Metal Extrusion

7/29/2019 Metal Extrusion

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Metal Extrusion

Extrusion is a metal forming process in which a work piece of a certain length and

cross section is forced to flow through a die of a smaller cross sectional area, thus

forming the work to the new cross section. The length of the extruded part will vary

dependant upon the amount of material in the work piece and the profile extruded.

Numerous cross sections are manufactured by this method. The cross sectionproduced will be uniform over the entire length of the extrusion. Starting work is

usually a round billet, and may be formed into a round part of smaller diameter, a

hollow tube, or some other profile. The basic principle of extrusion is illustrated in

figure 208.

Figure:208

In this case a round billet is forced through a die opening, creating a round part of

reduced diameter. The ram will continue to move forward pushing more of the billet

material through the die opening. As this occurs a continuous length of work will

emerge from the other side of the mold at a certain velocity relative to the speed of the


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ram. When manufacturing an extruded product, considerations to support and guide

the length of material as it exits the die are important. As the ram reaches the end of

its stroke a small portion of the billet stock can not be pushed through the die opening.

This last part of the work material is called the butt end. The product is cut at the die

opening to remove it from the butt end material. In manufacturing industry, methods

have been developed to extrude a wide variety of different materials. Some materialsare better suited for extrusion manufacture than others. Aluminum is an extremely

good material for extrusion. Copper, magnesium, zinc, tin, and some softer low

carbon steels can also be extruded with little complication due to the material. High

carbon steels, titanium, and various refractory alloys can be difficult to extrude.

Extrusion is capable of creating tremendous amounts of geometric change and

deformation of the work piece, more than other forming processes. Extrusion tends to

produce an elongated grain structure, usually considered favorable, in the part's

material in the direction that the work is extruded. Extrusion, in many instances, can

be considered a semi continuous manufacturing operation. Continuous because the

process will manufacture a continuous length of the same cross section. From this

length, individual discrete parts can be cut. It is semi continuous and not completely

continuous, (such as continuous casting), because the length of extruded product is

still limited by the amount of material in the work piece. The work piece must be

reloaded at the end of every cycle. Extrusion can also be a discrete manufacturing

process, producing a single part with every cycle. As in other forming operations, the

forces involved and the material flow patterns that occur during extrusion are of

primary concern in the analysis and development of this manufacturing process. The

many factors that effect material flow will be discussed.

Cold Extrusion Or Hot Extrusion

Extrusion is a forming process, and like other forming processes it can be performed

either hot or cold. The characteristics of hot forming and cold forming were discussed

in detail in the fundamentals of metal forming section.

Hot forming involves working a metal above its recrystallization temperature. Hot

working has many advantages in the improvement of the mechanical properties of the

part's material. Cast metal contains pores and vacancies throughout the material. Hot

working a metal will push and redistribute material, closing up these vacancies.


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Impurities in a material usually combine together in masses, forming solid inclusions

within the metal. These inclusions cause weakness in the surrounding metal. Hot

working causes these inclusions to break up and distributes them throughout the

material. Large, irregular, columnar grain structures are usually present in cast parts.

Hot working a metal will break up irregular structures and recrystallize to a finer

wrought grain structure. Mechanical properties of the part such as impact resistance,ductility, and strength characteristics are improved. If a hot extrusion is performed on

a cast work piece then the advantages of hot working will be imparted to the part.

However, most extrusions in manufacturing industry are performed on billets that

have already been hot formed, thus the mechanical advantages of hot forming have

already been imparted to the material.

In addition to the improved physical characteristics of the metal hot forming does

offer other advantages in a manufacturing process. A metal above its recrystallization

temperature is more easily manipulated than a cold metal. An increase in temperature

results in a corresponding decrease in strength and an increase in ductility, factors

more advantageous in the forming of the material.

When metals are worked above their recrystallization temperature strain hardening

does not occur, thus hot forming allows for a large amount of shape change. One of

the major disadvantages of hot forming of metals is the oxidation that occurs over the

surface of the hot work. This results in a layer of oxide scale build up on the external

surfaces of the work piece. Scale can effect surface finish and accuracy of the part as

well as increasing friction and wear at die metal interfaces. Heating to, and

maintenance of, high working temperatures, decreased tolerances, and increased die

wear, are all disadvantages of a hot forming manufacturing process over a cold one.

