1 History 2 Process
3.1 Forming internal cavities
3.2 Direct extrusion
3.3 Indirect extrusion
4 Die design 5 Materials
5.1 Metal 5.2 Plastic 5.3 Ceramic
6.1 Food 6.2 Drug carriers 6.3 Biomass briquettes
7 See also 8 References
8.1 Notes 8.2 Bibliography
9 See also
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The process begins by heating the stock material (for hot or warm extrusion). It is then loaded into the container in the press. A dummy block is placed behind it where the ram then presses on the material to push it out of the die. Afterward the extrusion is stretched in order to straighten it. If better properties are required then it may be heat treated or cold worked. The extrusion ratio is defined as the starting cross-sectional area divided by the cross-sectional area of the final extrusion. One of the main advantages of the extrusion process is that this ratio can be very large while still producing quality parts. Hot extrusion Hot extrusion is a hot working process, which means it is done above the material's recrystallization temperature to keep the material from work hardening and to make it easier to push the material through the die. Most hot extrusions are done on horizontal hydraulic presses that range from 230 to 11,000 metric tons (250 to 12,130 short tons). Pressures range from 30 to 700 MPa (4,400 to 101,500 psi), therefore lubrication is required, which can be oil or graphite for lower temperature extrusions, or glass powder for higher temperature extrusions. The biggest disadvantage of this process is its cost for machinery and its upkeep.
Hot extrusion temperature for various metals
Material Temperature [°C (°F)]
Magnesium 350–450 (650–850)
Aluminium 350–500 (650–900)
Copper 600–1100 (1200–2000)
Steel 1200–1300 (2200–2400)
Titanium 700–1200 (1300–2100)
Nickel 1000–1200 (1900–2200)
Refractory alloys up to 2000 (4000)
The extrusion process is generally economical when producing between several kilograms (pounds) and many tons, depending on the material being extruded. There is a crossover point where roll forming becomes more economical. For instance, some steels become more economical to roll if producing more than 20,000 kg (50,000 lb).
Front side of a four family die. For reference, the die is 228 mm (9.0 in) in diameter.
Close up of the shape cut into the die. Notice that the walls are drafted and that the back wall thickness varies.
Back side of die. The wall thickness of the extrusion is 3 mm (0.12 in).
Cold extrusion is done at room temperature or near room temperature.
The advantages of this over hot extrusion are the lack of oxidation,
higher strength due to cold working, closer tolerances, better surface
finish, and fast extrusion speeds if the material is subject to hot
Materials that are commonly cold extruded include: lead, tin,
aluminum, copper, zirconium, titanium, molybdenum, beryllium,
vanadium, niobium, and steel.
Examples of products produced by this process are: collapsible tubes,
fire extinguisher cases, shock absorber cylinders and gear blanks.
In March 1956, a US Patent was filed for "process for warm extrusion
of metal." Patent US3156043 A outlines that a number of important
advantages can be achieved with warm extrusion of both ferrous and
non-ferrous metals and alloys if a billet to be extruded is changed in
its physical properties in response to physical forces by being heated
to a temperature below the critical melting point. Warm extrusion
is done above room temperature, but below the recrystallization
temperature of the material the temperatures ranges from 800 to
1800 °F (424 to 975 °C). It is usually used to achieve the
proper balance of required forces, ductility and final extrusion
Surface cracking occurs when the surface of an extrusion splits. This is often caused by the extrusion temperature, friction, or speed being too high. It can also happen at lower temperatures if the extruded product temporarily sticks to the die. Pipe – A flow pattern that draws the surface oxides and impurities to the center of the product. Such a pattern is often caused by high friction or cooling of the outer regions of the billet. Internal cracking – When the center of the extrusion develops cracks or voids. These cracks are attributed to a state of hydrostatic tensile stress at the centerline in the deformation zone in the die. (A similar situation to the necked region in a tensile stress specimen) Surface lines – When there are lines visible on the surface of the extruded profile. This depends heavily on the quality of the die production and how well the die is maintained, as some residues of the material extruded can stick to the die surface and produce the embossed lines.
