UNIT -1 METAL CASTING PROCESS

CASTING Casting is one of the process used for making components of complicated shapes in larger quantity. It is the process of producing metal parts by pouring molten metal into the mould cavity of the required shape and allowing the metal to solidify.The solidified metal piece is called as “casting”.

Block diagram of casting process

Casting Methods Sand Casting High Temperature Alloy, Complex Geometry, Rough Surface Finish Investment Casting High Temperature Alloy, Complex Geometry, Moderately Smooth Surface Finish Die Casting High Temperature Alloy, Moderate Geometry, Smooth Surface

Types of pattern Solid or single piece pattern Split pattern Loose piece pattern Match plate pattern Sweep pattern Skeleton pattern Segmental pattern Shell pattern

Solid pattern These patterns are made of single solid piece without joints, partings or loose piece. This pattern is used for making a few large size simple castings.

Split pattern One pattern which is having complex geometry cannot be removed from mould if they are made by single piece. Generally split pattern is made into two parts. i.lower half ii.upper half These two parts are arranged together in correct position by pins “dowel pins”

Loose piece pattern If a pattern is made from a single piece having projections above the parting plane,it is impossible to withdraw it from the mould. In such case the pattern is made into solid piece and loose piece.

Match plate pattern These patterns are made in two pieces. One piece is mounted on one side and the other on the other side of a plate called match plate. Gates and runners are also attached to the plate along with the pattern. After moulding when the match plate is removed a complete mould with gating is obtained by joining the cope and drag together. The complete pattern with match plate is entirely made of metal, usually aluminium for its light weight and machinability.

Sweep pattern Sweep patterns are mainly used to generate surfaces of revolution like cylinder, cone, sphere in large castings. A sweep is a section or board made of wood or metal to the required cross section that is rotated about one edge to shape mould cavities having shapes of rotational symmetry.

Skeleton pattern For larger casting of simple shape, if a solid pattern is made of wood,it will require large amount of wood.Hence, it is very expensive.In such cases, a skeleton pattern is used instead of a full pattern.This shape is made of wooden strips with a lot of openings and fitted to end supports.Moulds for water pipes,turbine castings,pipe bends are made by skeleton pattern.

Segmental pattern This pattern is used for forming circular moulds. When making the mould using this pattern,a vertical spindleis fixed of the center of the drag box. The pattern is attached to the spindle.after ramming,the pattern is moved to the next segment. Thus, the full mould is completed.

Pattern materials Wood – teak wood, mahogany, white pine,etc.. Metal – cast iron,brass, aluminium, white metal etc.. Plaster Plastics Wax

Wood Teak wood, mahogany, white pine, rose wood… Laminated wooden sheets are used for accuracy,surface finish and long life. Metal spray coating upto 0.25mm thick may be given on wooden pattern.(zinc,aluminium) to avoid moisture absorption.

Advantages : 1.light in weight, cheap. 2.easy to work, repair. 3.easily smoothened by varnish and paints. Limitations : 1.It absorb water from sand. 2.It cannot be used for mass production

Metal Cast iron, Brass, Aluminium… Advantages : 1. long life, smooth surface. 2. mass production is possible. 3. does’nt absorb any water & free from corrosion. Disadvantages : 1. costlier and heavier 2. difficult to make desired shape.

Plaster Plaster of paris or gypsum cement… Plaster can be easily made into difficult shape. Compressive strength is high. It is not affected by moisture.

Plastics A plastic has many advatages over other materials. It is light in wight but strong. Not affected by moisture and smooth glassy surface. The following plastics are widely used for pattern making with the composition based on epoxy,phenol formaldehyde.. i. Poly acrylates ii.Poly ethylene iii.poly vinyle chloride….

wax Wax pattern primarily used in investment casting. the most commonly used waxes are i.paraffin wax ii.shelloc wax iii.microcrystalline wax It will not absorb moistures. Cost is very less,but it is used only for small patterns.

