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Pattern and Mold Making – Foundry

A foundry is a place that works on metal (or other types) casting in order to manufacture parts of different shapes, designs, and alloys. Metal casting involves pouring molten metal into a mould containing a cavity of the desired shape to produce a metal product. The casting is then removed from the mould and may be processed further. While metals are the primary choice for castings, plastics and ceramics can be moulded too. The foundry and indeed the casting process is very fundamental to the existence of most industries today. The castings produced by foundries are used in varied industries like the automobile, infrastructure, plumbing, and others. Castings find numerous uses, and foundries in India and other nations are integral to the development and progress of the nation. And while different types of castings are the main product of a foundry, the casting process in any foundry depends upon patterns and moulds ('molds' in American English) in order to produce the castings. We will use both ‘mold’ and ‘mould’ in this article.

The shape of castings is determined by the mold cavity, but something needs to shape the mold first, and that is where the pattern comes in. Patterns are a model for the object to be cast. A pattern is used to produce the cavity in the molding sand for pouring the molten-metal. During molding, sand is rammed against the patterns and in the core boxes to produce the cavity in which the molten metal will be poured. A pattern makes an impression on the mould, and once the molten metal is poured into the mold, the metal solidifies in the shape of the original pattern. Since metals are costly and melting them is an energy and time consuming job, it is very important to get the casting right the very first time. And pattern making is very important in the metal casting process since it is the replica of the project to be cast. Maintaining precise dimensions and designs is the key to successful pattern making. The casting patterns are used to press into the sand mold to create the hollow cavity that allowing molten metal to be poured into and forming the casting. Patterns are commonly made from wood or a metal like aluminum. Nowadays, even plastic is used for pattern making. Composite patterns are used as well, if the situation requires it. The type of pattern selected for a specific casting is also determined by the quantity of castings, the part complexity, and the type of moulding process used. Pattern design is influenced by the molding method as well. The molding system specifies the mounting arrangement of the pattern (as well as often limiting the material choice.

There are various types of patterns used in a foundry. A few of the most popular types of patterns include single piece, split, match plate pattern, cope and drag, multi-piece pattern and sweep pattern. Each pattern has unique features and specific purpose in manufacturing. A good pattern in a foundry must be lightweight, have dimensional stability, remain unaffected by ramming forces, should be easily removable from the mould, and should have a long life. A properly manufactured pattern reduces the overall cost of casting.

All modern foundries in India and elsewhere make use of computer-aided design (CAD) to design patterns. These systems can also be integrated with automated cutting tools that are controlled with computer-aided manufacturing (CAM) tools.

All casting processes use a mold, either permanent or temporary, which is a ‘negative’ of the desired shape. Once the metal is poured and has solidified it forms the ‘positive’ shape of the desired product.

A sand mold is formed by packing sand into each half of the mold (learn more about types of sand used in sand casting). The sand is packed around the pattern, which is a replica of the external shape of the casting. When the pattern is removed, the cavity that will form the casting remains. The top and bottom halves of the mold are known as the “cope” and “drag”, respectively.

The mould is formed in a mould box (flask), which is typically constructed in two halves to assist in removing the pattern. Sand moulds are temporary so a new mould must be formed for each individual casting. The bottom half of the mould (the drag) is formed on a moulding board. Cores require greater strength to hold their form during pouring. Dimensional precision also needs to be greater because interior surfaces are more difficult to machine, making errors costly to fix. Once the core is inserted, the top half of the mould (the cope) is placed on top. The interface between the two mould halves is called a parting line. Weights may be placed on the cope to help secure the two halves together, particularly for metals that expand during cooling. Mould designs include a gating system which is designed to carry molten metal smoothly to all parts of the mould. The gating system typically includes a sprue, gates, runners and risers. The sprue is where the metal is poured. Gates allow the metal to enter the running system. Runners carry the molten metal towards the casting cavity. Risers may have several functions including vents to allow gases to be released, reservoirs prior to the casting cavity to aid progressive solidification, and waste cavities to allow metal to rise from the casting cavity to ensure it is filled and to remove the first poured metal from the casting cavity, thus avoiding solidification problems.

Types of Moulds
The casting process is divided into two broad types of casting:
  • expendable-mold casting processes, and
  • nonexpendable-mold casting processes

Expendable-mold casting is a general classification that includes sand, plastic, shell, and investment (lost wax technique) moldings. All of these involve the use of temporary and non-reusable molds, and they all need gravity to help force the molten fluid into the casting cavities. In this process, the mold in which the molten metal solidifies must be destroyed in order to remove the casting. It is used only once. Nonexpendable mold casting differs from expendable processes in that the mold need not be destroyed after each production cycle. This technique includes at least four different methods: permanent, die, centrifugal, and continuous casting.

India is at the forefront of producing castings, machined components and alloys for castings. The pattern and mold making industry in India uses advanced technologies to maintain quality of castings. This is one of the reason US and European industries prefer castings from India.