A horizontal hydraulic press for hot aluminum extrusion (loose dies and scrap visible in foreground)
There are many different variations of extrusion equipment. They vary by four major characteristics:
Movement of the extrusion with relation to the ram. If the die is held stationary and the ram moves towards it then it is called "direct extrusion". If the ram is held stationary and the die moves towards the ram it is called "indirect extrusion". The position of the press, either vertical or horizontal. The type of drive, either hydraulic or mechanical. The type of load applied, either conventional (variable) or hydrostatic.
A single or twin screw auger, powered by an electric motor, or a ram, driven by hydraulic pressure (often used for steel and titanium alloys), oil pressure (for aluminium), or in other specialized processes such as rollers inside a perforated drum for the production of many simultaneous streams of material. Typical extrusion presses cost more than $100,000, whereas dies can cost up to $2000. Forming internal cavities
Two-piece aluminum extrusion die set (parts shown separated.) The male part (at right) is for forming the internal cavity in the resulting round tube extrusion.
There are several methods for forming internal cavities in extrusions. One way is to use a hollow billet and then use a fixed or floating mandrel. A fixed mandrel, also known as a German type, means it is integrated into the dummy block and stem. A floating mandrel, also known as a French type, floats in slots in the dummy block and aligns itself in the die when extruding. If a solid billet is used as the feed material then it must first be pierced by the mandrel before extruding through the die. A special press is used in order to control the mandrel independently from the ram. The solid billet could also be used with a spider die, porthole die or bridge die. All of these types of dies incorporate the mandrel in the die and have "legs" that hold the mandrel in place. During extrusion the metal divides, flows around the legs, then merges, leaving weld lines in the final product. Direct extrusion
Plot of forces required by various extrusion processes.
Direct extrusion, also known as forward extrusion, is the most common extrusion process. It works by placing the billet in a heavy walled container. The billet is pushed through the die by a ram or screw. There is a reusable dummy block between the ram and the billet to keep them separated. The major disadvantage of this process is that the force required to extrude the billet is greater than that needed in the indirect extrusion process because of the frictional forces introduced by the need for the billet to travel the entire length of the container. Because of this the greatest force required is at the beginning of process and slowly decreases as the billet is used up. At the end of the billet the force greatly increases because the billet is thin and the material must flow radially to exit the die. The end of the billet (called the butt end) is not used for this reason. Indirect extrusion In indirect extrusion, also known as backwards extrusion, the billet and container move together while the die is stationary. The die is held in place by a "stem" which has to be longer than the container length. The maximum length of the extrusion is ultimately dictated by the column strength of the stem. Because the billet moves with the container the frictional forces are eliminated. This leads to the following advantages:
A 25 to 30% reduction of friction, which allows for extruding larger billets, increasing speed, and an increased ability to extrude smaller cross-sections There is less of a tendency for extrusions to crack because there is no heat formed from friction The container liner will last longer due to less wear The billet is used more uniformly so extrusion defects and coarse grained peripherals zones are less likely.
The disadvantages are:
Impurities and defects on the surface of the billet affect the surface of the extrusion. These defects ruin the piece if it needs to be anodized or the aesthetics are important. In order to get around this the billets may be wire brushed, machined or chemically cleaned before being used. This process isn't as versatile as direct extrusions because the cross-sectional area is limited by the maximum size of the stem.
Constant-rate extrusion: A ram or plunger is used to pressurize the fluid inside the container. Constant-pressure extrusion: A pump is used, possibly with a pressure intensifier, to pressurize the fluid, which is then pumped to the container.
The advantages of this process include:
No friction between the container and the billet reduces force requirements. This ultimately allows for faster speeds, higher reduction ratios, and lower billet temperatures. Usually the ductility of the material increases when high pressures are applied. An even flow of material. Large billets and large cross-sections can be extruded. No billet residue is left on the container walls.
The disadvantages are:
The billets must be prepared by tapering one end to match the die entry angle. This is needed to form a seal at the beginning of the cycle. Usually the entire billet needs to be machined to remove any surface defects. Containing the fluid under high pressures can be difficult. A billet remnant or a plug of a tougher material must be left at the end of the extrusion to prevent a sudden release of the extrusion fluid.
Most modern direct or indirect extrusion presses are hydraulically
driven, but there are some small mechanical presses still used. Of the
hydraulic presses there are two types: direct-drive oil presses and
accumulator water drives.
