- The shell-mold casting process is also known as the shell molding process, is an expendable mold casting process that uses resin-covered sand to form the mold.
- In the shell-mold casting process, the mold is made like a shell i.e., the thin or hollow mold is used so that the weight of mold is less and handling of mold becomes easier.
- To manufacture the thin mold, the phenolic resin with a small addition of silica sand is used as the mold material.
- Making shell-mold casting requires the use of a metal pattern, oven, sand-resin mixture, dump box and molten metal.
- Shell-mold casting allows the use of both ferrous and non-ferrous metals, most commonly used molten metals are cast iron, carbon steel, alloy steel, stainless steel, aluminum alloys and copper alloys.
The shell-mold casting process consists of the following steps:
- Pattern creation – A two-piece metal pattern is created in the shape of the desired pattern required.
- Mold creation – First, each pattern half is heated to 175-370°C and coated with a lubricant to facilitate easy separation from the pattern formed. Next, the heated pattern is clamped to a dump box. The dump box is inverted, allowing this sand-resin mixture to coat the pattern. The heated pattern partially cures the mixture, which now forms a shell around the pattern. Each pattern half and the surrounding shell is cured to completion in an oven and then the shell is ejected from the pattern.
- Mold assembly – The two shell halves are joined together and securely clamped to form the complete shell mold. If any cores are required, they are inserted prior to closing the mold. The shell mold is then placed into a flask and supported by a backing material.
- Pouring – The mold is securely clamped together while the molten metal is poured from a ladle into the gating system and fills the mold cavity.
- Cooling – After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the desired casting.
- Casting removal – After the molten metal is cooled, the mold is broken and the casting is removed. Trimming and cleaning processes are performed to remove any excess metal from the feed system and to separate small sand particles from the mold.
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In the shell mold casting process in place of phenolic resin if the ceramic slurry is used as a mold material then it is called ceramic molding.
Features of Shell Molding Process:
- In the shell molding process thickness and weight of mold is less.
- The production rate of shell mold is very high.
- Because of the low porosity of mold, through riser must be used.
- It is useful for only circular and symmetrical jobs.
- The size of casting can be formed is limited to 30kg only.
- Highly complicated shapes cannot be formed using the shell mold casting method.
- This process produces more dimensional accurate casting compared to other sand casting processes.
- Lower draft angles can be used.
- The shell-mold casting method is used for making cylinders and cylinder heads for IC engines, automobile transmission parts, cast tooth bevel gears, gear blanks, refrigerator valve plates, small crankshafts, track rollers for crawlers tractors, transmission planet carriers etc.