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19 C - METAL BULK DEFORMATION - EXTRUSION


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EXTRUSION
COMPRESSION PROCESS THAT FORCES METAL THROUGH AN OPENING, like toothpaste out of a tube to form long pats with uniform cross section and enhanced strength characteristics due to better grain structure, parts like tubes and spark plugs. TOOTH PASTE METAL MAKES UNIFORM TUBES AND SPARK PLUGS.
*Types of extrusion
TYPES OF EXTRUSION can be classified by the following parameters: [I] Direct and indirect extrusion, [II] Cold, Warm, Hot extrusion, [III] Continuous or discrete extrusion. >>> Die TiC: DIRECTNESS, TEMPERATURE, CONTINUITY
Direct extrusion
RAM PUSHES BILLET THROUGH A DIE, pushing it against the friction with the walls, forcing it out of the die to produce a long part with a uniform cross section, sometimes with a thin layer inserted between the ram and the container's wall to reduce friction.
Indirect extrusion
HOLLOW RAM PUSHES METAL THROUGH ITSELF, applying less force due to a lack of friction with the walls, a problematic process for big lengths of extrusion
Hot extrusion
RAM EXTRUDES HOT METAL, applying less force, producing more complex shapes with greater size reductions, yet in order to overcome cooling and friction heating the die and lubricating the work with glass is required. WEAK RAM PENETRATES HOT AND LUBED CONTAINER
Cold and warm extrusion
RAM EXTRUDES COLD OR WARM BILLET, producing discrete and strain hardened parts with good surface finish and dimensional tolerance at high rates. COLD RAM QUICKLY MAKES PRECISE SPARK-PLUGS.
Continuous and discrete extrusion
ALL EXTRUSION PROCESSES ARE DISCRETE, differing only in their length, since none of them operates continuously and without interruption - at some point you need put your load into the extrusion machine.
*Analysis of extrusion
When calculating force needed for extrusion, you must take the friction with the die into account, the friction being bigger in the open die process. $$\begin{array}{ll} {\scriptstyle Stran\, ajusted\, for\, die\, friction:} & \epsilon_{x}=a+b\cdot ln\frac{A_{o}}{A_{f}}\\ {\scriptstyle a\, and\, b\, constants:} & a=0.8;\, b=1.2\div1.5\\ {\scriptstyle Average\, flow\, stress:} & \overline{Y_{f}}=\frac{K\epsilon_{x}^{n}}{1+n}\\ {\scriptstyle Pressure\, for\, indirect\, extrusion:} & p=\overline{Y_{f}}\epsilon_{x}\\ {\scriptstyle Pressure\, for\, direct\, extrusion:} & p=\overline{Y_{f}}\left(\epsilon_{x}+\frac{2L}{D_{0}}\right) \end{array}$$
Extrusion dies
DIE SHAPE AFFECTS EXTRUSION FORCE, since extruding a complex and shapy part is harder than extruding simple and round part, thus the shape factor comes into play, a factor relating between perimeter of extruded part $C_x$ and between perimeter of a circle with the same cross section as the extruded profile $C_c$ $$\begin{array}{1} K_{x}=0.96+0.02 \left(\frac{C_{x}}{C_{c}} \right)^{2.25}\\ p_{indirect}=K_{x} \overline{Y_{f}} \epsilon_{x}\\ p_{direct}=K_{x} \overline{Y_{f}} \left(\epsilon_{x}+ \frac{2L}{D_{0}}\right) \end{array}$$
Die angle
RIGHT DIE ANGLE REDUCES EXTRUSION FORCES. Low angle causes too much friction and steep angle causes redundant work, thus die angle must be positioned at a compromising position between the two, in order to reduce extrusion forces, yet the optimal angle changes based on the worked material, temperature and lubrication. ACCURATE ANGLE HELPS METAL FLOW.
