Extrusion

Manufacturing Process· I 203

3.4 EXTRUSI N

Extrusion may be defined as the forming of a material under compression, so that it isf~ced to flow out of a confined space through a suitable opening called the "die".

Usually, extrusion is used for producing cylindrical bars and tubes. However, productslike tooth-paste tubes, solid and tubular stocks with relatively complex sections can also bemanufactured by extrusion process readily extrudable metals, like aluminium. Extrusion isgenerally a hot working process because of large forces required. However, cold extrusion ispossible for many metals. The extrusion processes may be classified mainly into fourcategories-

1) Direct or Forward Extrusion2) Indirect or Backward or Inverted Extrusion3) Extrusion-Forging4) Impact Extrusion

DIRECT EXTRUSION

Figure 3-22a illustrates the process of direct extrusion. In this process a hot metal billet isplaced in the container of the press and then forced through the die by the hydraulically drivenram which advances into the container. Thus metal flow through the die occurs in the samedirection as that of the ram movement.

HOllOW RAMCONTAINER

(a)DirectExtrusion (b) IndirectExtrusion

Fig. 3-22. Methods of extrusion

The main advantage of this process is that it is the most simplest process and hencethe greatest tonnage of extruded metal cross-sections is produced by this method. However,in this process since there is relative motion between the container wall and the billet, frictionalforces are high and requires more power for operation. Also there is possibility of "extrusiondefect" in this method.

204 Manuiacturing Process· I

IN IIitECT EXTRUSI NFigure 3-22b shows the process of indirect extrusion. In this process the die is carned

by the hollow ram, which causes extrusion of the hot billet to take place in a direction oppositeto that of the ram movement. Some times, it is better to use a fixed ram with die and move thecontainer and billet. It can be seen that in indirect or backward extrusion the movement of theram and the product are in opposite directions.

In.this method since there is no relative movement between the billet and the containerwall, less mechanical energy is wasted in overcoming the frictional forces between the two.Hence, this process requires lesser power as compared to the direct extrusion processHowever, the process is complicated because of the hollow ram and reverse motion of theproduct.

EXTRUSION FORGINGThis method is combination of extrusion and forging. An example of this process is the

manufacture of poppet valves for intemal combustion engines. These valves can be made insingle operation from a heated steel slug in a press, as shown in figure 3-23. The heated steelslug is partly extruded through the die by the application of pressure from the punch to formthe shank of the valve, but sufficient metal is retained in the die to form the head. The punchis then raised and the valve is ejected from the die. The operation is very simple and fast, andcarried out at high temperature (about 11aa°C).

YPUNCHDIE Wi

Fig. 3-23. Extrusion forging

All the above 3 methods viz., direct extrusion, indirect extrusion and extrusion forgingare all hot extrusion processes. .

IMPACT EXTRUSIONThis is a cold extrusion process, similar to backward extrusion but carried out at higher

speeds. It is generally employed for the production of light metal collapsible and disposabletubes like tooth paste tubes, grease tubes, medical ointment containers, etc.

Manufacturing Process· ! 205

Fig. 3-24. Impact extrusion

This process uses heavily constructed mechanical presses. The arrangement of die andpunch in such a press is shown in figure 3-24. A small unheated slug of metal is placed in thedie cavity and the punch is driven rapidly into the die cavity. As there is no more place for themetal deformation in the cavity, the metal is forced upwards through the gap between thepunch and the die, forming a tube shaped part. As the punch is raised the extrusion is removedby an automatic stripping mechanism.

Advantages of impact extrusion1) The process is simple and economical.2) It is most suitable to produce collapsible tubes.3) Production cost is comparatively low.4) Surface finish is very good.5) The production rate is very high.

Limitations1) Its use is limited to soft metals like lead, tin, aluminium and copper.2) Tool wear is more because of cold working.3) The structure and the foundation is to be very strong and rigid to withstand high impact

forces. .'4) Lubrication is difficult in this process.

Applications

This process is more suitable for the production of disposable tubes in lead, tin andaluminium, used as containers for a wide range of domestic and industrial materials likeshaving cream, tooth paste, medical tube, grease containers, and milk containers.

