Casting Module PDF

Module 3 DE6310 Contents ½ Casting 3 Overview 3 Casting terminology 7 Sand casting (expendable mould) 8 Die-casting (permanent mould) 11 Shrinkage 12 Other metal casting...

Module 3 DE6310 Contents ½ Casting 3 Overview 3 Casting terminology 7 Sand casting (expendable mould) 8 Die-casting (permanent mould) 11 Shrinkage 12 Other metal casting processes 14 Design criteria 26  DE6310 Manufacturing Processes and Production Module 3: Casting Learning outcomes When you have completed this module will be able to: ½ define the meaning of foundry terms ½ describe briefly any of the foundry processes ½ recognise and describe various casting defects ½ be able to understand and choose the most appropriate casting process Overview There are many ways of shaping materials, one of the most important and versatile being casting. In fact casting is one of the oldest methods of manufacturing metals into useful products, dating back several thousand years B.C. Casting processes (several of which will be discussed in detail) offer a number of advantages over other manufacturing methods. ½ Complex and intricate shapes with internal hollow sections and cavities can be produced in a single operation. ½ Casting can produce small or very large components. ½ Most metals can be cast in or close to the final shape required with little or no further finishing work required. ½ Materials that are difficult to process with other methods can in most cases be cast. There are two commonly used methods of casting metals, sand casting and die-casting. Fundamentally, metal casting involves the solidification of a molten metal poured into a prepared mould or cavity and allowed to cool down and solidify, taking the shape of the mould. 05/23 Copyright © Te Pukenga 3  DE6310 Manufacturing Processes and Production continued... 05/23 Copyright © Te Pukenga 4  DE6310 Manufacturing Processes and Production De Garmo: 342–343 Copyright © John Wiley & Sons Inc. 05/23 Copyright © Te Pukenga 5  DE6310 Manufacturing Processes and Production Casting terminology continued... 05/23 Copyright © Te Pukenga 6  DE6310 Manufacturing Processes and Production De Garmo: 343–344 Copyright © John Wiley & Sons Inc. Sand casting (expendable mould) The casting process first starts with the manufacture of a pattern which closely resembles the final shape of the part to be cast, but is slightly larger to allow for shrinkage and/ or finishing allowances. The mould is made in two halves, so that the pattern can be removed from the mould after the sand/clay mould material has been packed around it. A core is a shape that is included in the mould to produce internal features such as holes and passageways. The parting line or separating surface is the interface between the two halves of the mould pattern, the top half of the mould being referred to as the cope and the bottom half the drag. Molten metal is then poured into the prepared mould through a pouring cup or feeder. Usually a gating system is included consisting of a series of passages which guide the molten metal to all sections of the mould cavity. The box (Fig. 11.3 from Kalpakjian) shows a typical sand mould. 05/23 Copyright © Te Pukenga 7  DE6310 Manufacturing Processes and Production Kalpakjian: Fig. 11.3, 292 Copyright © Addison-Wesley Publishing Co. Inc. Risers are included in the mould, which feed additional metal to the casting as it shrinks and cools down. Figure 11.3 (Kalpakjian) shows a blind riser and an open riser. One of the most important features of a sand mould is the draft or taper on the pattern or casting (see the following figure, Fig. 11.5 from Kalpakjian), which permits it to be withdrawn from the mould. Once the metal has solidified the cast is broken to retrieve the cast component. Kalpakjian: Fig. 11.5, 294 Copyright © Addison-Wesley Publishing Co. Inc. Typical sand cast products are engine blocks, machine bases, wheels, pump housings, railway wagon drawbars and bogie frames. 