SHELL MOULDING & INVESTMENT CASTING

Transcription

1 SHELL MOULDING & INVESTMENT CASTING SHELL MOULDING Thisistheprocessinwhichthesandmixedwiththermosettingresinisallowedtocomein contact with the heated metallic pattern plate. This is done 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 backup material and molten metal is poured into the mould. Dry and fine sand (90 to 140 GFN) that is completely free of clay is used for preparing the shell moulding sand. Too fine grain size requires large amount of resin, making the mould expensive. The synthetic resin used in the shell moulding is thermosetting resin, which gets hardened by heat. The most common resin is phenol formaldehyde resins. Combined with sand, they have very high strength and resistance to heat. The phenolic resin used in shell moulding usually are of two stage type, that is, the resin has excess phenol and acts as thermoplastic material. During coating with the sand, resin is combined with an catalyst (hexa-methylene-tetramine) in about 14-16% so as to develop thermosetting characteristics. The curing temperature of these would be around 150 Candthetimerequired wouldbe50to60seconds 1

2 Additives are added to sand mixture to improve the surface finish and avoid thermal cracking during pouring. Some of the additives are coal dust, pulverized slag, manganese dioxide, calcium carbonate etc. Some lubricants like calcium stearate and zinc stearate is added to resin sand mixture to increase the flowability of the sand and permit easy release of the shell from the pattern. First step is the preparation of the sand mixture in such a way that sand grains are thoroughly coated with resins. First the sand, hexa and additives are mixed in dry condition inside a Mueller for 1 minute. Then liquid resin is added and mixing is continued for another 3 minutes. Cold and warm air is introduced into Mueller and mixing is continued till all the liquid is removed from the mixture and coating of desired degree is obtained. Sand resin mixture is to be cured to 150 C temperature, so only metal patterns with the associated gating is used. The metal used for preparing patterns is grey cast iron, mainly due to its easy availability and excellent stability at the temperature. FIGURE 1: STEPS IN SHELL MOULDING 2

3 The metallic pattern plate is heated upto a temperature of 200 to 350 C depending on the type of the pattern. It is very important that the pattern plate be uniformly heated so that the temperature variation across the whole pattern is within 25 to 40 C depending upon thesizeofthepattern. A silicone-release agent is sprayed on the pattern and the match plate. The heated pattern is securely fixed in the dump box wherein the coated sand in an amount larger than required to form the shell of 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, causing the sand mixture to adhere to the pattern. When the 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. 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 requirement because overcuring and undercuring results in development of blowholes, burnout and even the shell may break. The shells thus prepared are joined either by mechanical clamping or by adhesive bonding. The resin used as an adhesive may be applied at 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 fittheexterioroftheshellisideal,butitistoo expensive.acast iron shot is generally prepared as it occupies any contour without applying any pressure on the shell. 3

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5 DISADVANTAGES OF SHELL MOULDING The patterns are very expensive and economical only if used in large scale production(above pieces). The size of the casting obtained by shell moulding is limited. Generally castings weighing up to 200 kg can be made, though in smaller quantity, castings up to a weightof450kgcanalsobemade. Highly complicated shapes cannot be obtained. More sophisticated equipment is needed for handling the shell moulding such as those required for heated metal patterns. APPLICATIONS OF SHELL MOULDING Cylinder and cylinder heads for air cooled IC engines, automobile transmission parts, cast tooth bevel gears, brake beam, transmission planet carrier, gear blanks, chain seat bracket, refrigerator valve plate, track rollers for crawler tractors etc. are some common applications of shell-mould castings. INVESTMENT CASTING This is the process where mould is prepared around an expandable pattern. The first step in this process is the preparation of the pattern for every castingtobemade.ametallicdieisusedwhichhasacavityofthecasting tobemade. Moltenwaxisusedaspatternmaterialandinjectedunderpressureof2.5 MPaintothedie(step1). The wax is allowed to solidify which would produce the pattern. The patternisejectedfromthedie(step2). Then the cluster of wax patterns are attached to the gating system by applying heat(step 3). 5

6 FIGURE 3: STEPS IN INESTMENT CASTING To make a mould, the prepared pattern is dipped into a slurry made by suspending fine ceramic materials in a liquid like ethyl silicate or sodium silicate(step4).theexcessliquidisallowedtodrainofffromthepattern. Dry refractory grains suchas fused silica or zircon are applied to give good outer surface on this liquid ceramic coating (step 5). A shell is formed around the wax pattern. The shell is cured and the process of dipping and fusing is continued with ceramic slurries of gradually increasing grain size. When the shell thickness of6to15mmisattained,themouldisreadyforfurtherprocessing. The next step is to remove the pattern from the mould, which is done by heating the mould to melt the pattern (step 6). The melted wax is completely drained through the sprue by inverting the mould. Any wax remaining in the mould is dissolved with the help of hot vapor of solvent (trichloro-ethylene). The moulds are then preheated to 100 to 1000 C, depending upon the size, complexity and metal of casting. This is done to reduce any last trace of wax. The molten metal is poured into the mould under gravity, under slight pressure, by evacuating the mould first(step 7). 6

7 ADVANTAGES OF INVESTMENT CASTING Complex shapes, which are difficult to produce by any other method are possible since the pattern is withdrawn by melting it. Very fine details and thin sections can be produced by this process, because the mould is heated before pouring. Very close tolerances and better surface finish can be produced due to the finegrainofthesandused. Sincetherearenopartingline,dimensionsacrossitdoesnotvary. Castings produced by this process are ready for use with little or no machining required. It is possible to control grain size, grain orientation and directional solidification in this process which results in controlled mechanical properties. DISADVANTAGES OF INVESTMENT CASTING Theprocessisnormallylimitedbythesizeandmassofthecasting.Theupper limitonthemassofacastingmaybeoftheorderof5kg. This is more expensive process because of larger manual labour involved in the preparation of the pattern and the mould. APPLICATIONS OF INVESTMENT CASTING The process is used for preparation of artifacts, jewellery and surgical instruments. Presently the products made bythis process arevanesand blades ofthe gas turbines, shuttle eye for weaving, paws and claws of movie cameras, waveguides for radar, bolts and trigger for firearms and impellers for turbo chargers. 7

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