206 Manufacturing Process- I

EXTRUSION EQUIPMENTSGenerally, extrusion operations are done in hydraulic presses. These are classified into

horizontal and vertical presses, depending upon the directiDn of travel of the ram. Verticalextrusion presses range in capacities from 3 to 20 MN. They give certain advantages overhorizontal presses like, easier alignment between the press ram and the tool, higher rate ofproduction, and lesser space area. But, on the other hand, they need high head room and toproduce longer extrusions a floor pit is also essential. Vertical presses, produce uniform coolingof the billet and hence symmetrically uniform deformation takes place.

In a horizontal press, the bottom of the billet which remains in contact with the container.gets cooled faster than the top surface, and hence may result in non-uniform deformations.Also, in these presses, warping of bars and non-uniform wall thickness of tubes may beresulted. Hence, in commercial applications to produce thin sections vertical presses are used,which produce uniform wall thickness and concentricity in the tubes. Horizontal extrusionpresses are used commercially to produce extrusions of bars, and other bigger sections.Usually, horizontal presses of the order of 15 to 50 MN are available.

The ram speed in the extrusions presses is also important, since high ram speeds are.required in high temperature operations. Ram speeds of 0.4 to 0.6 m/sec is used for extrudingrefractory metals, and requires a hydraulic accumulator. For aluminium and copper alloyswhich are susceptible to hot shortness lower ram speeds are to be used, in which case directlydriven hydraulic systems are sufficient.

The tooling and dies used in the presses are subjected to high stresses, thermal shocksand oxidation, and hence should be built with strong and robust structure. The ram is highlyloaded in compression, and it is protected from the hot billet oya follower pad. Since, extrusioncontainer is subjected to high pressures, it is built in two parts. A liner is shrunk into a massivecontainer to produce compressive pre-stresses in the inside surface of the liner. Since the linerand the follower pad are subjected to many thermal shock and load cycles, they need to bereplaced periodically.

EXTRUSION DIESTwo types of extrusion dies are generally used - flat faced die and conical die. Flat faced

dies (figure 3-25a) are used when the metal entering the die forms a dead zone and shearsintemally to form its own die angle. A parallel land on the exit side of the die makes the diestronger and permits reworking of the flat face on the entrance without increasing the exitdiameter. Conical dies have a conical entrance as shown in figure 3-25b. These are used forextrusion with sufficient lubrication. Reducing the die angle increases the homogeneity of thedeformation and lowers the extrusion pressure, but beyond certain point the friction in the diesurfaces becomes very high. Usually, for most operations the optimum half-die-angle is in therange of 45 to 60 degrees.

Manufacturing Process· I

Die entrance

207Die entrance

,

(a) (b)

Fig. 3·25. Extrusion dies

The other facilities required for extrusion process are billet-heating unit, automatictransfer equipment for the heated billet to the container. Provision for heating the extrusioncontainer may also be required. A hot saw is also necessary to cut-off the extrusion so thatdiscard can be removed from the die. Finally, a runout table is required for holding the extrudedproduct and straightner to correct minor warpage in the product.

METAL FLOW, DEFORMATION AND LUBRICATION IN EXTRUSIOThe pressure required to deform a metal in extrusion process is largely dependent on

the lubrication conditions. Also, mostly the extrusion defects are directly related to thedeformation process.

;-igure 3-26 shows the deformation patterns in extrusion. Figure 3-26a indicates the gridpattern in a billet before extrusion. Figure 3-26b shows a nearly homogeneous deformation,which is a feature of well lubricated billet hence low friction, as in indirect and hydrostaticextrusion. Deformation is uniform until close to the die entrance. This is the most desirabletype of deformation, which leads to lower extrusion pressure, die wear and minimum defects .

(a)

•••.... "- - / ----

Fig. 3·26. Metal flow I defonnation in extrusion

(e)(b)

Figure 3-26c represents the deformation with increased container-wall friction (withinsufficient lubrication). The grid patterns severely distorted in the comers of the die indicatethe formation of a "dead Zone", where the metal undergoes least deformation. The friction at

208 Manufacturing Process .\

the container-billet interface results in concentrated flow towards the centre. This is a commonfeature of direct extrusion and extrusion with insufficient friction. This situation may also arisewhen the billet surface is chilled by the cold walls of the container.

