ExtrusionExtrusion is a compression forming process in which the work metal is forced to flow through a die opening to produce a desired cross-sectional shape. Parts have constant cross-section Typical Products of Extrusion are Sliding Doors, tubing having various cross-sections, structural and architectural shapes and door and window frames.
MaterialsMetals that are commonly extruded include Aluminium: it is the most commonly extruded material. used to make s frames, rails, and heat sinks Copper: It is used to make pipe, wire, rods, bars, tubes, and welding electrodes. Lead and Tin: It is used to make pipes, wire, tubes, and cable sheathing. Molten lead may also be used in place of billets on vertical extrusion presses Zinc: Used to make rods, bar, tubes, hardware components, fitting, and handrails. Steel (1825 to 2375 F (1000 to 1300 C)) rods and tracks. Usually plain carbon steel is extruded, but alloy steel and stainless steel can also be extruded. Titanium (1100 to 1825 F (600 to 1000 C) aircraft components including seat tracks, engine rings, and other structural parts.
Plastic Plastics extrusion commonly uses plastic chips or pellets, which are usually dried in a hopper before going to the feed screw. A multitude of polymers are used in the production of plastic tubing, pipes, rods, rails, seals, and sheets or films Extrusion has application in food processing. Products such as certain pastas many breakfast cereals premade cookie,, jalebi some french fries certain baby foods dry pet food.
Billet Billet is the starting stock for the extrusion operation. Extrusion billet may be a solid or hollow form, commonly cylindrical, and is the length charged into the extrusion press container. Extrusion rates vary, depending on the alloy used and the shape of the die. A hard alloy, given a complex shape, may emerge from the press as slowly as one or two feet per minute; a soft alloy taking on a simple shape may be extruded at a rate of 180 feet per minute, or even faster.
Extrusion Process Extrusion process begins with billet, the aluminum material from which profiles are extruded. The billet must be softened by heat prior to extrusion. The heated billet is placed into the extrusion press, a powerful hydraulic device wherein a ram pushes a dummy block that forces the softened metal through a precision opening, known as a die, to produce the desired shape.
The extrusion process has been likened to squeezing toothpaste out of a tube. When pressure is applied at the closed end, the paste is forced to flow through the open end, accepting the round shape of the opening as it emerges. If the opening is flattened, the paste will emerge as a flat ribbon. Complex shapes can be produced by complex openings. Bakers, for example, use a collection of shaped nozzles to decorate cakes with fancy bands of icing. They are producing extruded shapes. As suggested by these toothpaste tubes, the shape of the extrusion (profile) is determined by the shape of the opening (die).
But we can’t make very many useful products out of toothpaste or icing and we can’t squeeze aluminum out of a tube with your fingers. we can squeeze aluminum through a shaped opening, however, with the aid of a powerful hydraulic press, producing an incredible variety of useful products with almost any shape imaginable. These photos show a new length of extrudate, just emerging from the press (left) and the production of a profile in progress (right).
Types OF Extrusion Direct Extrusion Indirect Extrusion Hydraulic Extrusion
Direct Extrusion Direct Extrusion (or) Forward Extrusion – In this type of extrusion Billet is placed in a chamber and forced through a die opening by a hydraulically-driven ram or pressing stem. Figure 15.1 Schematic illustration of the direct extrusion process.
Direct Extrusion Friction increases the extrusion force. Hollow section is formed using a mandrel.
Indirect ExtrusionIn the indirect process, the die is contained within the hollowram, which moves into the stationary billet from one end, forcingthe metal to flow into the ram, acquiring the shape of the die as itdoes so. Figure 15.3 Types of extrusion: (a) indirect; (b) hydrostatic; (c) lateral.
Indirect Extrusion Metal is forced to flow through the die in an opposite direction to the ram’s motion.
Difference In the direct extrusion process, the die is stationary and the ram forces the alloy through the opening in the die While In the indirect process, the die is contained within the hollow ram, which moves into the stationary billet from one end, forcing the metal to flow into the ram, acquiring the shape of the die as it does so.
Ram ForceVariation of Ram Force with ram stroke and die angle.
Extrusion Temperature Ranges forVarious Metals C Lead 200–250 Aluminum and its alloys 375–475 Copper and its alloys 650–975 Steels 875–1300 Refractory alloys 975–2200
Extrusion Defectsa) Centre-burst: internal crack due to excessive tensile stress at the centre possibly because of high die angle, low extrusion ratio.b) Piping: sink hole at the end of billet under direct extrusion.c) Surface cracking: High part temperature due to low extrusion speed and high strain rates.
Process Variables in Direct ExtrusionFigure 15.4 Process variables in direct extrusion. The die angle, reductionin cross-section, extrusion speed, billet temperature, and lubrication allaffect the extrusion pressure. Extrusion Ratio = Ao/Af Ao – cross-sectional area of the billet Af - cross-sectional area of extruded product
Factors Influencing the Forces Friction Material Properties Reduction In Area Speed Temperature Geometry Of The Die
Advantages of Extrusion processIt has ability to create very complex cross-sections andwork materials that are brittle.Material only encounters compressive and shear stresses.Form finished parts with an excellent surface finish.Minimize the cost of production.Wide variety of cross-sections can be made.