Pulp and Paper Machinery


Require Large Castings With the use of ceramic shell molds in the investment casting process, larger castings are becoming not only possible, but more economical to make. In many cases parts are now investment cast which previously were sand cast and required considerable machining. These heavily machined parts often have high rejection rates from defects uncovered during the machining operations. This is especially true for castings made from stainless and heat resistant alloys.


Two investment castings typical of the trend to large parts are components for a vorject system in a pulp mill. Vorject cleaners separate and reject foreign material such as sand, fine gravel and silicon carbide particles from the pulp stream. Components of the system are subject to corrosion, and some parts are subject to severe erosion. For example, figure 1 shows a core trap body. It is an investment casting in type 316 stainless steel and weighs about 24 pounds. Conversion from sand casting to investment casting for this component resulted in a better product and an overall cost saving of 30 per cent. Another example is a reject nozzle shown in figure 2. A study showed that switching from sand casting to investment casting would eliminate all machining. Furthermore, it was possible to experiment with different alloys to find the most suitable for the application. The final design called for Stellite No. 1 with the 316 stainless steel threaded bushing shrunk over the casting. The increase in service life was 1200 per cent. The investment casting alloy that provides this long life cannot be used with sand casting because of the subsequent machining necessary.


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