05/23 Copyright © Te Pukenga 8  DE6310 Manufacturing Processes and Production DeGarmo: 365 Copyright © John Wiley & Sons Inc. 05/23 Copyright © Te Pukenga 9  DE6310 Manufacturing Processes and Production Die-casting (permanent mould) Unlike sand casting, where the mould is used only once, die-casting uses the same mould for a large number of castings. Because of the repeated use, moulds are made of metal to withstand the wear and temperature changes associated with repetitive use. Therefore, only metals with a relatively low melting point such as light alloys and some steels are cast using this method. There are two types of die-casting, gravity and pressure die-casting. Gravity die-casting Gravity die-casting, like sand casting, has the molten metal poured into the mould, and due to the relatively high thermal conductivity of the mould, the metal must flow quickly to all areas of the mould before solidification starts to occur. This method of casting is used mainly for small, simple shapes in low- melting-point alloys such as those of copper, aluminium, zinc and magnesium. Typical gravity die-cast components are pistons, connecting rods and cylinder heads. Bolton Fig. 6.3, 105 Copyright © Butterworth-Heinemann Ltd Pressure die-casting Pressure die-casting is used when complex and intricate shapes requiring dimensional accuracy and good surface finish are needed. With pressure die-casting the molten metal is injected, under pressure, into a water-cooled mould. Like gravity die-casting, this method of casting is limited to relatively low- melting-point alloys such as those of aluminium and zinc. Bolton: Fig. 6.4, 105 Copyright © Butterworth-Heinemann Ltd. 05/23 Copyright © Te Pukenga 10  DE6310 Manufacturing Processes and Production Typical pressure die-cast components are toys, pump components, engine parts and domestic appliance parts. Advantages: Finish and surface texture is good, hence, machining and finishing costs are largely eliminated; complex and intricate shapes can be produced with high dimensional accuracy. Disadvantages: High cost of mould makes it unsuitable and uneconomic for small quantities. In general, the pressure die-casting process can be extremely economical, capable of producing both complex and intricate shapes with internal cavities or hollow sections in a single piece. Castings can range in size from as small as several mm and weighing just a few grams up to 10kg. Size of castings is limited by the consequent size of tooling and machinery required for the process. Shrinkage Due to their thermal expansion characteristics, metals shrink during solidification, causing dimensional changes, porosity, cavities and in some cases cracking. It is therefore necessary to make allowances for this contraction when designing the mould. The allowances for casting shrinkage during solidification, also called pattern makers shrinkage allowance, generally range from 8 mm/m to 20 mm/m, depending on the size of the casting. Kalpakjian: Table 12.1, 338 Copyright © Addison-Wesley Publishing Co. Inc. 05/23 Copyright © Te Pukenga 11  DE6310 Manufacturing Processes and Production In addition to shrinkage, several defects can develop in castings. Figure 10.12 (Kalpakjian) shows typical defects that can occur in castings, generally due to poor design and processing techniques. Kalpakjian: Fig. 10.12, 280 Copyright © Addison-Wesley Publishing Co Inc. 05/23 Copyright © Te Pukenga 12  DE6310 Manufacturing Processes and Production Other metal casting processes In addition to the more common casting methods outlined in the introduction above, we now need to look at other casting processes, their applications, advantages and limitations. The following diagram outlines the various casting processes. Kalpakjian: Fig. 11.2, 263 Copyright © Addison-Wesley Publishing Co. Inc. Most metals can be cast into the near-finished shape required, with little or no further finishing necessary. Complex and intricate parts are produced in a single operation, making it one of the most versatile of the manufacturing processes available. Kalpakjian: Table 11.1, 288 Copyright © Addison-Wesley Publishing Co. Inc. 05/23 Copyright © Te Pukenga 13  DE6310 Manufacturing Processes and Production Shell mould casting A shell mould casting has a number of significant advantages over sand casting. In addition to a superior surface finish, dimensional accuracy is good and manufacturing costs are low. The process consists of coating a preheated (325–600° F) ferrous metal or aluminium mould with a separating agent such as silicone. The coated pattern is then clamped to a box containing a mixture of fine sand and a thermosetting resin binder. The box is rotated, depositing a thin layer of the sand mixture, around 7 mm thick, onto the pattern. Alternatively the sand mixture can be blown onto the pattern. The pattern is then placed in an oven to complete the curing of the resin binding agent. Two half shells are bonded or clamped together to form the mould. continued... 