Choosing between hot extrusion or cold extrusion will depend on the specific details

of the manufacturing process. Some of the more difficult to form materials may have

to be worked hot. Some easy to extrude materials such as aluminum can be worked

either hot or cold depending upon other factors in the process. Hot extrusion is

generally preferred for larger parts, more extreme changes in shape, and extrusions

with more complex geometry. Cold extrusion is usually used for smaller parts, less

complex shapes, more workable materials, and the manufacture of discrete extrusions

that create a single part with each operation. Impact extrusion, a discrete

manufacturing process, is most often performed cold.

Advantages of cold extrusion over hot include not having to heat the work, higher

production rate, no oxidation and scale form on surfaces, greater geometric accuracy,

better surface finish, and the ability to strengthen the part by way of strain hardening.

In hot extrusion, like other hot forming operations, the heat transfer between the work

piece and the cooler surfaces of the die presents a problem during the manufacturing


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operation. In order to mitigate this issue, die used for extruding can be preheated to

lessen the temperature gradient. Lubricants also help in the reduction of heat transfer

between the part and the mold. With some particularly difficult to extrude materials

isothermal extrusion may be employed, this is similar in concept to isothermal

forging. In these instances the mold is maintained at or slightly below the temperature

of the work during the entire process.

Direct Extrusion Compared With

Indirect ExtrusionExtrusion operations in manufacturing industry can be classified into two main

categories, direct and indirect. Hollow extrusions, as well as cross sections, can be

manufactured by both methods. Each method, however, differs in its application of

force, and is subject to different operational factors.

Direct Extrusion

Direct extrusion is a similar extrusion method to the one illustrated in figure 208. In

direct or forward extrusion the work billet is contained in a chamber. The ram exerts

force on one side of the work piece, while the die through which the material is

extruded is located on the opposite side of the chamber. The length of extruded

product flows in the same direction that the force is applied.

Figure:209


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During direct extrusion, metal flow and forces required are effected by the friction

between the work piece and the chamber walls. Particularly in hot forming, oxide

scale buildup on the outer surfaces of the work piece can negatively influence the

operation. For these reasons, it is common manufacturing practice to place a dummy

block ahead of the ram. The dummy block is of slightly smaller diameter than the

chamber and work piece. As the extrusion proceeds the outermost surface of the work

is not extruded and remains in the chamber. This material will form a thin shell,

(called skull), that will latter be removed. Much of the skull will be comprised of the

surface layer of oxidized scale from the work metal.

Figure:210


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Hollow or semi hollow parts may be directly extruded, with the use of a mandrel

attached to the dummy block. A hole is created through the work parallel to the axis

over which the ram applies the force to form the extrusion. The mandrel is fitted

within this hole. Once the operation begins, the ram is forced forward. The extruded

material flows between the mandrel and the die surfaces, forming the part. The

interior profile of the extrusion is formed by the mandrel, while the exterior profile isformed by the die.

Figure:211

Indirect Extrusion

In indirect extrusion, the work piece is located in a chamber that is completely closed

off at one side. The forming die are located on the ram, which exerts force from the

open end of the chamber. As the manufacturing process proceeds the extruded product

flows in the opposite direction that the ram is moving. For this purpose the ram is

made hollow so that the extruded section travels through the ram itself. This process is

advantageous in that there are no frictional forces between the work piece and the

chamber walls. Indirect extrusion does present limitations. Tooling and machine set

up are more complicated, hollow rams are not as strong and less ridged, and support

of the length of the extrusion's profile as it travels out of the mold is more difficult.


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Figure:212

Indirect extrusion can also be used to produce hollow parts. In this process a ram is

forced into the work material. The ram gives the internal geometry to the tubular part,

while the material is formed around it. Difficulties in supporting the ram limit this

process and the length of tubular extrusions that may be manufactured.

