Shell moulding

Characteristics of lost foam casting Design freedom to produce complex shapes. 

Minimum finishing operations are needed, e.g. shot blasting, grinding. 

The process is used in the automotive industry in the production of cylinder heads, engine blocks, crankshafts and exhaust manifolds, in the electrical industry for electric motor frames, and for mains water valves to name but a few. 

The shell-moulding process uses a thin expendable mould or shell made from a fine silica or zircon sand bonded with a thermosetting resin. The fineness of the sand influences the surface of the finished casting. 

Patterns suitable for shell moulding can be made from any material capable of withstanding oven temperatures of about 400 °C and of transferring heat to the moulding material. The materials most widely used are cast iron and steel. 

When drying is complete the coated foam is suspended in a steel container which is vibrated while traditional green sand, with no binders or other additives, is poured around the coated pattern providing mechanical support to the thin ceramic layer. During the sand pouring operation, the vibration causes a layer of sand near the pattern surface to fluidise ensuring all voids are filled, and shapes supported, while the whole mass of sand is compacted. 

After compaction, molten metal is poured into the mould causing the foam to burn up and vaporise as the molten metal replaces the foam pattern, exactly duplicating all the features of the original pattern. Like other investment casting methods, for each new casting produced, a new pattern must be made hence the name lost foam casting. 

After solidification, the container is tipped over and the unbonded sand flows out together with the castings. Because there are no binders or other additives the sand is reclaimable. The castings are then subjected to further operations, e.g. removal of sprue, gate, risers, any machining required, heat treatment, etc. similar to other casting processes. 

Generally all ferrous and non-ferrous metals can be cast using this method but it is unsuitable for low- and medium-carbon steels due to the possibility of carbon pick-up. Because the foam pattern has to decompose to produce the casting, metal pouring temperatures above 540 are usually required so that casting of low-temperature metals is limited. 

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Shell moulding - Steel, iron and copper alloys... 

SHELL MOULDING PROCESS CAPA9 LITY MAP FOR ALUMINIUM AND MAGNESIUM ALLOYS... 

Liquid carbon dioxide is discussed on page 261. Carbon dioxide gas is commonly used for carbonating drinks, in fire extinguishers, for gas-shielding of welding and in shell moulding in foundries. Its physical and toxicological properties are summarized in Tables 8.5, 8.6 and 5.29. 

N)Anon, "Shell Moulding, Machinery Publg Co, Brighton,England(1955)... 

Hand-held electric tools, such as electric drills or hot-air blowers, usually have clam shell mouldings. When the two casing halves close, tongues or... 

After this, the shell is fired in a fiimaee at a high temperature. This bums out the residual wax and hardens the eeramic, leaving a one pieee eeramie shell mould, into which the molten metal is poured to form the casting. 

Ceramic shell moulding is a patented process (Replicast ) in which a polystyrene model is covered with a ceramic shell (2-3 mm thickness), based on ethyl silicate and refractory sand. The shell is hardened using ammonia and sintered at 1000 °C. The sintering cures the shell and bums away the polystyrene model. The metal is then poured into the shell. 

To successfully exploit post combustion to eliminate VOCs from off-gases, specific minimum concentrations are necessary. These limit values are compound specific and depend on the chosen technique. Post combustion is possible for the exhaust gas from shell moulding. Generally, the exhaust gas from the casting shop does not have high enough VOC levels to operate post combustion. One alternative to post combustion is to use exhaust air from the core blowers as combustion air for the cupola. 

INVESTMENT CASTING, LOST FOAM CASTING AND SHELL MOULDING... 

Figure 17.10. Shell-moulding pattern, mould, cores and casting...

Molten metal is poured into the prepared shell mould to form the casting, the surface of which will be free of blemishes as air and gases can readily escape due to the permeability of the shell. When the cast metal has solidified, the shell is broken away to reveal the casting. Figure 17.10 shows the metal pattern, shell mould, cores and resulting casting. 

Shell moulding allows intricate shapes to be produced with minimum taper or draft angle. An angle of to 1° is usually sufficient, compared with the 2° to 3° needed with green-sand casting. 

From what material is the expendable mould made in the shell moulding process. 

What are the requirements of the materials used to make patterns for shell moulding ... 

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