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.
CENTRIFUGAL CASTING Vs. FORGING AND STATIC CASTING
The facilities required for centrifugal casting are not elaborate but somewhat more expensive than those required for static casting. However, the smaller floor space requirements of centrifugal casting may offset its greater initial expense.
Most centrifugal foundries have complete testing facilities to meet customers high quality demands. On the average, initial equipment investment is highest for forging and for extruded material and lowest for static sand casting, with centrifugal lying between, competing with both.
CENTRIFUGAL CASTING PROCESS.
The process for centrifugal casting differs from static casting in that the mold itself is spinning during the time the casting is solidifying. Centrifugal castings are usually poured while the mold is spinning; however, for certain applications, particularly in the case of a vertical casting, it is sometimes preferable that the mold be stationary when pouring begins. The machine then accelerates the speed of rotating mold either during the filling of mold or after completion of pouring. In other cases, such as horizontal centrifugal casting, it is often desirable to have the mold rotating at a lower speed.
Centrifugal Castings are produced by pouring molten metal into a mold that is being rotated or revolved. Molten metal is poured into the spinning mold cavity and the metal is held against the wall of the mold by centrifugal force. The speed of the rotation and metal pouring rate vary with the alloy and size and shape being cast.
Speed pouring is followed by rapid acceleration to a higher speed during the solidification period. The application of centrifugal force to a metal as it solidifies can be used to achieve a dense sound 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.
CENRIFUGAL CASTING
The basic process is,
1. a mold is set up and rotated along a vertical, or horizontal (200-1000 rpm is reasonable ) axis.
2. The mold is coated with a refractory coating.
3. While rotating molten metal is poured in.
4. The metal that is poured in will then distribute itself over the rotating wall.
5. During cooling lower density impurities will tend to rise towards the center of rotation.
6. After the part has solidified, it is removed and finished.
Type metals cast are
1.Steel
2.Nickel alloys
3.Copper
4.Aluminum
Typical application are,
1.Train wheels
2.Jewelry
3.Seamless pressure tubes/pipes
Advantages,
1.Good uniform metal properties
2.No spruces/gates to remove
3.The outside of the casting is at required dimensions.
4.Lower material usage.
5.No parting lines
6.Low scrap rates
Disadvantages,
1.Extra equipment needed to spin mold
2.The inner metal of the part contains impurities.
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.
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.
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.
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.
TYPES OF MACHINES
INTRODUCTION
There are various types of casting machines used to produce centrifugal castings: horizontal axis machines, vertical axis machines, and face-plate machines. True centrifugal castings may be produced in either vertical or horizontal axis casting machines.
TYPES OF CENTRIFUGAL CASTING MACHINES
Different types of Centrifugal casting machines are :
1 Horizontal axis casting machines.
2 Vertical axis casting machines.
3 Vertical Roll machines.
4 Vacuum Machines.
5 Babbitting machines.
6 Face plate casting machines.
7 Inclined axis casting machines.
8 Over and Under casting machines.
The most commonly used casting machines are discussed below :
HORIZONTAL AXIS CASTING MACHINES
For true centrifugal casting, we assume that it is essentially a tube or cylinder with the bore concentric to the outside diameter. It is possible for the tube or cylinder to have various diameters on the outside, but it must have a uniform diameter on the inside. In other words, the inside diameter of the tube must be a cylinder. As a rule of thumb, one ordinarily uses a vertical machine to make castings whose length is less than two times its inside diameter. If the length is more than two times the inside diameter, the casting is usually made in a horizontal machine.
In the case of horizontal trunnion machines, the mold itself is mounted on the trunnions. If the mold is machined with the inside diameter of the mold concentric with the outside diameter of the mold, the castings produced will always be absolutely concentric with the mold. The wall section is the same throughout the circumference of the casting. It is usual to use horizontal machines to produce castings having a true cylinder as the inside diameter.
VERTICAL AXIS CASTING MACHINES
Vertical centrifugal casting machines are made in many different sizes, capable of handling loads up to at least 50 tons. Horizontal centrifugal machines are also capable of handling loads in this range or greater. There appears to be no limitation as to the weight or size of castings that can be produced centrifugally. The same basic criteria exist for small castings as for large castings. Accordingly, the vertical casting machines are made in several configurations. Some of them are mounted on the floor and are readily portable, while others for producing heavier castings are mounted below floor level in heavy concrete foundations. Various types of guards are used about the machines to protect the workmen from the possibility of any flying molten metal.
