INTRODUCTION

Prathik SAsst Prof

Department of aeronautical engg

Classification of manufacturing process

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Classification of manufacturing process

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Types of production systems

• Mass production / continuous flow process• Batch production• Job shop production

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Sand casting

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Sand casting

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Rolling

Hot-rolling

Cold-rolling

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Factors to be considered for selecting a production process

a) Shape and size to be produced - For products with simple shape, machining is best suited. But for complex and intricate shapes, casting is preferred. The size of the product is also an important factor. For example, 'long' products such as rails or 'thin' products such as car-body panels can be best made by forming process compared to others.

(b) Quantity to be produced - Both machining and casting can be used for producing large quantity products, but are not suitable for small quantity products, as they are not economical.

(c) Type of material - Materials possess various properties like ductility, hardness, toughness, brittleness etc. Hard materials cannot be machined easily. Brittle materials cannot be mechanically worked (Forming process). In such cases, casting is preferred. 8

Factors to be considered for selecting a production process

(d) Surface finish and dimensional accuracy - Casting with expendable moulds does not yield good surface finish. However, if casting process is selected, it should be followed by machining process to obtain the desired surface finish and dimensional tolerance.

(e) Quality and property requirements - A defect-free product with specific properties serve its purpose for long life. Properties of cast material are generally less when compared to that of mechanically worked materials. Also, casting gives a lot of defects. Hence, a process that gives better properties and quality should be selected.

(f) Cost of the product - Customers often demand for products with more features and performance at reduced prices. Hence, a low cost production process should be selected, but at the same time, see that no compromise is made in terms of quality.

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products made by machining process products made by casting process

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products made by forming process

CASTING PROCESS

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Terms involved in casting

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Terms involved in casting process• Mould box (flask) : It is usually a metallic

frame used for making and holding a sand mould. The mould box has two parts: the upper part called 'cope', and the lower part called 'drag'.

• Parting line/parting surface: It is the zone of separation between cope and drag portions of the mould in sand casting.

• Sprue: It is vertical passage through which the molten metal will enter the gate.

• Pouring basin: The enlarged portion of the sprue at its top into which the molten metal is poured.

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Terms involved in casting process• Gate/ingate: It is a short passage way

which carries the molten metal from the runner/ sprue into the mould cavity.

• Riser: A riser or feed-head is a vertical passage that stores the molten metal and supplies (feed) the same to the casting as it solidifies.

• Mould cavity: The space in a mould that is filled with molten metal to form the casting upon solidification.

• Core: A core is a pre-formed (shaped) mass of sand placed in the mould cavity to form hollow cavities in castings.

• Core print: It is a projection attached to the pattern to help for support and correct location of core in the mould cavity.

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Mould box (flask) Sprue

GateRiser

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Parts produced by casting

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STEPS INVOLVED IN MAKING A CASTING

The basic steps in making a casting are:(a) Pattern making(b) Mould preparation (including gating and riser)(c) Core making(d) Melting and Pouring(e) Cleaning and Inspection

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a) Pattern making• A pattern is a replica of the object to be cast.• It is used to prepare a cavity into which the molten

metal is poured.• A skilled pattern maker prepares the pattern using

wood, metal, plastic or other materials with the help of machines and special tools.

• Many factors viz., durability, allowance for shrinkage and machining etc., are considered while making a pattern.

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b) Mould preparation• Mould preparation involves forming a cavity by

packing sand around a pattern enclosed in a supporting metallic frame called 'flask' (mould box).

• When the pattern is removed from the mould, an exact shaped cavity remains into which the molten metal is poured.

• Gating and risering are provided at suitable locations in the mould.– Gating - Passage through which molten metal flows and enter the mould

cavity. – Risering - A reservoir of molten metal connected to the mould cavity to

supply additional metal so as to compensate for losses due to shrinkage, as the metal solidifies.

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c) Core making• In some cases, a hole or cavity is required in the

casting.• This is obtained by placing a core in the mould

cavity. • The shape of the core corresponds to the shape of

the hole required.

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d) Melting and Pouring• Metals or alloys of the required composition are

melted in a furnace and poured into the mould cavity.• Many factors viz., temperature of molten metal,

pouring time, turbulence etc., should be considered while melting and pouring.

