What is Iron Casting? Manufacturing Process of Cast Iron Castings

Posted by OTTOMMO CASTING on November 10th, 2020

Iron casting is the process of liquefying pig iron in a furnace and pouring the molten iron into a mold until it hardens. The iron thus obtained is used in various industrial applications. The cast iron is brittle, has a great strength-to-weight ratio, weak in tension, and a good compression strength. The composition of cast iron and the method of manufacture play a major role in determining the characteristics of this alloy.

Pig iron is the type of iron used for casting iron. The casting method is used to make parts and products that cannot be obtained by other manufacturing processes due to their unique design and size. Traditional casting methods comprise lost wax casting, plaster mold casting, and sand casting. The modern casting process is subdivided into two main categories: expendable and non-expendable casting. Expendable casting uses temporary, non-reusable molds. However, in the case of non-expendable casting, molds need not be changed every time.

Basic process involved

Over 70% of all iron castings is produced using sand casting. Cast iron manufactured through this process is produced in foundries.

1.Obtaining casting geometry and pattern making

Firstly, the required design is prepared by the engineers in consultation with the customers using CAD or other computer-aided designs. Based on the blueprint, the pattern is prepared. The pattern is a physical model of the casting used to make the mold. It is built using sand, metal, or plastic. When the pattern is withdrawn, the imprint provides the mold cavity.

This cavity is used to fill the molten iron casting. If the casting needs to be hollow, as in the case of certain parts like pipe fittings, extra patterns, such as cores, are used to form these cavities. It is important to remember the pattern must be slightly larger than the finished product, a difference known as contraction allowance.

2.Coremaking

Cores are patterns that are made of sand and placed into a mold cavity to form the interior surface of the casting. Hence, the casting obtained is the void space between the core and mold cavity surface. The cores are generally inserted into the casting box after removal of the pattern. Whenever possible, designs avoid the use of cores as they add up to the set-up time and hence incur a greater cost.

3.Molding

The next step in the process involves preparing the mold for receiving the molten iron. The mold along with the core with a supporting frame is placed around the pattern. The pattern is withdrawn leaving the mold cavity and core. Once the pattern is removed, the mold is closed.

4.Melting and pouring

The pig iron is liquefied in a capola furnace between 1,150 to 1,200 °C (2,100 to 2,190 °F), which is about 300 °C (572 °F) lower than the melting point of pure iron. When the iron melts at the desired temperature, it is transferred and poured into designed molds. The molten iron is left to harden.

5.Cleaning

When the cast iron solidifies, sand, scale, and excess metal are removed. The casting is separated from the mold and transported to the cleaning section. Burned-on sand and scale are removed to improve the surface appearance of the casting. Excess metal in the form of fins, wires, gates, etc. is also removed. Casting may undergo further processing like heat treatment as well as inspection and machining may be performed on cast iron.

When the process is complete and impurities are removed, the end product is not 100 percent iron. The chemical makeup like carbon and silicon make up the composition of cast iron in various percentages. Anything over two percent of silicon is known as gray cast iron, while a lesser amount produces white cast iron.

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