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Shell moulding is also similar to sand moulding except that a mixture of sand and 3

- 6% resin holds the grains together. Set-up and production of shell mould patternstakes weeks, after which an output of 5-50 pieces/hour-mould is attainable.

Aluminum and magnesium products average about 13.5 kg as a normal limit, but it

is possible to cast items in the 45-90 kg range. Shell mould walling varies from 3-10

mm thick, depending on the forming time of the resin.

There are a dozen different stages in shell mould processing that include:

1.  Initially preparing a metal-matched plate2.  mixing resin and sand3.  heating pattern, usually to between 505-550 K

4.  inverting the pattern (the sand is at one end of a box and the pattern at theother, and the box is inverted for a time determined by the desired thicknessof the mill)

5.  curing the shell and baking it

6.  removing investment

7. 

inserting cores8.  repeating for the other half

9.  assembling the mould10. pouring the mould11. removing casting12. cleaning and trimming

The sand-resin mix can be recycled by burning off the resin at high temperatures

Shell molding process offers better surface finish, better dimensional tolerances,and higher throughput due to reduced cycle times.

A heated (200 ºC / 392 ºF) metal pattern is covered with a mixture of sand andthermoset plastic. This causes a skin of about 3.5 mm (0.125 in) of sand/plastic

mixture to adhere to the pattern. This skin is removed from the pattern to form the"shell mold". The two halves of the shell mold are secured together and the metal ispoured in the shell to form the part. Once the metal solidifies, the shell is broken.

This process can produce complex parts with good surface finish 1.25 µm to 3.75 µm(50 µin to 150 µin) rms, and dimensional tolerance of 0.5 %. Size limits of 30 g to

12 kg (1 oz to 25 lb). Minimum thicknesses can be as low as 1.5 mm (0.062 in) to

6.25 mm (0.25 in), depending on the material. A good surface finish and good sizetolerance reduce the need for machining.

A fairly high capital investment is required, but high production rates can beachieved. The process overall is quite cost effective due to reduced machining andcleanup costs.

The materials that can be used with this process are cast irons, and aluminum and

copper alloys. Typical parts made with this process are connecting rods, gearhousings, lever arms etc.

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Shell moulding is a process well suited to rapid, automated, repetitive and high-

volume production. The most common method for producing shell moulds is to use a

dump box as shown in the diagram. The dump box is rotated through 3600 so thatthe sand contacts the heated surface. An organic thermosetting resin, such as phenolformaldehyde or furane is typically used (2.5-4.5% of sand volume) in conjunction

with a catalyst (11- 14% of resin volume). Catalysts include weak aqueous acids

such as ammonium chloride or hexamine, a white powder.

The thickness of the shell, typically around 10 mm, is determined by the contacttime between the sand and pattern. The mould is heated again to cure the sand,

causing it to harden. The mould is released from the pattern using ejector pins. Theentire cycle can be completed in a matter seconds, making it suitable for rapidproduction. Cores are added to the mould and the two halves of the mould are glued

and clamped together before the metal is poured. Moulds are relatively robust andcan therefore be stored for reasonably long periods of time. Depending on the coresused, spent sand can be reclaimed successfully using thermal reclamation.

Advantage of Shell Moulding Casting

1.  Good casting detail and dimensional accuracy are possible.

2.  Moulds are lightweight and may be stored for extended periods of time.

3.  Has better flexibility in design than die-casting.

4. 

Is less expensive than investment casting.5.  Capital plant costs are lower than for mechanized green sand moulding.

6.  Metal yields are relatively high.7.  Sand: metal ratios are relatively low.8.  Gives superior surface finish and higher dimensional accuracy, and incurs

lower fettling costs than conventional sand castings.

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What metals can be cast?

Virtually any metal that can be melted can and is being cast in foundries today. Themost common alloys used in metal casting are shown below. Metals are mostcommonly categorized as ferrous or non-ferrous.

Ferrous Metals

1.  Cast Iron2.  Grey Iron3.  Ductile Iron

4.  Malleable Iron

5.  Compacted Graphite Iron6.  Cast Steel

7.  Carbon & Low Alloy

8.  Corrosion Resistant9.  Heat Resistant10. Manganese

Non-Ferrous Metals

1.  Cast Aluminum2.  Cast Copper Alloy

3.  Bras

4.  Bronze5.  Zinc6.  Magnesium

7.  Titanium8.  Nickel9.  Cobalt

10. 

Tin