Process of Extrusion- Extrusion
- Types of Extrusion
- Classification based on temperature
- Extrusion
equipment
- Flow of metal during the process of extrusion
- Extrusion Parameters
- Defects
- Benefits_and_Limitations
- LINKS TO EXTRUSION MANUFACTURERS
- Extrusion – HPM, Corporation
- Metal Extrusion – Extrusion Dies, Incorporated
- Aluminum Extrusion – Extrusion Technology
- Polymer Extrusion – Verplas, Limited
Extrusion of Aluminum - An example
Extrusion Pressure calculations
A few advanced technical details
Process of Extrusion
Extrusion: Extrusion is the process by which a block of metal is reduced in cross section by forcing it through a die orifice under high pressure. In general, extrusion is used to produce cylindrical bars or hollow tubes, but shapes of irregular cross-sections may be produced from the more readily extrudable metals like aluminum.
Extrusion is a hot-working process which, like forging, rolling, etc., uses the good deformability of heated metallic materials for shaping them. The most important aspect of the process is that it enables considerable changes of shape to be achieved in a single operation and provides a means of dealing with metals and alloys whose physical structure renders them unsuitable for shaping by other methods. Besides, with extrusion it is possible to form complex sections that cannot be produced in other ways. Finally, extrusion also offers economic advantages in that the dies are relatively inexpensive and are interchangeable, so that one extrusion machine can be used for the production of a wide variety of sections.
A metal billet heated to the appropriate temperature is fed into the cylindrical container of the extrusion press and is forced by the action of a ram through a steel die whose orifice has the desired shape to produce the solid or hollow section. The metal emerges from the die as a continuous bar, which is cut to the required lengths. Extrusion products are therefore essentially “linear” in character, in the sense that shaping is confined to the cross section only. The process is therefore eminently suitable for the production of bar-like and tubular objects.
Most metals and alloys can be shaped by extrusion. At first the process was confined to nonferrous metals and has now in fact largely superseded other methods for the shaping of such metals. Cables sheathing, lead pipe and aluminum alloy structural sections are typical of such extrusion products. The extrusion of steel presented difficulties because of the heavy wear on the dies and the high working temperatures and stresses. However, these difficulties have been overcome, and extrusion is used, for example, in the production of stainless steel tubes.
For making tubular sections, a mandrel is arranged in the die orifice, and during extrusion the metal flows through the annular space so formed. Hollow billets are used for tubes, or solid billets are first pierced in the extrusion operation. Extrusion machines are generally hydraulic presses with capacities ranging from about 500 tons to about 7500 tons. Graphite grease is commonly used for lubrication between metal and tools.
There are four basic types of extrusion. They include the following:
(1)
Direct (forward) extrusion:
This is similar to forcing the paste through the opening of a toothpaste tube.
The billet slides relative to the container wall; the wall friction increases
the ram force considerably. A dummy block or pressure plate is placed at the
end of the ram in contact with the billet.
Direct extrusion
Direct extrusion, (b) hollow and (c) semi-hollow cross-sections
(2)
Indirect (reverse,
inverted or backward) extrusion: Here, the die moves toward the billet;
thus, except at the die, there is no relative motion at the billet-container
interface. As a consequence, the frictional forces are lower and the power
required for extrusion is less than for direct extrusion. In practice, a hollow
ram carries a die, while the other end of the container is closed with a plate.
Frequently, for indirect extrusion, the ram containing the die is kept
stationary, and the container with the billet is made to move
Indirect extrusion, (top) solid, and (bottom) hollow cross-sections.
(3) Hydrostatic
extrusion: In this process, the chamber is filled with a fluid that
transmits the pressure to the billet, which is then extruded through the die.
There is no friction along the walls of the container. Because the billet is
subjected to uniform hydrostatic pressure, it does not upset to fill the bore
of the container as it would in conventional extrusion. This means that the
billet may have a large length to diameter ratio (even coils of wires can be
extruded) or it may have an irregular cross section. Because of the pressurized
fluid, lubrication is very effective, and the extruded product has good surface
finish and dimensional accuracy. Since friction is nearly absent, it is
possible to use dies with very low semi-cone angle which greatly minimizes the
redundant deformation. The only limitaion with this process is the practical
limit of fluid pressure that may be used because of the constraint involving
the strength of the container and the requirement that the fluid not solidify
at high pressure.
Hydrostatic extrusion
(4)
Impact extrusion: It is a form of
indirect extrusion and is particularly suitable for hollow shapes. It is
usually performed on a high-speed mechanical press. The punch descends at a
high speed and strikes the blank, extruding it upwards. The thickness of the
extruded tubular section is a function of the clearance between the punch and
the die cavity. Although the process is performed cold, considerable heating
results from the high-speed deformation. Impact extrusion is restricted to
softer metals such as lead, tin aluminum and copper.
