WHAT IS PATTERN IN CASTING

WHAT IS PATTERN  IN CASTING? 

• Pattern is an important tool in the casting process. A pattern is the model of the part to be cast, it is considered as the replica of the part to be cast , except for the various allowances. it is slightly larger than the desired casting
•  the process of making a pattern is known as pattern making
• It is the first step in making a casting. a pattern may contain core, prints, runners , gates, and risers .
• casting is considered as the most ancient process of producing metallic component by using pattern
  
  

PATTERN MATERIALS

1. Quantity to be casted
2. Degree of surface finish
3. Type of moulding finish
4. Design of casting and possibility of its change in future

These some factors are considered while selecting material for pattern

which material are used for pattern making?

generally

• wood 
• metal
• plastics
• plaster
• wax

these materials are used for pattern making

WOOD 

• Wood is most commonly used material for pattern making because it is  economical, readily available, reasonally strong and durable
• It can also be worked easily
• When joined by glue no need to use of nails & screw
• wood pattern can be prepaired easily
• It also take good polish and be glued very effective

Advantage of wood 

1. it is cheap and easily workable
2. it can be easily shaped into different forms
3. it gives the best surface finish
4. it can be preserved for long time and can be painted
5. it can be repaired easily
6. it is light in weight

Limitations

1. it is affected by moisture, consequently change shape and size.
2. they have poor wear and abrasion resistance
3. they are weak compared to metal pattern
4. rough handling can damage wood

   

METALS 

• Metal patterns hold dimensions , do not warp , are not affected by by moisture and are more resistance, it is also easy to obtain good surface finish with metal
• metal pattern are made by first making a master pattern in wood and then casting with the pattern material , the pattern casting are machined to required surface finish and dimensional accuracy
• Aluminum, white metal , cast iron , mild steel , these materials are used for metal patterns
• When number of casting to be produced is large scale the wooden pattern and metal pattern are used

Advantage of metal pattern

1. They are more stronger and accurate than wooden pattern 
2. They do not absorb moisture , and retain their shape
3. They posses good resistance to wear , abrasion corrosion
4. when stored for longer period , they do not wrap

Limitation

1. there cost is more than wooden pattern
2. some metal get rusted and hence get spoiled 
3. they are heavier than wooden pattern 

PLASTICS 

• Plastics are intermediate between wood and metal in their utility as pattern material
• They can be cast in plaster moulds and finished to make pattern
• plastic pattern have reasonably good wear resistance , are not affected by moisture, take good finish , and light in weight
• because of their glossy surface , plastics patterns can be withdrawn very easily
• plastics pattern are used when production is in large scale but not large enough to warrant metal patterns
• Main disadvantage foe plastic pattern is that they are fragile and light sections may need metal reinforcement
• plastic pattern cannot withstand conditions of serve shock as in machine moulding

Advantage of plastic pattern

1. They are light in weight 
2. They do not absorb moisture
3. They are highly resistant to corrosion
4. They are stronger and stable than wooden pattern
5. They provide good surface finish 
6. They are easy to make

Limitation

1. They become soft when heated

PLASTER

• Plaster of paris or gypsum cement is also used for making patterns and core boxes, plaster is easy to cast and work with but brittle and not suitable when number of casting to be produced is large
• plaster  pattern can be prepared by pouring the plaster slurry in a mould prepared with the help of master pattern, alternatively , the pattern may be mad sweeping it into the desired shape with suitable tools
• Plaster can also be used for preparation of the follows boards for moulding of irregular parts
• wood , metals , plastics, plaster are materials for so called permanent of reusable pattern , temporarry and plaster or dispossable pattern used in same special casting process use , wax mercury, or polysterene, as pattern material.