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