Forward impact extrusion
Backward impact extrusion
(5) Polymer Extrusion: The extrusion of polymers differs from the extrusion of metals. The “billet” material is not in the form of bar stock, but rather in a pellet or powder form. This substance is added to a machine that utilizes a screw-type action to assist in mixing and pushes the plastic material through the die opening. The polymer is fed onto the rotating screw, heated, and mixed. The screw is made up of three sections:
· Feed Section - the stock is moved from the hopper and preheated.
·
Compression Section - the polymer is transformed into a
liquid consistency, air entrapped among the pellets is extracted from the melt,
and the material is compressed.
· Metering Section - the melt is homogenized and sufficient pressure is developed to pump it through the die opening.
As the melt reaches the die, it must pass through a screen pack, a series of wire meshes supported by a stiff plate (called a breaker plate) containing small axial holes. The screen pack functions to:
·
Filter the contaminants and hard lumps from the melt,
·
Straighten the flow of the polymer melt and remove its
memory of the circular motion imposed by the screw.
This melted polymer is finally forced through a tapered die/nozzle and the
extrudate is formed.
Polymer extrusion
Classification based on temperature:
Usually, because of the large forces required in extrusion, most metals are extruded hot under the conditions where the deformation resistance of the metal is low. Cold extrusion is also possible for many metals. On this basis we can classify extrusion as:
Hot extrusion: Hot extrusion is basically a Hot working process. Hot working is defined as working above the recrystallization temperature so that the work metal recrystallizes as it deforms. Hot working takes advantage of decrease in flow stress at higher temperatures to lower tool forces and, consequently, equipment size and power requirements. The principal variables, which influence the force required to cause extrusion, are:
(1) The type of extrusion
(2) The extrusion ratio
(3) The working temperature
(4) The speed of deformation, and
(5) The frictional conditions at the die and container wall.
Cold extrusion:
Cold extrusion is concerned with cold forming from rod and bar stock of small
machine parts. It is used for the manufacture of special sections and hollow
articles. The material is generally made to flow in the cold condition by the
application of high pressure, which forces it through the cavity enclosed
between a punch and a die. Both, forward extrusion and backward extrusion may
be used in simultaneous combination. Cold extrusion can be used with any
material that possesses adequate cold work ability - e.g., tin, zinc, copper
and its alloys, aluminum and its alloys and it is for these metals that the
process is more widely adopted. Low-carbon soft-annealed steel can also be
cold-extruded. If the product cannot be fully shaped in a single operation, the
extrusion process may be performed in several stages. The solid or
hollow products that can be made by cold extrusion are relatively limited in
size. The initial stock from which cold extrusions are produced consists of
round blanks, lengths cut from bars, or specially preformed blanks. The punches
and dies used in cold extrusion are subject to severe working conditions and
are made of wear-resistant tool steels - e.g., high-alloy chromium steels. To
reduce friction, the tool surfaces are
polished. In the cold extrusion of steel the blank may additionally be given a
phosphate coating to minimize friction. Precision cold forming can result in
high production of parts with good dimensional control and good surface finish.
Because of extensive strain hardening, it is possible to use cheaper materials
with lower alloy content. Extensive use is made of cold-formed low-alloy steels
in the automotive industry.
Most extrusions are made with hydraulic presses. Hydraulic extrusion presses are classified into horizontal and vertical presses, depending upon the direction of travel of the ram.
An Extrusion Press
· Vertical extrusion presses are generally built with capacities of 300 to 2000 tons. They have the advantage of easier alignment between the press ram and the tools, higher rate of production, and the need of less floor space than horizontal presses. However, they need considerable headroom, and to make extrusions of appreciable length, a floor pit is frequently necessary. Vertical presses will produce uniform cooling of the billet in the container, and thus symmetrically uniform deformation will result. In commercial operations the chief use of vertical presses is in the production of thin-wall tubing, where uniform wall thickness and concentricity are required.
· Horizontal extrusion presses are used for most commercial extrusion of bars and shapes. Presses with a capacity of 1500 to 5000 tons are in regular operation, while a few presses of 14000 tons capacity have been constructed. In a horizontal press, the bottom of the billet which lies in contact with the container will cool more rapidly than the top of the surface, unless the extrusion container is internally heated, and therefore the deformation will be no uniform. Warping of bars will result, and no uniform wall thickness will occur in tubes. A horizontal press is installed at Bowers Manufacturing Co.
