Shell Moulding – Steps , Advantages and Disadvantages
Shell moulding, also known as shell-mould casting, is an expendable mold casting process that uses a resin covered sand to form the mold. As compared to sand casting, this process has better dimensional accuracy, a higher productivity rate, and lower labor requirements. It is used for small to medium parts that require high precision.
Shell mold casting is a metal casting process similar to sand casting, in that molten metal is poured into an expendable mold. However, in shell mold casting, the mold is a thin-walled shell created from applying a sand-resin mixture around a pattern. The pattern, a metal piece in the shape of the desired part, is reused to form multiple shell molds. A reusable pattern allows for higher production rates, while the disposable molds enable complex geometries to be cast. Shell mold casting requires the use of a metal pattern, oven, sand-resin mixture, dump box, and molten metal.
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 part, typically from iron or steel. Other materials are sometimes used, such as aluminum for low volume production or graphite for casting reactive materials.
Mold creation – First, each pattern half is heated to 175-370 °C (350-700 °F) and coated with a lubricant to facilitate removal. Next, the heated pattern is clamped to a dump box, which contains a mixture of sand and a resin binder. 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 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 final casting.
Casting removal – After the molten metal has cooled, the mold can be broken and the casting removed. Trimming and cleaning processes are required to remove any excess metal from the feed system and any sand from the mold.
The shell process has several unique properties which makes it an important process in the foundries. These properties can be briefed as:
- Excellent Surface Finish: Shell sand process has the ability to produce castings with excellent surface finish and capacity to produce very fine detail.
- Dimensional Accuracy: The process has an ability to produce castings to tight dimensional tolerances. Due to this characteristic, machining allowance can be reduced which ultimately helps in reduction of the fettling and finishing costs. The shell process accommodates easily deep drawer patterns with less tapers than conversional production processes.
- Hollow Cores: With the shell sands hollow cores and thin profile moulds can be possible. This characteristic gives economics in sand usage & ease of handling. Hollow cores increase the permeability hence usage of very fine sands is also possible.
- Sand to Metal Ratio: This is a unique process that gives hollow cores and thin walled moulds, which results in substantial weight reduction and material savings. The normal sand to metal ratio is 1:1 which is much lower than other processes.
- Ease of Handling: The molds and cores made by shell molding have exceptional resistance to damage during storage and handling. They have a very high resistance to humidly and can be easily stored for long periods.
- Resistant to Moisture Pickup: The shell process has higher resistant to moisture and can be stored in humid conditions for months together. The resin used in shell molding process is very stable and moisture resistant.
- Excellent Flow-Ability: The dry coating on sand, gives better flowability and blowing ability compared to processes based on wet sand mixes. This property helps in producing intricate cores and moulds which can be blown to a greater density. e.g. cores for water jacket.
- Less Inclusions and High Thermal Stability: The shell sands are less prone to erosion by molten metal due to higher thermal stability of the phenolic resins. This unique characteristic helps in reducing defects like non-metallic burn-in and scabs etc.
- Lesser Pattern Wear: As most of the patterns are made from cast iron, very little or no-wear is observed which results in higher pattern life. This helps in producing large number of castings without much difficulty.
- High Cost of The Process: Phenolic resins used for the shell process is very costly. The percentage of resin usage is also very high as compared to other processes. Hence this process requires very tight control of the shell thickness else the competitiveness of the process will be sacrificed.
- High Tolling Cost: The shell process is thermoset in nature thereby requiring higher curing temperatures. The patterns used are of cast iron with smooth surfaces which have very low expansion coefficient.
- Cycle Time: Comparatively cycle time required for shell process in more than either cold box or Co2 processes.
- Limited Casting Weight: The shell molding process in best suited for use in small, intricate and light weight castings. The process can be effectively used for casting weights up to 80 kg.
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