Process
Generally, dry and fine sand (90 to 140
GFN) which is completely free of the clay is used for preparing the shell
moulding sand. The grain size to be chosen depends on the surface finish
desired on the casting. Too fine a grain size requires large amount of resin
which makes the mould expensive.
The synthetic resins used in shell moulding
are essentially thermosetting resins, which get hardened irreversibly by heat.
The resins, most widely used, are the phenyl formaldehyde resins. Combined with
sand, they give very high strength and resistance to heat. The phenolic resins
used in shell moulding usually are of the two stage type, that is, the resin
has excess phenol and acts like a thermoplastic material. During coating with
the sand, the resin is combined with a catalyst hexa-methylene tetramine in a
proportion of about 14 to 16% so as to develop the thermosetting
characteristics. The curing temperature for these would be around 150oC
and the time required would be 50 to 60 sec.
Additives may sometimes be added into the
sand mixture to improve the surface finish and avoid thermal cracking during
pouring. Some of the additives used are coal dust, pulverized slag, manganese
dioxide, calcium carbonate, and ammonium borofloride and magnesium
silicoflouride. Some lubricants such as calcium stearate and zinc stearate may
also be added to the resin sand mixture to improve the flowability of the sand
and permit easy release of the shell from the pattern.
The first step in preparing the shell mould
is the preparation of the sand mixture in such a way that each of the sand
grain is thoroughly coated with resin. To achieve this, first the sand, hexa
and additives, which are all dry, are mixed inside a Muller for a period of 1
min. Then the liquid resin is added and mixing is continued for another 3 minutes.
To this cold or warm air is introduced into the Muller and the mixing is
continued till all the liquid is removed from the mixture and the coating of
the grains is achieved to the desired degree.
Since the sand resin mixture is to be cured
at about 150oC temperature, only metal patterns with associated
gating are used. The metal used for preparing patterns is grey cast iron,
mainly because of its easy availability and excellent stability at temperatures
involved in the process. But sometimes-additional risering provision is
required as the cooling in shell mouldings is slow.
The metallic pattern plate is heated to a
temperature of 200 to 350 degrees depending on the type of pattern. It is very
essential that the pattern plate is uniformly heated so that the temperature
variation across the whole pattern is within 25 to 40 degrees depending on the
size of the pattern. A silicone agent is sprayed on the pattern and the metal
plate. The heated pattern is securely fixed to a dump box, wherein the coated sand
in an amount larger than required to form the shell of the necessary thickness
is already filled in.
Then the dump box is rotated so that the
coated sand falls on the heated pattern. The heat from the pattern melts the
resin adjacent to it thus causing the sand mixture to adhere to the pattern
When a desired thickness of shell is achieved, the dump box is rotated
backwards by 180 degrees so that the excess sand falls back into the box,
leaving the formed shell intact with the pattern. The average shell thickness
achieved depends on the temperature of the pattern and the time the coated sand
remains in contact with the heated pattern.
The shell along with the pattern plate is
kept in an electric or gas fired oven for curing the shell. The curing of the
shell should be done as per requirements only because over curing may cause the
mould to break down as the resin would burn out. The under curing may result in
blow holes in the casting or the shell may break during handling because of the
lack of strength.
The shells thus prepared are joined
together by either mechanical clamping or adhesive bonding. The resin used as
an adhesive may be applied to the parting plane before mechanical clamping and
then allowed for 20 to 40 seconds for achieving the necessary bonding.
Since the shells are thin, they may require
some outside support so that they can withstand the pressure of the molten
metal. A metallic enclosure to closely fit the exterior of the shell is ideal,
but it is too expensive and therefore impractical. Alternately, a cast iron
shot is generally preferred as it occupies any contour without unduely applying
any pressure on the shell. With such a backup material, it is possible to
reduce the shell thickness to an economical level.
Advantages
Shell moulding castings are generally more
dimensionally accurate than sand castings. It is possible to obtain a tolerance
of ± 0.25 mm for steel castings and ±0.35
mm for grey cast iron castings under normal working conditions.
A smoother surface finish can be obtained
in shell castings. This is primarily achieved by the finer size grain used. The
typical order of roughness is of the order of 3 to 6 microns.
Draft angles are lower than required in
sand castings. The reduction in draft angles may be between 50 to 75% which
considerably saves the material costs and the subsequent machining costs.
Sometimes, special cores may be eliminated
in shell moulding. Since the sand has a high strength the mould could be
designed in such a manner that the internal cavities can be formed directly
with the shell mould itself without the need of the shell cores.
Also, very thin sections of the type of air
cooled cylinder heads can be readily made by the shell moulding because of the
higher strength of the sand used for shell moulding.
Permeability of the shell is high and
therefore no gas inclusions occur.
Very small amount of sand needs to be used.
Mechanisation is readily possible because
of the simple processing involved in shell moulding.
