Centrifugal Casting

CENTRIFUGAL CASTING

1. INTRODUCTION

Centrifugal casting is the manufacturing process used to cast symmetrical objects having axis of revolution such as pipes, tubes gear blank etc. The centrifug centrifugal al casting casting process process uses uses rotati rotating ng molds molds to feed feed molten molten metal metal uniformly into the mold cavity. The centrifugal force of the rotating mold forces the molten against cavities of the mold under constant pressure until the molten metal has solidified. There are essentially two basic types of centrifugal casting machines the horizontal type, which rotates about horizontal axis and vertical type, which rotates about horizontal axis and vertical type, which rotates about a vertical axis. Differ Different ent types types of molds molds are used used in centri centrifug fugal al casting casting process. process. old old is designed according to various design considerations. Tubular casting produced in permanent molds by centrifugal casting usually have higher yields and higher mechanical properties.

College Of Engineering Pandharpur

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CENTRIFUGAL CASTING

2. CLASSIFICATION

Centrifugal casting process is classified as, True centrifugal casting: -

"t is used to produce cylindrical or tubular castings by spinning the mold about its own axis castings produced by this method will always have a true cylindrical bore of inside diameter regardless of shape or configuration. Castings produced in metal molds by this method have true directional cooling or solidification from the outside of the casting or solidification from the outside of the casting toward the axis of rotati rotation. on. This This direct direction ional al solidi solidifica ficatio tion n results results in the produc productio tion n of high high #uality #uality edfect$free casting without shrinkage.

Semicentrifugal Semicentrifugal casting: -

"t is used to produce castings with configurations determined entirely by the shape of the mold on all sides inside and out, by spinning the casting and mold about is own own axis. axis. Cores Cores may may be nece necessa ssary ry if the the casti casting ng is to have have hall hallow ow secti section ons. s. Directional solidification is obtained by proper gating. Casting that are difficult to produce statically can often produced by this method, because centrifugal force feeds the molten metal under pressure higher than static casting. casting. Thinner castings sections can be produced with this method than with static casting. Typical castings of this type include great blanks pulley sheaves, wheels, impellers and electric motor rotors.

Centrifuge Centrifugal Casting: -

"t has the widest field of application. "n this method the casting cavities are arranged arranged about the center axis of rotation rotation like the spokes spokes of a wheel, thus permitting permitting the production of multiple castings. Centrifugal force provides the necessary pressure on the molten metal in the same manner as in semicentrifugal casting. This casting method is typically used to produce valve bodies and bonnets, plugs, yokes, brackets and a wide variety of various industrial castings. College Of Engineering Pandharpur

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CENTRIFUGAL CASTING


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CENTRIFUGAL CASTING

. !ORI"ONTAL C#NTRIFU$AL CASTIN$

%rinci&le: -

'orizontal centrifugal casting is used to cast pieces having an axis of revolution. The techni#ue was the centrifugal force generated by a rotating cylindrical mold to throw the molten metal against the mold wall and from the desired shape. #'ui&ment: -

( horizontal centrifugal machine has following importing important parts. !)

old

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*oller track

&)

+nd plate

)

-pinning pot

)

Cooling track

/)

-pout

0)

'ousing

%r(cess Details: -

olten metal can be introduced into the mold at one end, at both ends, or through channel of variable length. 1ouring rate widely according to the size of the casting being produced and the metal being used. 2or example, a pouring rate of !.! t %.% tons3min. has been used to cast/ alloy steel tubes of $$mm in outside diameter with $mm thick walls. 1ouring rates that are too slow can result in gas porosity while high rates slow solidification are one of the main causes of longitudinal cracking.

Cast tem&eratures: -

The degree of superheat re#uired to produce a casting is a function of the metal or alloy being poured. The following empirical formula has been suggested as general guideline to determine the degree of super heat. 4 5 %.  T 6 !!$


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CENTRIFUGAL CASTING

7here the length of spiral fluidity 8in mm) and

 T

is the degree of superheat

8in degrees centigrade) old Temperatures 9 "nitial mold temperatures vary over a wide range according to the metal being cast, the mold thickness, and the wall thickness of tube being cast. "nitial mold temperature does not affect the structure of the resultant casting as greatly as the process parameters discussed above do.


