The Investment Casting Process

THE PROCESS

Whatever the size or the complexity of the component, the same basic manufacturing process is employed. RPC add to this a unique understanding of the materials used and utilise advanced technology for the adjustment and control of crucial production stages in order to meet customers precise requirements, time and time again.

1 DESIGN

Following Consultation with RPC's Project Engineers, a drawing is supplied to RPC by the customer, defining the geometry, it's required finish and acceptance criteria. Depending upon complexity, ceramic or soluble cores are a requirement. Tooling is then designed with adequate feeding incorporated wherever possible. From this tooling, wax patterns are produced by injecting wax under pressure. Specifically designed wax injection presses are used, as precise control of the wax temperature and the pressure under which it is injected are crucial.

2 WAX ASSEMBLY

In most cases, a one piece wax is produced from the tool and this is mounted onto a wax frame. In some cases, to accommodate the design, a wax pattern is produced in two or more parts and is sub assembled by precision wax welding. Where necessary additional gates and runners are added to help the flow of molten metal during casting, allowing the ceramic mould to fill quickly, and unwanted air and gasses to escape.

3 SHELL MANUFACTURE

The wax assembly is invested (coated) with a ceramic shell by repeated dipping in ceramic slurries and alternately in dry beds of sands and clays. The most crucial stage of the shelling process is the primary coating, where a very fine grade of textured sand is used. This in turn results in a casting with excellent reproduction detail and the designated surface finish. Where heavy or thick sections are a feature of any part, additional coatings are invested to give added strength to the shell, thus eliminating bulging or distortion of the casting. To complete this stage, the ceramic shell is heated under pressure in an autoclave and the wax removed by melting, leaving a hollow shell.

4 CASTING

Having removed the wax, the shell is ready for firing, which pre-heats and prepares it to receive the specific alloy. Induction melting units are used to melt the alloy to controlled temperatures - the desired melting point. Strict control of the shell firing temperature is a further critical factor in the casting process. Having melted and de-gassed the molten metal, the pre-heated shell is filled to the required level and left to solidify. The cooling rate is strictly controlled to ensure metal purity and casting integrity.

5 FINISH

When fully cooled, the ceramic shell is removed mechanically from the cast assembly. All cast parts are then removed from the runner system by conventional engineering methods. RPC offer a choice of finishes, achieved by using a variety of blasting techniques.

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