05/23 Copyright © Te Pukenga 14  DE6310 Manufacturing Processes and Production DeGarmo: 380–381 Copyright © John Wiley & Sons Inc. 05/23 Copyright © Te Pukenga 15  DE6310 Manufacturing Processes and Production Composite moulds Composite moulds utilise the characteristics of two or more different mould materials and are generally used in casting complex and intricate shapes in aluminium alloys, such as impellers for large turbines. Moulding materials include plaster, metal, graphite and sand- plastic mixtures. Composite moulds can increase the strength and dimensional accuracy of the mould and improve surface finish. Kalpakjian: Fig. 11.13, 301 Copyright © Addison-Wesley Publishing Co. Inc. Expendable pattern casting A polystyrene pattern is used in the expendable pattern casting process, also known as full mould or lost foam casting. The procedure involves placing polystyrene beads into a preheated die. The beads expand, taking the shape of the die cavity. After the die has been allowed to cool down, it is opened, and the polystyrene pattern is removed. The pattern is then coated with a water-based refractory, allowed to dry and positioned in a flask. A fine unbonded sand is packed around the polystyrene pattern, taking care not to damage it, and the sand is usually vibrated into place. Without removing the polystyrene pattern, molten metal is poured into the mould, which melts and vaporises the pattern that fills the mould cavity, replacing the space previously occupied by the polystyrene pattern. This process has a number of advantages over other casting methods. It can be used for any size of casting in both ferrous and nonferrous material. Polystyrene is relatively cheap and is easily formed into complex and intricate patterns. Cores, parting lines and in most cases risers are not required. Accuracy and surface finish is excellent, with little or no further finishing required. It is very economical for lengthy production runs. On the down side, the die can be quite expensive. 05/23 Copyright © Te Pukenga 16  DE6310 Manufacturing Processes and Production Unit DE5303 De Garmo: Fig. 14-32, 395 Copyright © John Wiley & Sons Inc. Typical expandable pattern cast components are machine bases and a variety of automotive components such as cylinder heads, crankshafts, and engine blocks. Plaster mould casting The plaster mould process uses plaster of paris with various additives to improve its strength and permeability. The resulting compound is mixed with water to form a slurry, which is poured over a prepared pattern. After the plaster sets, the pattern, normally made of metal, is removed and the mould is force-dried to remove all excess moisture. The two halves of the mould are assembled to form the mould cavity and the metal is poured into the mould. Only nonferrous alloys with a low melting point, such as copper, zinc, aluminium and magnesium, can be cast with this process. Dimensional accuracy is excellent, as is the surface finish. 05/23 Copyright © Te Pukenga 17  DE6310 Manufacturing Processes and Production De Garmo: 388–389 Copyright © John Wiley & Sons Inc. Typical components are locks, gears, tooling and valves. 05/23 Copyright © Te Pukenga 18  DE6310 Manufacturing Processes and Production Ceramic mould casting Similar to the plaster mould process, ceramic mould casting uses a refractory material, such as a mixture of zircon and aluminium oxide mixed with a bonding agent for the mould. The process is also referred to as cope and drag investment casting. As with the plaster mould process, the slurry is poured over the pattern. When the moulds have set they are removed from the wooden or metal pattern. To remove all moisture the moulds are oven dried, then assembled in readiness for pouring. Because the refractory mould has high temperature resistance, stainless steels, tool steels and other high- temperature alloys can be cast with this method. The process is capable of producing intricate shapes with excellent dimensional accuracy and surface finish. Typical ceramic mould castings are impeller blades, cutters, moulds for the manufacture of plastic or rubber components and dies. Kalpakjian: Fig. 11.14, 305 Copyright © Addison-Wesley Publishing Co. Inc. 05/23 Copyright © Te Pukenga 19  DE6310 Manufacturing Processes and Production continued... 