Basic Manufacturing Process - Ch 6.1


Q.1    Briefly describe casting its advantages and limitations also define patterns.
Ans.    Casting: -

        Casting is a manufacturing process by which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. Casting materials are usually metals or various cold setting materials that cure after mixing two or more components together; examples are epoxy, concrete, plaster and clay. Casting is most often used for making complex shapes that would be otherwise difficult or uneconomical to make by other methods.
    The casting process is subdivided into two main categories: expendable and non-expendable casting. It is further broken down by the mold material, such as sand or metal, and pouring method, such as gravity, vacuum, or low pressure.
Advantages: -
    Casting process has several advantages over other methods a manufacture:
1.     It is cheaper and most economical method for producing complicated shapes. There is no limit to the size, shape and intricasy of the articles that can be produced by casting.
2.     One of the attractive features of casting is its ability to form any shape in one operation. This results in saving of time, and labour costs.
3.     Casting simplifies construction in many cases, e.g., object may be cast in a single piece which would otherwise require to be manufactured in several pieces which are subsequently assembled e.g., machine beds, ship’s propellers etc.
4.     Casting process can be used for producing a shape with certain desired mechanical properties by suitable control of alloy composition, grain structure and heat treatment.
5.     Certain metals and alloys such as highly creep resistant metal-based alloys for gas turbine can not be worked mechanically and can be cast only.
6.     A high degree of reproducibility is possible. Identical castings can be produced in large numbers.
7.     Casting is best suited for composite components requiring different properties in different sections. These are made by incorporating prefabricated inserts in a casting. Some examples are : aluminium conductors into slots in iron armatures for electric motors, wear resistant skins onto shock resistant components, steel screw threads in zinc die castings etc.
8.     Properties such as machinability and vibration damping capacity, good bearing qualities, strength and lightness are obtained more favourably in castings of certain light metal alloys.
9.     In many cases the casting reduces machining time, as fcr example, die castings are produced with high accuracy and close dimensional tolerance.
Limitations of Casting Process: -
1.     When only a few castings are required, the casting proess is uneconomical due to the expenditure involved in making patterns.
2.     Special purpose casting processes like die casting, centrifugal casting, etc. which can give high accuracy are expensive and can be used only in the case of mass production.
3.     Sand casting process, cannot produce dimensionally accurate components.
4.     The castings are susceptible to internal porosity from shrinkage and liberated gas from molten metal.
5.     Processes like shell moulding and investment casting require high skill of operation and fine control at every stage of production.
Pattern: -                                                                                                                 (AKTU - 2008 - 09)
    Patterns are required to make moulds. The mould is made by packing suitable moulding material, such as moulding sand around the patterrn. When the pattern is withdrawn, the imprint provides the mould cavity, which is ultimately filled with molten metal to become casting.
    A pattern may be defined as a full size model of the desired casting which when packed or embedded in a suitable moulding material, produces a cavity called mould. This mould, when filled with molten meta1 forms the desired casting after solidification of the molten metal. The pattern very closely conforms to the shape and size of the desired casting, except that it carries pattern allowances to compensate metal shrinkage, provide sufficient metal for machined surfaces, and faci1ilitate moulding. The process of making a pattern is known as pattern making.

Q.2.    What is Master Pattern?                                                                              (AKTU - 2012 - 13)
Ans.    Master Pattern: -

        Metal patterns are cast from wooden pattern called as a master pattern. The master pattern forms the mould in to which molten metal is poured to cast the metal pattern. The metal pattern is then used to produce the final casting. Therefore double shrinkage should be provided on master pattern. For example, if the metal pattern is to be made of aluminium and the castings are to be of brass, the shrinkage allowance on the wooden master pattern will have to the 18 mm per meter for aluminium, plus 14 mm per meter for brass, making a total of 32 mm per meter.
       For making master pattern mahogony is preferred because of its durability.