A good lubricant for hot-extrusion should have a low shear strength and high tempera-ture stability. For hot-extrusion of steel and nickel-based alloys the common lubricant is moltenglass. To use this lubricant, the billet is heated in an inert atmosphere and coated with glasspowder and then put into the container. The glass coating serves both as the lubricant and asa thermal insulation to reduce the heat losses at the containerwalls. Also, a glass pad IS placedat the face of the die. and acts as the main source of lubricant for the metal getting deformed.The glass film formed around the metal under extrusion will be around 25 micron thick. whichalso depends on the temperature of the billet and the ram speed. For better results of theextruded products, the lubncant film should be continuous and complete.

DEFECTS IN EXTRUDED PRODUCTSThe important defects that occur in an extrusion process are discussed here.

a) Extrusion Defect

This isthe most common defect in extrude oducts. This is caused due to inhomogeneousdefol.mation of the metal, ma be u 0 insufficient lubrication at the container-bi let interfaceor due to chilling of the metal at the container walls. Because of this. the hot metal at the centreOfthe bille.!..!:!!oves faster than the periphery and the d~d outer metal zone extends down~After about two-thirds of the billet"is extruded 0 er surtace.of he.billet moves to sthe centre and extrudes through the die. The oxide skin from the outer surface thus gets intothe internal surface and form ox~ringers whJ£b..£aLlse th~nal eip'ing and is generallknown as the 'extrusion defect'. The tendency for the formation of the extrusion defect is higherWit greater wall friction --

Formation of extrusion defect

In direct extrusion, high frirtion between the billet and the container and the greater distancethat must be travelled by particles near the ram surface result in non uniform flow of metal.Even if the billet is heated above the temperature required, its skin becomes chilled relativeto its core by contact with the cold walls of the container. Thus the hot core is more easilyextruded, than the chilled skin which tends to remain in the container [Figure 3-27(i)]. Asextrusion proceeds the outside skin begins to buckle [Figure 3-27(ii)] and is ultimately drawninto the stream of extruded metal [Figure 3-27 (iii) and (iv)], so that the resultant rod is badly'piped' over a considerable portion of its rear length.

,

In indirect extrusion, the absence of movement between the billet and the container resultsin less turbulence in the not yet extruded part [Figure 3-26b], as compared to that in the directextrusion process. This results ill reduced extrusion defect.

Manufacturing Process· I '2090'

(iv)

Fig. 3-27. Fonnation of extrusion defect

lhere are various ethods of overcoming the extrusion detect. Une meextr~lon operation 0

Fig. 3-28. Fonnation of skull

c) Surface CrackingS ace cracks of different types may be formed in extruded products ba the

nature of deformatiOn. These are formed .as- a result of longitudinal tensile stresses as theextrusion Rasses through the die. In hot extrusion, cracking is usually inter-granular an IS

associated with hot shortness. The major and common cause of fhis de ect is the nigh ram- -

210 Manufacturing Process· I

given extrusion---

/ HYDROSTATIC EXTRUSIONThe recent development in extrusion forming processes is the 'Hydrostatic Extrusion'.

In this process, the billet is surrounded by a liquid on all sides except at its front end which ispointed and passes through a cone-shaped die, as shown in Figure 3-29. The liquid transmitspressure applied to the ram. This pressure not only forces the billet forward into the die butalso compresses the billet circumferencially, since there is no friction between the billet andthe container much lowerworking loads are required than in normal direct extrusion, and henceno chance for extrusion defect. The working fluid used is the mineral oil to which ,10% of thehigh pressure lubricant molybdenum disul- phide is added·

Highpressure

liquid

. ~t"--.......,=--=~n~.,-_sealing" ring

,~~~-T--7'--- Billet

~j;-+.-r-r-r-,~- DieV//l_~r-T'-:-- Sealing

ring

Fig. 3-29. Hydrostatic extrusion

Manufacturing Process -I 211

Advantages ./

1) Materials with limited ductility can be extruded.2) There is no friction between the billet and the container and hence requires less power.3) Because of the absence of friction. there is no possibility of extrusion defect.4) High speed steel and cast iron can be extruded easily.