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CENTRIFUGAL CASTING

S&ee) (f r(tati(n: -

:enerally the mold is rotated at a speed that creates a centrifugal force ranging from 0 to !%$g speed of rotation is varied during the casting process. The cycle can be divided into three parts; < (t the time of pouring, the mold " rotating at a speed sufficient to throw the molten metal against the mold wall. (s the metal reaches the opposite end of the mold, the speed of rotation is increased. -peed of rotation is held constant for a time after pouring the time at constant aped varies with mold type, metal being cast, and re#uired wall thickness. The ideal speed of rotation causes rapid adhesion of the molten metal to the mold wall with minimal vibration. -uch conditions tend to result in a casting with a uniform structure. (s the molten metal enters the mold a pressure gradient is established across the tube thickness by centrifugal acceleration. Too low a speed of rotation can cause sliding and result in poor surface finish. Too high speed of rotation can generate vibrations, which can result in circumferential segregation.


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CENTRIFUGAL CASTING

*. +#RTICAL C#NTRIFU$AL CASTIN$

%rinci&le: -

"n vertical centrifugal casting uses the centrifugal force generated by a cylindrical mold about vertical axis throw the molten metal against the mold wall and form the desired shape. #'ui&ment: -

( vertical centrifugal casting machines have following important parts. !)

old

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(dapter table

&)

2unnel

)

-pinner machine

%r(cess )etails: Casting Insi)e Diameters: -

7hen making castings on a vertical centrifugal casting machine, the inside diameter 8bore) of the casting will be tapered in accordance with the following formula.

n 5%/

h r!% 9 r%%

7here n is speed of rotation, r! is the inside radius at the top of the casting, r% is the inside radius at the bottom of the casting and h is casting height.


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CENTRIFUGAL CASTING


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CENTRIFUGAL CASTING

S&ee) (f r(tati(n: -

To establish a temperature gradient of the molten metal from the outside diameter towards the centre of rotation. "t is usually necessary for the mold to be spinning when the metal is poured. "n some cases, in order to eliminate defects such as erosion and dirt in sand molds, it is desirable to pour at a slow speed of rotation. 'owever, true centrifugal castings having a wall section of !%.0mm or less must be poured at spinning speed because the metal in this thin section. >omographs are available for determining the proper speed of rotation for centrifugal casting. +#uation used to calculate spinning speed. : 5 $,$$$$!%Dn 7here g is the centrifugal force, D is the inside diameters of the casting, and n is the speed of ration.


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CENTRIFUGAL CASTING

,. %OURIN$ T#C!NIU#S

%(uring Tecni'ues In Centrifugal Casting

2or permanent molds, the metal is generally poured about $ $ C higher than the temperature used for the same casting if poured statically in a sand mold. This is because of the more rapid chilling effect of permanent molds. The pouring rates re#uired for successful permanent mold centrifugal casting are #uite high compared to those for static casting in sand molds. "t is particularly important that the initial rate of pour at the beginning be very high to prevent cold laps and cold laps and cold shuts. 2or most types of centrifugal castings weighing less than  kg, a pour rate of about ? kg3s, is recommended. 2or castings weighing up to $ kg, an initial pour rate of g to %& kg3s is recommended. 7hen pouring into spinning mold, it is recommended. 2or castings weighing more than $kg, pour rate of  to ?$ kgs are recommended. 7hen pouring into spinning mold, it is important to introduce the molten metal into the mould in such a way as to prevent or minimize turbulence of the molten metal, which can cause splashing, spraying, of droplets and can result in undesirable defects. (lthough many vertical centrifugal castings can be poured directly into the mold from the ladle to produce a #uality centrifugal casting it is more often desirable to use a pouring funnel. 7ith a pouring funnel, the nozzle can be lined to the re#uired diameter so that, with a certain raises height of molten metal in the funnel, a controlled pour rate can be obtained for a particular casting weight. "n addition, with a pouring funnel, the entry of molten metal into the mold can be made to impinge upon the body of the mold with initial metal floe in the direction of mold rotation. This type of pouring will provide superior casting #uality by minimizing or eliminating any upsetting turbulence in the flow of molten metal that might cause defects. #/tracti(n (f castings: -