Figure:213


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Extrusion Practice For Manufacturing

Extrusion practice in manufacturing industry must take into consideration a variety of

factors, many of which will be specific to each particular operation. The type of

material, size of work piece, geometric cross section of extruded part, ram speed,temperature of work, and type of extrusion process, are all important elements in the

design and analysis of an extrusion operation. The main goal is to enact the right

metal flow through the correct application of force. The force is applied through a ram

powered by some sort of press. Most extrusions are performed horizontally, by

hydraulic presses. Hydraulic presses can deliver a constant force at a constant speed

over a long stroke, making them ideal for extruding parts; however, in some instances

mechanical presses may be used. The ram's speed effects the forces involved during

the operation. Ram speeds can be as low as a few feet every minute, or may be as high

as 15 feet per second, though most are under 2 feet per second. The length of extruded

product in common manufacturing practice is generally up to 25 feet, but much longer

lengths, as high as 90 feet have been created. Many of the extruded sections produced

in industry require bending or straightening after the completion of the extrusion

process. When performed correctly extrusion can be very economical for both small

and large batch production.

Material Flow During Extrusion

During an extrusion process, material from a work piece of a certain cross section is

forced to flow through a die of smaller cross section, forming an extruded part. It is

important to understand the flow of material that occurs as the part is being formed. In

some ways it is similar to fluid flowing from one channel into another channel of

decreasing width. The material is deformed and forced to flow together as it

progresses towards and through the die. As the work travels through the die the outer

layers are deformed more than the ones closer to the middle. The outer sections

further from the central axis will experience greater material displacement and will

have more turbulent metal flow characteristics. The material closer to the center will

move faster through the mold, meaning it will have the higher velocity relative to the

die. With square die, which are die with 90 degree angles sections in the material

close to the mold opening but adjacent to the die may not move. These areas, termed

dead zones, or dead metal zones, are indicative by stagnation of metal flow. Note that


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there will be a type of shearing of the material occurring between layers, at the

interfaces of dead zones.

Extrusion Ratio

The extrusion process is capable of creating a tremendous amount of metal

deformation of the work. The size of the cross section of the work billet may be much

larger than the size of the cross section of the extruded part. For example in figure 214

the starting work billet has a certain diameter, say 10 inches. It is formed into a round

extrusion with a diameter of 5 inches. We can relate the size of the work's crosssection with that of the extruded part by comparing their diameters. It can be said that

the extrusion has a diameter of 1/2 the original work, thus measuring the cross

sectional reduction that occurred during the process.

Figure:214

This is an easy relationship to make, since both the work and the extrusion are round.

If the work and the extruded part have a different profile, another means will beneeded to relate their sizes. For example in figure 215 a round billet is extruded into a

smaller u channel profile.

Figure:215


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To relate the cross section of the work piece to that of the extruded product the

extrusion ratio was established. The extrusion ratio is the ratio of the area of the

work's cross section (Ao) to that of the extrusion's cross section (Af). The extrusionratio, or reduction ratio can be expressed as (Ao/Af).

Figure:216

Obviously since the starting work's cross section will be greater than that of the

extrusion, the extrusion ratio will always be more than 1. In manufacturing industry,

extrusion ratios typically range from about 4 to 100, although they can be even higher

in certain special cases.


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Extrusion Shape Factor

The exact geometric profile of an extrusion's cross section will have an effect on the

force required to extrude the work. A circle cross section requires the least amount of

work to extrude. Generally the more complex a shape, the more force that will beneed to extrude a cross section of that shape. In order to quantify the effect that

different cross sectional profiles have on extrusion force requirements, the extrusion

shape factor was established. The lower the shape factor, the lower the pressure

needed to extrude that cross section. A circle profile has a shape factor of 1, the shape

factor increases as the part becomes more complex. The actual shape factor

calculation is relative to the ratio between the perimeter of the extruded cross section

and the perimeter of a circle of the same area.

Circumscribing Circle Diameter

As noted, the geometry of an extruded profile is a large factor in force requirements

for that manufacturing operation. As in all processes, there are always limitations on

the size of parts that may be manufactured based on the physical natures of the

process. The work material is an important characteristic in determining the sizelimitations for an extruded part. Stronger materials require more pressure to form,

therefore the maximum size of an extrusion will be lower for more difficult to shape

metals.

