Metal Casting : Production

 Metal Casting :

Casting is a process in which the liquid molt
en metal is poured into the casting cavity whose shape is same as that of the casting to be produced and allowed it to solidify .

After Solidification the casting can be taken out by breaking the mould .

                             Metal Casting

Advantages of casting :

1. Any shape can be produced .

2. Any material can be cast .

3. Requiring tools are simple and inexpensive .

4. Heavy equipment can be cast easily in required size rather than fabricating them by joining several smaller pieces .

Limitations

Dimension accuracy and surface finish is poor .

Steps in casting :

1. Pattern Making

2. Mould and core making

3. Pouring and solidification

4. Fetting

5. Inspection

1. Pattern Making :

Patten is the replica of casting .

Pattern size =casting size + - allowance

Allowances :

1. Shrinkage Allowance ( SA)

2. Machining Allowance

3. Draft Allowance

4. Shake Allowance

5. Distortion Allowance / Bending Allowance

Shrinkage Allowance :

Three stage -

1. Shrinkage of molten metal when reducing from pouring temperature to freezing temperatures .

2. Shrinkage of molten metal during  freezing .

3. Shrinkage solid metal when reducing from freezing temperature to room temperature .

* In casting process aluminium have highest liquid shrinkage that is 6 % .

* Gray cast - iron have a negative liquid shrinkage and solid solid shrinkage is positive .

* Total shrinkage  ( solid + liquid ) will be highest in steels .

Machining Allowance :

Extra material provided on the pattern is called machining allowance .

Machining allowance is provided for good surface finish .

Draft Allowance :

Making the vertical surface of pattern .

Easy removal of pattern from mould .

Shake Allowance :

Shake allowance is provided to avoid mold damages taking place due to adhesiveness property of moulding sand .

Pattern is made by using materials like wax , mercury , polystyrene .

Distortion Allowance / Bending Allowance :

It is mainly required in casting of U or V shaped casting .

Types of Pattern :

1. Solid or single piece pattern

2. Split pattern 

3. Match plate pattern

4. Gated pattern 

5. Sweep pattern

6. Skeleton pattern 

7. Loose piece pattern

8. Follow board pattern

9. Shell pattern 

                           Types of pattern

Patten Materials :

1. Wood

2. Metals

3. Plastics

4. Wax 

5. Mercury

Mould Making :

Three basic elements :

1. Silica sand particles ( 75- 80 %)

2. Clay ( 15 - 20 % )

3. Water or sodium silicate (6 -8%)

4. Additives ( 2% )

Additives : 

1. Saw dust or wood powder :

Increasing porosity and collapsibility .

                                 Porosity

2. Coal powder :

Increasing refractoriness . 

3. Starch or dextrin :

Increasing strength , resistance .

Saw dust or wood powder

Coal powder

Starch or dextrin

Types of sands :

1. Green sand : 

Silica sand + clay + water 

                               Green Sand 

2. Dry sand :

Silica sand + clay + clay + sodium silicate

                                  Dry Sand 

3. Loam sand :

Green sand or dry sand with 50 % clay .