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.
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.
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.
|Material||Temperature [F° (C°)]|
|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 become more economical to roll if producing more than 50,000 lb.
Materials that are commonly cold extruded include: lead, tin, aluminum, copper, zirconium, titanium, molybdenum, beryllium, vanadium, niobium, and steel.
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.
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 and flows around the legs, leaving weld lines in the final product.
Typical extrusion presses cost more than $100,000, whereas dies can cost up to $2000.
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 the 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 because of this reason.
The disadvantages are:
The advantages of this process include:
The disadvantages are:
Direct-drive oil presses are the most common because they are reliable and robust. They can deliver over 5000 psi (34.5 MPa). They supply a constant pressure throughout the whole billet. The disadvantage is that they are slow, between 2 and 8 ips (51 to 203 mm/s).
Accumulator water drives are more expensive and larger than direct-drive oil presses, plus they lose about 10% of their pressure over the stroke, but they are much faster, up to 15 ips (381 mm/s). 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.
Hydrostatic extrusion presses usually use castor oil at pressure up to 200 ksi (1380 MPa). Castor oil is used because it has good lubricity and high pressure properties.
Magnesium and aluminium alloys usually have a 30 μin. RMS or better surface finish. Titanium and steel can achieve a 125 μin. RMS.
In 1950, Ugine Séjournet, 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.
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.
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.
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. Some Play-Doh toy products also make use of the extrusion process.
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.
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.
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.
|Material||Minimum cross-section [sq. in.]||Minimum thickness [in.]|