Another method used in manufacturing industry to quantify the geometry of an

extrusion's profile, particularly with regard to size, is the circumscribing circle

diameter. The circumscribing circle diameter is simply the diameter of the smallest

circle that the profile of the extruded cross section can fit. Aluminum is one of the

easiest to shape metals for extrusion. The range of circumscribing circle diameters for

extruded aluminum parts in industrial production, typically spans from 1/4 inch to 10

inches, although much larger aluminum parts have been extruded in certain

operations. Figure 217 shows some different cross sectional profiles produced by

extrusion, with their circumscribing circles and circumscribing circle diameters.

Figure:217


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Extrusion Die

Extrusion die used in manufacturing extruded sections must have certain mechanical

characteristics. Die must be strong and hard, capable of holding their dimensional

accuracy throughout the high stresses created during the process. They must also be

resistant to wear, which is always an issue when extruding in large quantities. Dies for

hot extrusion must have high thermal resistance and be able to maintain strength andhardness at elevated temperatures. Tool steels are a common type of material for

extrusion molds. Dies may be coated to increase wear resistance. Carbides are

sometimes used for a mold material, carbides do not wear easy and can provide

accurate part dimensions.


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Extrusion die angle is an important factor in the process, as it is a large determinant in

the flow of material. The amount of force necessary to form a certain cross section

will vary with different die angles. A lower angle will create more friction at the work

mold interface. Friction is a factor that increases the force necessary to extrude a part.

High die angles create more material movement particularly in the outer regions away

from the center. The greater metal displacement gives a greater turbulence in themetal flow. Increased turbulence in the flow also increases the amount of force

necessary for the operation. All factors must be calculated in the design of an

extrusion process.

Figure:218

The optimum die angle will balance out the more extreme friction of lower die angles

with the more extreme turbulence of the higher die angles, and be somewhere between

the two extremes. The exact optimum die angle is difficult to determine for any

process due to the influence of other operational factors, such as temperature and

lubrication. The manufacturing engineer must try to provide the best angle based on

all the considerations of a given extrusion operation.


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Lubrication

Lubrication is used in manufacturing industry to assist in metal flow over the work

mold surfaces as a part is being extruded. Soaps, oils, graphite immersed in oil, and

many other special lubricants are all used in manufacturing industry to extrude parts.Some materials can be problematic in that they tend to stick to the tooling. To prevent

sticking a softer metal may be used for lubrication. In this case, the softer metal will

bejacketedaround a the work. For manufacturing practice, particularly in high

temperature processes, molten glass is often employed as an effective lubricant in the

extrusion of tougher materials.

Extrusion DesignDesign of an extrusion manufacturing operation involves consideration of many

different factors. The primary goal is to create a process that enacts a smooth and

effective flow of material. In general, force necessary is minimized as much as

possible. Higher forces require greater capacity and energy. Higher forces also

increase the chances of part defects, die wear and die breakage. Friction has an

important roll when extruding a part and should be maintained at an optimum level.

By examining and understanding the causes of extrusion defects, the process engineercan design a particular operation to mitigate the chances of defects occurring. Defects

fall into different categories and have several root causes. Most defects are similar,

less specifically, in that they are created by improper material movement and stress

distributions. Control of operational factors will allow for control of forces and metal

flow, producing efficient, defect free extrusions. In addition to operational factors, the

geometric profile of the extrusion also effects the manufacture of the part.


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Troubleshooting Defects

Extrusion defects that occur during the manufacturing process generally fall into three

basic categories. Internal breakage, particularly in the center, surface cracking, and

piping.

Center Cracking

Internal breakage; common names used in manufacturing industry for this type of

defect are center cracking, cheveron cracking, arrowhead fracture, and centerburst. As

the work piece is being extruded through the die, stresses within the work break thematerial causing cracks to form along the central axis of the extruded section. Center

cracking is a difficult defect to detect since it occurs within the material of the part.

Figure 219 shows an extrusion subject to centerburst, the part has been cut in a half

section so that the defect may be observed.