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e) Cleaning and Inspection• After the molten metal has solidified and cooled, the

rough casting is removed from the mould, cleaned and dressed.

• This involves removing cores, adhered sand particles, gating and riser systems, fins, blisters etc., from the casting surface.

• then sent for inspection to check for dimensions or any defects like blow holes, cracks etc.

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Procedure for making the casting

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Components Produced by Casting Process

• Casting is the first step and the primary process for shaping any material.

• All materials have to be cast before it is put to use.• The ingots produced by casting process are used as raw

material for secondary processes like machining, forging, rolling etc.

• More than 90 % of all manufactured goods and capital equipment use castings for their manufacture.

• To list the components produced by casting is an endless process. A few major components produced by casting are given below.

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Components Produced by Casting Process• Automotive sector - Nearly 90 % of the parts in automobiles are-

manufactured by castings. A few parts include brake drum, cylinder, cylinder linings, pistons, engine blocks, universal joints, rocker arm, brackets etc.,

• Aircraft - Turbine blades, casing etc.• Marine propeller blades.• Machining - Cutting tools, machine beds, wheels and pulleys,

blocks and table for supports etc.• Agriculture and rail road equipments.• Pumps and compressors frame, bushings, rings, pinion etc.• Valves, pipes and fittings for construction work.• Camera frames, parts in washing machine, refrigerators and air-

conditioners.• Steel utensils and a wide variety of products.

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Advantages of casting process• Large hollow and intricate shapes can be easily cast.• Quick process, and hence suitable for mass production.• No limit to size and shape. Parts ranging from few millimeters to

meters and few grams to tons can be cast efficiently and economically.

• Better dimensional tolerances can be obtained by good casting practice.

• Castings exhibit uniform properties in all the directions - longitudinal, lateral and diagonal.

• The casting process is usually the cheapest process.• Unrelieved internal stresses are absent in cast components.• Certain metals and alloys can be manufactured by means of

casting only, e.g., Phosphor-Bronze.33

Limitations of casting process• Presence of defects in cast parts is a major disadvantage.• Casting process is not economical for small number of parts.• Properties of cast materials are generally inferior when

compared to those made by machining or forging process.• Casting process mostly deals with elevated temperatures.• There are limitations regarding thin sections.• Casting process is not suitable for very small number of

components.

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INTRODUCTION TO PATTERN MAKING

• A pattern is a mold forming tool in the hands of foundry men.• A pattern is a model or the replica of the object to be cast.• Except for the various allowances a pattern exactly

resembles the casting to be made.• A pattern is required even if one object has to be cast.• A pattern may be defined as a model or form around which

sand is packed to give rise to a cavity known as mold cavity in which when, molten metal is poured, the result is the CAST OBJECT.

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Difference between pattern and casting• The main difference between a pattern and the casting is their dimensions.• A pattern is slightly larger in size as compared to the casting, because a

pattern,– carries Shrinkage allowance, it may be of the order of 1 to 2 mm/ 100

mm.– is given a Machining allowance to clean and finish the required surfaces.– carries a Draft allowance of the order of 1 and 3 degrees for external

and internal surfaces respectively– carries core prints.

• A pattern may not have all holes and slots which a casting will have. Such holes and slots unnecessarily complicate a pattern and therefore can be drilled in the casting after it has been made.

• A pattern may be in two or three pieces whereas a casting is in one piece.• A pattern and the casting also differ as regards the material out of which they

are made.36

Functions of a patterns• A pattern prepares a mold cavity for the purpose of making a casting.• A pattern may contain projections known as core prints if the casting

requires a core and need to be made hollow.• Runner, gates and risers (used for introducing and feeding molten

metal to the mold cavity) may form a part of the pattern.• A pattern may help in establishing locating points on the mold and

therefore on the casting with a purpose to check the casting dimensions.

• Patterns establish the parting line and parting surfaces in the mold.• A pattern may help position a core (in case a part of mold cavity is

made with cores), before the molding sand is rammed.• Patterns that are properly made and having finished and smooth

surfaces, reduce casting defects.• Properly constructed patterns minimize overall cost of the castings.

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Pattern materials

• The following factors assist in selecting proper pattern material:The number of castings to be produced. Metal patterns are

preferred when the production quantity is large.The desired dimensional accuracy and surface finish required

for the castings.Nature of molding process i.e., sand casting, permanent mold

casting, shell molding, investment casting etc. Method of molding i.e., hand or machine molding.Shape, complexity and size of the casting.Type of molding materials i.e., sand etc.The high probability of changing the casting and hence the

pattern in near future.

