Precision parts manufactured at a cost-effective entry price are a prize contract for any manufacturer and something of an ongoing challenge. There is also the steady demand to maintain tolerances for size, gauge, performance and lately there is the challenge of fending off emerging technologies that promise more efficiency, more accuracy and quick production. For producers of castings these challenges have never been more relevant.
The foundry division of Weir Minerals Africa, the African head office for multi-national company the Weir Group, a supplier to the global mining industry, is not immune to these trials that all foundries face when producing castings but the company has decided to address the situation by implementing the Replicast process into its foundry.
Replicast is a moulding and casting process, created to meet demand for tailor made castings and improve quality and dimensional accuracy at the same time.
The foundry division of Weir Minerals Africa has implemented the Replicast process into its foundry
According to the developers – Castings Technology International – the technology replaces conventional wooden patterns with dimensionally precise replicas made from expanded polystyrene made by injection moulding. Once the polystyrene replica is complete, it is dipped several times in a special slurry and different grades of ceramic to create the mould for casting. Using inert ceramic shells reduces the possibility of hydrogen defects, which can be a problem with many high-alloy steels and nickel-based alloys.
The disposable, high-quality, expanded polystyrene patterns are used to facilitate the manufacture of exceptionally thin, highly refractory ceramic shell moulds without parting lines or cores. With suitable support, these high-quality moulds can be used to produce relatively large components with excellent ‘as-cast’ integrity and surface finish. Combined with zero draft and high dimensional accuracy, these features offer substantial benefits to customers and foundries alike.
Castings Technology International (Cti), based in the Advanced Manufacturing Park at Catcliffe, near Sheffield in the UK, developed the Replicast process in which the polystyrene is fully burnt out before casting. This allows a wide range of alloys to be cast in the mould – from ultra-low carbon stainless steel to nickel-based alloys as the pick-up of carbon from. The EPS pattern is eliminated whilst the advantages of the lost foam process are retained.
Replicast compared with other processes
In the Replicast process a ceramic shell is built up on the polystyrene pattern. On firing the ceramic, all traces of the polystyrene are removed before the mould is embedded in the supporting un-bonded sand. In many respects, the process is very similar to the lost wax or investment casting process but with certain key advantages. For example, polystyrene is lighter and relatively insensitive to temperature; patterns can be produced in very thin or thick walls (2mm to 200mm); complex shapes can be formed by gluing sections, affording the design freedom of the lost foam process; the physical properties of polystyrene allow a much thinner ceramic shell to be produced because the polystyrene is removed from the mould, a higher density pattern can be produced.
In the Replicast process a ceramic shell is built up on the polystyrene pattern
Cti report that superior dimensional accuracy and surface finish can be achieved, equivalent to those from lost wax casting and the lighter patterns and ceramic moulds allows much larger castings to be produced than would realistically be possible in any lost wax operation. Castings weighing in excess of 500kg, with a poured weight in excess of one ton are possible.
Compared with sand casting, the process can achieve a weight reduction commonly as high as 25%, a 50% reduction of feed metal is typically obtained, 40% more castings per melt can be expected and fettling and finishing costs can be reduced by more than 30%.
New designs can be produced in half the time it would take if a conventional wooden pattern had to be made, modifications can be made just as rapidly, and it is far easier to reverse engineer a product, using a Co-ordinate Measuring Machine to collect the data to produce a CAD design from which the sacrificial pattern can be made.
Weir Isando plant installation
“Reducing component turnaround time while improving quality and reducing waste are the main motivators for us implementing the Replicast process,” explained Umar Smith, plant manager at Weir Minerals Africa’s Isando facility.
“The development enables the company to produce more components at a time and more quickly. This will assist in meeting growing customer demand, while also reducing rework and wastage.”
Umar Smith, plant manager at Weir Minerals Africa’s Isando facility
The Replicast process is not the first time that the Weir Group has implemented it at one of its plants. Already in 2002 Weir Minerals introduced the process into its facility in Todmorden, West Yorkshire, UK.
“As part of our Project Vuka, this new process in our Isando facility allows us to cast multiple small components per batch rather than just one at a time. We can also reduce our knock-out times from days to just a couple of hours.”
“There are two phases to the Replicast process, which uses polystyrene to create the moulds. The first phase is the polystyrene moulding process, which occurs after the polystyrene beads have been expanded. The second phase is where the ramming, pouring and demoulding takes place.”
“In contrast to the traditional moulding line, where a resin and a catalyst are used to bind sand, the Replicast process uses silica sand of 30-35 AFS grade together with the polystyrene mould. The system involves a vacuum bin, from which all the air is removed to compress the sand,” explained Smith.
“The absence of resin and catalyst, as well as having no clamping process, results in less scrap being produced, and therefore brings operational savings. The quality of castings is also raised, with a better surface finish and fewer defects.”
“Other advantages are that the geometrical stability of components is improved, as there is less fettling of the finished product thereby reducing dimensional variation between the same components. This in turn contributes to the reliability of the equipment using those components.”
Cti report that superior dimensional accuracy and surface finish can be achieved with the Replicast process, equivalent to those from lost wax casting and the lighter patterns and ceramic moulds allows much larger castings to be produced than would realistically be possible in any lost wax operation. Castings weighing in excess of 500kg, with a poured weight in excess of one ton are possible
“The foundry will also realise significant environmental benefits as a result of using no chemicals in the sand. This aligns well with our corporate sustainability goals, ensuring that our processes are not only compliant but continuously reduce our environmental impact. Our new moulding systems ensure that fewer gases are emitted during the casting process and there are zero emissions of harmful substances such as benzene.”
“The new technology is also leading to less frequent disposal of silica sand, and the sand itself is more environmentally friendly as it contains no resin or acid.”
“We will be casting high-chrome components weighing up to 250kg using the Replicast process.”
Smith complimented the local foundry equipment manufacturing and engineering company that was involved in the installation of the process into the Isando plant.
“A remarkable aspect of the development was the fact that it was done with our local skills and largely during the serious Covid-19 pandemic period. Despite this and the logistical challenges created by the pandemic, it was successfully implemented on time and within budget.”