Commercially available casting pulling tongs 82ig. 0) are recommended for extracting vertical centrifugally cast castings. These pullers engage onto the inside diameter of the casting and are used to lift the casting from the mold. College Of Engineering Pandharpur

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CENTRIFUGAL CASTING


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CENTRIFUGAL CASTING

0. SOLID#FICATION

"n centrifugal casting, heat is removed from the solidifying casting only through the water
The mold, including the mold material, its thickness, and initial mold temperature.

•

The thickness and thermal conductivity of the mold wash used.

•

Casting conditions, including degree of superheat, pouring rate and speed of

rotation. •

(ny vibration present in the casting system.

Termal As&ects (f S(li)ificati(n: -

"t appears that the mold
The as
Aery thin, fine columnar skin.

•

7ell
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ore or less fine e#uiaxed structure. "n the case of steels that solidify as ferrite, the columnar areas may bye

nonexistent if superheat and mold wash thickness are low. "n steels that solidify as austenite, it is relatively easy to obtain well

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CENTRIFUGAL CASTING

. OLDS

(l)s use) in centrifugal casting: -

olds consist of four parts, the shell the casting spout, roller tracks and end heads. The mold assembly is placed on interchangeable carrying rollers that enable the use of different mold diameters and fine adjustments. olds are cooled by a water spray, which can be divided into severed streams for selective cooling. Different types of molds are generally used according to the geometry and #uantity of castings needed and the characteristic of the metal or allow being cast.

#/&en)a3le (l)s: -

+xpendable molds lined used in centrifugal cooling, especially for producing relatively few casting. ( single mold case can be used with different thickness of sand linings to produce tubes of various diameters within a limited range. :reen -and olds; < :reen sand is commonly used as the liner in expendable molds. Aarious mixtures and binders are used for example, a mixture of /$B silica sand and $B calcined and crushed asbestos or sand bound with resin.

Dry -and molds; < a) Dry sand molds can also be employed in this cases, the sand is pressed down around a pattern having the same dimensions as the casting. 'ardening is sometimes accomplished with carbon dioxide.

%ermanent (l)s: -

The most common materials for permanent molds are steel, copper and graphite.


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CENTRIFUGAL CASTING

-teel olds; < "t is used to cast large #uantities and for some casting alloys that re#uire specific solidification conditions. -teel molds are sensitive to thermal shock, alumina or circonial 9 base mold sprays are used to lesser thermal shocks to the mold and to improve the mold surface, old coatings are also important in regulating the solidification rates of some casting materials. Copper olds; < These are sometimes used for their high thermal conductivity. Their relatively high cost and the difficulty of calculating the correct dimensions of these molds limit their field of application. :raphite olds; < @ecause of their relatively low cast, graphite molds can be an economical alternative to sand in the production of small #uantities of parts. :raphite is the mold material of choice in the casting of =$B a bronze, high phosphorus brass, and other copper alloys. :raphite has excellent thermal conductivity and resistance to thermal shock.


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CENTRIFUGAL CASTING

4. OLD D#SI$N

etal (l) Design: -

achining allowances for casting depend of melting practices, the condition and thickness of the mold insulation coating, the volumetric shrinkage of the metal being cast, and the susceptibility of the metal to causing defects such as dross formation. 7ith minimum allowances, the mold can be refashioned to allow for more machining allowance as determined by experience and the specific melting practices used, 1atternmakers shrinkage is the shrinkage allowance built into the pattern to compensate for the change in dimensions caused by the contraction of the cast metal as it cools. This shrinkage allowance is the factor with which the mold maker is concerned.