Disadvantages.)./"

1) Only cold extrusion IS possible.2) Construction and workina mechanism is complicated.3) Suitable only for limited shapes.

THE EXTRUSION OF TUBESTubes can be produced by extrusion, using either a hollow billet or a solid billet. In order

to extrude a tube, a mandrel should be passed axially through the billet so that its tip lies inthe aperture of the die thus forming an annular gap through which the metal is extruded. Figure3-30(i) shows how a tube can be extruded usmg a hollow cast or bored billet. Here the mandrelis fixed to the end of the ram and passes though a hole in the pressure pad. VVhen the extrusionbegins. the tip of the mandrel moves into the mouth of the die just before pressure is exertedon the billet. The mandrel can be fitted as shown in Figure 3-30(ii). In this the mandrel is floatingand better concentricity of the resultant tube is obtained.

PressurePad FixedMandrel

Fig. 3-30. Extrusion of tubes from hollow billet.

212 Manulacturing Process· I

The production of hollow billets is expensive and the internal bore may be oxidized whileheating, resulting in inaccessible and invisible defects in the finished tube. Therefore. a methodfor the production of tubes by solid billets is advantageous.

In this process the solid billet is first pierced by a mandrel and then extruded by the rampressure. Generally, the mandrel is actuated by a separate press, moves coaxially with theram, and is independent of its motion [Figure 3-31a & b]

Fig. 3-31. Extrusion of tubes from solid billets

.THE HOOKER PROCESS

This is a special extrusion process, similar to direct method, but employs impactextrusion under cold working conditions.

Hooker extrusion is generally carried out in a crank press. Figure 3-32 shows thearrangement of punch and die in a Hooker process. In this usually preformed cupped blank isplaced in the die and as the punch descends, metal is forced down between the body of thepunch and the die. This produces a tubular extrusion as shown in figure.

Manufacturing Precess- I 213

(i)

Extruded shell

Fig. 3-32. Extrusion by Hooker process

(ii)

(iii)

This process is useful for the production of small brass cartridge cases, copper tubesfor radiators and heat exchangers. and other short tubular components. This process isrestricted to soft materials like lead, tin, aluminium and copper.

214 Manulacturing Process .\

3.5 DRAWING OF RODS. WIRES & TUBES

Drawing is a cold working process and involves pulling the material to be drawn througha hole in a hard steel or carbide block called "die" and reduce the diameter of the material.lndrawing, tensile forces are applied by way of pulling the rod at the exit end of the die. Plasticflow is caused by compression forces arising from the reaction of the metal at the die surface.The process of reducing the diameter of a material by successive drawing operations is called"Bar, rod orwire drawing" depending upon the, diameterofthe drawn product. When a hollowtube is drawn through a die without using any mandrel, it is called "tube sinking". If amandrel/rod is used to support the inside diameter of the tube, it is called "tube drawing"

Only highly ductile materials can be cold drawn. Drawing is generally used for themanufacture of wires but it is also useful for bar and tube manufacture ..

Though the principles of drawing of bars, rods and wires are same, the equipments usedare different. Rods, bars and tubes which cannot be coiled but to be kept straight, aremanufactured on drawbenches. The rod is pointed with a swager, passed through the die, andclamped to the jaws of the drawhead. The drawhead is driven by mechanical or hydraulicmeans. Drawbenches can be of capacities upto 1 MN and 30 m of runout, and draw speedsare ranging from 150 to 1500 mmlsec.

Wires are those products which have diameter less than 5 mm and can be coiled easily.It uses a coil of hot-rolled rod. Instead of drawbench, it consists of a circular bull block whichcoils the drawn wire as it pulls it through the die. The capacities depend on the number of diesand the draw speeds are in the range of 10-30 mlsec.

Advantages of Drawing Process1) Ductile materials can be drawn down to very small diameters and to exact sizes.2) The surface finish of a cold drawn product is superior to that of a hot rolled or extruded

material.

