11) What is pattern? Discuss different types of pattern materials and different types of patterns.

Pattern

            A model or a form around which sand is packed to give rise to a cavity known as mould cavity in which when molten metal is poured, the result is the cast object.

Difference between pattern and casting

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

Because a pattern:

1)      Carries shrinkage allowance, it may be of the order of 1 to 2 mm/ 100 mm.

2)      Is given a machining allowance to clean and finish the required surfaces.

3)      Carries a draft allowance of the order of 1 to 3 degrees for external and internal surfaces respectively.

4)      Carries core print. 

In addition

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

6)      A pattern may be in two or three pieces whereas a casting is in one piece.

7)      A pattern and the casting also differ as regards the materials out of which they are made.

Materials for making pattern:

1)      Wood –

Advantages – a) inexpensive

                        b) Easily available in large quantity

                        c) Easy to machine and shape

                        d) Easy to acquire complex and large pattern shape

                        e) Light in weight

                        f) Good surface finish

                        g) Long time preservation

                        i) Produce very strong joints

                        j) Can be repaired easily.

Limitation:     1) susceptible to shrinkage and swelling

              2) Possesses poor wear resistance

Applications:

            Wooden patterns are used where the number of castings to be produced is small and the pattern size is large.

Woods used for making pattern:

a)      While pine

b)      Mahogany

c)      Teak

d)     Deodar

e)      Shisham

f)       Kail

g)      Maple

h)      Birch

i)        Cherry

2) Metals:

                  a) aluminium and aluminium alloys

                              - Aluminium patterns find uses as big (sized) pattern. Their light weight      keeps the pattern handy

                  b) Steel

                  c) Cast iron

                  d) Brass

                  e) Tin base alloy – white metal

                                              - lining stripping plates (in machine molding)  

3)  Plastic

4) Plaster – small and intricate patterns, core boxes

5) Wax – investment casting process

6) Polystyrene patterns

Types of patterns:

a)      One piece (solid) pattern –stuffing box of steam engine

b)      Split pattern – tap and water stop cocks

c)      Loose piece pattern

d)     Match plate pattern – piston rings of I.C.engine

e)      Cope and drag pattern

f)       Sweep pattern- kettles of cast iron

g)      Gated pattern

h)      Skeleton pattern – casting for turbine casing, water pipes, chutes, L-bends etc.

i)        Segmental pattern – big gears, wheel rims etc.

j)        Flow board pattern