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 are 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, concrete and foodstuffs.
Hollow cavities within extruded material cannot be produced using a simple flat extrusion die, because there would be no way to support the center barrier of the die. Instead, the die assumes the shape of a block with depth, beginning first with a shape profile that supports the center section. The die shape then internally changes along its length into the final shape, with the suspended center pieces supported from the back of the die.
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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]
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.[2]
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 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 12,000 short tons). Pressures range from 30 to 700 MPa (4,400 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 [°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).[2]
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 and gear blanks.
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 properties.[3]
There are many different variations of extrusion equipment. They vary by four major characteristics:[1]
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 aluminum), 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.
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.[1] 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 and flows around the legs, leaving weld lines in the final product.[4]
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 need 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.[5]
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:[6]
The disadvantages are:[6]
In the hydrostatic extrusion process the billet is completely surrounded by a pressurized liquid, except where the billet contacts the die. This process can be done hot, warm, or cold, however the temperature is limited by the stability of the fluid used. The process must be carried out in a sealed cylinder to contain the hydrostatic medium. The fluid can be pressurized two ways:[6]
The advantages of this process include:[6]
The disadvantages are:[6]
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).[7]
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.[7]
Hydrostatic extrusion presses usually use castor oil at pressure up to 1400 MPa (200 ksi). Castor oil is used because it has good lubricity and high pressure properties.[8]
Metals that are commonly extruded include:[9]
Magnesium and aluminium alloys usually have a 0.75 µm (30 μin) RMS or better surface finish. Titanium and steel can achieve a 3 micrometres (120 μin) RMS.[1]
In 1950, Ugine Séjournet, of France, invented a process which uses glass as a lubricant for extruding steel.[10] 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]
Plastics 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.
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.[11]
Extrusion has application in food processing. Products such as certain pastas, many breakfast cereals, Fig Newtons, premade cookie dough, Murukku, Sevai, Idiappam, jalebi, some french fries, certain baby foods, dry pet food and ready-to-eat snacks are mostly manufactured by extrusion. In the extrusion process, raw materials are first ground to the correct particle size (usually the consistency of coarse flour). The dry mix is passed through a pre-conditioner, where other ingredients are added (liquid sugar, fats, dyes, meats and water depending on the product being made), steam is injected to start the cooking process. The preconditioned mix is then passed through an extruder, and then forced through a die where it is 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 process can induce both protein denaturation and starch gelatinization, depending on inputs and parameters. Extruders using this process have a capacity from 1–25 tonnes per hour depending on design.
As with other forms of cooking, extrusion achieves the following nutritionally:
Extrusion is also used to modify starch and to pellet animal feed.
Extrusion through nano-porous, polymeric filters is being used to manufacture suspensions of lipid vesicles liposomes or Transfersomes for use in pharmaceutical products. The anti-cancer drug Doxorubicin in liposome delivery system is formulated by extrusion, for example.
The extrusion production technology of fuel briquettes is the process of extrusion screw wastes (straw, sunflower husks, buckwheat, etc.) or finely shredded wood waste (sawdust) under high pressure when heated from 160 to 350 °C. The resulting fuel briquettes do not include any of the binders, but one natural - the lignin contained in the cells of plant wastes. The temperature during compression, causes melting of the surface of bricks, making it more solid, which is important for the transportation of briquettes.
The design of an extrusion profile has a large impact on how readily it can be extruded. The maximum size for an extrusion is determined by finding the smallest circle that will fit around the cross-section, this is 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 60 cm (24 in) diameter circumscribing circles for aluminium and 55 cm (22 in). diameter circles for steel and titanium.[1]
The complexity of an extruded profile can be roughly quantified by calculating the shape factor, which is the amount of surface area generated per unit mass of extrusion. This affects the cost of tooling as well as the rate of production.[14]
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 0.4 mm (1/64 in) and for steel corners should be 0.75 mm (0.030 in) and fillets should be 3 mm (0.12 in). The following table lists the minimum cross-section and thickness for various materials.[1]
| 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) | 1.00 (0.040) |
| Magnesium | <2.5 (0.40) | 1.00 (0.040) |
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