Introduction to Extrusion• Process is similar to

squeezing toothpaste out of a toothpaste tube

• Material is compressed in a chamber and the deformed material is forced to flow through a shaped opening in a die.

• The die opening corresponds to the cross section of the required product.

• It is basically a hot working process, however, for softer materials cold extrusion is also performed.

• In general, extrusion is used to produce long parts of uniform cross-sections

Extruded ItemsRailings for sliding doorsWindow framesTubing having various cross-sectionsAluminum laddersNumerous structural and architectural shapes

Extruded MetalsCommon metals used in extraction process:AluminumCopperSteelsStainless steelsMagnesiumLead

Other metal alloys can be extruded with various levels of difficulty

Extrusion RatioIt is the ratio of area of cross-section of the billet (A0) to the area of cross-section of the extrude (Af).

About 40: 1 for hot extrusion of steel, 400: 1 for aluminum

Extrusion DiesSimplest cross section shape = circular die orifice This process allows designers and engineers freedom to create products in complex and intricate shapes, since the end product can be extruded as a final piece instead of requiring multiple ones that must be assembled.Shape of die orifice affects ram pressure As cross-section becomes more complex, higher pressure and greater force are required.

A solid die, as shown here, produces shapes without any enclosed voids and/or semi-hollow conditions

Extrusion Die Features

(a) Definition of die angle

(b) Effect of die angle on ram force.

Process parametersThe force required for

extrusion depends on:The strength of the billet materialThe extrusion ratio Ao/Af

Friction between the billet and the chamber and die surface

The geometric variables in extrusion are:

The die angleThe ratio of the cross-sectional area of the billet to that of the extruded product, Ao/Af

The temperature of the billetThe speed at which the ram travelsThe type of lubrication

Types of Extrusion1. Direct extrusion

2. Indirect extrusion

3. Impact extrusion and

4. hydrostatic extrusion.

Direct ExtrusionAlso called forward extrusionMetal flows in the same direction as that of the ram. Because of the relative motion between the heated billet and the chamber walls, friction is severe. Friction is reduced by using molten glass as a lubricant in case of steels at higher temperatures. At lower temperatures, oils with graphite powder is used for lubrication. Starting billet cross section is usually round, but final shape is determined by die opening

Indirect ExtrusionAlso called backward extrusion and reverse extrusion Metal flows in the opposite direction of the ram. It is more efficient since it reduces friction losses considerably. The process, however, is not used extensively because it restricts the length of the extruded component. Lower rigidity of hollow ram and difficulty in supporting extruded product as it exits die

Impact ExtrusionSimilar to indirect extrusion. The punch is made to strike the slug at high speed by impact load. The slug gets indirectly extruded on to the punch and to give a tubular section.The length of the tube formed is controlled by the amount of metal in the slug or by the blank thickness. Hollow sections such as cups, toothpaste containers are made by impact extrusion.Usually metals like copper, aluminium, lead are impact extruded.

Hydrostatic ExtrusionContainer is filled with a fluid. Extrusion pressure is transmitted through the fluid to the billet. Friction is eliminated in this process because of no contact between billet and container wall. Brittle materials can be extruded by this process. Greater reductions are possible by this method. Pressure involved in the process may be as high as 1700 Mpa.

Hydrostatic Extrusion …Pressure is limited by the strength of the container, punch and die materials. Vegetable oils such as castor oil are used. Normally this process is carried out at room temperature. A couple of disadvantages of the process are: leakage of pressurized oil and uncontrolled speed of extrusion at exit, due to release of stored energy by the oil. This may result in shock in the machinery. This problem is overcome by making the punch come into contact with the billet and reducing the quantity of oil through less clearance between billet and container.

Cold ExtrusionCold extrusion could produce parts with good surface finish, high strength due to strain hardening, improved accuracy, high rate of production. However, the process requires higher pressure and tools are subjected to higher stresses. Used with low‐strength metals such as lead, tin, zinc, and aluminum Proper lubrication is necessary for preventing seizure of tool and workpiece. To produce collapsible tubes for toothpaste, medications, and other creams; small "cans" for shielding electronic components and larger cans for food and beverages.

Hot ExtrusionHot extrusion can be employed for higher extrusion ratios. Difficult to work metals are usually hot extruded or warm extruded Inhomogeneous deformation can occur due to die wall chilling of the billet. Metal may get oxidized. The oxide layer can increase friction as well as the material flow. Glass is used as lubricant for hot extrusion. Molybdenum disulfide or graphite are the solid lubricants used in hot extrusion. Canned extrusion using thin walled cans made of copper or tin is usually used for extruding highly reactive metals and metal powders.

Extrusion Presses1. Horizontal presses2. Vertical presses

Horizontal Extrusion Presses

Horizontal presses up to 50 MN capacity are being used.

Used for most commercial extrusion of bars and shapes.

Disadvantages: Deformation is non‐uniform due to different temperatures between top and bottom parts of the billet

Vertical Extrusion PressesVertical presses are of capacity ranging from 3 to 20 MN. Tubular extrusions are mostly done in vertical presses

Advantages: Easier alignment between the press ram and tools. Higher rate of production.Require less floor space than horizontal presses. Uniform deformation, due to uniform cooling of the billet in the container. Requirements: •Need considerable headroom to make extrusions of appreciable length. •A floor pit is necessary

General Advantages of Extrusion

Variety of shapes possible, especially in hot extrusionLarge deformations can take place without fractureExtrusion results in better grain structure, accuracy and surface finish of the components. Close tolerances possible, especially in cold extrusion In some operations, little or no wastage of material Many shapes (than rolling)Since the die geometry remains constant throughout the process extruded products have a constant cross-section