05/23 Copyright © Te Pukenga 20  DE6310 Manufacturing Processes and Production DeGarmo: 389–390. Copyright © John Wiley & Sons Inc. Investment casting Investment casting is used for the manufacture of high-precision, complex and intricately shaped castings in high-melting-point metals. The pattern is made of wax, hence the process is sometimes referred to as the lost wax process, although plastic patterns can also be used. The pattern is produced by injecting molten wax into a metal die. It is then dipped in a slurry of refractory material, dried, and allowed to harden. This thin film of smooth refractory material will ensure that a fine smooth surface finish and accurate detail are achieved. After the initial coating has dried and hardened the pattern is coated repeatedly with a refractory/ silica sand mixture to increase its thickness. The mould is then placed upside down in an oven, where the wax melts and runs out. Metal is then poured into the mould and allowed to solidify. The mould is then broken to remove the casting. This process is used for producing relatively small, complex and intricate shapes in a variety of ferrous and nonferrous metals and alloys. It is normal practice to join several patterns together to form a cluster or tree, thereby increasing the rate of production. Typical investment casting components are turbine blades, gears, cams and valves. 05/23 Copyright © Te Pukenga 21  DE6310 Manufacturing Processes and Production Kalpakjian: Fig. 11.16, 306 Copyright © Addison-Wesley Publishing Co. Inc. Centrifugal casting Centrifugal casting, as its name suggests, uses inertial forces caused by rotation to deposit molten metal into the mould cavity. This process produces hollow cylindrical parts such as pipes, bushings, cylinder linings, pressure vessels, brake drums and bearing rings. The mould is made of iron, steel, graphite or dry sand and can be coated with a refractory lining to increase the mould life. As shown in Fig. 11.28 (Kalpakjian), molten metal is poured into the rotating mould. The outer mould surface is normally circular but can be shaped to any profile, including square. The inner surface will always be cylindrical of course. Cylindrical parts ranging from 12 mm up to 3 m in diameter and up to 15 m long can be cast using this process, with wall thicknesses of 6 mm to 120 mm. Quality castings with good dimensional accuracy can be produced with this process. 05/23 Copyright © Te Pukenga 22  DE6310 Manufacturing Processes and Production Kalpakjian: Fig. 11.28, 318 Copyright © Addison-Wesley Publishing Co. Inc. As with most other manufacturing processes, computer aided design and manufacturing techniques are being introduced to foundry operations, and many of the basic foundry operations such as grinding and shot blasting can utilise robotic automation. Rotary casting (rotational moulding) Rotational casting is the process used primarily for the manufacture of thin- walled plastic containers such as trash cans, buckets, and footballs. Complex hollow shapes with wall thickness as slight as 0.4 mm can be produced with this process. Semicentrifugal casting Semicentrifugal casting is somewhat similar to centrifugal casting, the molten metal being forced into the mould cavity by centrifugal force. It is used to cast parts with rotational symmetry such as wheels with spokes. 05/23 Copyright © Te Pukenga 23  DE6310 Manufacturing Processes and Production De Garmo: Fig. 15-11, 410 Copyright © John Wiley & Sons Inc. Centrifuging In centrifuging, the molten metal is poured from the centre and forced into the mould by centrifugal forces. As illustrated in Fig. 11.29b (Kalpakjian), moulds are placed a specific distance from the axis of rotation. The distance from the centre to the mould is critical, as this determines the properties of the casting. Kalpakjian: Fig. 11.29, 319 Copyright © Addison-Wesley Publishing Co. Inc. 05/23 Copyright © Te Pukenga 24  DE6310 Manufacturing Processes and Production Design criteria continued... 05/23 Copyright © Te Pukenga 25  DE6310 Manufacturing Processes and Production continued... 05/23 Copyright © Te Pukenga 26  DE6310 Manufacturing Processes and Production Waters: 33–35 Copyright © T F Waters 05/23 Copyright © Te Pukenga 27  DE6310 Manufacturing Processes and Production Key points continued... 05/23 Copyright © Te Pukenga 28  DE6310 Manufacturing Processes and Production continued... 05/23 Copyright © Te Pukenga 29  DE6310 Manufacturing Processes and Production continued... 05/23 Copyright © Te Pukenga 30  DE6310 Manufacturing Processes and Production continued... 05/23 Copyright © Te Pukenga 31  DE6310 Manufacturing Processes and Production Waters: 33–39 Copyright © T F Waters 05/23 Copyright © Te Pukenga 32  DE6310 Manufacturing Processes and Production Activity 1 Waters: 40 Copyright © T F Waters Compare your answers with the feedback provided. 05/23 Copyright © Te Pukenga 33  DE6310 Manufacturing Processes and Production Feedback on Activity Activity 1 continued … 05/23 Copyright © Te Pukenga 34  DE6310 Manufacturing Processes and Production Waters: 40-41 Copyright © T F Waters 05/23 Copyright © Te Pukenga 35