Shell molding: Process, Advantages and disadvantages

Shell molding

Shell moulding, also known as shell-mold casting, is a process of expendable mold casting using resin-covered sand to form the mold. This method has greater dimensional precision, a greater rate of productivity, and reduced employment costs relative to sand casting. It is used for tiny to medium components that require high precision. Shell mold casting is a sand-like metal casting method in which molten metal is poured into an expendable mold. The mold, however, is a thin-walled shell produced by adding a sand-resin combination around a motif in shell mold casting. The model is utilized to create various shell molds, a metal element in the shape of the required portion. A reusable model enables greater manufacturing prices, while the disposable molds allow the casting of complicated geometries. Shell mold casting needs a metal model, furnace, combination of sand-resin, dump tank, and molten metal to be used.

Shell mold casting enables both ferrous and non-ferrous metals to be used, most frequently using cast iron, coal steel, alloy steel, stainless steel, aluminum alloys and metal alloys. Typical components are small to medium in size and involve elevated precision such as equipment housings, heads of the cylinder, linking wires and arms of the lever.

The method of shell mold casting consists of the following stages: 

Model formation-A two-piece metal model is developed in the form of the required portion, typically from iron or steel. Sometimes other products are used, such as low-volume aluminum or reactive casting graphite.

Creation of molds–First, each half of the pattern is heated to 175-370 ° C (350-700 ° F) and covered with a lubricant for easy extraction. The heated model is then fastened to a dump box containing a combination of sand and a resin binder. The dump box is reversed enabling the model to be covered by this sand-resin blend. Partially the heated model cures the blend, which now forms a shell around the structure. Each quarter and adjacent shell model is healed in an oven to complete and the shell is then removed from the structure.

Mold installation-The two shell halves are linked together to create the full shell mold and safely secured. If any cores are needed, they will be inserted before the mold is closed. Subsequently, the shell mold is put in a flask and backed by a fabric.

Pouring-The mold is securely clamped while the molten metal is poured into the gating system from a ladle and fills the mold cavity. Pouring-The mold is securely clamped together while the molten metal is poured into the gating system from a ladle and fills the mold cavity.

Cooling-The molten metal may cool and solidify into the final casting form after the mold has been completed.

Casting extraction-The mold may be fractured and the casting withdrawn after the molten metal has dried. To remove any excess metal from the feed system and any sand from the mold, trimming and cleaning processes are required.

Examples of shell shaped products are gear housings, heads of the cylinder and coupling wires. It is also used to create molding centers of high precision.

Process

The shell mold method comprises of six stages.

1. Fine silica sand coated in dense (3–6 percent) phenolic resin and fluid catalyst thermosetting is poured, blown, or fired on a warm model. Usually made of cast iron, the pattern is heated to 230 to 315 ° C (450 to 600 ° F). For a few minutes, the sand is permitted to rest on the structure to allow partial healing of the sand.
2. The pattern and sand are then reversed, leaving only the "shell" free of the pattern. The shell size is 10 to 20 mm (0.4 to 0.8 in) depending on the setup time and temperature.
3. Together the model and shell are put in an oven to complete the sand healing. The shell now has a 350-450 psi (2.4-3.1 MPa) tensile resistance.
4. Then the hardened shell is removed from the structure.
5. Using a thermoset adhesive, two or more shells are then combined to form a mold by clamping or gluing. You can then use this completed mold instantly or store it almost forever.
6. The shell mold is put inside a flask for painting and encircled by shot, sand, or gravel to strengthen the shell.

A shell molding device is the computer used for this method. It prepares the model, the sand combination is applied, and the shell is baked.

Details

Setting up and manufacturing of shell mold patterns takes weeks, after which an output of 5–50 pieces / hour mold is attainable. Common materials include cast iron, aluminum and metal alloys. Aluminum and bronze products average about 13.5 kg (30 lb) as a standard threshold, but it is feasible to place objects within the range of 45–90 kg (100–200 lb). The thinest cross-section castable is 1.5 to 6 mm (0.06 to 0.24 in) depending on the metal. The minimum draft is 0.25 to 0.5 degrees.

Typical tolerances are 0.005 mm / mm or in / in due to the sand compound being intended to decrease scarcely and using a metal model. Because a thicker sand is used, the cut ground value is 0.3–4.0 micrometers (50–150 μin). The resin also helps to create a very soft texture. In particular, the method generates very coherent casts from one casting to the next.

It is possible to recycle the sand-resin blend by boiling the resin at elevated temperatures.

Advantages and disadvantages

Advantages

• For mass production, shell molding can be fully automated.
• High productivity, small labor costs, excellent surface finishes, and method accuracy can pay more than for themselves if they reduce machining expenses.
• Because of the lack of moisture in the shell, there are few issues owing to oxygen, and the small gas that is still present can quickly flee through the fragile shell. Some of the resin binder breaks out on the shell surface when the metal is washed, making it simple to shake out.
• With very nice surface finish, elevated production rate, low labor cost (if automated), complex forms and nice details can be created.
• Low tooling expense produced a small scrap.
• It is possible to produce very big components and complicated forms.
• Low tooling and equipment cost
• It is possible to recycle scrap.
• It is feasible to have short lead time.

Disadvantages

• The gating scheme must be component of the structure because the structure forms the whole mold, which can be costly.
• The resin is costly for the sand, although not much is needed as only a shell is created.
• High price of machinery.
• Poor resistance of the material.
• Possible high porosity.
• Often needed secondary machining.
• If performed manually, high labor costs.

Applications

Cylinder heads, connecting rods, Engine blocks and manifolds, machine bases