Figure:219

To understand the causes of center cracking it is important to understand the flow of

the material that is occurring while extruding a profile. As mentioned in the material

flow section, the outer layer of the extrusion will experience more deformation, and

also more material displacement and more turbulent flow than the areas towards thecentral region. The difference between the material movement of the outer regions

compared to that of the central region is critical. If the material displacement

occurring in the outer areas is of a much greater magnitude than that which is

occurring in the central area, then the difference will cause high stresses to develop

within the material. The greater the difference in flow characteristics between regions,


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the greater the stresses that will occur. If the stress level becomes too high then

material breakage will occur in the form of internal cracks.

In manufacturing practice, selection of die angle will be a primary factor in preventing

center cracking. High die angles will favor center cracking in an extrusion more than

low die angles will. The reason being the greater flow turbulence created in the outerregions of the work. Higher fiction in general will tend to reduce the chances of center

cracking. Cracks will propagate over inclusions, therefore the higher the amount of

inclusions in the work material, the more favorable the conditions for the occurrence

of this extrusion defect. High extrusion ratios are less likely to promote center cracks

than lower ones.

Surface Breakage

Surface breakage defect on a metal extrusion, is breakage on the surface of the part.

Most surface defects are in the form of cracks that extend from the surface, into the

parts material to varying degrees. These cracks usually occur along the grain

boundaries of the metal. The primary cause of surface cracking defect in extrusion

manufacture is excessive stresses on the surface of the part's material. Friction is a

large factor in controlling surface breakage, while manufacturing an extruded section.Increased friction will create a more favorable environment for surface cracking.

Lubrication can help reduce friction, so can an increased die angle.

When designing a manufacturing process it is important to balance all factors such as

friction. Lower friction may create better conditions at the surfaces between the work

and die. However, if friction forces are too low, a more turbulent outer flow may

result in center cracking. Another important factor in surface breakage defect is the

hardness of the parts material. The speed a which the part is extruded is also a

consideration. Higher extrusion speeds will create conditions more favorable to

material breakage.

Conditions in which the work material sticks to the extrusion die can be a cause of

surface cracking. Work sticking to tool surfaces can sometimes be a problem in many

different kinds of manufacturing operations, especially with some specific materials.

It is a common case that when the extrusion sticks to the die, pressure builds up

behind the material. Enough force breaks loose the work, causing cracks in the metal.


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The part shoots forward a small distance, then it sticks to the die again. This cycle

repeats itself as the part is being extruded. The cracks will appear at spaced intervals

around the parts peripheral. This is somewhat reminiscent of the appearance of a

bamboo tree, thus this particular manufacturing defect is termed bamboo defect.

Surface cracking of an extrusion need not have a mechanical cause, often the rootcause of cracking of a metal being extruded will be thermal stresses. High thermal

gradients between the work and die interface can cause the extrusion's surfaces to

loose heat rapidly. Heating molds and lubrication can help mitigate thermal gradients

at surfaces. The work billet should be heated to the best temperature considering the

operational variables. Extrusion speed and friction are critical factors in controlling

the thermal characteristics of this type of manufacturing process. Higher extrusion

speeds can not only generate greater mechanical forces, but more heat as well.

Increases in heat generation during the process can crack the part material.

Piping Defect

Piping, also called tailpipe or fishtailing is a defect common when manufacturing

sections by direct extrusion. The use of a dummy block and good surface preparation

of the work can help avoid piping. Piping occurs in the work material at the endopposite to the die. Piping is a result of improper metal flow during the extrusion

operation. Piping manifests itself as a funnel shaped void of material at the end of the

work. As mentioned before, material flow is a very important consideration in a

forming operation. The way to solve a piping problem is to enact a smooth metal flow

during the extrusion process. The combination of friction and thermal gradients acting

at the die work interface needs to be observed, and their cumulative effect on the

metal flow occurring during the process determined. The manufacturing engineer

must control the different process variables to achieve the smoothest material flow

possible during extrusion.