Extrusion presses
EXTRUSION PRESSES USUALLY PUSH WITH HYDRAULICS, sometimes impacting the part with mechanical means, oriented in a horizontal or vertical manner, depending on the process needs.
*Other extrusion processes
OTHER EXTRUSION PROCESSES include impact extrusion and hydrostatic extrusion. HARD PLUNGER SPLASHES MOIST CONTAINER.
Impact extrusion
HARD PUNCH IMPACTS COLD METAL, violently beating it into shape by impacting a cold blank in a die with a hard punch, forming tooth paste tubes and battery cases, creating them at very high rates of production in this important commercial process. FAST PUNCHES BEAT BATTERY COVERS FOR ENERGIZER BUNNIES.
Hydrostatic extrusion
LUBRICATING FLUID DIRECTLY FORCES FRAGILE METAL THROUGH DIE, the ram pushes on the fluid, building up pressure, pressure forces the billet of a brittle metal that would shatter in regular extrusion out of the die, the processes lengthened by the work billet preparation - you must taper it to make a plug to prevent the liquid escaping from the die. PRESSURIZED OILS EXTRUDE FRAGILE BUTT-PLUG INTO A DILDO.
*Defects in extruded products
DEFECTS IN EXTRUDED PRODUCTS can form due to large deformations involved in forcing material through a narrow hole, thus conditions such as [1] Centerburst - cracks at the center of hte part, [2] Piping - sinkhole on the end of extruded product, [3] Surface cracking - cracks on the surface. >>> CuPS: CENTERBURST, PIPING, SURFACE_CRACING
Centerburst
LARGE TENSILE DEFORMATIONS CRACK PART ALONG CENTERLINE, a defect occurring due to [1] High die angles, [2] Low extrusion ratios, and [3] impurities in work metal, a defect also known as "arrowhead fracture", "center cracking", and "chevron cracking". >>> STEEP ANGLES AND NARROW HOLES CRACK DIRTY METAL.
Piping
FRICTION WITH CONTAINER WALLS CREATES SINKHOLE ON BILLET, a phenomena preventable by dummy block with a slighter smaller diameter than that of a billet.
Surface cracking
HIGH TEMPERATURE CRACKS PART, a defect often occurring and high extrusion speeds that produce high strain rates and generate heat, a defect also caused by high friction and rapid chilling of extruded parts. DON'T BE A FAST FOOL - COOL DOWN YOUR TOOL.
WIRE AND BAR DRAWING
WIRE OR BARD PULLED THROUGH A DIE, the die reducing their diameter by pulling them on one end and compressing them, with the die, on the other, thus the tensile stress must must never exceed yield stress. TENSILE STRESS COMPRESSES WITH DIES and must be lower than $\sigma_{Y}$
Bar drawing
DRAWING A SINGLE BAR, usually accomplished in a single-draft operation by pulling the stock through one die opening.
Wire drawing
CONTINUOUS DRAWING OF A LONG COIL of wire through one or several dies. Wire sizes up to 0.03[mm] can be drawn.
