mm and, with a standard 7-10ppi foam, offer approximately 2.5 tonnes of safe filtration capacity in ductile iron through a single filter. However, most foundries resort to using multiple smaller filters to meet their capacity requirements. Unfortunately, this entails the cost of additional filters and the potential need to enlarge moulds to accommodate this expanded filter area. This results in increased sand and binders, additional weight to life, and possibly higher expenses for filter holder units and extended setup time during moulding.
Over the years, a common question has been: “Why can’t we have a filter that can handle double the weight of iron for a given size?” The simple answer lies in the limitations of the foam used to make the filters, with 10ppi as the coarsest option available. Although the filter material can be manipulated to be more or less adhesive to oxide particles in the metal, the differences in ceramic composition typically account for only a twenty per cent variation in iron filtration capacity, far from doubling it! Enter additive manufacturing, where you can create engineered foams with remarkable freedom. A lattice structure can be designed equivalent to, say 5ppi, and filtration and flow modification can be fine-tuned for increased capacity. The added benefit is that since a digital model is produced, integrating it into casting simulations becomes potentially straightforward. This is not just a fanciful idea – it is entirely feasible and is expected to become increasingly accessible in the market. Over time, the costs associated with this technology are expected to decrease significantly but right now it is still relatively expensive.
SIMPLE SOLUTIONS
However, in the foundry world, simplicity often prevails. There are other cost effective solutions that can be explored. CAT International co-founder Mike Leaney takes up the story: “Whilst we at CAT International Ltd are deeply immersed in 3D printing filters and the development of ceramic 3D printing in general, we also recognise that the technology used in our foam filters puts us in a unique position to present an alternative yet simple solution.” CERACAT is a ceramic bonded filter with carbon that boasts impressive strength compared to purely carbon bonded filters and is much more cost effective due to its lower firing temperature during production.
This heightened strength allows CAT to incorporate 6-8mm diameter straight holes into the company’s foam filters. Leaney says: “It is a very simple idea but by doing so, we effectively double the filtration capacity, and when executed carefully in relation to the total open area of the holes to the runner area on the input side of the filter, there is no discernible compromise in filtration quality. “The incorporation of straight holes in our foam filters generates enough back pressure to ensure good flow through the body of the foam, while the added straight holes facilitate free flow as the foam structure gradually becomes obstructed during the pouring process.
“In essence, CAT can offer a filter with double the capacity of conventional foam filters, all at a cost of less than half that of a 3D printed filter. We call this version CERACAT MAX HC-High Capacity.
HIGH CAPACITY
One example of how CERACAT MAX HC foam filters have proven their value was when a UK based customer began making a large ductile iron flask for the containment of nuclear waste with a high specification for an iron casting. This was proving difficult to achieve. They approached CAT to come up with a simple, reliable way to incorporate a filter/device that could take three to four tonnes of SG iron throughput and be placed as close to the ingates as possible. They were using ceramic gating but in a relatively simple configuration due to tight constraints in the moulding box. The predominant requirement was for flow control. Whilst the existing CERACAT MAX filters could be used, they would typically only provide a throughput of 2.25-2.5 tonnes. Leaney explains: “We know that CERACAT MAX filters have a very high mechanical strength. For years we have supplied high chrome iron foundries hitting these filters in a direct pour application from ten tonne ladles with a 70mm nozzle and typically a one metre drop, without breakage.
“By introducing holes (in the case of the D300-8mm diameter by 50mm deep) we could increase the capacity significantly. However, by doing it in a calculated way in relation to the ingate area of the filter holder and down sprue, we can ensure flow through the ceramic foam structure without jetting through the holes. If blocking of the ceramic foam starts to occur in the latter part of the pour the open hole area allows a consistent flow to remain.” The composition of the CERACAT ceramic gives it a high affinity to collect non metallic particles, it is in effect very ‘sticky’, so a high degree of filtration still occurs.
LARGER AND LARGER
In conclusion, the iron filtration landscape is undergoing a remarkable transformation, with the advent of additive manufacturing and innovative solutions like CERACAT MAX HC foam filters further advancing what is possible in the production of large iron castings. While the allure of new technologies is undeniable, it’s reassuring to know that cost effective alternatives are also available, ensuring that foundries can adapt to evolving demands without incurring excessive expense.
Contact: Mike Leaney, CAT International Ltd, Tel: +44 (0) 1889 271755, email: [email protected] web: www.cat-intl.com