Q.3    Which materials are generally making use of pattern and what do you mean by pattern allowances.
Related Questions -
Q.    What are the allowances provided, when making a pattern?                         (AKTU - 2011 - 12)
Q.    What is pattern? Explain various allowances provided in pattern making and the reasons for it.                                                                                                                            (AKTU - 2012 - 13)    
Ans.    Pattern Materials: -
        The pattern should be strong enough to withstand rapping, and abrasive action of sand and should have well finished surfaces to produce smooth and accurate castings. Pattern may be made of wood, metal, plastic, plaster of paris, wax etc.
(1) Wood: -
    Wood is the most common material for making pattern. The advantages of wood as a pattern making material are:
(a)     It is cheap and readily available.
(b)     It can be cut and fabricated into numerous forms by glu bending and curving.
(c)     Good surface finish can be easily obtained.
(d)     It can be preserved for a fairly long time by applying preservative like shellac varnish.
(e)     As compared to most of the metals, it is light in weight.
The disadvantages of wood as a pattern material are:
(a)     It wears out quickly as a result of sand abrasion.
(b)     It is readily affected by moisture.
(c)     Because of the above reasons, the life of wooden pattern is short. Therefore, wooden patterns are used when only L castings are required.
Characteristics of wood to be used for making patterns:
(a)     It should be strong enough to resist wear
(b)     It should be easy to work.
(c)     It should have a reasonable cost.
    Teak wood is most commonly used for making patterns. The types of woods which may be used for patterns are: sal, shisham, pt deodar, mahogany etc.
(2) Metals: -
    Metal is used as a pattern making material only when number of castings are to be made. The advantages of metal as pattern material are:
(a)     They have much longer life as compared to wooden patterns.
(b)     They do not change their shape when subjected to moist conditions.
(c)     Freedom from warping in storage.
(d)     Resistant to wear and more strong as compared to wooden pattern.
(e)     Close dimensional accuracy and better surface finish can be obtained on the castings.
Disadvantages: -
(a)     They are costlier than wood.
(b)     For giving different shapes and finish they need machining. This also adds to their cost.
(c)     As compared to wood, most of them are heavy.
Metals used for patterns include cast iron, brass, aluminium, etc.
(3) Plaster of Paris: -
    It is very useful material for making pattern and core boxes which are small in size and have iregular or intricate shapes. It has a high compressive strength and can be easily work with wood tools. When the gypsum cement is mixed with water, it form plastic mass capable of being cast into mould. The special characteristic of gypsum cement is that ; it expands during solidification. Thus, cement of proper coefficient expansion is selected, the effect shrinkage of casting can be aumatically neutralised.
(4) Plastics: -
    Plastics can be used advantageously as a pattern making material because of their following specific characteristics:
(1)     It has a smooth and glossy surface due to which it can very easily withdrawn from the mould without damaging it.
(2)     It is light in weight.
(3)     It possesses high resistance to wear.
(4)     High resistance to corrosion.
(5)     Low solid shrinkage.
(6)     High strength.
(7)     It can be easily worked.
(8)     It is comparatively cheap.
(5) Wax: -
    Wax patterns are used in investment casting method. The wax patterns are made by pouring molten (heated wax) in metallic die. These moulds are made in two parts and kept cooled by circulating water around it. After the wax sets on cooling the die parts are separated and the wax pattern is taken out.
Pattern Making Allowances: -
    Although the pattern is used to produce a casting of the desired dimension, it is not dimensionally indentical with the casting. For metallurgical and mechanical reasons, a number of allowances must be provided on the pattern to get dimensionally correct casting.
(1) Shrinkage (Contraction) Allowance: -
    When the molten metal is poured in the mould it cools and solidifies. The metal contracts in size when it cools and solidifies. To compensate this shrinkage the pattern must be made larger than the finished casting. Therefore, shrinkage allowance is the amount by which the pattern must be made larger than the desired casting.
To compensate this shrinkage a shrink rule is used in laying out the measurement for the pattern.
(2) Machining Allownaces: -
    A casting may require machining all over or on certain specified portion. Therefore, additional metal must be provided on these surfaces so that there will be some metal to machine. The corresponding surface of the pattern must be made larger.
    The amount of this allowance depends upon the metal of the casting, method of machining to be used, method of casting used, size and shape of the casting and the degree of surface finished required.
    Ferrous metals need more machining allowance than non ferrous metals.
(3) Distortion Allowances: -
    This allowance is applicable only to irregular shaped castings which are distorted in the process of cooling. It is a result of uneven metal shrinkage. For example, a casting in the form of a letter U will be distorted with the legs diverging, instead of remaining parallel. To compensate this an opposite distortion is provided in the pattern so that the effect is neutralised and correct casting is obtained. For making the U shaped casting, the legs of the pattern are made convergent but as the casting cools after its removal from the mould, the legs straighten up and remain parallel.
(4) Rapping Allowances: -
    When the pattern is to be withdrawn from the mould, it is first rapped or shaken, by striking over it from side to side, so that its surfaces may be free from the adjoining walls of the mould. As a result of this the cavity in the mould is slightly increased. To compensate this a negative allowance is to be provided by making the pattern slightly smaller. In small and medium size casting, rapping or shake allowance is generally neglected.

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