Never exceed yield stress
BEYOND YIELD POINT YOU'RE JUST PULLING ROPE. Operation depends on pulling the stock though a die, thus if you'll exceed yield stress you'll just be deforming the material rather than pulling it through the die
*Analysis of drawing
METAL EXPERIENCES HIGHER STRESS DUE TO FRICTION with container walls and the die, thus when calculating drawing force, you must account for it $$\begin{array}{ll} {\scriptstyle Change\, in\, work\, size:} & {\scriptstyle r=\frac{A_{0}-A_{f}}{A_{0}}}\\ {\scriptstyle Draft:} & {\scriptstyle d=D_{0}-D_{f}}\\ {\scriptstyle True\, strain:} & {\scriptstyle \epsilon=ln\frac{A_{0}}{A_{f}}=ln\frac{1}{1-r}}\\ {\scriptstyle Average\, flow\, stress:} & {\scriptstyle \overline{Y_{f}}=\frac{K\epsilon^{n}}{1+n}}\\ {\scriptstyle Stress\, in\, part} & {\scriptstyle \sigma=\overline{Y_{f}}\epsilon=\overline{Y_{f}}ln\frac{A_{0}}{A_{f}}}\\ \begin{array}{c} {\scriptstyle Stress\, ajusted\, for}\\ \,{\scriptstyle die\, and\, friction} \end{array}: & {\scriptstyle \sigma_{d}=\overline{Y_{f}}\left(1+\frac{\mu}{tan\alpha}\right)\phi ln\frac{A_{0}}{A_{f}}}\\ {\scriptstyle Deformation\, factor:} & {\scriptstyle \phi=0.88\pm0.12\frac{D}{L_{c}}}\\ {\scriptstyle Average\, diameter:} & {\scriptstyle D=\frac{D_{0}+D_{f}}{2}}\\ {\scriptstyle Contact\, length\, with\, die:} & {\scriptstyle L_{c}=\frac{D_{0}-D_{f}}{2\cdot sin\alpha}}\\ \mathbf{{\scriptstyle Draw\, force}} & {\scriptstyle \mathbf{F=A_{f}\sigma_{d}=A_{f}\overline{Y_{f}}\left(1+\frac{\mu}{tan\alpha}\right)\phi ln\frac{A_{0}}{A_{f}}}} \end{array}$$
Maximum reduction per pass
In order not to exceed yield strength of metal multiple dies are used to gradually reduce bar or wire thickness. MAXIMUM REDUCTION PER PASS FOR BARS: 50%, AND MAXIMUM REDUCTION PER PASS FOR WIRES: 30%
*Drawing practice
Drawing usually forms cold parts in cold working process, creating different types of cross sections: round, square, and others, creating electrical wires, wire stock for nails and hangers, rod stock for nails, rivets, screws, springs, and tube stock for pipes, a process that has: [1] Close dimensional control, [2] Good surface finish, [3] Improves mechanical properties due to grain flow, [4] Adaptable for economical batch and mass production. ACCURATE DIMENSIONS FINISH DRAW CHEAP GRAIN FLOW.
Drawing equipment
Bars squeeze through dies on a draw bench - a table with an actuator that pulls the bars through dies, and wire goes through series of consecutive dies, sometimes annealed to relief work hardening, the drums called capstans pulling each section of wire through it's corresponding die, sometimes lubricating the wire to allow for easy tension. SLIDING CYLINDERS AND ROTATING DRUMS LUBRICATE, SHAPE, AND ANNEAL BAR AND WIRE STOCK
Draw bench
CYLINDER PULLS BARS THROUGH DIES, the bars transfixed to the carriage that pulls them through dies in the die stand.
Capstan
Motor driven drum to pull wire through die.
Draw dies
TOOL STEEL OR CARBIDE DIE CHANNELS AND SHAPES STOCK, allowing easy entry and lubrication at the wide entry region, compressing the stock when it passes through the approach region angled at 6° to 20° to form the part, squeezing through the bearing surface that determines the final diameter of the drawn product, and finally escaping the die through back relief, a zone angled at 30° to allow easy exit. DIAMOND DIES SQUEEZE HARD BARS.
Preparation of work
ONLY AN ANNEALED, CLEANED, POINTED PART CAN GO THROUGH. The preparations prevent die and stock damage while shaping stock to penetrate die. [1] Annealing - heating and cooling the stock to increase ductility and remove strain hardening; [2] Cleaning - wage slaves remove dirt from stock to prevent die and stock damage; [3] Pointing - wire or bar go through thinning by swagging, rolling, or turning, in order to fit through the die and start the drawing operation. CLEAN, SOFT, NARROW ROPE MOVES IN LUBRICATED HOLE.
*Tube drawing
MACHINE PULLS FORTH A TUBE, reducing it's diameter, sometimes using a fix mandrel or a floating plug to control the wall thickness. METAL BUTPLUG SPREADS STEEL FLAPS