manufacturing process - extrusion of metal

MANUFACTURING PROCESSMANUFACTURING PROCESS(BMFG 2323)(BMFG 2323)

LECTURE 6LECTURE 6

~ EXTRUSION OF METAL~

Prepared and presented by: Masjuri Bin Musa @ Othman

Faculty of Mechanical Engineering(Department of Innovation & Engineering Design)

Universiti Teknikal Malaysia Melaka

EXTRUSION PROCESSEXTRUSION PROCESS- The process of forcing a billet thru’ a die to produce constant solid or hollow cross section.

- Products are long pieces and cut into desire length e.g. pipes, railings for sliding doors/windows,

aluminum ladder, automotive power train application, etc.

- Types: Direct, indirect, hydrostatic, and impact.

- In extrusion process, a large deformation takes place without fracture due the material is

under high triaxial compression during the process.

- Commonly extruded materials: aluminum, copper, steel, magnesium, lead.

- Extruded process can be done under room temperature or at elevated temperature depending

on the ductility of the material.- Extrusion at room temperature – combination with forging operations which known as cold

extrusion process.

EXTRUSION OF METALEXTRUSION OF METAL 1

@jurie 2007 – Lecture 6

Process Variables in Direct ExtrusionProcess Variables in Direct Extrusion

Figure shows a process variables in direct extrusion. The die angle, reduction in cross-section, extrusion speed, billet temperature, and lubrication all affect the extrusion pressure.





Direct ExtrusionDirect Extrusion

Schematic illustration of the direct extrusion process. A billet is placed in a container and forced thru’ a die opening by a hydraulically pressing stem. The die shape will determine the shape of the product.



ExtrusionsExtrusionsFigure: Extrusions, and examples of products made by sectioning off extrusions.



THE PROCESSTHE PROCESSDirect (Forward extrusion)Direct (Forward extrusion)

- The most common process.-A billet is place in the chamber (container) and forced through a die opening by a hydraulically driven ram (pressing stem or punch).

- Die opening can be in round shape or may also have various shapes.

- Dummy block – to protect the tip of the pressing stem (punch), especially during hot extrusion.



Indirect extrusion (reverse/inverted/backward extrusion)Indirect extrusion (reverse/inverted/backward extrusion)

- The die moves toward the unextruded billet



Hydrostatic extrusionHydrostatic extrusion- Usually is carried out at room temperature.- The billet is smaller in diameter than the chamber which is filled with fluid (vegetable oil, castor oil).- The pressure is transmitted to the billet by ram typically 1400MPa.-Because of the high pressure in the chamber, it transmits some of the fluid to the die surfaces which reduces friction.- This process is uneconomical because – the complexity of the tooling, long cycle times required.



Impact ExtrusionImpact Extrusion-Almost similar to indirect extrusion – particularly included cold extrusion.-The punch descends rapidly on the blank, which is extruded backward.

Figure shows a schematic illustration of the impact-extrusion process. The extruded parts are stripped by the use of a stripper plate, because they tend to stick to the punch.

- Part’s max. diameter: ≈ 150mm.- Produce thin walled tubular sections; having thickness-to-diameter ratios as small as 0.005.



Examples of Impact ExtrusionExamples of Impact Extrusion

Figure shows (a) Two examples of products made by impact extrusion. These parts may also be made by casting, by forging, or by machining; the choice of process depends on the dimensions and the materials involved and on the properties desired. Economic considerations are also important in final process selection. (b) and (c) Impact extrusion of a collapsible tube by the Hooker process.



HOT EXTRUSIONHOT EXTRUSION- In hot extrusion process, since the billet is hot, it will develops an oxide films.

- This film can be abrasive and it can effect the flow pattern of the material.

- It also will effect the surface finish of the product.

-In order to avoid the formation of oxide films on the hot extruded product, the dummy block is placed

ahead of the ram where by this dummy block is made a little smaller in diameter than the container.-As a result of this, a thin shell (skull) consisting mainly of the outer oxidized layer of the billet is left in the container and the skull is removed later from the chamber.



COLD EXTRUSIONCOLD EXTRUSION- It is a combination of direct/indirect extrusion together with forging.

Figure shows two examples of cold extrusion. Thin arrows indicate the direction of metal flow during extrusion.

- This process uses slugs cut from cold-finished or hot rolled bars, wire or plates.

-Slugs which diameter less than 40mm are sheared, and sometimes their ends are squared off by

using the upsetting method, machining or grinding process.- For larger diameter of slugs, they will machined from bars into specific lengths.



Advantages of cold extrusion processAdvantages of cold extrusion process

1. Improved mechanical properties e.g. strength and hardness.

2. Good control of dimensional tolerances, reducing or no further finishing.

3. Improved surface finish.

4. High productivity over machining.

- In cold extrusion process, tooling design and the selection of appropriate tool and die materials are crucial for example the punch hardness usually ranges between 60 and 65 HRC and the die hardness between 58 and 62 HRC.



