In centrifugal casting process, the molten metal poured at the centre of a rotating mold or die. Because of the centrifugal force, the lighter impurities are crowded towards the centre of the mold. For producing a hollow part, the axis of rotation is placed at the centre of the desired casting. The speed of rotation is maintained high so as to produce a centripetal acceleration of the order of 60g to 75g. The centrifuge action segregates the less dense non-metallic inclusions near to the centre of rotation that can be removed by machining a thin layer. No cores are therefore required in casting of hollow parts although solid parts can also be cast by this process.
The centrifugal casting is very suitable for axisymmetric parts. Very high strength of the casting can be obtained. Since the molten metal is fed by the centrifugal action, the feeding is simple. Both horizontal and vertical centrifugal castings are widely used in the industry. Figure 25 schematically shows a set-up for horizontal centrifugal casting process. Figure 25 typically shows large pipes that are made using centrifugal casting process.
Figure 25 Centrifugal casting process
Features of centrifugal casting
• Castings can be made in almost any length, thickness and diameter.
• Different wall thicknesses can be produced from the same size of mold.
• No need for cores.
• Resistant to atmospheric corrosion, a typical situation with pipes.
• Mechanical properties of centrifugal castings are excellent.
• Only cylindrical shapes can be produced with this process.
• Size limits are up to 3 m diameter and 15 m length.
• Wall thickness range from 2.5 mm to 125 mm.
• Tolerance limit: on the OD can be 2.5 mm on the ID can be 3.8 mm (0.15 in).
• Surface finish ranges from 2.5 mm to 12.5 mm.
.Casting Defects
Figure 26 schematically shows various defects that are arise or occur during casting, especially sand casting processes, some of defects are indicated as follow.
Shrinkage
Shrinkage of molten metal as it solidifies is an important issue in casting. It can reduce the 5-10% volume of the cast. Gray cast iron expands upon solidification due to phase changes. Need to design pattern and mold to compensate of this amount of volume decreasing is important. Shrinkage defect can be reduced by good design the riser and pattern and molds.
Porosity
Porosity is a phenomenon that occurs in materials, especially castings, as change of state from liquid to solid during the manufacturing process. Casting porosity which either effects the surface finish or as a leak path for gases and liquids. The pouring temperature should be maintained properly to reduce porosity. Adequate flux of metal, controlling the amount of gas-producing and materials in the molding, core making and sand mixes can help in minimizing this defect.
Hot tear
Hot tears are internal or external discontinuities or crack on the casting surface, caused by rapid contraction directly after the metal solidified. They may be produced when the casting is poorly designed and sudden sectional changes occur. Incorrect pouring temperature and improper placement of gates and risers can also create hot tears. Method to prevent hot tears may improving the casting design, achieving directional solidification and even rate of cooling, selecting proper mold and materials to suit the cast metal, and controlling the mold hardness in relation to other components of sand.
Figure 26 Schematic of various casting defects..
Blowhole
Blowholes are smooth round holes that are clearly on the surface of the casting. To prevent blowholes, moisture content in sand must be well adjusted, sand of proper grain size should be used, ramming should not be too hard and venting should be adequate.
Shift
A shift results in a mismatch of the sections of a casting usually as a parting line. This defect can be prevented by ensuring proper place of the pattern for die parts, molding boxes, and checking of pattern locating pins before use.
Warped casting
Warping is an undesirable deformation in a casting which occurs during or after solidification. Wrap edge occurs in large and flat sections especially. Wrap edge may also be due to insufficient gating system that may not allow rapid pouring of metal or due to low green strength of the sand mold or inadequate tolerance in the pattern / mold cavity.
Metal Penetration and Rough Surfaces
This defect appears as rough external surface of the casting. It may be caused when the sand has too high permeability, large grain size, and low strength. Soft ramming may also cause metal penetration.
Fin
A thin projection of metal, this not part of casting, is called a fin. Fins occur at the parting of the mold or core sections. Molds and cores incorrectly assembled will cause the fin. Insufficient weighing of the molds or improper clamping of flasks may again produce the fin defect.
Cold Shut
A cold shut is a defect in which a discontinuity is formed due to the imperfect fusion of two streams of metal in the mold cavity. The reasons for cold shut may be too thin sections and wall thickness, improper gating system, damaged patterns, slow and discontinuity pouring, poor fluidity of metal caused by low pouring temperature, improper alloy composition, etc.
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