Extrusion
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Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section. The two main advantages of this process over other manufacturing processes is its ability to create very complex cross-sections and work materials that are brittle, because the material only encounters compressive and shear stresses. It also forms finished parts with an excellent surface finish.[1]
Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). The extrusion process can be done with the material hot or cold.
Commonly extruded materials include metals, polymers, ceramics, and foodstuffs.
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[edit] History
In 1797, Joseph Bramah patented the first extrusion process for making lead pipe. It involved preheating the metal and then forcing it through a die via a hand driven plunger. The process wasn't developed until 1820 when Thomas Burr constructed the first hydraulic powered press. At this time the process was called squirting. In 1894, Alexander Dick expanded the extrusion process to copper and brass alloys.[2]
[edit] Process
The process begins by heating the stock material. 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. Afterwards the extrusion is stretched in order to straighten it. If better properties are required then it may be heat treated or cold worked.[2]
[edit] Hot extrusion
Hot extrusion is done at an elevated 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 250 to 12,000 tons. Pressures range from 5,000 to 100,000 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.[1]
| Material | Temperature [F° (C°)] |
|---|---|
| Magnesium | 650-850 |
| Aluminium | 650-900 |
| Copper | 1200-2000 |
| Steel | 2200-2400 |
| Titanium | 1300-2100 |
| Nickel | 1900-2200 |
| Refractory alloys | up to 4000 |
The extrusion process is generally economical when producing between several pounds and many tons, depending on the material being extruded. There is a crossover point where rolling becomes more economical. For instance, some steels becomes more economical to roll if producing more than 50,000 lb.[2]
[edit] Cold extrusion
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, good surface finish, and fast extrusion speeds if the material is subject to hot shortness.[1]
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, automotive pistons, and gear blanks.
[edit] Warm extrusion
Warm extrusion is done above room temperature, but below the recrystallization temperature of the material. It is usually used to achieve the proper balance of required forces, ductility and final extrusion properties.[3]
[edit] Equipment
There are many different variations of extrusion equipment. They vary by four major characteristics:[1]
- Movement of the extrusion with relation to the ram. If the die is held stationary and the ram moves towards it then its called "direct extrusion". If the ram is held stationary and the die moves towards the ram its 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 (for steel alloys and titanium alloys for example), oil pressure (for aluminum), or in other specialized processes such as rollers inside a perforated drum for the production of many simultaneous streams of material.
There are several methods for forming internal cavities in extrusions. One way is to have the mandrel integrated into the ram. 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.[1] Another method is using whats known as a "spider die, porthole die and bridge die". During extrusion, the metal divides and flows around the supports for the internal mandrel. (This is much like water in a river flowing around a large rock and rejoining downstream.)
Typical extrusion presses cost more than $100,000, whereas dies can cost up to $2000.
[edit] Extrusion defects
- Surface cracking - When the surface of an extrusion splits. This 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 cause 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.)
[edit] Materials
[edit] Metal
Metal extrusion is used by industry for various purposes such as:
- Copper pipe for plumbing
- Aluminium extrusion profiles for tracks, frames, rails, mullions, and extrusion-type heat sinking devices
- Steel rods or track
- Titanium aircraft components including seat tracks, engine rings, and other structural parts
Magnesium and aluminium alloys usually have a 30 μin. RMS or better surface finish. Titanium and steel can achieve a 125 μin. RMS.[1]
In 1935,[citation needed] Ugine Sejournet, of France, invented a process which uses glass as a lubricant for extruding steel. The Ugine-Sejournet, or Sejournet, process is now used for other materials that have melting temperatures higher than steel or that require a narrow range of temperatures to extrude. The process starts by heating the materials to the extruding temperature and then rolling it in glass powder. The glass melts and forms a thin film, 20 to 30 mils (0.5 to 0.75 mm), in order to separate it from chamber walls and allow it to act as a lubricant. A thick solid glass ring that is 0.25 to 0.75 in (6 to 18 mm) thick is placed in the chamber on the die to lubricate the extrusion as it is forced through the die. A second advantage of this glass ring is its ability to insulate the heat of the billet from the die. The extrusion will have a 1 mil thick layer of glass, which can be easily removed once it cools.[3]
Another breakthrough in lubrication is the use of phosphate coatings. With this process, in conjunction with glass lubrication, steel can be cold extruded. The phosphate coat absorbs the liquid glass to offer even better lubricating properties.[3]
[edit] Plastic
Plastic extrusion commonly uses plastic chips or pellets, which are usually dried in a hopper before going to the feed screw. The polymer resin is heated to molten state by a combination of heating elements and shear heating from the extrusion screw. The screw forces the resin through a die, forming the resin into the desired shape. The extrudate is cooled and solidified as it is pulled through the die or water tank. In some cases (such as fibre-reinforced tubes) the extrudate is pulled through a very long die, in a process called pultrusion.