Cold Extruded Spark PlugCold Extruded Spark Plug

Figure shows a cross-section of the metal part showing the grain flow pattern. First, a slug is sheared off the end of a round rod. Then, is cold extruded with a blind hole. Then material at the bottom of the blind hole is punched out, producing the small slug.



Metal flow in extrusionMetal flow in extrusion- The metal flow in extrusion is very important because its influence the quality and the mechanical

properties of the extruded product.- The flow pattern is determined by the basic principle that energy is minimized in any such process.- The material flows longitudinally, much like compressible fluid flow in a channel, and this cause

why the extruded products have an elongated grain structure.

Figure shows types of metal flow in extruding with square dies. (a) Flow pattern obtained at low friction, or in indirect extrusion. (b) Pattern obtained with high friction at the billet-chamber interfaces. (c) Pattern obtained at high friction, or with cooling of the outer regions of the billet in the chamber. This type of pattern, observed in metals whose strength increases rapidly with decreasing temperature, leads to a defect known as pipe, or extrusion defect.



Extrusion defectsExtrusion defects- The extrusion defects happened depending on the workpiece material condition and the process

involved.- This defects significantly will affect the strength and product quality.- Generally, there are three principal of extrusion defects:

i) Surface cracking.ii) Pipe defect.iii) Internal cracking.

1. Surface cracking1. Surface cracking

- This type of defect occurs when extrusion temperature, friction, or speed is too high, and this caused the surface temperature rise significantly.

- However, surface cracking may also occur at lower temperatures, where it has been attributed to periodic sticking of the extruded product along the die land due to the extrusion pressure increase rapidly.

- To overcome: lowering the billet temperature and the extrusion speed.



2. Pipe defect (Tailpipe, Fishtailing)2. Pipe defect (Tailpipe, Fishtailing)

- The flow metal pattern in extrusion tends to draw surface oxides and impurities towards the center of

the billet.

- One third of the extruded length of a product may contain this type of defect and need to be cut off

as scrap.

- Can be minimized by: modifying the flow pattern to be more uniform, such as by controlling the

friction and minimizing the temperature gradients.

- The surface impurities can also be removed by machining the billet’s surface.



3. Internal cracking3. Internal cracking

- Occurred at the center of the extruded product.- Also known as center cracking, center-burst, arrowhead fracture or chevron cracking.- Happened due to hydrostatic tensile stress at the centerline in the deformation zone in the die.

(a)(b)

Figure shows (a) Chevron cracking (central burst) in extruded round steel bars. Unless the products are inspected, such internal defects may remain undetected, and later cause failure of the part in service. This defect can also develop in the drawing of rod, of wire, and of tubes. (b) Schematic illustration of rigid and plastic zones in extrusion. The tendency toward chevron cracking increases if the two plastic zones do not meet. Note that the plastic zone can be made larger either by decreasing the die angel or by increasing the reduction in cross-section (or both).

DRAWING PROCESSDRAWING PROCESS 18


DRAWING PROCESSDRAWING PROCESS- The cross-section of a long rod/wire is reduced by pulling thru’ a die.

- Extrusion vs drawing: Pushed vs pulled.

- The die angle influences the drawing force and the quality of the drawn product.

- Typical products are welding electrodes, electrical wiring, cables, tension-loaded structural members, and stringed musical instruments.

Figure: Process variables in wire drawing. The die angle, the reduction in cross-sectional area per pass, the speed of drawing, the temperature, and the lubrication all affect the drawing force, F.

DRAWING PROCESSDRAWING PROCESS 19


Examples of Tube-Drawing OperationsFigure: Examples of tube-drawing operations, with and without an internal mandrel. Note that a variety of diameters and wall thicknesses can be produced from the same initial tube stock (which has been made by other processes).

SHEET METAL FORMING PROCESSSHEET METAL FORMING PROCESS 20


SHEET METAL FORMING PROCESSSHEET METAL FORMING PROCESS

- Advantages: light weight and versatile shape.

-Common material used for sheet metal are low carbon steels because of its low cost and good

strength and formability characteristics; eg; aluminum as beverage cans, packaging, kitchen utensils,

and those parts for corrosion resistance.

Shearing ProcessShearing Process- Before a sheet metal part is made, a blank of suitable dimensions first is removed from a large sheet

usually in a form of coil, by using shearing technique.

- This sheet is cut off by subjecting it to shear stresses, generally using a punch and a die.

- Shearing process starts with the deformation of cracks on both the top and bottom edges of the

work piece.

- These cracks eventually meet each other and complete separation will occurs.- Observation from this process:• Rough fracture surfaces due to the cracks.• Smooth and shiny burnished surfaces on the hole.



Figure shows a) Schematic illustration of shearing with a punch and die, indicating some of the process variables. Characteristic features of (b) a punched hole and (c) the slug. Note that the scales of the two figures are different.

SHEARING



-The major processing parameters in shearing are:• The shape of the punch and die.• The speed of punching.• Lubrication.• The clearance, c, between the punch and the die.

-The clearance is a major factor in determining the shape and the quality of the sheared edge.-As the clearance increase, the zone of deformation become larger, and the sheared edge become more rougher.-The edge quality can be improved with increasing punch speed (can be as high as 10 to 12m/s).



