Different types of patterns used in casting

Pattern: 

A replica of the  the final casting, used to form the mold cavity.        

A pattern is an element used for making cavities in the mould, into which molten metal is poured to produce a casting. It is not an exact replica of the casting desired.

There are certain essential differences. It is slightly larger than the desired casting, due to the various allowances (shrinkage allowance, machining allowance etc.).

Pattern materials:

1.     Wood:

Advantages:

1.       Light in weight

2.       Comparatively inexpensive

3.       Good workability

4.       Lends itself to gluing and joining

5.       Holds well varnishes and paints

6.       Can be repaired easily

                                Disadvantages:

1.       Inherently non uniform in structure

2.       Posses poor wear and abrasion resistance

3.       Can not withstand rough handling

4.       Absorbs and gives off moisture, so that it varies in volume, wraps and thus changes its mechanical properties

2.      Metals:  

Advantages:

1.       More durable and accurate in size than wooden patterns

2.       Have a smooth surface

3.       Do not deform in storage

4.       Are resistant to wear, abrasion, corrosion and swelling

5.       Can withstand rough handling

                                Disadvantages:

1.       Expensive as compared to wood

2.       Not easily repaired

3.       Heavier than wooden patterns

4.       Ferrous patterns can get rusted

The common metals used for pattern making are:

1.       Cast iron

2.       Brass

3.       Aluminum

4.       White metal

3.     Plaster gypsum cement

4.     Plastic compound

Advantages:

1.       Facilitates the production process.

2.       Makes it more economical in cost and labor.

3.       Plastic patterns are highly resistant to corrosion, lighter and stronger than wood patterns.

4.       Molding sand sticks less to plastics than to wood.

5.       No moisture absorption.

6.       Smooth surface of patterns.

7.       Plastic patterns are Strong. And so they are dimensionally stable.

5. Wax

Wax is the best material for surface finish.