The following are the major defects, which are likely to occur in sand castings
- Gas defects
- Shrinkage cavities
- Molding material defects
- Pouring metal defects
- Mold shift
|Fig Casting Defects|
A condition existing in a casting caused by the trapping of gas in the molten metal or by mold gases evolved during the pouring of the casting. The defects in this category can be classified into blowholes and pinhole porosity. Blowholes are spherical or elongated cavities present in the casting on the surface or inside the casting. Pinhole porosity occurs due to the dissolution of hydrogen gas, which gets entrapped during heating of molten metal.
The lower gas-passing tendency of the mold, which may be due to lower venting, lower permeability of the mold or improper design of the casting. The lower permeability is caused by finer grain size of the sand, high percentage of clay in mold mixture, and excessive moisture present in the mold.
- Metal contains gas
- Mold is too hot
- Poor mold burnout
These are caused by liquid shrinkage occurring during the solidification of the casting. To compensate for this, proper feeding of liquid metal is required. For this reason risers are placed at the appropriate places in the mold. Sprues may be too thin, too long or not attached in the proper location, causing shrinkage cavities. It is recommended to use thick sprues to avoid shrinkage cavities.
Molding Material Defects
The defects in this category are cuts and washes, metal penetration, fusion, and swell.
Cut and washes
These appear as rough spots and areas of excess metal, and are caused by erosion of molding sand by the flowing metal. This is caused by the molding sand not having enough strength and the molten metal flowing at high velocity. The former can be taken care of by the proper choice of molding sand and the latter can be overcome by the proper design of the gating system.
When molten metal enters into the gaps between sand grains, the result is a rough casting surface. This occurs because the sand is coarse or no mold wash was applied on the surface of the mold. The coarser the sand grains more the metal penetration.
This is caused by the fusion of the sand grains with the molten metal, giving a brittle, glassy appearance on the casting surface. The main reason for this is that the clay or the sand particles are of lower refractoriness or that the pouring temperature is too high.
Under the influence of metallostatic forces, the mold wall may move back causing a swell in the dimension of the casting. A proper ramming of the mold will correct this defect.
Particles of slag, refractory materials, sand or deoxidation products are trapped in the casting during pouring solidification. The provision of choke in the gating system and the pouring basin at the top of the mold can prevent this defect.
Pouring Metal Defects
The likely defects in this category are
- Mis-runs and
- Cold shuts.
A mis-run is caused when the metal is unable to fill the mold cavity completely and thus leaves unfilled cavities. A mis-run results when the metal is too cold to flow to the extremities of the mold cavity before freezing. Long, thin sections are subject to this defect and should be avoided in casting design.
A cold shut is caused when two streams while meeting in the mold cavity, do not fuse together properly thus forming a discontinuity in the casting. When the molten metal is poured into the mold cavity through more-than-one gate, multiple liquid fronts will have to flow together and become one solid. If the flowing metal fronts are too cool, they may not flow together, but will leave a seam in the part. Such a seam is called a cold shut, and can be prevented by assuring sufficient superheat in the poured metal and thick enough walls in the casting design.
The mis-run and cold shut defects are caused either by a lower fluidity of the mold or when the section thickness of the casting is very small. Fluidity can be improved by changing the composition of the metal and by increasing the pouring temperature of the metal.
The mold shift defect occurs when cope and drag or molding boxes have not been properly aligned.
The defects in a casting may be due to pattern and moulding box equipment, moulding sand, cores,gating system or molten metal. Some of the defects are:
1: Mould shift
It results in a mismatching of the top and the bottom parts of the casting , usually at the parting line.
It is an enlargement of the mould cavity by molten metal pressure resulting in localized or general enlargement of the casting.
3: Fins and Flash
These are thin projections of the metal not intended as a part of casting. These usually occurs at the parting line of the mould.
4: Sand Wash
It usually occurs near the in the gates as rough lumps on the surface of a casting.
It is a crack or breakage in the casting on the surface of the work piece, which results from un equal contraction of the metal during solidification.
6: Hot Tear
It is an internal or external ragged discontinuously in the metal casting resulting just after the metal has solidified.
7: Sand Blow or Blow Hole
It is smooth depression on the outer surface of the casting work piece.
8: Honeycombing or Slag holes
These are smooth depression on the upper surface of the casting. They usually occur near the ingates.
These are patches of sand on the upper surface of the casting component.
10: Cold Shut and Misruns
These happens when the mould cavity is not completely filled by the molten and insufficient material or metal.
11: Run-outs and Bust-outsThese permit drainage of the metal from the cavity and result in incomplete casting.