Extrusion

Introduction:

Extrusion is a process that forces metal to flow through a shape-forming die. The metal is plastically deformed under compression in the die cavity. Extrusion processes can be carried on hot or cold. Extrusion differs from drawing in that the metal is pushed, rather than pulled under tension.

Forward extrusion

Forward extrusion is sometimes known as the Hooker process. In this process the confined metal is forced to flow downward in the direction of the punch travel. 

The process is generally used to produce thin-walled tubular parts with heavy flanges, straight tubular shapes, and extrusion of stepped; multiple diameters. 

Forward extrusion is best applied to parts having an outer diameter of 25.4 mm (1 in) or more. The production of rods and other solids shapes is also possible with forwards extrusion. 

In forward extrusion the work piece is placed in a close-fitting die. The punch is forced downward, displacing the metal through a restricted opening in the bottom of the die. The metal is forced to flow considerable distance beyond the end of the punch. Cupped or tubular parts of the punch extension serve as a mandrel. This controls the wall thickness and inner contour of the extruded parts.

Backward extrusion

Backward extrusion is a process that forces the metal confined in the cavity to flow in a direction opposite to that of the punch travel. 

The slug (workpiece) is contained in a closed die. The descending punch enters the slug. The pressure displaces the metal upward through the opening between the punch and die. This is generally used for extruding symmetrically shaped parts having a closed end.

Combined extrusion

Combined extrusion uses a combination of forward extrusion and backward extrusion. The metal is confined inside a matrix between the lower and upper punches. This forces the metal to flow both up and down. The extruded part is lifted from the die on the upward stroke of the slide by a lift out on the bed of the press. Some aspects of combined extrusion are: 

1. it is fast 
   
2. it can complete parts in few steps 
   
3. it can produce large quantities with low unit costs 
   
4. it wastes little material 
   
5. it can make parts with small radii 
   
6. it requires mirror tooling 
   

Design Considerations

Limit the irregularities of shape as much as the function of the part allows. Metal flows less readily into narrow and irregular die sections, making distortion and other quality problems more likely to occur. 

Many extrusion shops and metals suppliers provide standard shapes that might serve the designer. A good rule of thumb is to always use standard cross sections when possible. 

Tolerances are advised to be liberal enough to avoid secondary drawing operations, if possible. 

With all metals, and particularly with steel and less easily extruded metals, it is recommended to avoid extreme changes in section thickness. 

Sample Parts

The extrusion process can be used to manufacture building and automotive trim, window frame members, tubing, aircraft structural parts, railings, flashlight cases, aerosol cans, military projectiles, and fire extinguishers.

Technological advances have allowed extrusion companies to use this process in applications that were considered too difficult few years ago, as the figures below show.

Materials used in extrusion:

Metals and alloys: Brass, copper, lead, aluminum, steel, magnesium, tin, titanium, and zinc.

Thermoplastics: ABS, Acrylic, Butyrate, Flexible Vinyl, PETG Co-Polyester, Polycarbonate, Polyethylene- High & Low Densities, Polypropylene, Polystyrene, Polyurethane, Rigid Vinyl, Thermoplastic Elastomer.

Following is a table of materials and their ranking. The ranking indicates the material suitability for the extrusion process. 

Material Ranking

Cast Iron 50

Carbon Steel 80

Alloy Steel 80

Stainless Steel 80

Aluminum & Alloys 100

Copper & Alloys 100

Zinc & Alloys 80

Magnesium & Alloys 100

Titanium & Alloys 50

Nickel & Alloys 80

Refractory Metals 0

Thermoplastics 100

Thermosets 0

Ceramics 50

PhotoPolymers 0

Wood (dry) 0

A value of zero means that the corresponding material is never used with this process, a ranking of 100 means that it is excellent for use with this process.

Properties of extruded materials:

Deformation is greater in the outer zones of a bar than it is at the center, particularly when the extrusion ratio is low. The center receives only light deformation.

The improvement in tensile strength of Al 1100 resulting from extrusion with 3:1 ratio is 5000 psi (before extrusion) to 19000 psi (after extrusion). In general, the yield strength is increase about four times the initial strength.

In general, the inside surfaces of backward extruded parts are 5 to 25 Rockwell-B Hardness harder than the outside surfaces. The hardness of material alloys which undergo this backward extrusion process is decreased by 20 to 10 Rockwell-B Hardness. However in forward extrusion the outer surfaces are harder than the inner surfaces.

Advantages:

1. The tooling cost is low, as well as the cost due to material waste ( it has high material utilization). 
   
2. Intricate cross sectional shapes, hollows and with undercuts can be produced. 
   
3. The hardness and the yield strength of the material are increased. 
   
4. In most applications, no further machining is necessary. 
   

Disadvantages:

1. High tolerances are difficult to achieve. 
   
2. The process is limited to ductile materials. 
   
3. Extruded products might suffer from surface cracking. It might occur when the surface temperature rise significantly due to high extrusion temperature, friction, or extrusion speed. Surface cracking might also occur at low speeds due to periodic sticking of the extruded product along the die land. 
   
4. Internal cracking might also occur. These cracks are attributed to a state of secondary tensile stresses at the centerline of the deformation zone in the die. 
   

Drawing of Sheet and Plates

Drawing is a plastic deformation in which a flat sheet or plate is formed into a recessed, three-dimensional part with a depth several times the thickness of the metal. 

