Centrifugal Casting Process
Published on Mar 13, 2020
Centrifugal casting process is a subject which is emerging in the present foundries. The reason for this seminar being chosen is to get theoretical knowledge of centrifugal casting process and machines.
In this seminar all centrifugal casting methods, processes and the machines being used are discussed. An installation design, different types of molds and pouring process are also studied. This subject is becoming very popular in the casting of pressure controlling parts such as valves and actuators.
Casting is the process whereby liquid or molten material is poured into a mold and permitted to solidify. The mold and casting are separated from each other. Molds are frequently made of sand or other ceramic materials, although they may also be made of metal or carbon (graphite). The latter type of mold, whether metal or graphite, is referred to as “permanent mold” because it is reusable for making a large number of castings. Ceramic or sand molds, by contrast, produce only one cast of metal and are then destroyed; new molds must be made for each cycle of casting.
2.0 METHOD OF CENTRIFUGAL CASTING
There are to basic classes of centrifugal casting machines: Vertical and horizontal; These terms are related to the position of the rotation axis of the machines.
There are three methods for the utilization of centrifugal force for casting. All are referred to as “centrifugal casting’’, though more specifically we can distinguish between them as follows.
2.1 METHOD I- TRUE CENTRIFUGAL CASTING
Where no core is used, essentially all of the heat is extracted from the molten metal through the outer mold wall. The poor thermal conductivity of the air in contact with the internal diameter results in little heat loss from this direction. Thus, we obtain perfect direction solidification from the outside inward and grain growth is typically columnar. Because of favorable thermal gradients, in addition to the outward centrifugal force acting upon the molten each successive increment of metal to solidity is fed by the residual liquid metal in contact with it until solidification is complete. Under proper conditions, shrinkage porosity is non-extent.
This type of casting is “true centrifugal casting”. Perfect directional solidification exists with little or no effort on the part of the operator. It is due to the conformation of the mold. True directional solidification usually exists in all cylindrical castings.
2.2 METHOD II- SEMI-CENTRIFUGAL CASTING
This is very similar to Method I; however, due to the irregular contour of the internal bore, it is necessary to use a core. Solidification occurs in both in ward and outward direction, with the consequent problem of centerline soundness. Feeding, however, is enhanced by centrifugal force and is equivalent to the use of very high risers. Gates of various types may be used, some of which serve as a riser and others for the directional f metal into mold cavity. Method II is adaptable to a wide variety of cast parts such as jaw clutches, sheaves, gear blanks, casing heads, and flanges.
Centrifugal casting is also used for the production of another type of casing, one which has very thin metal section. Such casting would be very difficult to pour statically, because the molten might solidify before completely filling the mold cavity. The molten metal would not run into the very thin casting sections; thus, the casting would not be complete. By using centrifugal force in pouring the metal into the spinning mold, the additional force added to the normal static pressure head causes the metal to flow into the thin or narrow casting section.
2.3 METHOD III- CENTRIFUGE, OR PRESSURE CASTING
In this method (usually done vertically but sometimes horizontally) , there is a central spruce at the axis of rotation of the mold. Mold cavities are clustered about the central spruce in a symmetrical array, each connected to the spruce by one or more radial gates. Since conditions are not designed to promote ideal directional solidification, it is necessary to apply skill and ingenuity in gating so as to promote solidification from remote points of the casting towards the gate (which also acts as the riser). Usually only small castings such as union, valve bodies, gates, plugs, and intricate parts are cast by this method.
Most metals and alloys, when changing from the liquid to the solid state, undergo a reduction in volume referred to as “solidification shrinkage”. The extent of this shrinkage is often quite significant, usually mounting to 5 % or more. Unless precautions are taken in the production of casting, this type of shrinkage can manifest itself either as localized or distributed cavities inside the solidified casting.
Unless the mold cavity is initially made larger than the size of the specified finished casting, the part will be undersize. Thus, an allowance must be made for the contraction of the casting after is has solidified and subsequently cools to room temperature. This is known as the “pattern allowance” or “ patternmaker’s shrinkage allowance and depends on pattern geometry, metal cast, and method of casting.
Raining is harmful and can be caused by molten metal being poured at too high a temperature. It is very detrimental to the production of centrifugal casting, as it causes oxidation of the metal and is very likely to cause the formation of cold shuts, laps, or other types of similar defects on the outside diameter of the centrifugal tube.
Except in very specialized cases, such as the De Lavaud process, the metal is introduced at one end of the mold and caused to flow by centrifugal action throughout the mold length. If the spinning speed is adequate and if the fluidity of the molten metal is satisfactory, the metal will distribute itself uniformly in section throughout the mold before solidification.
If the temperature of the molten metal is too high or if the fluidity of the molten metal is too great, the metal will not readily accelerate to the speed of the mold. In a horizontally spinning mold, it will not have sufficient rotational velocity. The force of gravity will tend to make part of the metal fall from the top of the mold down into the body of the metal at the bottom of the mold . This phenomenon in centrifugal casting is called “raining ”.
If raining occurs, several correctional measures can be taken, depending on which effect is causing the raining .It is possible that the fluidity is too high, which could be caused by the metal being at an excessively high temperature. Therefore, reduction in the temperature of the metal would be advisable. The mold coating might be too smooth, in which case a rougher mold coating could be applied. This would permit the metal to gain velocity more readily because of the increased friction between the molten metal and the spinning mold.
Casting can be removed from metal molds by using a long bar with a flange at the far end. The bar is inserted into the bore of the casting, hooks onto the end, and pulls the casting out of the mold. In some cases, it may be necessary to use a hydraulic ram to force the casting out of the mold. However, this is only necessary if an incorrect type of mold coating is used that does not permit an easy release of the casting from the mold. Generally, the castings are very free and can be easily removed from the mold.
Various types of pulling tongs can be used for casting removal and can be operated either mechanically or pneumatically. These tongs are inserted into one end of the bore of the casting and clamped against the inside diameter of the pipe. This permits the pipe or tube to be readily removed from the mold. For the smaller sizes, the casting removal device is similar to a reverse ice tong, and can be used by an operator to pull the casting out of the mold very easily by hand.
The parameters in this report are introduced and discussed to get a brief and “to the point” behaviors of centrifugal casting machines & centrifugal casting process. As the whole industrial zone is switching towards silicon wave of automation it is but essential to work for the core technological issues in mechanical industries. Now onwards, power sector will have a tremendous demand and responsibility so industries like ONGC, BHEL and others who are in the manufacturing of products such as valves, actuators have adopted this centrifugal casting technology.
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