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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.
Shell mold casting allows the use of both ferrous and non-ferrous metals, most commonly using cast iron, carbon steel, alloy steel, stainless steel, aluminum alloys, and copper alloys. Typical parts are small-to-medium in size and require high accuracy, such as gear housings, cylinder heads, connecting rods, and lever arms.
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.
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.
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.
The mold is securely clamped together while the molten metal is poured from a ladle into the gating system and fills the mold cavity.
After the mold has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting.
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.
Because shell molding uses phenolic resin as the sand binder, so the smooth and hard surfaces of sand molds make the castings have good surface smoothness. The following photo could be taken as sample for the surface quality.
Moreover, this process have less sand residue during production, so could reduce some iron casting defects, such as sand inclusion, sand holes and air holes.
This molding material is a type of hard mold, so there will be less swell of sand molds, so the dimensional tolerance will be smaller. This advantage will be very useful for producing high accuracy rough castings, and reduce machining cost.
Less than 5mm wall thickness will be taken as very thin as for sand castings. Only shell molding process could produce these cast products.
In addition, hot shell and core molds are made by molding machines, so it could produce the castings with complex structures, especially complex inside structures.
Since the main works have been completed by the molding machines, so this process could be operated by women workers, and there is no special skill required. This is very different with green sand casting process.
The phenolic resin sand is more expensive than green sand and furan resin sand, and it could not be recyclable. Therefore, shell molding castings will have higher prices.
This process needs to use metal patterns (iron patterns), which will be more costly. So, it is not suitable for producing small quantity castings and small orders.
The shells and cores of castings are produced by shell molding machines. These machines have dimensional limitation. So, most of shell molding castings will be less than 400mm length, and less than 20kg weight. Too long or too heavy can not be produced by this process.
This process offers better dimensional accuracy, higher productivity and lower labor requirements than sand casting. It is typically used for small to medium sized parts that require high precision.
Shell casting is used for ferrous and non-ferrous metals. The most commonly used are cast iron, carbon steel, alloy steel, stainless steel, aluminum alloys and copper alloys. Typical parts cast using this process are small to medium sized and require high precision, such as gearboxes, cylinder heads, connecting rods and lever arms.
The inner surface of the shell mold is very smooth and rigid. This allows the liquid metal to flow easily through the mold cavity during casting, resulting in castings with very good surface finishes. Shell casting enables the manufacture of complex parts with thinner cross-sections and smaller protrusions than wet sand casting.
Manufacturing with the shell mold process also imparts high dimensional accuracy. tolerances of 0.010 inch (0.25 mm) are possible. When cast by this process, further machining is usually not required.
Shell sand molds are less permeable than wet sand molds and the binder may generate significant amounts of gas when in contact with the molten metal being poured for casting. For these reasons, shell molds should be well ventilated.
The cost of shell casting is increased by the cost of the thermosetting resin binder, but reduced by the fact that only a small amount of sand is used compared to other sand casting processes.
The shell casting process can be easily automated.
The special metal models required for shell casting are expensive, making it a less desirable process for short runs. However, for high volume production, manufacturing by shell casting may be economical.
For many years, Zen Young Foundry has been providing customized metal casting services for various industries and project types.
Mining & Engineering Machinery Casting Parts
If you are looking for a solution that fits your custom metal Sand casting project, Zen Young can help you produce perfect metal parts through a reliable casting process. We always welcome special material and difficult parts and would like to help our comstomers utmost.
Zen Young have capabilities to produce Cast iron，Stainless steel，Copper and Aluminum parts from start to finish. Whether you have a design ready for casting, or you need a custom casting design, we can help
We work with our customers to design patterns and molding methods for each custom casting. Original castings or ready-to-use castings can be provided. Secondary finishing operations, including powder coating, are additional options. Contact us with any questions about the casting process and also for more information on how our casting services can meet your project requirements.