Basically, a vertical machine is one having spindle or shaft with a set of bearings, driven at one end by some type of motor, and having a face- plate or table at the other end. In case of a true vertical centrifugal casting, there will be some vertical taper on the inside surface of the casting due to the earth’s gravitational force. At the speed normally used for the rotation of the mold, the amount of this taper is generally 1/8 inch (3mm) per foot. This small amount of taper is usually imperceptible unless the length of the casting is great. For this reason there is a practical limitation on the height of castings made in vertical machines.
INSTALLATION DESIGNS
INTRODUCTION
All centrifugal casting machines require a relatively heavy, firm foundation. It is highly desirable that the mold be perfectly dynamically balanced. However, as a practical matter, it is not possible to have perfectly balanced mold on a machine. The casting machines are designed to accept a relatively large amount of imbalance in the molds and even in the casting. The machines are mounted on heavy bases, usually of steel or cast iron, and then on very heavy concrete foundations. Sometimes small vertical or horizontal machines are mounted on a structural steel base, which can then be bolted down so as to allow the machine to be moved from one place to another for experimental or varied work.
VERTICAL CASTING MACHINE INSTALLATION
Vertical centrifugal casting machines are generally mounted in a concrete pit with the table of the machine at floor level. In some cases, particularly with the large vertical machines, the table is located below floor level. When the table is located below floor level, the top of the mold will also be below the floor level.
GUARDS
This configuration utilizes the pit itself as guard for protection from flying molten metal, etc. However, for smaller machines it is convenient for the table to be located at floor level so that molds can be easily placed on or removed from the machine. In this case, a guard surrounding the machine would be used during the pouring operation.
Various types of guards are available. Some raise themselves up and rotate so as to present an area of the machine table to the workmen for mold removal and replacement. In other cases, a simple bail-type guard is used and is later removed by means of a crane or hoist.
HORIZONTAL CASTING MACHINE INSTALLATION
Horizontal centrifugal casting machines are usually mounted at floor level; sometimes slightly lower. The elevation above floor is determined by the need for easy accessibility. The centerline of a machine should be elevated or lowered depending upon the ease of pouring, attaching and removing end plates, mold preparation, and casting removal. Pipe machines are generally located somewhat above floor level so that the center of rotation of the mold is at a convenient height for the operator to place the bell cores in one end of the mold. In most cases, a permanent mold has to be internally coated with mold coating. This can be performed more conveniently if the mold centerline is located above floor level.
GUARDS
From a safety standpoint it is desirable for the machine to be enclosed to protect the workers, particularly during the pouring operation . It is possible for a run-out to occur from one or both ends of spinning mold, which would cause a pinwheel of molten metal to be ejected from the end of the mold. A suitable guard is quite important.
The guards are made of steel. Some have a hinged top so that the molds can be readily removed and replaced. In other cases, the entire guard is lifted off the machine to replace the mold. For very small machines it is usually possible to remove the mold from under the guard without removing the guard from its normal position.
MOLDS
MOLD TYPES
Two categories of molds are used in centrifugal casting : expendable molds and permanent molds. Each will be considered in more detail below.
EXPENDABLE MOLDS
The expendable molds are usually made of sand, ceramic, or plaster. With such materials, it is necessary to completely destroy the mold to remove the casting. The methods for producing such molds for centrifugal casting are very similar to those used in ordinary static sand casting or in the “lost wax process”.
In centrifugal casting, expendable molds find only limited use. They are primarily used when a very slow rate of cooling is required for metallurgical reasons, or because the casting is so large that a permanent mold would not be economically justified. This type of molding method is quite popular for the production of large, tubular products, such as stern tube bearing and ship shaft liners.
Current centrifugal practice is to produce everything possible in a permanent mold of one type or another. However, there are specialized cases where the use of permanent molds is not feasible. These include designs configurations where the casting cannot be extracted from a permanent mold due to back drafts or protuberances, or simply where production requirements are very small. Accordingly, it would not be economical or possible to produce the casting in a permanent mold. In such cases, sand-lines molds are used.