(l) 5all Tic6ness: -

7all thickness may vary depending on the specific e#uipment and application involved. old wall thickness is independent of casting wall thickness for water< cooled steel molds.

#n) %late Tic6ness: -

+nd plate thicknesses depend on casting wall thickness and end plate material. Commonly used and end plate material are (-T ( &/, steel, !$! steel, or !$!= steel plate.

#n) %late Recess: -

The end plate recess in the mold provides an effective to prevent leakage of the molten metal from the mold. The amount of end plate recess used depends on mold inside diameter and can be as high as the mold inside diameter plus %mm 8! in.) for mold inside diameters of !$mm 8/ in.) or more. College Of Engineering Pandharpur

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CENTRIFUGAL CASTING

#n) %late (l) L(c6 Design: -

Aarious removable end plates are shown in 2ig. These end plate types are recommended for all vertical molds. 1referred end plate types are those shown in 2ig 8a), 8b) and 8c). old end plate diametric clearance fit into the mold recess diameter should be a minimum of !.% mm. This clearance is effective for end plates upto about &=$ mm 8! in.) in diameter. n large end plates, clearances as large as %. mm. 8&3 &% in.) may be necessary for easy removal and assembly. The standard draft angle of & $ will permit easy removal and assembly.

#n) %late 7e)ge an) %in Design: -

7edges for holding the end plate secure in the mold are principally of two different types; the tapered wedge and the tapered pin. The maximum allowable stress for tapered wedges and pins is /? pa.

(l) A)a&ter Ta3les: -

( mold adapter table is usually furnished or re#uired with a vertical centrifugal casting machine to facilitate attachment of the molds to the machine. (lso furnished or re#uired with the mold adapter table is a mold centering boss or index boss3plug for aligning and centering the mold so that it is concentric with the spinner shaft of the machine.


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CENTRIFUGAL CASTING


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CENTRIFUGAL CASTING

8. D#F#CTS

Segregati(n 9an)ing: -

"t occurs only in true centrifugal casting, generally where the casting wall thickness exceeds $to 0 mm. "t rarely occurs in thinner
"n a horizontal achine, raining can occur if the mold is rotated at too low speed or if the metal is poured into the mold too fast. "n this phenomenon, the metal actually rains or falls the top of the mold to the bottom. +i3rati(n Defects: -

Aibration can cause a laminated casting. "t can beheld to a minimum by proper mounting, careful balancing of the molds, and fre#uent inspection of rollers, bearing, and other vital parts.


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CENTRIFUGAL CASTING

1. A%%LICATIONS

A&&licati(ns: -

The flexibility of the centrifugal casting process, in terms of both materials and the wide range of part sizes that can be produced, has led to the application of centrifugally cast parts in many industries. -ome of the most common applications are outlined briefly in this section. "ron and -teel industry; < Centrifugally cast parts are used in the production of iron and steel for continuous casting rollers, rolling mill rolls, furnace rollers, special pipelines, winding spools, and other applications. 1etroleum production; < ff shore production platforms in the oil and gas industry use centrifugally cast tubes in various application. 'ot extruded bimetallic tubes are used in pipelines and gathering systems. ther (pplications; < ther applications for centrifugally cast tubes include hydraulic cylinders, rollers for glass production, pipelines for the transport of abrasive materials, rollers in the pulp and paper industry, tubes for the chemical processing industry, foundation piles, and building columns. :ears, disked wheels, propellers and pulleys also produced by centrifugal casting.


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CENTRIFUGAL CASTING

11. CONCLUSION

Centrifugal casting id the most economical method of producing a superior #uality tubular or cylindrical casting with regard to casting yield, cleaning room cost and mold cost. Tubular castings produced in permanent molds by centrifugal casting usually have higher yields and higher mechanical properties than castings produced by the static casting process. (n economic advantage of centrifugal casting is the elimination or minimization of gates and risers.


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CENTRIFUGAL CASTING

12. R#F#R#NC#S

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Centrifugal Casting

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