As a punch descends into a die or the die moves upward over a punch, the metal assumes the configuration of the mating punch and die tooling. 

We distinguish hot drawing and cold drawing. 

Hot drawing is used for forming relatively thick-walled parts of simple geometry, usually cylindrical. The thickness of the material reduces considerably as it moves through the dies. This process is used in forming components such as oxygen tanks and large artillery shells. 

Cold drawing uses relatively thin metal, changes the thickness very little or not at all, and produces parts in various shapes. 

  

Method of cup drawing or hot forming

The ram force F, or the input force exerted on the ram, during a forward extrusion process can be calculated. The value of the calculated force depends on the model used. 

The input power is the power supplied by the ram force as it moves with a velocity uo:

Pinput = F uo 

This total input power is transformed into:

1. Ideal power consumed by the plastic deformation. 
   
2. Frictional power dissipated due to friction along the die angle. 
   
3. Redundant work due to inhomogeneous deformation. 
   

In general, the total ram force depends on the above three components. There is a die angle, see figure below, for which the ram force is minimum. However, unless each component of the powers is know as a function of the die angle, it is very hard to determine the optimum angle.

 

Ideal Deformation

Let Ao be the cross-sectional area of the billet (material before being extruded), and let Af be the cross-sectional area of the extruded piece. An extrusion ratio is defined as:

R = Ao / Af 

Then, the absolute value of the true stain,  , is given by:

x = ln(R) 

If Y denotes the yield stress of the perfectly plastic material, the energy dissipated in plastic deformation per unit volume is:

E = Ydef

The power due to plastic work of deformation is:

Pplastic = uo Ao E = uo Ao Y  

In the ideal case, we assume that the total power input is equal to the power due to plastic work of deformation. Recall that the input power is:

Pinput = F uo = p Ao uo 

Where p is the extrusion pressure at the ram. Equating Pplastic and Pinput, we find that:

p = Y ln(R)

Note that the value of extrusion pressure is equal to the area under the true stress / true strain curve for the material.

Ideal Deformation and Friction

When friction at the die-billet interface is accounted for, the power input is equal to the sum of the plastic deformation and the frictional power.

Because the billet is forced through a die with a substantial reduction in its cross-section, a dead zone in the metal flow pattern develops at the die exit region. 

 

We assume that the material flow in that region takes place at a 45 degrees, this is an "effective die angle", and that the friction stress is equal to the shear yield stress k = Y/2 of the material. The power dissipated due to friction along the die angle is:

Pfriction = (uo/cos(45)) Ao (Y/2)  

Equating the power input to the sum of the power of plastic deformation and the power of friction force:

p Ao uo = uo Ao Y  + (uo/cos(45)) Ao (Y/2)  

It follows immediately that the extrusion pressure is:

p = 1.7 Y ln(R)

In this analysis, the force required to overcome friction at the billet-container interface was neglected. It can be easily calculated if we assume again that the frictional stress is equal to the shear yield stress of the material, k, and we let AL denotes the lateral surface of the billet remaining in the die, then an additional ram pressure, pf, due to wall friction is given by:

pf Ao = k AL = (Y/2) AL 

Thus, the total extrusion pressure becomes:

p = Y( 1.7 ln(R) + AL / 2Ao )

 

Actual Forces

The derivation of analytical expressions, including friction, die angle, and redundant work due to inhomogeneous deformation of the material, can be difficult. Consequently, a convenient empirical formula has been developed:

p = Y(a + b ln(R))

where a and b are constants determined experimentally. Approximate values for a and b are 0.8 and 1.2 to 1.5, respectively.

For strain hardening materials, Y in the above expressions should be replaced by the average flow stress.

Flow of metal during the process of extrusion

When a billet of material is forced through a die, with a substantial reduction in its cross-sectional area, the metal flow pattern in extrusion is important. Typically, three different metal flow patterns have been observed during the process of extrusion depending upon the prevailing conditions. The conditions under which the different flow patterns are obtained are as follows.

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The most homogeneous flow pattern is obtained when there is no friction at the billet-container-die interfaces. This type of flow occurs when the lubricant is very effective or with direct extrusion. 

When friction along all interfaces is high, a dead-metal zone develops. As a result a high-shear area appears as the material flows into the die exit, somewhat like a funnel. This configuration may indicate that the billet surfaces could enter the high shear zone and be extruded, causing defects in the extruded product.

The high shear zone extends farther back. This extension can result from high container wall friction, which retards the flow of the billet or materials in which the flow stress drops rapidly with increasing temperature. In hot working, the material near the container walls cools rapidly, subsequently increasing the strength. Thus, the material in the central regions flows toward the die more easily than that at the outer regions. As a result, a large dead metal zone forms and the flow is inhomogeneous. This flow pattern leads to a defect known as a pipe or extrusion defect.

Thus the two factors that greatly influence metal flow in extrusion are the frictional conditions at the billet-container-die interfaces and thermal gradients in the billet.

Materials property change during the extrusion process

After the extrusion process the properties of materials will change, for example hardness, strength and grain size.

Results :

Initial diameter      25 mm

Final diameter        10 mm

Initial area = Ao = pi*Do^2 /4 = 490.874 mm^2

Final area  = Af = pi*Df^2 /4 = 78.54 mm^2

Extrusion ratio = Ao/Af = 6.25

Strain = ε = ln(Ao/Af) = 1.83

Extrusion reduction in area = (Ao-Af)/Ao*100% = 84%