Ram speed has to be given a special consideration, as high ram speeds are a necessity for high-temperature extrusion where there is a problem of heat transfer from the billet to the tools. High ram speeds of 1000 to 1500 in/min may be used in extruding refractory metals, and this necessitates a hydraulic accumulator system with the press. While, on the other hand, aluminum and copper alloys are prone to hot shortness so that the ram speed must be restricted to a few inches per minute. In this case, direct-drive pumping systems are adequate. The dies and tooling used in extrusion must withstand considerable abuse from the high stresses, thermal shock, and oxidation. The general extrusion tooling assembly is designed for easy replacement of damaged parts and for reworking and reuse of components of the tooling.
Flow of metal during the process of extrusion:
Because
the billet 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:
(a) 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.
(b) 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.
(c) 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 & hence
increases 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.
Breakthrough Pressure: This is defined mostly in case of direct extrusion. This id the maximum extrusion pressure at which the metal begins to flow through the die. As the billet extrudes through the die, the pressure required to maintain flow progressively decreases with decreasing length of the billet in the container.
Extrusion Ratio: It is defined as the ratio of the initial billet cross-sectional area to the final cross sectional area. It is essentially a measure of the strain which the billet undergoes during the extrusion process. Large extrusion ratios require higher pressures from the presses, particularly in case of cold extrusion processes. It is denoted by R and is given by the expression:
R = Ao/Af
where, Ao is the cross-sectional area of the billet before extrusion and Af is the cross-sectional area of the billet after extrusion.
Fractional reduction in area: It is given by the following expression: r = 1 - Ao/Af
Extrusion Pressure: The extrusion pressure is directly related to the natural logarithm of the extrusion ratio. Therefore, the extrusion force may be be expressed as:
P = k Ao ln(Ao/Af )
where, k = the extrusion constant.
The extrusion constant is an overall factor which accounts for the flow stress, friction, and inhomogeneous deformation. Most metals are extruded hot so as to take the advantage of the decrease in flow stress or deformation resistance with increasing temperature.
Surface
defect: If the extrusion
temperature, friction, or extrusion speeds are too high, surface temperatures
rise significantly and can lead to surface cracking and tearing ( fir-tree
cracking or speed cracking). These cracks are intergranular and are
a result of hot shortness. This can be avoided by using lower temperatures and
speeds. However, surface cracking may also occur at low temperatures and has
been attributed to periodic sticking of the extruded product along the die
land. Because of this sticking, the extrusion pressure increases rapidly
Extrusion defect: Some types of metal flow tend to draw surface oxides and impurities toward the center of the billet. This defect is known as extrusion defect, pipe, tailpipe or fishtailing. A considerable portion of the metal can be rendered useless as an extruded product because of this. This defect can be reduced by modifying the flow pattern to a less inhomogeneous one, such as by controlling friction and minimizing temperature gradients. Another method is to machine the surface of the billet prior to extrusion to eliminate scale and impurities.
Internal cracking: The center of an extruded product can develop cracks (also known as centerburst, center cracking, arrowhead fracture or chevron cracking) due to a state of hydrostatic tensile stress at the centerline of the deformation zone in the die. This situation is similar to the necked region in an uni-axial tensile test specimen. The major variables influencing this are the die angle, extrusion ratio and friction. Experimentally, it was observed that for the same reduction, as the die angle becomes larger, the deformation across the part becomes more inhomogeneous. Also, smaller the die, the longer is the contact length. The size and depth of the deformation zone increases with increasing contact length.
Centerburst: A defect that propagates from an internal crack in the billet as a result in tensile stresses along the centerline of the workpiece.
Piping: The formation of a sink in the end of a billet.
Benefits and Limitations: Extrusion is beneficial for many reasons. It can be used to create a variety of shapes, the grain structure and strength properties are enhanced, close tolerances can be held, continuous lengths are produced, and as is often the case, there is only minimal waste. Extrusion performed at high speeds is even more beneficial, resulting in good surface finish, product straightness, good flow of metal, materials maintain mechanical properties, and close dimensional tolerances are held.
Extrusion is an excellent way of producing identical products
of equal cross-sectional area. The
advancement of technology will only better the process. Finite element packages already allow
simulations to replace expensive testing.
Extrusion is often cheaper than forging for larger quantities of equal
parts. This is primarily due to a
forged part requiring more machining than an identical extruded part. There are, however, reduced tooling costs
associated with forging compared to that of extrusion. Therefore, for fewer parts, the tooling
costs for forging prove to be less expensive than that of extrusion.
Relative cost for manufacturing an aircraft part
The major limitation with extrusion is that the final cross-sectional area is the same throughout the entire length of product. This may require that additional machining be done to meet design criteria. Product defects are another concern with extrusion, as with any metal forming operation.