DIFFERENCES BETWEEN METAL FORMING PROCESS AND CASTING PROCESS Metal casting process Casting is a manufacturing process where a solid is melted, heated to proper temperature (sometimes treated to modify its chemical composition), and is then poured into a cavity or mould, which contains it in the proper shape during solidification. Thus, in a single step, simple or complex shapes can be made from any metal that can be melted. The resulting product can have virtually any configuration the designer desires. OBTAINING THE CASTING GEOMETRY-The traditional method of obtaining the casting geometry is by sending blueprint drawings to the foundry. This is usually done during the request for quotation process. However, more and more customers and foundries are exchanging part geometry via the exchange of computer aided design files. PATTERNMARKING-The pattern is a physical model of the casting used to make the mould. The mould is made by packing some readily formed aggregate material, such as moulding sand, around the pattern. When the pattern is withdrawn, its imprint provides the mould cavity, which is ultimately filled with metal to become the casting. If the casting is to be hollow, as in the case of pipe fittings, additional patterns, referred to as cores, are used to form these cavities. COREMAKING-Cores are forms, usually made of sand, which are placed into a mould cavity to form the interior surfaces of castings. Thus the void space between the core and mould-cavity surface is what eventually becomes the casting. MOULDING-Moulding consists of all operations necessary to prepare a mould for receiving molten metal. Moulding usually involves placing a Differences Working Principle Metal forming process The plastic deformation of a metal in order to produce a useful shape. Metal forming is done at elevated or hot-working temperatures with processes such as forging, or it is done at coldworking room temperatures with processes such as stamping or bending.
Metal-forming processes: Processes that cause changes in the shape of solid metal articles via plastic (permanent) deformations. Drawing: Metal forming process whereby the workpiece is a shaped longitudinal prism that undergoes a reduction and change in its cross section area and shape while being pulled through a shaped converging die. Extrusion: Metal forming process whereby the workpiece is placed in a chamber with an opening and is forced to escape through the opening, usually being pushed out by a mandrel. Forging: Metal forming process whereby the workpiece is placed between an anvil and a hammer and subjected to compressive force between them. Rolling: Metal forming process whereby the workpiece is a longitudinal prism, which is placed between two opposing circular rolls that rotate in opposite directions, drag the workpiece along, and force it to reduce in cross section. Metal forming performed after is normally the primary
moulding aggregate around a pattern held with a supporting frame, withdrawing the pattern to leave the mould cavity, setting the cores in the mould cavity and finishing and closing the mould. MELTING AND POURING-The preparation of molten metal for casting is referred to simply as melting. Melting is usually done in a specifically designated area of the foundry, and the molten metal is transferred to the pouring area where the moulds are filled. CLEANING-Cleaning refers to all operations necessary to the removal of sand, scale, and excess metal from the casting. The casting is separated from the mould and transported to the cleaning department. Burned-on sand and scale are removed to improve the surface appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and gates, is removed. Castings may be upgraded by welding or other procedures. Inspection of the casting for defects and general quality is performed. OTHER PROCESS-Before shipment, further processing such as heattreatment, surface treatment, additional inspection, or machining may be performed as required by the customer's specifications. -the most intricate of shapes, both external and internal, may be cast. As a result, many other operations, such as machining, forging and welding can be minimized or eliminated. -due to their physical properties, some metals can only be cast to shape (since they cannot be hotworked into bars, rods, plates, or other shapes) from ingot form as a preliminary to other processing. -construction may be simplified. Objects may be cast in a single piece which would otherwise require assembly of several pieces if made by other methods. -casting is a process highly adapted to the requirements of mass
processes of extraction, casting, and powder compaction and before the finishing processes of metal cutting, grinding, polishing, painting, and assembly. With few exceptions, the bulk of the products of the metal fabrication industry are shaped by forming or a combination of forming and other processes like metal cutting or joining. Forming operations are classified as those processes where the desired shape is achieved by imparting plastic deformation to the workpiece in the solid state. Classification by (1) product, (2) material, (3) forming temperature, and (4) Nature of deformation (sheet metal versus bulk deformation) can also be helpful. However, the boundaries between categories are not perfectly defined.
-no or very small loss of material -little or no scrap -increase in ductility (hot forming of cast ingots) -increase in strength and hardness -high production rate, generate final shape in short time -better mechanical and metallurgical properties (strength, toughness, grain size)
production. Large numbers of a given casting may be produced very rapidly. -large and heavy objects may be cast when they would be difficult or uneconomic to produces otherwise. -limitation on mechanical properties -porosity -surface finish -dimensional accuracy -safety hazards to humans -environmental concerns -more uniform properties from a directional standpoint -strength and lightness in certain light metal alloys, which can be produced only as castings. -good bearing qualities are obtained in casting metals
-equipment expensive because of the large forces involved -suited for a large number of parts only -Large capital expenditure because of heavy presses and die -near net-shaping forming
METAL CASTING (EXPANDABLE AND NON EXPANDABLE MOULD) Expandable mould Expendable mould casting or one use mould is a generic classification that includes sand, plastic, shell, plaster, and investment (lost-wax technique) mouldings. All of these involve the use of temporary and non-reusable moulds, and need gravity to help force molten fluid into casting cavities. In this process the mould is used only once. Non-Expandable mould Non-expendable mould casting differs from expendable processes in that the mould need not be reformed after each production cycle. This technique includes at least four different methods: permanent, die, centrifugal, and continuous casting. This form of casting also results in improved repeatability in parts produced and delivers Near Net Shape results. TYPES OF CASTING PROCESS FOR EXPANDABLE AND NON EXPANDABLE MOULD CASTING Expandable mould Sand Casting A sand casting or a sand moulded casting is a cast part produced by forming a mould from a sand mixture and then pouring molten liquid metal into the cavity in the mould. The mould is then cooled until the metal has solidified. In the last stage the casting is separated from the mould. There are six steps in this process: 1. Place a pattern in sand to create a mould. 2. Incorporate a gating system. 3. Remove the pattern. 4. Fill the mould cavity with molten metal. 5. Allow the metal to cool. 6. Break away the sand mould and remove the casting
Figure: Sand casting overview Equipment Mould In sand casting, the primary piece of equipment is the mould, which contains several components. The mould is divided into two halves - the cope (upper half) and the drag (bottom half), which meet along a parting line. Both mould halves are contained inside a box, called a flask, which itself is divided along this parting line. The mould cavity is formed by packing sand around the pattern in each half of the flask. The sand can be packed by hand, but machines that use pressure or impact ensure even packing of the sand and require far less time, thus increasing the production rate. After the sand has been packed and the pattern is removed, a cavity will remain that forms the external shape of the casting. Some internal surfaces of the casting may be formed by cores. Cores are additional pieces that form the internal holes and passages of the casting. Cores are typically made out of sand so that they can be shaken out of the casting, rather than require the necessary geometry to slide out. As a result, sand cores allow for the fabrication of many complex internal features. Each core is positioned in the mould before the molten metal is poured. In order to keep each core in place, the pattern has recesses called core prints where the core can be anchored in place. However, the core may still shift due to buoyancy in the molten metal. Further support is provided to the cores by chaplets. These are small metal pieces that are fastened between the core and the cavity surface. Chaplets must be made of a metal with a higher melting temperature than that of the metal being cast in order to maintain their structure. After solidification, the chaplets will have been cast inside the casting and the excess material of the chaplets that protrudes must be cut off. In addition to the external and internal features of the casting, other features must be incorporated into the mould to accommodate the flow of molten metal. The molten metal is poured into a pouring basin, which i