3) Mechanical properties can be easily controlled by controlling the degree of cross sectionalreduction in the final cold drawing operation.

4) Operation is simple compared to other processes.5) There is no problem of surface defects as drawing is a cold working process.

Manulacturing Process· I 215

ROD DRAWINGThe process involves cleaning the materials, pointing the rod and actual drawirg in a

die. This operation is explained below.

Cleaning Before DrawingFor steel rods that are previously hot rolled or extruded. surface cleaning IS required.

For this. the material is immersed in a dilute solution of hydrochloric or sulphuric acid. This iscalled as 'Pickling'. This removes any scale, oil. grease etc., present on the material surface.It is then washed with water and dipped in an emulsion of slaked lime and water. Thisneutralizes the acid and excess lime dries on the material surface and forms a base to absorbthe minerai oil or grease, whichever is the lubricant. Soap can be also be used as lubricant,but results in dull surface of the finished product.

Pointing or Tagging the Rod and DrawingSince the rod is drawn in straight lengths, the operation carried out on a drawn-bench

which must be long enough to accommodate the length of the rod required. A draw-benchconsists of a die held rigidly in steel frame and a 'dog' which grips the end of the rod and pullsit through the die. The dog runs on rails to the required length of the rod.

To operate the bench the end of the rod must be pointed or 'tagged'. For this purpose.the rod is first forced through the die so that it projects out of the die. This projected part IS

rigidly held by the dog (jaw) of the draw bench and pulled by the moving chain. driven eithermechanically or hvdraulically.

Drawing carriage

-Draw bench

-../

Fig. 3-33. Arrangement of rod drawing

216 Manufacturing Process ·1 .

Figure 3-33shows the simple arrangement of rod drawing operation. In this the die iskept immersed in a lubricant while drawing the rod, such a process is called "wet drawing".Instead, if the die is kept in open and grease or soap powder is used as lubricant, it is calledas "dry drawing".

WIRE DRAWINGWire drawing starts with hot rolled 'wire rod'. Similar to rod drawing. the rod IS first

cleaned by pickling and washed. It is then coated with lime or plated WIth thin layer of copperor tin. The lime neutralizes any acid, and serves as an absorber and carrier of the lubricantduring dry drawing. The lubricant may be either grease or soap powder. For wet drawingoperation the die itself is immersed in a lubricant. The schematic of wire drawing operation isshown In figure 3-34.

Fig. 3-34. Wire drawing Jaws

For drawing of fine wire a large number of draw blocks are used, with the wire passingthrough a number of dies until it is reduced to its final size in one continuous operation (Figure3-35). For fine wires reductions per pass of 15 to 25% are used, while for coarse wires thereductions per pass may be 20 to 50%. In drawing operations, depending upon the metal andthe reductions, intermediate anneals may be required to restore ductility.

Patenting of Steel WireHigh carbon steels (above 0.25% C) in the hot rolled condition generally possess

insufficient ductility for drawing to fine wires, and if drawn the product will be too brittle for use.Therefore, a heat treatment process known as 'patenting' is generally used, both beforedrawing and as a final treatment for the drawn product.

Patenting is a high temperature austempering process, in which the wire is heated aboveits upper critical temperature (around 975°C) and then rapidly cooled in a lead bath maintainedat around 530°C. In the double lead bath patenting process, the wire passes continuouslythrough a lead bath maintained above the upper critical temp. of the wire. then through asecond cooler lead bath. This treatment gives a tough bainitic structure to the wire, which isresponsible for increasing the yield strength and retaining sufficient ductility for considerabledeformation.

Manulacturing Process· I 217

Fig. 3-35. Continous wire drawing

DIES FOR DRAWING

Figure 3-36 shows the important features of a wire drawirlg die. It consists of a taperedhole, with smooth working surface of high strength and wear resistance. The bell-mountedentrance AB of the die will never be in contact with work, but serves as a reservoir for lubricantcarried in by the work.

The tapered portion BC is the actual working surface where plastic deformation takesplace, and hence must be carefully designed and prepared. The angle of taper is critical, anddepends both on the material used for the die and the metal to be drawn. Its surface shouldbe polished to reduce friction to a minimum.