Direct-drive oil presses are the most common because they are reliable
and robust. They can deliver over 35 MPa (5000 psi). They supply a
constant pressure throughout the whole billet. The disadvantage is
that they are slow, between 50 and 200 mm/s (2–8 ips).
Accumulator water drives are more expensive and larger than
direct-drive oil presses, and they lose about 10% of their pressure
over the stroke, but they are much faster, up to 380 mm/s (15
ips). Because of this they are used when extruding steel. They are
also used on materials that must be heated to very hot temperatures
for safety reasons.
Material Minimum cross-section [cm² (sq. in.)] Minimum thickness [mm (in.)]
Carbon steels 2.5 (0.40) 3.00 (0.120)
Stainless steel 3.0–4.5 (0.45–0.70) 3.00–4.75 (0.120–0.187)
Titanium 3.0 (0.50) 3.80 (0.150)
Aluminium < 2.5 (0.40) 0.70 (0.028)
Magnesium < 2.5 (0.40) 1.00 (0.040)
Materials Metal Metals that are commonly extruded include:
Sectional view of a plastic extruder showing the components
Sectional view of how a caterpillar haul-off provides line tension
Main article: Plastics extrusion
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Main article: Food extrusion
Elbow macaroni is an extruded hollow pasta.
With the advent of industrial manufacturing, extrusion found application in food processing of instant foods and snacks, along with its already known uses in plastics and metal fabrication. The main role of extrusion was originally developed for conveying and shaping fluid forms of processed raw materials. Present day, extrusion cooking technologies and capabilities have developed into sophisticated processing functions including: mixing, conveying, shearing, separation, heating, cooling, shaping, co-extrusion, venting volatiles and moisture, encapsulation, flavor generation and sterilization. Products such as certain pastas, many breakfast cereals, premade cookie dough, some french fries, certain baby foods, dry or semi-moist pet food and ready-to-eat snacks are mostly manufactured by extrusion. It is also used to produce modified starch, and to pelletize animal feed. Generally, high-temperature extrusion is used for the manufacture of ready-to-eat snacks, while cold extrusion is used for the manufacture of pasta and related products intended for later cooking and consumption. The processed products have low moisture and hence considerably higher shelf life, and provide variety and convenience to consumers. In the extrusion process, raw materials are first ground to the correct particle size. The dry mix is passed through a pre-conditioner, in which other ingredients may be added, and steam is injected to start the cooking process. The preconditioned mix is then passed through an extruder, where it is forced through a die and cut to the desired length. The cooking process takes place within the extruder where the product produces its own friction and heat due to the pressure generated (10–20 bar). The main independent parameters during extrusion cooking are feed rate, particle size of the raw material, barrel temperature, screw speed and moisture content. The extruding process can induce both protein denaturation and starch gelatinization, depending on inputs and parameters. Sometimes, a catalyst is used, for example, when producing texturised vegetable proteins (TVP). Drug carriers
This section needs expansion. You can help by adding to it. (August 2008)
For use in pharmaceutical products, extrusion through nano-porous,
polymeric filters is being used to produce suspensions of lipid
vesicles liposomes or transfersomes with a particular size of a narrow
size distribution. The anti-cancer drug
Equal channel angular extrusion
^ a b c d e f g h i Oberg et al. 2000, pp. 1348–1349
^ a b c Backus et al. 1998, pp. 13-11–12, Hot extrusion
^ Grazioso, Charles G.; Mulder, Gerard W. (1956-03-09). "Process for
warm extrusion of metal". Google. Retrieved 2017-08-16.
^ a b c Avitzur, B. (1987), "
Backus, Robert G.; Boshold, R. F.; Johannisson, Thomas G.; Noble, Paul
D.; Pfeffer, Jerome B.; Schiebold, Ted A.; Spearman, J. E. (1998)
. "Drawing, extruding, and upsetting". In Wick, Charles;
Benedict, John T.; Veilleux, Raymond F. Tool and manufacturing
engineers handbook. vol. 2 (4th ed.). SME.
Oberg, Erik; Jones, Franklin D.; Horton, Holbrook L.; Ryffel, Henry H.
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