Figure:220


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Extrusion Design For ManufactureWhen designing an extrusion process the geometric profile of the extruded section is

an important factor, effecting the forces involved, metal flow, die wear, and part

quality. Some cross sections, when extruded, will produce undesirable force

distributions and metal flow. If possible it is desirable to design for manufacture. This

section will discuss the effect of different features of a profile, and how to optimize

the cross section for extrusion manufacture.

First it is important to understand that the nature of the forces involved in extrusion

are fundamentally different than those of other forming processes. For example, in aprocess such as forging, the forces acting on the operation will vary throughout the

process, stress will change as the forging stroke progresses. With extrusion, (although

in direct extrusion the is somewhat of a linear decrease in the necessary force, due to

decreasing friction with billet length), the forces acting are generally consistent

throughout the bulk of the operation.

Some components need to have a very specific geometry and must be manufactured

that way. If parts may be geometrically altered in such a way as to ease their

manufacture without changing their ability to perform their desired function, then they

should be redesigned for manufacture. Symmetry is generally preferred in extrudedcross sections, particularly with regard to open areas of the cross section. Open areas

in an extrusion's cross section will complicate the process by changing the forces

involved. Die wear and breakage is a primary consideration of a manufacturing

process designer when designing an extrusion operation. Controlling material flow is

essential in metal forming manufacture. In addition the stresses applied to the die as a

result of this metal flow must be calculated and the mold must be designed to handle


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these forces. Some areas of the die may be subject to more stresses than others. Some

areas of a die may not be able to handle as much stress as others. The geometry of the

mold must be considered carefully with respect to the forces created, as the part is

being extruded.

Excessive stresses in places that can not handle it, can cause rapid wear and diebreakage. Extrusion die tongues cause a weaker area in the die's geometry. The

location of die tongues and the forces they are subject to are an important factor in die

design.

Figure:221

Symmetry provides for more balanced forces, and helps avoid overstressing areas of

the die. Hollow areas within the cross section, in particular, should be balanced.

Figure:222


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Usually, larger hollow areas cause more difficulties during the extrusion operation.

Reducing the area of hollow sections, if possible, may ease the manufacturing

operation. If hollow sections can be eliminated completely, in favor of a single solid

extrusion, it would be a better design for manufacture.

Figure:223

Thin walled extrusions are often produced in manufacturing industry, however, there

are limits to the minimum wall thickness. Minimum wall thickness of an extrusion

depends a large part on the material being extruded. Minimum wall thickness for


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aluminum manufactured extrusions is usually about .040 inches. A common steel

extrusion may have a minimum wall thickness of three times that of aluminum.

Standard wall thickness throughout the profile is desirable, if not, the wall thickness

should be as balanced as possible.

Figure:224

One way to improve the design of an extrusion process that requires a cross section of

walls of different thickness, is to smooth the transition between thick to thin wallswith a large radius.

Figure:225


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The manufacturing designer should avoid sharp corners in an extrusion's profile, if at

all possible.

Figure:226

As mentioned earlier, die tongues are critical areas for mold wear and breakage.

Rounding certain corners of an extrusion can strengthen die tongues.


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Figure:227

Impact ExtrusionImpact extrusion is a discrete manufacturing process in which a part is extruded

through the impact of a die with the work piece. The part is formed at a high speed,

and over a relatively short stroke. In standard extrusions, the force to form the work is

commonly delivered by way of a hydraulic press. In impact extrusions, mechanical

presses are most often employed. The force used to form standard extrusions is

usually delivered over a horizontal vector, producing a long continuous product. Force

used to form impact extrusions is usually delivered over a vertical vector, producing a

single part with each impact of the punch. Impact extrusion is most often performed

cold, occasionally with some metals and thicker walled structures the work is heated

before impact forming it. This process is best suited for softer metals, aluminum is agreat material for forming by impacting.

