Name the three types of centrifugal process. Draw the neat sketch of the process that is used to produce multiple castings and explain the process
Answers
Various types of centrifugal casting methods are in use. The main principle behind any centrifugal casting is metal that is poured in to a rotating mold exactly at center is thrown outward due to the action of centrifugal force. The main advantage of centrifugal casting is the impurities get accumulated exactly at the center. The types of centrifugal castings are
1) True centrifugal casting
2) Semi centrifugal casting
3) Centrifuge casting
True centrifugal casting
This casting technique is employed when axis symmetrical objects with uniform diameter are to be produced. Core is not employed in these castings
Procedure
A cylindrical mold is made to rotate about its axis at certain speed. This speed is maintained in such a way that the metal which is poured at the center is exactly thrown to the walls of the mold. In this casting solidification starts from outside to inside. Wall thickness of mold is regulated by amount of liquid that is being poured. The impurities present in the molten metal due to its less weight and can be machined easily. The molds of centrifugal casting are made by metal (or) sand lined. The quality of mold depends on the quality required.
Advantages of true centrifugal casting
1) Relatively very light impurities move inwards towards center. So they can be removed easily thus helping in producing sound castings.
2) These castings have a directional solidification starting from outside to inside.
3) Central core is not required for making a hole or pipe
4) Employment of gates and risers is not required
5) This technique is best suited for the mass production of symmetrical objects.
6) Castings yield is very high in some cases it is even equal to 100%
Disadvantages of true centrifugal casting
1) Centrifugal castings require very high investments.
2) Skilled labors are to be employed for this process.
3) Only some shapes can be generated by this casting process.
Applications
1) Bearings for machines
2) Pipes
3) Liners for IC engines
4) Rings and other annular components
Semi-centrifugal casting
This is also similar to the true centrifugal casting but there is a modification in it. These castings consist of a central core. Semi centrifugal casting is employed for casting gear blanks, sheaves, and wheels etc. low spinning speeds can be used speeds ranging between 180 to 200 RPM is recommended. These castings are normally prepared in vertical machines. The mold cavity is arranged with in the mold so that its central axis is vertical and concentric with the axis of rotation. Gating system is employed in the casting for having a directional solidification.
Advantages
1) In this number of molds can be stacked together, one over the other can be fed by a common central sprue in order to produce more than one casting at a time.
2) Similar to true centrifugal casting it ensures purity and density of the product poorer structure forms the center of the casting so this can be machined off.
Centrifuge casting
This type of casting method is used for casting unsymmetrical castings in groups. This is done in groups for obtaining balance of whole casting. In these castings the axis of mold doesn’t coincide with each other to induce the pressure on molds the casting rotated at the center. Feeding to the mold cavities is done by central sprue by the action of centrifugal forces. When castings are generated in multiple layers by placing one over the other then it is called stack molding.
Advantages of centrifugal casting
1) Ensures better quality.
2) Castings when generated on large scale will be economical.
3) These castings generate high yield
4) The castings obtained by this method cannot be satisfactorily obtained by other casting methods.
5) Cleaning and fettling cost of the castings can be reduced.
6) Denser castings having physical properties similar to those of forged products
7) Rejection percentage is very low
8) Easy way to achieve directional solidification