The material for the mold can be silicate-C02 sand, dry sand, chamotte, or any type of fairly strong ceramic mold material. No special requirements are necessary for these materials for use in centrifugal casting, it is almost imperative that a suitable mold wash be applied to all surface that come into contact with the molten metal in order to reduce mold erosion. Exclusive mold erosion does not usually occurs because the speed of rotation are generally rather low when producing centrifugal castings in sand.
PERMANENT MOLDS
A permanent mold is one mode of metal or graphite and can be used repeatedly for the production of many casting of the same form. Permanent molds are divided into two rough classification viz molds made graphite or carbon and molds of metal as steel, cast iron, or copper.
MOLD PRE-TREATMENT
After a mold has been made it requires a pre-treatment process before being put into service. The pre-treatment requires that the mold be heated to a temperature of from 200 to 300 F/93 to 149 C. The inside of the mold is then swabbed with a saturated solution of ammonium persulfate or other mild etchant. This will very slightly etch the inside of the mold as well as clean the mold of all oil or grease. Sometimes the inside of the mold is lightly blast-clean to provide lightly rough finish. After the treatment, the mold should be cleaned thoroughly with fresh water.
The purpose of this pre-treatment is to remove all oil or grease that may be on the mold as well as to slightly roughen the mold surface. This will permit the mold wash that will be used to have better adherence to the mold. This treatment need not be performed each time the mold is put in service, but only on a few mold, or on a mold which has in some way accumulated oil or grease on the inside diameter.
POURING
POURING TEMPERATURES
Although some emphasis has been placed on the importance of the actual pouring temperature, it is not most important factor when producing a centrifugal casting. The speed of rotation and the technique of introducing molten metal into the spinning molds are of equal, if not greater, importance. Nevertheless, a few things regarding temperature should be mentioned here.
When pouring metal casting in permanent molds, either statically or centrifugally, in most cases the metal pouring temperatures are exactly the same , However ,in a few cases it is necessary to pour centrifugal casting at a higher or lower temperature than that used for static casting .Because of the fact that the mold is spinning during the centrifugal casting process, the molten metal is supplied with additional velocity over that which exists during the static casting process. The result is that the metal has more mobility and additional kinetic energy. At the same time, it is possible for the metal to become super-cooled when pouring into permanent molds. This account for the fact that centrifugal casting can, in many cases, be poured at lower metal temperature than static 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. The speed of rotation could be increases, which would tend to assist the metal reaching a higher spinning speed more quickly because of the increased friction effect.
POURING EQUIPMENT
When pouring into either a horizontally or vertically spinning mold, the metal should be introduced so that it has initial velocity in the direction of the molds rotation. This can be done fairly easily by utilizing a right -angle nozzle spout in the runner pot. The spout effectively diverts the flow of the molten mental from a direction in line with the axis of rotation to one at nearly 90 degrees to the axis of rotation. By utilizing this re-directed flow and the force of gravity, the flowing mental can be diverted to impinge upon the mold in a direction closer to that of the direction closer to that of the direction of rotation. Since the initial velocity of the molten metal is appreciable, it can more readily be accelerated to the spinning speed of the mold and ,therefore , inhibit slippage and mold erosion.
When the initial molten metal is introduced into the mold, it should enter the mold quickly and more or less as mass in one location. The centrifugal force will drive. the molten metal longitudinally, causing it to flow down the length of the mold and form a tube. When this occurs, there will be an absence of cold shuts and the metal will flow smoothly from the pouring end of the mold to the far end of the mold.
POURING RATES
Pouring rates for centrifugal castings are generally appreciably higher than those used in static casting. It is found that for heavy casting weighing 400 kg or more, pouring rates from 400 to 800 kg or less a pouring rate of only 8 kg per second can be obtained. This is because is practically impossible to have a higher pouring rate for a small quantity of metal. A 40 kg casting should be poured in about 5 seconds.
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. This casting removal device can also used to carry the casting a short distance to put it in a tool box or into storage.
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.
There are further details to study as this gives the explanation in brief.
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