It is a process in which, China precision machining the sand mixed with a thermosetting resin allowed to come into contact with a heated metallic pattern plate, so that a thin and strong shell of mould is formed around the pattern. Then the shell is removed from the pattern and the cope and drag are removed together and kept in a flask with the necessary back up material and molten metal is poured into the mould.
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 phenol formaldehyde resins. Combined with sand, they have 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. China precision machining During coating with the sand the resin is combined with a catalyst such as hexa-methylene-tetramine (hexa) in a proportion of about 14 to 16% so as to develop the thermosetting characteristics.
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, pulverised slag, manganese dioxide, calcium carbonate, ammonium borofluoride and magnesium silicofluoride. 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 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 Mueller for a period 1 min. Then the liquid is added and mixing is continued for another 3min. To this cold or warm air is introduced into the Mueller and the mixing is continued till all the liquid is removed from the mixture and coating of the grains is achieved to the desired degree.
The metallic pattern plate is heated to a temperature of 200 to 350 ºC depending on the type of the 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 ºC depending on the size of the pattern. A silicone release agent is sprayed on the pattern and the metal plate. The heated pattern is securely fixed to a dump box, as shown in
Fig. 6.7 (a), wherein the coated sand in an amount larger than required to form the shell of necessary thickness is already filled in.
Fig. 6.7 Shell moulding procedure
When a desired thickness of shell is achieved, the dump box is rotated backwards by 180 so that the excess sand falls back into the box, leaving the formed shell intact with the pattern as in Fig. 6.7 (d). 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 actual shell thicknesses required depends on the pouring metal temperature and the casting complexity. This may normally be achieved by trial and error method.
Fig.6.8 Shell mould ready for pouring
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 is too expensive and therefore impractical.
Alternatively, a cast iron shot is generally preferred as it occupies any contour without unduly applying any pressure on the shell. China precision machiningWith such a backup material, it is possible to reduce the shell thickness to an economical level.