What is Hot Extrusion Process ? Types, Advantages, Applications

In this article we will learn about Hot Extrusion Process, its different types, how it works and what are its applications.

What is Hot Extrusion ?

Hot Extrusion is a Process to provide the desired form of the finished part through a billet or slug of metal which is forced by high pressure through an orifice.

Hot extrusion is a popular method for creating objects with a fixed cross-sectional profile. This extrusion process is carried out at a higher temperature, which prevents the materials from hardening and simplifies the procedure of pushing the material through the die.

In general, horizontal hydraulic presses with capacities ranging from 250 to 12,000 tons are used to perform a large number of hot extrusions. The pressure range here is 30 to 700 MPa (4,400 to 102,000 psi), so lubrication is required. Glass powder can be used to lubricate higher temperature extrusions, while graphite or oil can lubricate lower temperature extrusions. Though hot extruded materials are highly preferred, there are a few drawbacks to hot extrusion, including the cost of extrusion machines and their maintenance.

Am everyday analogy is the squeezing of toothpaste from a collapsible tube. Because of the large forces required in Extrusion, most metals are extruded hot under conditions where the deformation resistance of the metal is low.

However, cold extrusion is possible for many metals and is rapidly taking an important commercial position.

Hot Extrusion Working

The reaction of the extrusion billet with the container and die results in high compressive stresses which are effective in reducing the cracking of the materials during primary breakdown from the ingot. This is an important reason for greater use of extrusion in the working of metals difficult to form, e.g., stainless steels, nickel-base alloys, and molybdenum.

Most hot extrusion is done on horizontal hydraulic presses especially constructed for this purpose. Common sizes are rated from 250 to 5500 tonnes. Temperatures of billets are 350°C to 425°C for magnesium, 425°C to 475°C for aluminium, 650°C to 1300°C for copper alloys, and 1200°C to 1300°C for steel. Pressure normally vary from 4998 to 7038 kgf per sq cm (69*10′ N/m²). Lubrication of the extrusion chamber, die, and ram is necessary and is ordinarily achieved by mopping with oil supported graphite. Vegetable oils are better than petroleum oils for this purpose.

The extrusion of steel at high temperatures is most successfully done by using a glass which, at the temperatures involved, acts as a lubricant.

Types of Hot Extrusion

Metal is extruded in a number of basic ways as follows:

1. Direct or Forward Extrusion

Direct extrusion illustrated in Fig. 7.9 employs a press-operated ram and a cylinder or container into which the workpiece is placed for confinement. A dummy block is used between the ram and the hot metal. With application of ram pressure, the metal first plastically fills the cylindrical shape, and it is then forced out through the die opening until a small amount remains in the container. It is then sawed off next to the die and the butt end removed.

2. Indirect or Backward Extrusion

Indirect extrusion is similar to direct extrusion except that the extruded part is forced through the hollow ram as shown in Fig. 7.10. It involves no friction between the metal billet and container walls, because the billet does not move the container. Compared with direct extrusion, less total force is required, but the equipment used is mechanically more complicated in order to accommodate the passage of the extruded shape through the center of the hollow ram.

3. Tube Extrusion

It is a form of direct extrusion but uses a mandrel to shape the inside of the tube. After the heated billet is placed inside the container, the die containing the mandrel is pushed through the billet. The ram then advances and extrudes the metal through the die and around the mandrel as shown in fig. 7.11

4. Impact Extrusion

Extrusions are also made by striking slugs of metal and forming them by high impact. This is essentially a cold working operation.

Hot Extrusion Temperature For Various Metals

Applications of Hot Extrusion

Most commercial metals and their alloys, such as steel, copper, aluminium, magnesium, and nickel, are directly extruded at elevated temperatures. Rods, tubes, moulding trim, structural shapes, brass cartridges, lead-covered cables, aircraft parts, flooring strips, and many hardware items such as window sash, door trim, etc., are typical products of extrusion.

The hot extrusion process is widely used to manufacture products from Aluminium, Copper along with their alloys. Some of the products that are developed using this process are Electrical Wires, Bars and Tubes.

What is Hot Extrusion Used For ?

Hot extrusion is the production process used to form special steel profiles with a constant longitudinal cross-section. It acts through a force called “compression”. This process can be employed to manufacture solid or hollow profiled bars, with complex geometries and a fixed cross-section, in a single step.

Multiple Choice Questions (MCQ) on Hot Extrusion Process

1. Which of the following is true for hot extrusion?

A. Less force is required in forward extrusion as compared to backward extrusion
B. More force is required in forward extrusion as compared to backward extrusion
C. Same force is applied in both forward and backward extrusion
D. Any of the above

Ans : B

2. Which of the following is also called as forward hot extrusion?

A. Direct extrusion
B. Indirect extrusion
C. Impact extrusion
D. Hydrostatic extrusion

Answer: A
Explanation: Direct extrusion is also called as forward hot extrusion. This method is most widely used and maximum production is done by this method in extrusion.

3. Backward extrusion is another name for which of the following?

A. Direct extrusion
B. Indirect extrusion
C. Impact extrusion
D. Hydrostatic extrusion

Answer: B
Explanation: Direct extrusion is also called as backward extrusion. In this method, the metal is extruded back into the plunger.

4. Extrusion process is not suitable for which of the following?

A. Steel alloys
B. Non-ferrous alloys
C. Stainless steel
D. Iron

Answer: D
Explanation: Extrusion process is not suitable for any of the iron alloys, neither is it suitable for iron metal. For extruding stainless steel or other non-ferrous alloys or steel alloys, extrusion process can be used.

This was all about the Hot Extrusion process where we learnt about the meaning of the process along with its applications and the different types of extrusion processes. If you liked this article please do share it with your friends also.

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