The section CO is cylindrical, which must be of adequate length, since the working part ofthe die wears, and C moves nearer to 0, but the diameter of the wire will remain withinspecifications.

218 Manufacturing Process ·1

Fig. 3-36 .. Wire drawing die

Portion DE is called as 'relief. Its function is to provide reinforcement for the workingsection of the die and prevent the circular edge from breaking or pulling away.

Die MaterialsDies can be made from a variety of materials, like chilled cast iron, high carbon steels and

alloy steels. Carbon and alloy steel dies have the advantage that they can be forged so that. as the die hole wears to an extent where the resulting wire is over size, the hole can be

hammered up and then reamed to the correct size. Chilled cast iron dies are used for theproduction of low quality materials.

Tungsten carbide dies are widely used nowadays because of their long life. As tungstencarbide is expensive and also some-what brittle, only the working part of the die is made fromtungsten carbide and this is held in a mild steel block. The schematic arrangement of a carbidedie held in a mild steel block is shown in Figure 3-37.

Fig. 3-37. Tungsten-carbide die

Manufacturing Process ·1 219

In addition to long life, tungsten-carbide dies consume less power due to lesser frictionand result in a fine surface finish of the product. The initial cost of the tungsten carbide die isconsiderably higher than that of a steel die and holes cannot be hammered as they wear.Instead they can be used to the next working size.

Optimal Cone Angle, Dead Zone Formation-and Shaving

In a drawing die, when the cone angle is too small, the length of contact between the wireand the die is more and causes high frictional losses. If the cone angle is large, distortion ofthe material will be more, resulting in many surface defects. But, there exists a cone angle inbetween, which results in minimum draw force for a given reduction, called 'Optimal ConeAngle'. This optimal cone angle depends upon the reduction, the lubrication and materialsinvolved.

DIE DIE

DIEa) Sound now b) Dead zone formation

Fig. 3-38. Effects of cone angle in drawing

c) Shaving

Figure 3-38 shows the effects of cone angle In drawing operation. VVhenthe cone angle isoptimum, sound flow of metal occurs (Figure 3-38a). VVhenmaterial is drawn through dies withcone angles above 'that of optimal angle, the meta1shears within itself to develop a dead metalzone adhering to the die, which no longer takes part in the metal flow (Figure 3-38b). Thisphenomenon is called as "Dead zone formation".

When the dead zone material doesn't adhere to the die but starts to move backward, withpeeling off as in metal cutting operation. This is called 'Shaving' in drawing operation. Shavingwill occur with too large cone angles, and the core of the billet no longer deforms, but movesthrough the die with no change in diameter and with exit velocity same as the entrance velocity(Figure 3-38c).

220 Manufacturing Process -\

Effect of Back Pull in Drawing

Back pull is the force that is exerted in the direction opposite to that of draw pull in a drawmqprocess. Back pull exists mainly due to frictional forces acting on the draw blocks of multipledrawing machines. A back pull materially increases the drawing force. However. a minimumamount of back pull is desirable, due to the following reasons. It reduces the wall pressure inthe die, and reduces the friction, thus die wear is appreciably decreased. This increases thedie life.

TUBE DRAWINGTube drawing is similar to wire drawing and uses draw benches and dies. However. to

reduce the wall thickness and to control the inside dia, the inside surface of the tube must besupported while drawing. This is accomplished by inserting a mandrel or plug inside the tubepositioned at the die throat. The mandrel may have either a cylindrical or tapered cross section.Hence, there are basic methods of tube drawing-

a) Sinking or drawing without a mandrelb) Plug drawing or drawing with a fixed mandrelc) Drawing with a.floating mandreld) Drawing with a moving mandrel

a} Tube sinking or Drawing without a MandrelTubes which have wall thickness greater than the diameter can be drawn without a

mandrel. Reductions of up to 35% can be obtained in steel tubes by this method, which isknown as "tube sinking" (see figure 3-39). One end of the tube is swaged so that can bethreaded through the die and drawn similar to a rod.

