Extrusion and its types with Advantages & Disadvantages

Extrusion, Hot Extrusion, Cold Extrusion, Direct Extrusion, Indirect Extrusion, Impact Extrusion

Principle of Extrusion:

            The process of forcing the metal to flow through the die opening to produce a desired cross sectional shape.

            This process can be linked to squeezing toothpaste out of a toothpaste tube. During extrusion, the metal is subjected to plastic deformation and it undergoes reduction and elongation. It is most widely used in fabricating solid and hollow sections from ferrous, non-ferrous metals and their alloys. The section of the product depends on the shape of the die. Vegetable oils, and oil with graphite are widely used as lubricants. Extrusion is generally considered as closed die forging. The difference is that, the main body of the metal is the product and flash is cut away and almost no slug remaining. But in extrusion the flash itself is the product and there is a slug remaining in the die.

The various types of extrusions are,
1.Hot Extrusion
2.Cold Extrusion
3.Direct Extrusion
4. Indirect Extrusion
5. Impact Extrusion.

Hot Extrusion:

                It is the process of enclosing the heated billet or slug of metal in a closed cavity and then pushing it to flow from only one die opening so that the metal will take the shape of the opening.

            The pressure is applied either hydraulically or mechanically. Extrusion process is identical to the squeezing of toothpaste out of the toothpaste tube. The extrusion setup consists of a cylinder container into which the heated billet or slug of metal is loaded. On one end of the container, the die plate with the necessary opening is fixed. From the other end, a plunger or ram compresses the metal billet against the container walls and the die plate, thus forcing it to flow through the die opening, acquiring the shape of the opening. The extruded metal is then carried by the metal handling system as it comes out of the die. Tubes, rods, hose, casing, brass cartridge, molding-trims, structural shapes, aircraft parts, gear profiles, cable sheathing etc., are some typical products of extrusion.

 Advantages & Disadvantages of Hot Extrusion 

Advantages of Hot Extrusion:

• Large reduction can be achieved.

• Very complex shapes can be produced.

• The forces involved are compressive.

• Relatively brittle material can be extruded.

• Excellent dimensional tolerances can be maintained.

• Very high surface finishing.

Disadvantages of Hot Extrusion:

• It creates in-homogeneous deformation conditions.

• Extrusion of steel requires high pressure and temperature.

• Heat generated during extrusion may cause surface cracks.

Cold extrusion:

              Similar to hot extrusion except that the metal worked possess plasticity, which is necessary for deforming without heating it. The metals that are cold extruded will have a high degree of ductility. It is carried out to improve physical properties of the metal. Generally used to produce discrete parts, often in finished form. Metals like aluminium alloys, copper, brass, and bronze are extruded. Aluminium is the most ideal metal for extrusion [hot and cold].

Direct or forward extrusion:

               The ram pushes the billet in a stationary container or cylinder through a die of desired shape.

            Relative movement takes place between the billet and the container. The deformed metal [product] flows through the die opening in the same direction to the ram motions [forward].  The work piece [hot billet] is placed in the container and it is confined. A dummy block is placed between the ram and the hot metal. Press operated ram pushes the hot metal to fill the cylinder first, and it is then forced through the die opening. At the end of the extrusion process, a small piece of metal called butt end scrap remains in the contains and cannot be extruded. It is then sawed off next to the die and the butt end removed. As the ram moves in the direction towards the die, the metal is forced out through the annular clearance and form a tube.

Indirect or Backward extrusion:

                  Similar to direct extrusion, except the product is extruded through the hollow ram. The die is located in front of the hollow stem and pushed against the billet by forward movement of the container closed at the back.

            No relative movement between the billet and the container. No friction between the metal billet and container walls because the billet does not move the container. The deformed metal [product] flows through the die opening in the direction opposite to the ram motions [backward]. Less total force is required compared to direct extrusion. The force is independent of billet length. But the equipment used is mechanically more complicated. To reduce frictional forces, a hydraulic press is used, so that it can also act as lubricant. Force in indirect extrusion is 25 to 30% less than direct extrusion. Because of the large forces required in cold extrusion, most metals are hot extruded.

Impact extrusion:

                Impact extrusion is a different manufacturing process in which a part is extruded through the impact of a die with the work piece.

            The product is formed at a high speed, and over a relatively short stroke. Impact extrusion is most often performed cold, occasionally with some metals and thicker walled structures the work is heated before impact forming it. This process is best suited for softer metals, aluminum is a great material for forming by impacting. During the manufacturing operation of impact extrusion, a work piece is placed in a mold and struck with great force causing the metal to rapidly flow into position in an instant.

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