Shell Molding

WE PRODUCE SHELLMOLDED GRADED ci AND sg iRON cASTINGS AND

PROTOTYPE CASTINGS WITH STATE OF THE ART 3D PRINTING TECHNOLOGIES

Introduction

Shell molding

Shell molding, also known as shell-mold casting, is an expendable mold casting process that uses resin covered sand to form the mold. As compared to sand casting, this process has better dimensional accuracy, a higher productivity rate, and lower labour requirements. It is used for small to medium parts that require high precision.Shell moulding was developed as a manufacturing process during the mid-20th century in Germany. It was invented by German engineer Johannes Croning.
Shell mould casting is a metal casting process similar to sand casting, in that molten metal is poured into an expendable mould. However, in shell mould casting, the mould 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 moulds. A reusable pattern allows for higher production rates, while the disposable moulds enable complex geometries to be cast.
Shell mould casting requires the use of a metal pattern, oven, sand-resin mixture, dump box, and molten metal.Shell mould casting allows the use of both ferrous and non-ferrous metals, most commonly using cast iron, carbon steel, alloy steel, stainless steel, aluminium 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.

Shell Molding 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 aluminium 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) & coated with a lubricant to facilitate removal. Next, the heated pattern is clamped to a dump box, which contains a mixture of sand & a resin binder.
Mould assembly - The two shell halves are joined together & securely clamped to form the complete shell mould. If any cores are required, they are inserted prior to closing the mould. The shell molding is then placed into a flask & supported by a backing material.
Pouring - The mould is securely clamped together while the molten metal is poured from a ladle into the gating system & fills the mould cavity.
Cooling - After the mould has been filled, the molten metal is allowed to cool & solidify into the shape of the final casting.
Casting removal - After the molten metal has cooled, the mould can be broken & the casting removed. Trimming & cleaning processes are required to remove any excess metal from the feed system & any sand from the mould.

Shell molding offers several benefits for companies, some of which include the following:

Although it requires minimal labor, shell molding is a somewhat complicated process that requires multiple steps. First, the sand is thoroughly mixed with resin, which acts as a binding agent. Next, the sand is poured into a heated mold — with the mold typically reaching temperatures of 400 to 700 degrees Fahrenheit. A heated mold is important because the heat triggers a reaction with the resin-covered sand. When the sand comes into contact with the hot mold, a shell is formed on the inner surface of the mold.
After the shell has been formed, any excess sand is removed from the mold and either discarded or saved for other applications. The shell is then removed from the mold, which typically occurs using ejector pins. The ejector pins are built into the mold itself, allowing companies to easily remove the newly created shell without damaging it.

Shell molding is a molding process invented by German engineer Johannes Croning in the mid-1900s. Like other molding processes, it relies on a mold to create a new object often of a base material. Shell molding is unique, however, because it uses resin-covered sand as a base material. When the sand is poured into the heated mold, it fills the mold cavity while reacting with the heat. This reaction causes some of the resin-covered sand to form a shell inside the mold, which is removed using ejector pins.

In Shell molding Process Fine silica sand that is covered in a thin (3–6%) thermosetting phenolic resin & liquid catalyst is dumped, blown, or shot onto a hot pattern.
The pattern & sand are then inverted so the excess sand drops free of the pattern, leaving just the "shell". Depending on the time & temperature of the pattern the thickness of the shell is 10 to 20 mm (0.4 to 0.8 in).
The pattern & shell together are placed in an oven to finish curing the sand. The shell now has a tensile strength of 350 to 450 psi (2.4 to 3.1 MPa).
The hardened shell is then stripped from the pattern.

Shell molding can be completely automated for mass production.
The high productivity, low labor costs, good surface finishes, & precision of the process can more than pay for itself if it reduces machining costs.
There are few problems due to gases, because of the absence of moisture in the shell, and the little gas that is still present easily escapes through the thin shell.
Complex shapes and fine details can be formed with very good surface finish, high production rate, low labor cost (if automated).
Low tooling cost, Little scrap generated.
Very large parts and complex shapes can be produced.
Many material options.
Low tooling and equipment cost.
Scrap can be recycled.
Short lead time possible.

The gating system must be part of the pattern because the entire mold is formed from the pattern, which can be expensive.
The resin for the sand is expensive, although not much is required because only a shell is being formed.
High equipment cost.
Poor material strength.
High porosity possible.
Secondary machining often required.
High labor cost if done manually.