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 360 degrees, so that the sand contacts
the heated surface. An organic thermosetting resin, such as phenol
formaldehyde 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 shell moulding, typically around 10 mm, is determined by
the contact time between the sand and pattern. The shell moulding is heated again
to cure the sand, causing it to harden. shell moulding is released from the
pattern using ejector pins. The entire cycle can be completed in a
matter seconds, making it suitable for rapid production. Cores are added
to the shell mould and the two halves of the mould are glued and clamped
together before the metal is poured.
Shell Moulds are relatively robust and
can therefore be stored for reasonably long periods of time. Depending
on the cores used, spent sand can be reclaimed successfully using
thermal reclamation.
Advantages of Shell Molding |
Good
casting detail and dimensional accuracy are possible.
Moulds
are lightweight and may be stored for extended periods of time.
Has
better flexibility in design than die-casting.
Is
less expensive than investment casting.
Capital
plant costs are lower than for mechanized green sand moulding.
Metal
yields are relatively high.
Sand:metal
ratios are relatively low.
Gives
superior surface finish and higher dimensional accuracy, and incurs
lower fettling
costs than conventional
sand castings.
