Pattern in Casting

 

What is pattern? 



The Mechanical post welcomes you back! Today we’ll talk about pattern, what materials are used for the making, its advantages, limitations and types.

What is pattern in casting ?

Patterns are used to make moulds for the casting into which the molten metal would be poured. Patterns are the copy of the product which is intended to be casted. However, it is not an exact replica of the casting desired. There are certain essential difference.

It is slightly larger than the desired casting, due to various allowances that is shrinkage allowance, machining allowance etc. and it may have several projections or bosses called cold prints. It may also have extensions to produce runners and gates during the moulding process.


Desired characteristics of pattern


  • Secure the desired shape and size of the casting.
  • Cheap and readily repairable. 
  • Simple in design for ease of manufacture. 
  • Light in mass and convenient to handle.
  • Have high strength and long life in order to make as many moulds as required.
  • Retain its dimensions and rigidity during the definite service life.
  • Its surface should be smooth and wear resistant.
  • Able to withstand rough handling.

The common materials used in pattern making include wood, metal and plastic. Each material has its own advantages, limitations and field of application. Also, the required accuracy, strength and life of a pattern depends upon the quantity of castings to be produced. Based on the above factors, we can choose the material of pattern as follows:

  • For short run production wood is the suitable material.
  • For large scale and mass production metal being more durable than wood though is costlier but however can survive in the long run.
  • For batch production plastics for example epoxy, resins and also from gypsum and cement are preferred.


    Wooden pattern

    The wood used for pattern making should be properly dried and season. It should not contain more than 10% moisture to avoid wrapping and distortion during subsequent drying. It should be straight grained and free from knots.

    Advantages of using wood

    • Light in weight.
    • Comparatively inexpensive. 
    • Good workability. 
    • Lends itself to gluing and joining. 
    • Holds well varnishes and paints. 
    • Can be repaired easily.

    Disadvantages or limitations

    • Inherently non uniform in structure. 
    • Possess poor ware and abrasion resistance. 
    • Cannot withstand rough handling. 
    • Absorbs and gives of moisture, so that it varies in volume, wraps and thus changes its mechanical properties. These drawbacks, however, can be remedied by drying and seasoning it and then giving coats of water proof varnishes and paints.

    Types of wood commonly used in pattern making


    White pine

    It is the most widely used wood, because of it straight grain and light weight and because it is soft, easy to work and unlikely to wrap.

    Mahogany

    It is harder and more durable than white pine. Can be work easily if straight grained. It is less likely to wrap than some of the other Woods.

    Maple birch and cherry

    These woods are harder and heavier than white pine. They tend to wrap in large sections, so should be used for small patterns only. They should be carefully treated, because they pick up moisture readily.

    Metal

    Metal pattern can be cast from a master wood pattern or may be machined by the usual methods of machining. Metal patterns are usually used in machine moulding.

    Advantages of using metal

    • More durable and accurate in size than wooden patterns.
    •  Have a smooth surface. 
    • Do not the deform in storage. 
    • Are resistant to wear, abrasion, corrosion and swelling. 
    • Can withstand rough handling.                                                                                       

    Limitations.


    • Expensive as compared to wood. 
    • Not easily repairable. 
    • Heavier than wood patterns. 
    • Ferrous metals are prone to rusting.


    Common metals used for pattern making are


    Cast iron. 

    With fine grain can be used as a pattern material. It has low corrosion resistance unless protected. Heavier and difficult to work. However, it is cheaper and more durable than other metals. 
                                                                      

    Brass.                      

    May be easily work and built up by soldering for brazing. It has a smooth closed pore structure. It is expensive, therefore, generally used for small cast parts.                                                                                                                       

    Aluminium.                                                                                                    

    It is the best pattern material, because it is easily worked, light in weight and is resistant to corrosion. It is however, subjected to shrinkage and by abrasive action.            
                                                                                                        

    White metal.                                                                                                   

    It has low shrinkage, can be easily cast, has low melting point, it is light in weight and may be built up by soldering. However, it is subjected to wear by abrasive action of the sand.

      Plastics

      The use of plastics for pattern material is in following advantages
      • Facilitates the production process. 
      • Makes it more economical in cost and labour. 
      • Plastic patterns are highly resistant to corrosion, lighter and stronger than wood patterns. 
      • Moulding sand sticks less to plastics than to wood .
      • No moisture absorption.
      • Smooth surface of patterns. 
      • Strong and dimensionally stable.

      Various plastics make good materials for the production of patterns. These are the compositions based on epoxy phenol formaldehyde and polyester resins.
      In most wide use our old-curing plastics based on epoxy resins and arcylates.

      Plastic patterns are manufactured by extrusion, injection moulding and lamination processes.

      Finishing of patterns

      After the patterns are made, they should be finished by sanding so that tool marks and other irregularities are erased. Then they should be applied with 2 to 3 coats of shellac. Shellac fills up the pores and imparts a smooth finish.
      The finish of the casting depends on the finish of the pattern. If the pattern is to be preserved for a long period and if a colour scheme is to be used for good quality enamel paint should be selected to spray or brush paint it.

      Pattern allowances

      The difference in the dimensions of the casting and the pattern is due to the various allowances considered while designing a pattern for a casting. These allowances are discussed below:

      Shrinkage allowance

      Since metal shrinks on solidification and contracts further on cooling to room temperature, linear dimensions of patterns are increased in respect of those of the finished casting to be obtained. This is called the shrinkage allowance. It is given as mm/m. Typical values of shrinkage allowance for various metals are:
         
      C.I., Malleable iron    =  10mm/m.
      Brass, Cu, Al              =  15mm/m.
      Steel                           =   20mm/m.
      Zinc, Lead                  =   25mm/m.


      While laying out a pattern, the dimensions are taken from a Pattern makers rule, called “shrink scale“, which is longer than a standard scale by the shrinkage value for the appropriate metal.

      Machining allowance

      Machining allowance or finish allowance indicates how much larger the rough casting should be over the finished casting to allow sufficient material to insure that machining will “clean up” the surfaces. This machining allowance is added to all surfaces that are to be machined.

      The amount of finish allowance depends on the material of the casting, its size, volume of production, method of moulding, configuration of the casting, the position the wall surface occupies in the mould and during pouring. Machining allowance is larger for hand moulding as compared to machine moulding. The largest allowances are taken for the surface is located in the cope half of the mould, since they are liable to contamination due to slag.

      Types of pattern

      • One piece or solid pattern. 
      • Stock split or parted pattern. 
      • Loose piece pattern. 
      • Gated patterns. 
      • Match plate pattern. 
      • Cope and drag pattern. 
      • Sweep patterns. 
      • Skeleton patterns. 
      • Segmental pattern.
      • Follow board pattern.

      Colour scheme for patterns


      Frequently, a print of the finished part is not furnished with a pattern. As a result, the foundry man is not able to take the necessary precautions to produce the best results. Many mistakes may be eliminated by indicating the functions of the various parts of the pattern with proper colours. The common colour scheme as per Indian standards (IS 1513 – 1917 ) is given as follows:

      • Red = Surface to be machined
      • Black = Surface as cast
      • Yellow = core prints and seats
      • Yellow/red stripes = Loose piece
      • Yellow/black stripes = Stop off

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