Determine different defects in casting. State their causes and remedies
1. Blow holes: It is smooth sound cavities produced in a casting due to entrapped bubbles of gases, steam.
Causes:-
i) Excessive moisture in the sand.
ii) low permeability of sand
iii) Sand grains are too fine
iv) Sand is rammed too hard
v) Venting is insufficient
Remedies:-
i) Moisture content of the sand must be well.
ii) Sand of proper grain size should be used.
iii) Ramming should not be too hard.
iv) Vent holes should be provided.
2. Mis-run and cold shut:- When molten metal fails to fill the entire cavity of the mould, incomplete casting is obtained. This defeat is called mis-run and imperfect fusion of two stream of molten metal in the mould cavity results in a discontinuity called cold-shut.
Causes:-
i) Too thin sections and wall thickness.
ii) Improper gating systems.
iii) Damaged pattern.
iv) Slow and intermediate pouring.
v) Pour fluidity of metal.
vi) Improper ally composition.
Remedies:-
i) Use hotter metals
ii) Frequent inspection and replacement of pattern.
iii) Proper design of gating and raiser
iv) Use of chills and padding.
3. Drop: - This is an irregular deformation of the casting produced when a portion of the sand drops into the molten metal.
Causes:-
i) It is caused due to low strength
ii) soft ramming
iii) Insufficient reinforcement of hanging section
Remedies:
i) These can be controlled by adopting proper moulding, gating and melting techniques.
4. Dirt: - Presence of particles of dirt and sand in the casting.
Causes:-
i) improper handling of mould
ii) Presence of sand slag particles in molten metal
Remedies:-
i) Proper handling of mould
ii) Adopting proper moulding, gating and melting techniques.
iii) Proper design of gating and raiser
iv) Use of chills and padding
5. Shifts: - It is a misalignment of top and bottom parts of mould at parting line. This results in mismatch of the casting, incorrect dimension, incorrect location of holes.
Causes:-
i) misalignment of pattern parts, due to worn or damaged patterns
ii) misalignment of moulding box or flask equipment
Remedies:-
i) ensuring proper alignment of the pattern, moulding boxes
ii) correct mounting of pattern on pattern plates etc
6. Fins and flash: - It is a thin metal projection on casting.
Causes:-
i) incorrect assembly of moulds and cores
ii) Improper clamping of the mould
iii) excessive rapping of the pattern
iv) insufficient weight on the top part of the mould
Remedies:-
i)These can be controlled by adopting proper moulding, gating and melting techniques.
ii) insufficient weight should be placed on the top part of the mould
7. Swell: - It is un-intentional enlargement found on the casting surface due to liquid metal pressure.
Causes:-
i) improper ramming
ii) low strength of mould
iii) Pouring the metal too rapidly
Remedies:-
i) Proper ramming of sand
ii) uniform flow of molten metal into the mould
8. Run-out: - This defect occurs when molten metal leaks out to the mould during pouring. It results in incomplete casting.
Causes:-
i) defective moulding boxes
ii) inadequate mould weights
iii) excessive pouring pressure
Remedies:-
i) The corrective measures taken in respect of the above reasons will prevent this defect.
9. Warpage: - This is unintentional and undesirable deformation of casting produced during solidification of metal.
Causes:-
i) inadequate and improper gating, runners and risers
ii) continuous large flat surface on casting, indicate a poor design
Remedies:-
i) This defect can be eliminated by modifying the casting design and proper directional solidification.
10. Hot tears (Hot Cracks):- These are internal or external cracks resulting immediately after the solidification of metal.
Causes:-
i) abrupt changes in section
ii) poor design
iii) incorrect pouring temperature
Remedies:-
i) abrupt change in section should be avoided
ii) Pouring temperature should be correct
iii) there should be even rate of cooling
11. Core shift
12. Shrinkage
13. Core blow
14. Scabs
15. Pour short
16. Metal penetration
17. Rough surface finish