Defects during the casting process are not uncommon. Under practical conditions, all casting products contain voids, inclusions and other imperfections which contribute to a normal quality variation. Such imperfections begin to be regarded as true defects or flaws only when the satisfactory function or appearance of the product is in question: consideration must then be given to the possibility of salvage or, in more serious cases, to rejection and replacement.
Classification
The general origins of defects lie in three sectors:
1. the casting design,
2. the technique of manufacture–the method,
3. the application of the technique–‘workmanship’.
Standardization and optimisation of all aspects of the production technique offers the best way-out against such troubles. More specific precautions can be taken in those cases where there is a known susceptibility to a particular defect, whilst the radical approach of design modification may need to be considered in the extreme cases which do not respond to changes in foundry technique. It should be clear that any approach to the elimination of casting defects must be on an economic basis.
1. Shaping faults arising in pouring.
2. Inclusions and sand defects.
3. Gas defects.
4. Shrinkage defects due to volume contraction in the liquid state and during solidification.
5. Contraction defects occurring mainly or wholly after solidification.
6. Dimensional errors.
7. Compositional errors and segregation.
Significance of analysis of defects
Casting defects, whatever their cause, cannot be considered in isolation; their significance can only be established in relation to the function of the casting. Behaviour under service stresses and environment is in most cases the important reason behind the appearance.
What are the reasons of their occurrence? The first one is the
quality and the parameters of moulding and core mixtures. Number of the
defects was affected by temperature and humidity of ambient air, both in
store yards and in moulding mixture preparation shop. Consequently, it is
reflected in moisture content of silica sand and moulding mixture. This fact
can affect the quality of moulding and core mixtures and their prone to
disintegration by molten metal flow.
The second one is the presence of complicated design of the cavity in the mould,
containing thin walls, profiles, runners. From this fact follows the necessity
to design parameters of moulding mixtures from the mould complexity
point of view.
The third one is chemistry and pouring temperature of molten metal.
Viscosity of poured metal is extremely important.
