15- Metal Extrusion And Drawing

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extrusion

Back 1

a cylindrical billet is forced through a die in a manner similar to squeezing toothpaste from a tube, PUSHING a material through a die for the purpose of reducing its cross-sectional area, makes window frames, railings for sliding doors

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cold extrusion

Back 2

extrusion at room temp and combined with forging operations, makes fasteners and components for automobiles, bicycles, motorcycles

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drawing

Back 3

cross section of solid rod, wire, or tubing is reduced or changed in shape by pulling it through a die, used for shafts, spindles, raw materials for fasteners like bolts and screws, PULLING a material through a die for the purpose of reducing or changing its cross-sectional area

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rod vs. wire

Back 4

rod = larger cross section
wire = rod that has been drawn through a die at least once or has diameter small enough to be coiled

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types of extrusion

Back 5

direct/forward extrusion, indirect, hydrostatic

Front 6

direct extrusion

Back 6

a billet is placed in a chamber and forced through a die opening by a hydraulically driver ram, dummy block protects tip of pressing stem

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indirect extrusion

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the die moves toward the unextruded billet, advantage of having no billet-container friction since there is no relative motion, used on material with very high friction (high strength steels)

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hydrostatic extrusion

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billet is smaller in diameter than the chamber (which is filled with liquid), and pressure is transmitted to the fluid by a ram, fluid pressure results in triaxial compressive stressed acting on the work-piece and thus improved formability

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extrusion ratio, R

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Ao/Af, ratio of the cross-sectional area of the billet to that of teh extruded product

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extrusion force, F

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= (Ao)k*ln(Ao/Af), k=extrusion constant

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extrusion constant

Back 11

measure of the strength of the material being extruded and the frictional conditions

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metal flow in extrusion

Back 12

important because of impact it has on quality and mechanical properties of extruded product

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dead-metal zones

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where metal at the corners is essentially stationary

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coaxial extrusion or cladding

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coaxial billets are extruded together provided that the strength and ductility of the two metals are compatible, ex: copper clad with silver

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stepped extrusions

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produced by extruding the billet partially in one die then in one or more larger dies

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lateral extrusion

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used for the sheathing of wire and coating of electric wire with plastic

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hot extrusion

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for metals with insufficient ductility at room temp, causes die wear from heat, to avoid oxide forming: dummy block is slightly smaller

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skull

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thin shell made up of the outer oxidized layer of the billet, left over in the container as a result of small dummy block, later removed from chamber

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square dies (shear dies)

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used in extruding nonferrous metals like aluminum, develop dead-metal zones

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how is tubing made?

Back 20

by extruding from a solid or hollow billet

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what kind of dies are used to extrude hollow cross sections?

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porthole die, spider die, bridge die

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is lubrication important in hot extrusion? if so, why?

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yes, because of its effects on: material flow, surface finish and integrity, product quality, extrusion forces,
ex: glass is excellent lubricant for steels

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sejournet process

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circular glass pad is placed in the chamber at the die entrance, hot billet conducts heat to pad where a thin layer of glass begin to melt and acts as a lubricant at the die interface as the extrusion progresses

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jacketing or canning

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for metals that have a tendency to stick to the container and the die, billet is enclosed in a thin-walled container made of softer metal, acts as low-friction interface, prevents contamination of billet by the environment, used for extruding reactive metal powders

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cold extrusion

Back 25

denotes a combination of operations, such as direct and indirect extrusion and forging, makes components in automobiles, motorcycles, bicycles

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cold extrusion process

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uses slugs cut from cold-finished bars, wire, or plates

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force in cold extrusion, F

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F=1.7(Ao)(Yavg)E, Ao=cross-sectional area of blank, Yavg=average flow stress, E=true strain

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advantages of cold extrusion

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-mech properties
-good control of dimensional tolerances
-improved surface finish

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is lubrication necessary in cold extrusion? if so, why?

Back 29

yes, because of the possibility of sticking between workpiece and tooling, uses phosphate-conversion coating and soap or wax

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impact extrusion

Back 30

like indirect extrusion, punch descends rapidly on the blank, which is extruded backwards

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hydrostatic extrusion

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pressure required in the chamber is supplied via a piston through an incompressible fluid medium surrounding the billet, room temp, uses vegetable oil as fluid, used to extrude brittle materials

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principal extrusion defects

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surface cracking, pipe, internal cracking

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surface cracking

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caused by hot shortness, occurs when temp, friction, or speed is too high, avoided by lowering the billet temp and extrusion speed

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bamboo effect

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surface cracking at low temps that is attributed to periodic sticking of the extruded product along the die land, resembles the surface of a bamboo stem

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pipe

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the tendency of the metal flow pattern in extrusion to draw surface oxides and impurities toward the center of the billet (like a funnel)

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extrusion equipment

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horizontal hydraulic press, vertical hydraulic presses are used for cold-extrusion, crank-joint presses, knuckle-joint presses

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drawing

Back 37

cross section of a long rod or wire is reduced by PULLING

Front 38

what is the difference between extrusion and drawing?

Back 38

in extrusion the material is PUSHED through a die and in drawing it is PULLED through a die

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drawing force, F

Back 39

=(Yavg)(Af)*ln(Ao/Af)

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where does the optimum die angle occur?

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where drawing force is a minimum

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wedge-shaped dies

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used for drawing flat strips, fundamental deformation mechanism in ironing

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sizing pass

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light reduction (in drawing)

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patenting

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heat treating of drawn wire, makes high-carbon wires for springs and for musical instruments

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bundle drawing

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increase productivity, draw a hundred or more wires simultaneously

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profile drawing

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involves various stages of deformation to produce the final profile

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die materials for drawing

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tool steels and carbides, diamond dies used for drawing fine wire

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basic methods of lubrication in wire drawing

Back 47

wet drawing, dry drawing, metal coating, ultrasonic vibration

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wet drawing

Back 48

dies and rod are immersed completely in lubricant

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dry drawing

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surface of rod to be drawn is coated with lubricant by passing it through a stuffing box

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stuffing box

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box filled with lubricant

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ultrasonic vibration

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vibrations used to reduce forces

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defects in drawing

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center cracking, seams

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seams

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longitudinal scratches or folds in a material

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equipment for drawing

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draw bench, bull block

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draw bench

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contains a single die, used for single length drawing of straight rods

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drum or bull block

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rotated to draw very long rods and wire