01-06-2015, 04:19 PM
Abstract
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.
Introduction
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.
Shrinkage
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
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 Removal
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.
Conclusion
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.