Pattern allowance Patterns are not made into exact size of the castings to be produced.Patterns are made slightly larger than required castings.This extra size given on pattern is called pattern allowance.If the allowances are not given on the pattern, the casting will become smaller than the required size

Types of allowance Shrinkage allowance Machining or finish allowance Draft or taper allowance Distortion or camber allowance Rapping or shake allowance

Shrinkage allowance The metal shrinks on solidification and contracts further on cooling to room temperature.To compensate this, the pattern is made larger than the required shape. C.I 10.4mm ALUMINIUM 17mm BRASS 15.3mm STEEL 20.8mm

Machining allowance The surface finish obtained in sand castings is generally poor (dimensionally inaccurate), and hence in many cases, the cast product is subjected to machining processes like turning or grinding in order to improve the surface finish. During machining processes, some metal is removed from the piece. To compensate for this, a machining allowance should be given in the casting.

MetalDimension (inch)Allowance (inch) Cast iron Up to to to Cast steel Up to 6 6 to to Non ferrous Up to 8 8 to to

Draft allowance When the pattern is to be removed from the sand mold, there is a possibility that any leading edges may break off, or get damaged in the process. To avoid this, a taper is provided on the pattern, so as to facilitate easy removal of the pattern from the mold, and hence reduce damage to edges.

The taper angle provided is called the Draft angle. The value of the draft angle depends upon the complexity of the pattern, the type of molding (hand molding or machine molding), height of the surface, etc.

Distortion allowance During cooling of the mold, stresses developed in the solid metal may induce distortions in the cast. This is more evident when the mold is thinner in width as compared to its length. This can be eliminated by initially distorting the pattern in the opposite direction.

Types of moulding sand Green ‑ sand molds - mixture of sand, clay, and water; “Green" means mold contains moisture at time of pouring Dry ‑ sand mold - organic binders rather than clay And mold is baked to improve strength Skin ‑ dried mold - drying mold cavity surface of a green ‑ sand mold to a depth of 10 to 25 mm, using torches or heating lamps

Properties of moulding sand Porosity or permeability Plasticity or flowability Adhesiveness Strength or cohesiveness Refractoriness Collapsibility

Core making A core is a body made of sand which is used to make a cavity or hole in a casting. The shape of the core is similar to the required cavity in the casting to be made. Core making materials: Core sand Binders Additives

Types of cores According to the state of core i. Green sand core ii. Dry sand core According to the position of the core in the mould i. Horizontal core ii. Vertical core iii. Balanced core iv. Hanging core v. Drop core

Furnaces for Casting Processes Furnaces most commonly used in foundries: Cupolas Crucible furnaces Electric ‑ arc furnaces Induction furnaces

Cupolas Vertical cylindrical furnace equipped with tapping spout near base Used only for cast irons Although other furnaces are also used, the largest tonnage of cast iron is melted in cupolas The "charge," consisting of iron, coke, flux, and possible alloying elements, is loaded through a charging door located less than halfway up height of cupola

Crucible Furnaces Metal is melted without direct contact with burning fuel mixture Sometimes called indirect fuel ‑ fired furnaces Container (crucible) is made of refractory material or high ‑ temperature steel alloy Used for nonferrous metals such as bronze, brass, and alloys of zinc and aluminum Three types used in foundries: (a) lift ‑ out type, (b) stationary, (c) tilting

Crucible Furnaces Figure Three types of crucible furnaces: (a) lift ‑ out crucible, (b) stationary pot, from which molten metal must be ladled, and (c) tilting- pot furnace.

Electric ‑ Arc Furnaces Charge is melted by heat generated from an electric arc High power consumption, but electric ‑ arc furnaces can be designed for high melting capacity Used primarily for melting steel

Induction Furnaces Uses alternating current passing through a coil to develop magnetic field in metal Induced current causes rapid heating and melting Electromagnetic force field also causes mixing action in liquid metal Since metal does not contact heating elements, environment can be closely controlled to produce molten metals of high quality and purity Melting steel, cast iron, and aluminum alloys are common applications in foundry work

Shell casting

Steps in shell ‑ molding (1) a match ‑ plate or cope ‑ and ‑ drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin. (2) box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell; (3) box is repositioned so that loose uncured particles drop away (4) sand shell is heated in oven for several minutes to complete curing; (5) shell mold is stripped from the pattern;

