Lesson 04 - What is Pattern ?

What is pattern?

• Pattern is the principal tool during the casting process.

• A pattern is a model or the replica of the object (to be casted)

• It 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.

• A pattern prepares a mold cavity for the purpose of making a casting

 

Objective of a Pattern

Pattern prepares a mould cavity for the purpose of making a casting.

• Pattern possesses core prints which produces seats in form of extra recess for core placement in the mould.

• It establishes the parting line and parting surfaces in the mould.

• Runner, gates and riser may form a part of the pattern.

• Properly constructed patterns minimize overall cost of the casting.

• Pattern may help in establishing locating pins on the mould and therefore on the casting with a purpose to check the casting dimensions.

• Properly made pattern having finished and smooth surface reduce casting defects

 

Pattern Materials

Wood: Inexpensive, Easily available, Light weight, easy to shape, good surface finish, Poor wear resistance, absorb moisture, less strength, not suitable for machine moulding, easily repaired, warping, weaker than metallic patterns.
• Ex. Shisam, kail, deodar, Teak wood, maogani.

Metal: less wear and tear, not affected by moisture, metal is easier to shape the pattern with good precision, surface finish and intricacy in shapes, withstand against corrosion and handling for longer, excellent strength to weight ratio,• metallic patterns are higher cost, higher weight and tendency of rusting.
• preferred for production of castings in large quantities with same pattern.
• Ex. cast iron, brass and bronzes and aluminum alloys

Plastic: Plastics are getting more popularity now a days because the patterns made of these materials are lighter, stronger, moisture and wear resistant, non sticky to molding sand, durable and they are not affected by the moisture of the molding sand.
• fragile, less resistant to sudden loading and their section may need metal reinforcement.
• Ex. phenolic resin, foam plastic

Plaster: Intricate shapes can be made, good compressive strength, expands while solidifying, less dimensionally accurate.

Wax: Good surface finish, high accuracy, no need to remove from the mould, less strength.

 

Factors Effecting Selection of Pattern Material

1. Number of castings to be produced. Metal pattern are preferred when castings are required large in number.

2. Type of mould material used.

3. Kind of molding process.

4. Method of molding (hand or machine).

5. Degree of dimensional accuracy and surface finish required.

6. Minimum thickness required.

7. Shape, complexity and size of casting.

8. Cost of pattern and chances of repeat orders of the pattern

Location: United States

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