Pattern Materials: Wood, Metal, Plastic, Plaster, Wax

Pattern Materials

Selection of pattern material depends on the following factors.

Number of castings to be produced

Required dimensional accuracy and surface finish.

Process used like, sand, investment, shell, etc.

– Moulding, hand/machine.

– Shape, complexity and size of casting

Materials used for Pattern are

(1) Wood 

(2) Metal 

(3) Plastic 

(4) Plaster and 

(5) Wax

1. Wood

Types

– White pine

– Mahogany

– Teak

– Deodar, etc.

Advantages

– Inexpensive

– Easy availability

– Light in weight

– Easy to machine

– Easy to join for obtaining complex shapes

Limitations

– Shrinkage and swelling

– Poor wear resistance

– Easily absorb moisture and result in change of shape and size.

– Weak compared to metal patterns.

Applications

Used where large size patterns produced for small number of castings.

2. Metal

These are cast from wooden patterns or machined.

Types

(i) Al and Al alloys

(ii) Steel

(iii) CI

(iv) Brass (Cu – Zn)

Advantages

– Stronger and accurate than wooden pattern.

– More life than wood.

– Greater resistance to abrasion

– Greater wear resistance

– Surface finish obtained is good.

– Stable in different environments.

Limitations

– More expensive as compared to wooden.

– Heavier than wooden (Ferrous pattern).

– Get rusted easily (Ferrous Patterns).

– Can’t be easily repaired (Al) or machined.

Applications

Generally used where large number of castings are to be produced.

Aluminium and Al alloys

Advantages

– Best pattern material

– Good resistance to corrosion, swelling

– Good machinability, wear resistance and strength.

– Do not rust

– Pattern cost moderate

Limitations

– Do not withstand to rough handling

– Poor repairability

Applications

For any step patterns :

Cast-iron

Advantages

– Less expensive

– Durability is high

– Resistance against abrasive action of sand.

– Easily machined

Limitations

– More weight

– It is hard and brittle, so it can be broken easily

– Gets rusted in presence of moisture.

Steel

Advantages

– Good machinability

– High wear resistance

– High strength

– Good repairability

Limitations

– High weight

– Poor resistance to rust and corrosion.

3. Plastic

Advantages

– Durable

– Smooth cast surface

– Moisture resistant

– No markable change in shape and size during use.

– High abrasion resistance

– Good resistance against chemicals

Limitations

– Fragile and so may need metal reinforcements.

– Not satisfactory under shock (m/c moulding / jolting)

4. Wax

Advanatges

– Very good surface finish

– High accuracy in cast product.

Applications

Investment casting process

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