LPSC AND REPLICAST®: a winning combination for high quality casting components in steel or super alloys
by J. L. Comarteau |
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Summary
SEVA is specialised in the design and manufacture of components and moulds for the glass industry. At a Cti conference1 SEVA explained how it has developed and used the "Low Pressure Steel Casting" (LPSC) Process to produce high quality steel and super alloy castings in its integrated foundry.
To enhance the LPSC's ability to manufacture innovative-shaped parts, SEVA decided to use this process with Replicast® ceramic shell (CS) moulds. This will be referred to as the L+R process.
Process Principals
Replicast® is the Registered Trade Mark of Castings Technology International. It is a patented process and may only be used in accordance with the terms and conditions of the licence granted to the licensee.
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The LPSC process is adapted to all kinds of conventional sand moulds as well as Replicast® moulds The process involves the use of an inert gas to push molten metal of constant temperature, from a furnace into a mould sited above the furnace. Mould filling is regulated by gas pressure and controlled by a PLC.
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Combined Advantages
MOULDING ACCURACY
Complex Components
The Replicast® technique enables cast components of complex geometry to be produced. The technique is particularly well suited for large size industrial components (more than 200mm) of significant weight (more than 15kg). The process can easily replace complex fabricated/forged assemblies. In addition, casting solutions offer substantial benefits when the material is difficult to weld or machine.
Dimensional Accuracy and Surface Finish
The process provides a good surface finish (Ra = 6 - 7 µm), clearly superior to that of sand and shell moulded castings. Because it uses a replica of the required article, the castings are produced with an exceptionally tight dimensional tolerance. In addition to weight saving benefits, this enables a reduction of subsequent machining operations.
Free Design
Compared to sand moulding, Replicast® can eliminate the inconvenience of core making (seams, joints ...). It makes the creation of new component forms and designs possible, while integrating fine details such as bosses, holes and lettering.
QUALITY STEEL
Metallurgical Quality
The counter-gravity process allows a smooth, regular and consistent filling of the mould cavity and this is ensured by the controlled pressurised flow of metal. The chemically inert ceramic shell ensures castings free of inclusions and oxidation defects. A minimised alloy superheat ensures that the metal structure is finely grained.
Thin Wall Casting
The LPSC process allows the manufacture of thin wall components (down to 2mm.) and larger components with thin sections that are impossible to cast using conventional processes.
Reliability and Reproducibility
The computer assistance in the LPSC process enables reliable and consistent castings to be produced.
Industrial Applications
SEVA's FOUNDRY EXPERIENCE
SEVA was granted a license to operate Replicast® by Castings Technology International in 1996. Over the next two years, SEVA's foundry experts adapted Replicast® and LPSC one to each other to operate the "L+R" Process industrially.4 Nickel-based glass industry tooling manufacture is the main activity of SEVA and such tooling is now regularly produced by the L+R Process. For these kinds of parts, the main benefits are high quality levels and a reduction in machining operations. In addition to this significant improvement in the main casting activity, new parts have been designed for other industrial markets.
INDUSTRIAL APPLICATION: STAINLESS STEEL OPEN IMPELLER
Design Concern: A Complex Part with Large Stainless Steel Open Impeller
Mixed flow and spiral vane impellers are difficult to cast by conventional sand mould techniques when the vanes are large and skewed: complex shaped cores are required and their assembly decreases the dimensional accuracy and the surface finish of the parts. One of these complex-shaped parts has been studied by SEVA for ABS PUMP6. The LPSC and Replicast® combination has allowed this impeller to be cast in one piece, instead of welding the vanes onto the impeller hub.
Consistent Dimensions and Surface Finish
Expensive welding operations are eliminated due to the dimensional accuracy of the Replicast® ceramic moulds and reproducing the impeller as a one part casting. The orientation of the vanes on the hub is precise and consistent (which is important for the pump's efficiency) and the machining allowances are reduced.
The other benefit of the ceramic moulds is to give an excellent surface finish. Difficult sand mould grinding operations are not required: a fine sand blasting is enough to provide a Ra from 6 to 12 microns on the vanes surface, which reduces pump hydraulic friction losses.
Pump Efficiency Improvement
Reduction of hydraulic friction losses, more precise vane orientation, and more accurate geometry, all combine to significantly improve the pump efficiency (several %).
Looking to the Future
All these characteristics lead to the conclusion that, today, the L+R Process is best suited to the manufacture of complex components (in particular quite large parts with varying wall thicknesses), cast in high value alloys (such as stainless steel, nickel-based or cobalt-based alloys), with significant machining operations and with high quality requirements. The first two years allowed the foundry of SEVA to make substantial improvements in its main activity, and to manufacture value-added new parts, which are keys to open new markets, whenever stainless steel or super alloys are required. Due to the new possibilities given to the designers, a redesign of the components led by the customer and the foundryman will significantly increase, in many cases, the benefits brought by the L+R Process. The fields of interest have been explored for such a short time that the potential of the Process has not yet been identified, and we have great expectations for the future.
Notes and References
- A.REMY "Low Pressure casting Process for steels and super alloys" Cti CONFERENCE, May 1995
- M.C.ASHTON "The Replicast® ceramic shell process".
Metals and Materials Journal, January 1991
- M.C. ASHTON "A case for the development and evaluation of a system for the counter-gravity (pressure) pouring of commercial quality and high integrity cast steel products".
J.of SCRATA Nr 72 , December 1986
- SEVA's patent pending.
- With the first foam pattern, more than 400 parts have already been cast. The main dimension of these parts is 300 mm. Successful tests have been led for 600 mm. large parts, which is a clear demonstration that the L+R Process is particularly competitive for large net-shaped parts .
- ABS PUMP, an international pump manufacturer specializes in particular in the paper industry. The ABS PUMP Head-quarters are based in SWEDEN.
Acknowledgements
The author is grateful to Messrs L. IRHFELT and L. OLOFSSON of ABS PUMPS and to Dr M. C. ASHTON and all the experts at Cti for their helpful contributions.
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