Following defects occur in casting made in green sand because of its high moisture content and low strength :
- Blowholes : These are caused by excessive moisture, low permeability, fine grains, gas producing ingredients, hard ramming and improper venting. Hence these factor should be properly controlled to eliminate blowholes.
- Drop: Drop in casting occur when a portion of the sand drops into the molten metal from the top part of mould or other overhanging section. This defect is caused by low strength and low hardness of the mould or by not properly reinforcing the mould. This defect is remedied by proper moulding.
- Dirt: In this defect, particles of dirt and sand are embedded in the casting due to crushing of mould, due to mishandling of sand and slag particles. They may be prevented from entering the mould cavity by proper fluxing and use of dirt traps.
2. Defects due to mold and core making techniques.
- Shifts : It is a misalignment of the top and bottom parts of the mold due to which an inaccurate casting is produced. This may be caused by not properly locating the pins of pattern plate when using half patterns. This also may be caused if moulding box is twisted. To avoid this, box pins should be properly located and worn pins removed.
- Fins : It is an undesired projection of metal found along the parting line. These defects are caused if mould parts are not clamped tightly and some opening are lift out between mould parts. To avoid this, sufficient weight should be placed on the top part of the mould so that the two parts tightly together.
- Crush : It is an irregularly shaped cavity projection on the casting used by displacement of the sand mould joints or core print which usually occurs when mould is being closed. These defects may be caused by badly made mould joints and excessive pressure on the sand surface. Too large cores on the small cores on too small cores joints also produced cracks. This may be avoided by making good mould joints and the correct length of cores and core points.
- Swell : It is a local displacement of the sand in the mould which is caused by the pressure of the molten metal. A swell is usually caused by soft ramming of the mould and may be avoided by ramming harder and evenly.
3. Defects due to casting process.
- Blow holes and Gas holes : Blow holes are cavities in the casting which may be spherical, flattened or elongated. These are caused by the evolution of gas during solidification of the castings. As the gas follows the path of least resistance, the blow holes usually appear in the cope portions of the castings.
Gas holes are those which appear on the casting when the casting is machined or cut into sections.
(a) Due to sufficient core point or outlet provided for venting, the gas cannot escape the mould as fast as it is generated in the core during the pouring operation. As a result, a gas pressure is built up in the cores and the gas escape through the core at its weakest point. This escaped gas tend to gravitate to molten metal. Metal escape from the space between the core and the mould and the caps, and over the vent opening. To avoid this use crush grooves on core print to seal off metal or provide crush around core to insure vent in core to remain open.
(b) Whenever a casting containing have metal sections and whenever the casting design prevents adequate metal pressure, gas holes are formed because without adequate metal pressure, the gas will not be forced through the light sections and therefore, it will tend to congregate in the heavier portions of metal in the casting. To avoid this use of external of internal chills or risers or, if possible, by redesigning the casting to either reduce heavy sections or provide proper gating.
(c) Sometimes, due to excessive amount of moisture at the parting line, gas may accumulated the parting line which may caused blow holes. Gas accumulation in this area may also occur due to hard ramming. These defects may be minimized by checking the permeability of the sand, leading vents through mould by drying the area.
(d) In some cases where shallow copes are used, moulds have to be rammed hard by hand, to hold the sand in the cope part of the mould. Due to this, the mould will not allow easy escape of gases. When the molten metal comes in the contact with the cope surface of the mould. The send resultant trapped gases then cause blow holes. These defects may be minimizes by raising sand permeability and by reducing the height of sprue risers.
(e) When flask wall is too close to mould surface, the ramming may be too hard and strong and easy escape of gas would be difficult, which will cause gas holes.
- Misruns or cold shuts : Misrun casting is an arc which lacks completeness due to failure of the metal to fill the mould cavity.
Cold shut casting is one which a definite discontinuity exists due to fluidity of metal being too low. This occurs at a place where metals meet in the flow from two sides.
4. Defects due to impurities in molten metal.
It is an external defects causing a number of cavities in close promixity. It is caused by dirt or swarf held in the molten metal, imperfect skimming and poor quality of molten metal. They may be internal or external cavities/cracks immediately after solidification. Metal, having low strength undergo widened shrinkage causing high stresses and the metal will fail, resulting in tear.
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