-o~_[J

Fig. 3-39. Tube sinking Fig. 3-40. Drawing with a fixed mandrel

b) Plug Drawing or Drawing with a Fixed MandrelIn this method, the mandrel or plug is short in length, and is held in position in the mouth

of the die by means of a tie-rod attached to it and to a fixed support at the opposite end of thedraw-bench, similar to wire drawing. The rod is detachable, so that it can be drawn backwardsand aside in order that the tube shell can be threaded over it.

Manufacturing Precess- I 221

The tagged end of the tube is then pushed through the die and gripped by the dog while theback end of the mandrel rod is anchored to its support. The tube is then drawn through annulusformed between the die opening and the mandrel as shown in figure 3-40. This method can beused to draw longer tubes since the mandrel is fixed.

c) Drawing with a Floating MandrelFor tubes of small diameter, drawing with fixed mandrel requires a very fragile mandrel rod

Also, the length of the tube that can be drawn is limited by the leng1l\of mandrel rod. However.in such cases, a floating mandrel can be used and there Will be no limit to the length of thetube drawn. The contour of the plug is so designed that the plug adjusts itself to the correctposition during drawing (Figure 3-41). This method is suitable for the manufacture of small diatubes in which the wall thickness is greater than the bore.

Fig. 3-41. Drawing with a floating ma'ndrel .Fig. 3-12. Drawing with a moving ",andrel

d) Drawing with a Moving Mandrel

In this method the mandrel used is made of heat treated alloy steel rod equal in length tothe finished tube and having a diameter equal to the bore of the tube. The rod is not fixed tothe draw bench, but moves through the die along with the tube as shown in Figure 3-42.

In this method the frictional loss is low, and there is geed lubricatini effect. But in thismethod there is problem of stripping the mandrel from the drawn tube, and also limited to.tubesbelow 6 mm diameter.

DEFECTS IN DRAWN WIRES AND RODSDefects in drawn wires and reds can be due to. two reasons - one, the defects in the

starting material itself, like seams, slivers, pipe, scale, etc.; two, improper deformatien precesslike excessive drawing without patenting treatment, without the use of proper lubricant, etc.

The most common drawing defect in wires and rods is centre burst or chevro.ncrackinq,which is also.called "cupping".

222 Manufacturing Process· !

These are the internal cracks developed as a result of secondary tensile stresses whichusually occur with large values of h/L ratio (i.e, ratio of mean thickness to the length of thedeformation zone). Acco'tdin~ to upper bound analysis, the cupping fracture occurs due to lowdie angles (IX.) at low reductlens.

The other common defect in drawn products is the surface cracks. These result mainlyfrom heavy die friction in ~xtrusion. This can be due to two reasons - lack of lubricaticn andexcessive drawing with the loss of ductility.

The defects resulting from oxide layers and seams on the basic materials are theinclusion of these unwanted materials into the surfaces of the drawn products.

All these defects can be avoided by the use of proper lubrication, low h/L ratio, frequentpatenting and cleaning of the raw material before drawing.

RESIDUAL STRESSES IN RODS, WIRES AND TUBESTwo different types of residual stresses are introduced in cold-drawn rods and wires. The

extent of residual stress induced depends mainly on the amount of reduction. For reductionsper pass of less than 1%, the longitudinal residual stresses are compressive at the surface,and tensile at the axis, the radial stresses are tensile at the axis and drop off to zero at thefree surface, while circumferential stresses are same as the longitudinal residual stresses. Inthe case of larger reductions the residual stress distribution is the reverse of the above case.In this, the longitudinal stresses are tensile at the surface and compressive at the axis, theradial stresses are compressive at the axis and the circumferential stresses follow the samepattern as the longitudinal stresses. The first type of residual stress pattem is usually noticedin drawing operations where the deformation is localised in the surface layers.

In tubes produced by tube sinking, the longitudinal residual stresses are tensile on the •outer surface and compressive on the inner surface of the tube. The residual stresses in thecircumferential direction follow the same pattern, while the stresses in the radiaf direction arenegligible.

Residual stresses in tubes produced by drawing over a plug and mandrel are similar tothat of tube sinking. However, in this case, a substantial reduction in the level of residual stresscan be achieved by tandem drawing, in which a small reduction (2%) is to be produced by asecond die immediately following the main reduction.