Selection of pattern materials

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Materials for making patterns• Patterns may be constructed out of the following

materials. (a) Wood (b) Metal(d) Plastic (e) Plaster (POP)(f) Wax

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WOOD• The most common materials for making patterns for sand casting is the wood.Advantages

– Inexpensive.– Easily available in large quantities.– Easy to machine and to shape to different configurations and forms– Easy to join to acquire complex and large pattern shapes– Light in weight– Easy to obtain good surface finish– Wooden patterns can be preserved for quite long times with the help of suitable

wood preservatives.Limitations

– Wooden patterns are susceptible to shrinkage and swelling.– They possess poor wear resistance.– They are abraded easily by sand action.– They absorb moisture, consequently get deformed and change shape and size.– They cannot withstand rough handling.– They are weak as compared to metal patterns.

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Metal• Metal patterns are employed where large number of

castings have to be produced from the same patterns.• Metal patterns are cast from wooden patterns.• The different metals and alloys used for making patterns

are,– Aluminium and Aluminium alloys– Steel– Cast Iron– Brass– White Metal

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Advantages of Metal Patterns– Unlike wooden patterns, they do not absorb moisture. They retain their

shape.– They are more stronger and accurate as compared to wooden patterns.– They possess life much longer than wooden patterns.– They can withstand rough handling.– They do not distort– They possess greater resistance to abrasion. They have accurate dimensional

tolerances. They are far stable under different environments.– It is easy to obtain smooth surface finish.– They possess excellent wear resistance and strength to weight ratio.

Limitations of Metal Patterns– Expensive as compared to wood patterns.– Are not easily repaired e.g. (Aluminium patterns).– Ferrous patterns get rusted.– They (ferrous patterns) are heavier than wooden patterns,– They cannot be machined so easily as wooden ones.

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Plastic• Advantages

– Durable– Provides a smooth surface.– Moisture resistant.– A plastic pattern does not involve any appreciable change in its size or shape.– Lightweight.– Wear and corrosion resistant.– Provides good surface finish.– It possesses low solid shrinkage.

• Limitations– Plastic patterns are fragile and thus light sections may need metal

reinforcements.– Plastic patterns may not work well when subject to conditions of severe

shock as in machine moulding.43

PLASTER• Advantages

– can be easily worked by using wood working tools.– Intricate shapes can be cast without any difficulty.– It has high compressive strength (up to 285 kg/cm2)

• Disadvantages– Can be used for small castings only

• Plaster Patterns Material: Plaster patterns may be made out of Plaster of Paris or Gypsum cement.

• Applications: Plaster is used for making (i) Small and intricate patterns, and (ii) Core boxes.

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WAX • Advantages

Wax patterns provide very good surface finishThey impart high accuracy to the castings.After being molded, the wax pattern is not taken out of

the mold like other patterns; rather the mold is inverted and heated; the molten wax comes out and/or is evaporated. Thus there is no chance of the mold cavity getting damaged while removing the pattern.

• Applications• Wax patterns find applications in Investment

casting process.

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PATTERN ALLOWANCES• A pattern is always larger in size as compared to

the final casting, because it carries certain allowances.

• The various pattern allowances are below(a) Shrinkage or contraction allowance.(b) Machining or Finish allowance.(c) Draft or Taper allowance.(d) Distortion or camber allowance.(e) Shake or rapping allowance.

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Shrinkage Allowance• Almost all cast metals shrink or contract volumetrically after solidification and

therefore to obtain a particular sized casting, the pattern is made oversize by an amount equal to that of shrinkage or contraction.

• Different metals shrink at different rates because shrinkage is the property of the cast metal or alloy.

• The metal shrinkage depends upon• The cast metal or alloy.• Pouring temperature of the metal or alloy.• Casting dimensions (size).• Casting design aspects.• Molding conditions (i.e., mold materials and molding methods

employed). • Cast iron poured at higher temperatures will shrink more than that poured at

lower temperature.• Wood patterns used to make metallic patterns are given double allowance; one

for the shrinkage of the metal of the pattern and the other for that of metal to be cast. 47