A multitude of polymers are used in the production of plastic tubing, pipes, rods, rails, seals, and sheets or films.
[edit] Ceramic
Ceramic can also be formed into shapes via extrusion. Terracotta extrusion is used to produce pipes. Many modern bricks are also manufactured using a brick extrusion process.[4] Some Play-Doh toy products also make use of extrusion.
[edit] Food
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Please help improve this article or section by expanding it. Further information might be found on the talk page or at requests for expansion. (January 2008) |
Extrusion has found great application in food processing. Products such as pastas, breakfast cereals, Fig Newtons, prefab cookie dough, Sevai, Idiappam, jalebi and ready-to-eat snacks are now manufactured by extrusion. Krispy Kreme doughnuts are also manufactured by extrusion to keep the doughnuts uniform in shape and size. Softer foods such as meringue have long been "piped" using pastry bags. Extrusion is also used with grains such as wheat, corn, and rice. In feed industry it is used for process with floating and slow sinking feed.
[edit] Design
The following guidelines should be followed to produce a quality extrusion. The maximum size for an extrusion is determined by finding the smallest circle that will fit around the cross-section (called the circumscribing circle). This diameter, in turn, controls the size of the die required, which ultimately determines if the part will fit in a given press. For example, a larger press can handle 25 inch diameter circumscribing circles for aluminium and 22 in. diameter circles for steel and titanium.[1]
Thicker sections generally need an increased section size. In order for the material to flow properly legs should not be more than ten times longer than their thickness. If the cross-section is asymmetrical, adjacent sections should be as close to the same size as possible. Sharp corners should be avoided; for aluminium and magnesium the minimum radius should be 1/64 in. and for steel corners should be 0.030 in. and fillets should be 0.125 in. The following table lists the minimum cross-section and thickness for various materials.[1]
| Material | Minimum cross-section [sq. in.] | Minimum thickness [in.] |
|---|---|---|
| Carbon steels | 0.40 | 0.120 |
| Stainless steel | 0.45-0.70 | 0.120-0.187 |
| Titanium | 0.50 | 0.150 |
| Aluminium | <0.40 | 0.040 |
| Magnesium | <0.40 | 0.040 |
[edit] See also
[edit] References
- Schmid, Serope Kalpakjian and Steven R. Manufacturing Engineering and Technology, Fifth Edition. Pearson Prentice Hall, 2006.
- ^ a b c d e f g h i Oberg, E.; et al. (1996). Machinery's Handbook, 25th ed., Industrial Press Inc., 1348-1349.
- ^ a b c Drozda, Tom; Wick, Charles; Bakerjian, Ramon; Veilleux, Raymond F. & Petro, Louis, Tool and manufacturing engineers handbook, vol. 2, SME, pp. Chapter 13: 11-12, ISBN 0872631354, <http://books.google.com/books?id=9ty5NPJ0UI4C>.
- ^ a b c Avitzur, B. (1987), “Metal forming”, Encyclopedia of Physical Science & Technology, vol. 8, San Diego: Academic Press, Inc., pp. 80-109.
- ^ Brick manufacturing process
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