Advantages of Shell Mould Casting Good surface finish High dimensional tolerance towards automation Castings weighing upto 450 kgs can be cast by this process Thin sections (upto 0.25 mm) can be cast by this process Limitations Patterns are expensive Castings weighing more than 450 kgs cannot be made Highly complicated shapes cannot be made Applications Cast iron, Aluminium and copper alloys are cast by this process

Investment casting (lost wax casting) (a) Wax pattern (injection molding) (b) Multiple patterns assembled to wax sprue (c) Shell built  immerse into ceramic slurry  immerse into fine sand (few layers) (d) dry ceramic melt out the wax fire ceramic (burn wax) (e) Pour molten metal (gravity)  cool, solidify [Hollow casting: pouring excess metal before solidification (f) Break ceramic shell (vibration or water blasting) (g) Cut off parts (high-speed friction saw)  finishing (polish)

Centrifugal casting Schematic illustration of the centrifugal casting process. Pipes, cylinder liners, and similarly shaped parts can be cast by this process.

The Centrifugal accelerating force is high almost 70 to 80 times that of gravity which helps in pushing the molten metal to the walls of the mold Outer shape is controlled by the mold contour while the inner shape is controlled by the amount of the molten metal poured in. There are horizontal as well as vertical axis machines. Only short tubes aremanufacture in the vertical machines

Die casting Designed to hold and accurately close two mold halves and keep them closed while liquid metal is forced into cavity Two main types: 1. Hot ‑ chamber machine 2. Cold ‑ chamber machine

©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Hot-Chamber Die Casting Metal is melted in a container, and a piston injects liquid metal under high pressure into the die High production rates parts per hour not uncommon Applications limited to low melting ‑ point metals that do not chemically attack plunger and other mechanical components Casting metals: zinc, tin, lead, and magnesium

Figure : Cycle in hot ‑ chamber casting: (1) with die closed and plunger withdrawn, molten metal flows into the chamber (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification.

Cold ‑ Chamber Die Casting Machine Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure into die cavity High production but not usually as fast as hot ‑ chamber machines because of pouring step Casting metals: aluminum, brass, and magnesium alloys Advantages of hot ‑ chamber process favor its use on low melting ‑ point alloys (zinc, tin, lead)

Figure : Cycle in cold ‑ chamber casting: (1) with die closed and ram withdrawn, molten metal is poured into the chamber

Figure : Cycle in cold ‑ chamber casting: (2) ram forces metal to flow into die, maintaining pressure during cooling and solidification.

Advantages of die casting: Economical for large production quantities Good accuracy and surface finish Thin sections are possible Rapid cooling provides small grain size and good strength to casting Disadvantages: Generally limited to metals with low metal points Part geometry must allow removal from die

A casting that has solidified before completely filling mold cavity Figure : Some common defects in castings: (a) misrun General Defects: Misrun

Two portions of metal flow together but there is a lack of fusion due to premature freezing Figure : Some common defects in castings: (b) cold shut General Defects: Cold Shut

Metal splatters during pouring and solid globules form and become entrapped in casting Figure : Some common defects in castings: (c) cold shot General Defects: Cold Shot

Depression in surface or internal void caused by solidification shrinkage that restricts amount of molten metal available in last region to freeze Figure : Some common defects in castings: (d) shrinkage cavity General Defects: Shrinkage Cavity

Balloon ‑ shaped gas cavity caused by release of mold gases during pouring Figure : Common defects in sand castings: (a) sand blow Sand Casting Defects: Sand Blow

Formation of many small gas cavities at or slightly below surface of casting Figure : Common defects in sand castings: (b) pin holes Sand Casting Defects: Pin Holes

When fluidity of liquid metal is high, it may penetrate into sand mold or core, causing casting surface to consist of a mixture of sand grains and metal Figure : Common defects in sand castings: (e) penetration Sand Casting Defects: Penetration

A step in cast product at parting line caused by sidewise relative displacement of cope and drag Figure : Common defects in sand castings: (f) mold shift Sand Casting Defects: Mold Shift