Figure:228


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The Mechanics And Design Of ImpactExtrusions

During the manufacturing operation of impact extrusion, a work piece is placed in a

mold and struck with great force causing the metal to rapidly flow into position in an

instant. The forces acting on the machinery are very high, particularly on the punch

and die. Tooling must have sufficient impact resistance, fatigue resistance, and

strength for metal forming by impact. There are three types of impact extrusions,

forward, reverse, and combination. The different categories are based on the kind ofmetal flow that occurs during the process. In forward extrusion, metal flows in the

same direction that force is delivered. In backward extrusion, the metal flows in the

opposite direction that the force is delivered. In combination the metal flows in both

directions. Backward extrusion is shown in figure 228, forward and combination are

illustrated as follows.


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Figure:229

Figure:230


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When designing an impact extrusion process it important that the part geometry be

symmetrical about the punch. In addition it is essential to the proper forming of the

extruded component, that the punch die delivers a precise blow concentric to the

work.

Figure:231


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The end of the punch is better designed not to be completely flat, to avoid slipping at

the punch work interface. Sometimes a center point recess can help keep the punchconcentric to the work.

Figure:232

Another factor when manufacturing by this process is the proportion between an

extrusion's length and its internal diameter. Long punches with relatively small

diameters may fail during manufacture. Hollow thin walled tubes, closed on one end,are often produced in manufacturing industry by backward impact extrusion. It is

good practice to design the extrusion so that the bottom is at least 20% thicker than

the sides, to help prevent material breakage.

Figure:233


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Impact Extrusion In Modern

Manufacturing Industry

Impact extrusion is used to manufacture a variety of parts, such as components for

machinery and appliances. Impacted parts of complex geometries can be produced as

long as the part is symmetrical over the axis by which it is formed. One of the best

utilizations of this type of operation is in the production of hollow tubes with one end

partially or completely closed. Hollow tubes may be formed with internal and external

geometry. The wall thickness of a part manufactured by impact extrusion may vary

along its length, so may its interior and exterior diameters. Extrusions created by this

process do not need to have a circular cross section. Noncircular but symmetrical parts

are also formed. The impact extrusion manufacturing process is very effective at

forming flanges on tubular parts. Flanges with varying diameters and geometricattributes may be formed along the length of the extruded section. Bosses and hollow

tubes concentric to the extrusion axis can be formed within hollow tubes by impact

extrusion.

Many of the parts formed by impacting in industry will require further manufacturing

processes such as forging, ironing, and machining before completion. Impact


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extrusion can work harden a part, this may or may not be desirable. If necessary, a

component may be annealed before further processing occurs. Favorable grain

structures and good surface finish are some possible advantages of manufacturing by

impact extrusion.

Hydrostatic Extrusion

In hydrostatic extrusion the work piece is held in a sealed chamber surrounded by

pressurized liquid. Hydrostatic extrusion is actually a form of direct extrusion. Theforce delivered through the ram is what pressurizes the liquid. The liquid applies

pressure to all surfaces of the work billet. When the ram moves forward, it is the force

from the incompressible fluid that pushes the work through the die, extruding the part.

Figure:234


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A critical aspect of manufacturing by this process is setup. The work billet must first

be tapered to fit through the die opening, thus creating a seal. This is done before

adding the liquid in, order to prevent leaking. Since the liquid is under great pressure,

this taper must be precise to create a robust bond.

Figure:235

Many different shapes may be produced this way, using a variety of materials. Asignificant advantage of hydrostatic extrusion, is that essentially all friction at the

work chamber interface is eliminated. Another advantage is that the hydrostatic

pressure will increase the ductility of the work, enabling brittle materials to be

extruded easier. Liquid pressure from all directions also greatly decreases the chances

of buckling of the work. Hydrostatic extrusion may be performed at room or elevated

temperatures depending upon the process. When performed hot, the liquid will

insulate the work from thermal gradients between the container and work material. An

advanced variation of this process, is called fluid to fluid extrusion. This process is

basically the same, except that the part is extruded into a second chamber also

containing pressurized liquid. The liquid in the second chamber is of a lower pressurethan the first. Several different kinds of liquids are used when manufacturing by

hydrostatic extrusion, including oils, waxes, melted polymers, and molten glass.

Hydrostatic extrusion has not had much use in manufacturing industry, due to the

complicated equipment and procedures, work preparation, long cycle times, and

dangers of working with hot, high pressure liquid.


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Metal Extrusion