Casting processes - Resin Shell Moulding

The shell moulding process is a precision sand casting process capable of producing castings with a superior surface finish and better dimensional accuracy than conventional sand castings. These qualities of precision can be obtained in a wider range of alloys and with greater flexibility in design than die-casting and at a lower cost than investment casting. The process was developed and patented by Croning in Germany during World War II and is sometimes referred to as the Croning shell process.

The fundamental feature of the process is the use of fine-grained, high purity sand that contributes the attributes of a smooth surface and dimensional accuracy to moulds cores and castings alike. In conventional sand moulding the use of such fine sand is precluded because it would dramatically reduce mould permeability. This has the effect of retarding the escape of air and mould gases, causing short0run castings or castings containing gas defects. However, the distinguishing feature of the shell moulding process is that the mould is literally a shell, being in the region of only 10mm (0.4in) thick. It was the ability to produce such a thin shell mould, which made the process a revolutionary development in metal founding. The coincident development of plastics, like Bakelite, which were based on thermosetting resins such as phenol formaldehyde, provided the basis for shell moulding. In shell moulding the fine sand is coated with a thermosetting resin which provides the relatively high strength required enabling a thin section, or shell, mould to be produced.

The requirement that the mould should accurately replicate the pattern detail and dimensions if a precision casting is to be produced is also met by the shell moulding process. This is achieved because the resin bond is developed whilst the mould is in contact with a heated pattern plate. Furthermore, the mould is separated from the pattern without the need to enlarge the cavity, as is the case in green sand moulding. These features apply equally to the production of cores by the process. A further improvement in casting accuracy can be obtained if zircon sand is used instead of silica sand. That arises because the expansion of zircon sand, caused by the heat of the cast metal, is both lower and more predictable than that of silica sand. Foundry production of castings by the process is comparatively straightforward and the process lends itself readily to close control, with the advantage of consistency in the castings produced.

The advantages and disadvantages of the process are summarised below (18,19):

Advantages

• lower capital plant costs, when compared with mechanised green sand moulding
  
  
• capital outlay on sand preparation plant is not essential
  
  
• good utilisation of space
  
  
• low sand to metal ratio
  
  
• mould coatings are unnecessary
  
  
• lightweight moulds are produced which are readily handled and have good storage characteristics
  
  
• skilled labour is not required
  
  
• shells have excellent breakdown at the knockout stage
  
  
• lower cleaning an fettling costs
  
  
• castings have a superior surface finish and dimensional accuracy, when compared with green sand moulded castings

Disadvantages

• the raw materials are relatively expensive
  
  
• the size and weight range of castings is limited
  
  
• the process generates noxious fumes which must be effectively extracte

From Precision Casting Processes by A J Clegg. Reprinted by permission of Butterworth Heinmann