Fine BlankingFine Blanking(a) (b)

Figure show (a) Comparison of sheared edges produced by conventional (left) and by fine-blanking (right) techniques. (b) Schematic illustration of one setup for fine blanking.

- Can produced very smooth and square edges.-A v-shaped stinger or impingement mechanically locks the sheet tightly in place and prevents the type of distortion of the material.



Slitting

Figure shows slitting with rotary knives. This process is similar to opening cans.

- Using a pair of circular blades.-The blades follow either a straight line, a circular path, or a curve path and normally the cutting edges will have burr.-If not performed properly, slitting operations can cause various distortions of the sheared edges.



Shaving and Shear AnglesShaving and Shear Angles

Figure shows schematic illustrations of the shaving of a sheared edge. (a) Shaving a sheared edge. (b) Shearing and shaving, combined in one stroke.



Figure shows an examples of the use of shear angles on punches and dies.

BevelingBeveling

- Suitable for shearing thick sheets because it reduce the force at the beginning of the stroke, reduces the operation’s noise level due to the smoother operation.



BendingBending

Figure shows (a) and (b) The effect of elongated inclusions (stringers) on cracking, as a function of the direction of bending with respect to the original rolling direction of the sheet. (c) Cracks on the outer surface of an aluminum strip bent to an angle of 900. Note the narrowing of the tope surface due to the Poisson effect.

(a) (b)

(c) - The outer fibers of the material are in tension, while the inner fibers are in compression.

- Because of the Poisson effect, the width of the part has become smaller in the outer region and larger in the inner region than the original width.



SpringbackSpringbackFigure shows a springback in bending. The part tends to recover elastically after ending, and its bend radius becomes larger. Under certain conditions, it is possible for the final bend angle to be smaller than the original angle (negative springback).

Figure shows the methods of reducing or eliminating springback in bending operations.

- Springback in forming operations usually is compensated for by overbending the part (figure a and c).

- Another method is to coin the bend area by subjecting it to highly localized compressive stresses between the tip of the punch and the die surface.



Bending OperationsBending Operations

Figure shows the common die-bending operations, showing the die-opening dimension, W, used in calculating bending forces.



Bending in a four slide machineBending in a four slide machine

- For short pieces, the bending process can be done on the lateral movements of the dies which controlled and synchronized with the vertical die movement to form the part into desired shapes.

Roll bendingRoll bending

- In this particular process, plates are bend by using a set of rolls.- By adjusting the distance between the three rolls, various curvatures can be obtained.

Figure below shows an examples of various bending operations.



Bending in a Press BrakeBending in a Press Brake

Figure shows (a) through (e) Schematic illustrations of various bending operations in a press brake. (f) Schematic illustration of a press brake.



Press Brake FormingPress Brake Forming

- Sheet metal or plate can be bent easily with simple fixtures using a press machine and suitable for

small quantity of production runs.

- The process can be done automatically (low cost and high production runs) or manually.

- Sheets or narrow strips that are 7m or even longer usually are bent in a press brake.

- This particular machine utilizes long dies in a mechanical or hydraulic press.

- The application of tooling on this machine are very simple which only involved the motions of up

and down.

- Die’s materials: common are carbon steel or grey iron, others: carbides.



Bead FormingBead FormingFigure below shows (a) Bead forming with a single die. (b) Bead forming with two dies, in a press brake.

- The sheet metal is bent into the cavity of a die.

- The bead imparts stiffness to the part by increasing the moment of inertia of the section.

- This process improve the appearance of the part as well as eliminate the exposed sharp edges.



FlangingFlanging

Figure shows various flanging operations. (a) Flanges on a flat sheet. (b) Dimpling. (c) The piercing of sheet metal to form a flange. In this operation, a hole does not have to be prepunched before the punch descends. Note, however, the rough edges along the circumference of the flange. (d) The flanging of a tube; note the thinning of the edges of the flange.



FlangingFlanging

- This is a process of bending the edges of sheet metals, usually to 900.

-In shrink flanging, the flange is subjected to compressive hoop stresses, which, if excessive, can

cause the flange periphery to wrinkle.

-On the other hand, in stretch flanging, the flange periphery is subjected to tensile stresses that, if

excessive can lead to cracking along the periphery.

Tube bending and formingTube bending and forming- For this process it needs a special tooling because of the tendency for buckling and folding.

- The assistance of sand or mandrel: to prevent the tube from buckling inward.

- Thick tube to be formed to a large bend radius can be bent safely without the use of this particular plugs.



Tube BendingTube Bending

Figure show methods of bending tubes. Internal mandrels, or the filling of tubes with particulate materials such as sand, are often necessary to prevent collapse of the tubes during bending. Solid rods and structural shapes can also be bent by these techniques.



Honeycomb StructuresHoneycomb Structures

Figure shows the methods of manufacturing honeycomb structures: (a) Expansion process; (b) Corrugation process; (c) Assembling a honeycomb structure into a laminate.

manufacturing process - extrusion of metal

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