Extrusion: Hot Extrusion, Forward or Direct extrusion, Backward or Indirect extrusion, Cold Extrusion, Hooker extrusion, Hydrostatic extrusion, Impact extrusion, Cold extrusion forging

Extrusion

Extrusion is a process in which the metal is subjected to plastic flow by enclosing the metal in a closed chamber in which the only opening provided is through a die. The material is usually treated so that it can undergo plastic deformation at a sufficiently rapid rate and may be squeezed out of the hole in the die. In the process the metal assumes the opening provided in the die and comes out as a long strip with the same cross-section as the die-opening. Incidentally, the metal strip produced will have a longitudinal grain flow. The process of extrusion is most commonly used for the manufacture of solid and hollow sections of nonferrous metals and alloys e.g., aluminium, aluminium-magnesium alloys, magnesium and its alloys, copper, brass and bronze etc. However, some steel products are also made by extrusion. The stock or the material to be extruded is in the shape of cast ingots or billets. Extrusion may be done hot or cold. The cross-sections of extruded products vary widely.

Some advantages of extrusion process are described below:

• The complexity and range of parts which can be produced by extrusion process is very large.

• Dies are relative simple and easy to make.

• The extrusion process is complete in one pass only. This is not so in case of rolling, amount of reduction in extrusion is very large indeed. Extrusion process can be easily automated.

• Large diameter, hollow products, thin walled tubes etc. are easily produced by extrusion process.

• Good surface finish and excellent dimensional and geometrical accuracy is the hall mark of extruded products. This cannot be matched by rolling.

Pressure required for extrusion depends upon the strength of material and upon the extrusion temperature. It will reduce if the material is hot. It will also depend upon the reduction in cross-section required and the speed of extrusion.

There is a limit to the extrusion speed. If extrusion is done at a high speed, the metal may crack. The reduction of cross-sectional area required is also called “extrusion ratio”. There is a limit to this also. For steel extruded hot, this ratio should not exceed 40 : 1, but for aluminium extruded hot it can be as high as 400 : 1.

EXTRUSION PROCESSES

Extrusion processes can be classified as followed:

(A) Hot Extrusion

(i) Forward or Direct extrusion.

(ii) Backward or Indirect extrusion.

(B) Cold Extrusion

(i) Hooker extrusion.

(ii) Hydrostatic extrusion.

(iii) Impact extrusion.

(iv) Cold extrusion forging.

A. Hot Extrusion Processes

(i) Forward or direct extrusion process: 

In this process, the material to be extruded is in the form of a block. It is heated to requisite temperature and then it is transferred inside a chamber as shown in Fig. In the front portion of the chamber, a die with an opening in the shape of the cross-section of the extruded product, is fitted.

The block of material is pressed from behind by means of a ram and a follower pad. Since the chamber is closed on all sides, the heated material is forced to squeeze through the die-opening in the form of a long strip of the required cross-section.

Forward or direct extrusion process:  In this process, the material to be extruded is in the form of a block. It is heated to requisite temperature and then it is transferred inside a chamber as shown in Fig. In the front portion of the chamber, a die with an opening in the shape of the cross-section of the extruded product, is fitted.  The block of material is pressed from behind by means of a ram and a follower pad. Since the chamber is closed on all sides, the heated material is forced to squeeze through the die-opening in the form of a long strip of the required cross-section.
Forward or direct extrusion

The process looks simple but the friction between the material and the chamber walls must be overcome by suitable lubrication. When extruding steel products, the high temperautre to which the steel has to be heated makes it difficult to find a suitable lubricant.

The problem is solved by using molten glass as a lubricant. When lower temperatures are used, a mixture of oil and graphite is used as a lubricant. At the end of the extrusion process, a small piece of metal is left behind in the chamber which cannot be extruded. This piece is called butt—end scrap and is thrown away. To manufacture a tubular rod, a mandrel of diameter equal to that of tube—bore is attached to the ram. This mandrel passes centrally through the die when the material is extruded. The outside diameter of the tube produced will be determined by the hole in the die and the bore of tube will be equal to mandrel diameter. The extrusion process will then called “tubular extrusion”.

Backward or indirect extrusion:

This process is depicted in Figure. As shown, the block of heated metal is inserted into the container/chamber. It is confined on all sides by the container walls except in front; where a ram with the die presses upon the material. As the ram presses backwards, the material has to flow forwards through the opening in the die. The ram is made hollow so that the bar of extruded metal may pass through it unhindered. 

Backward or Indirect extrusion

This process is called backward extrusion process as the flow of material is in a direction opposite to the movement of the ram. In the forward extrusion process the flow of material and ram movement Were both in the same direction. The following table compares the forwards (direct) and backwards (Indirect extrusion process).


Cold Extrusion Processes

Hooker extrusion process: 

This process is also known as extrusion down method. It is used for producing small thin walled seamless tubes of aluminium and copper. This is done in two stages. In the first stage the blank is converted into a cup shaped piece. In the second stage, the walls of the cup one thinned and it is elongated. The process can be understood by referring to Figure. This process is a direct extrusion process.

Cold Extrusion Processes Hooker extrusion process:  This process is also known as extrusion down method. It is used for producing small thin walled seamless tubes of aluminium and copper. This is done in two stages. In the first stage the blank is converted into a cup shaped piece. In the second stage, the walls of the cup one thinned and it is elongated. The process can be understood by referring to Figure. This process is a direct extrusion process.
Hooker extrusion

Hydrostatic extrusion: 

This is a direct extrusion process. But the pressure is applied to the metal blank on all sides through a fluid medium. The fluids commonly used are glycerine, ethyl glycol, mineral oils, castor oil mixed with alcohol etc. Very high pressures are used – 1000 to 3000 MPa. Relatively brittle materials can also be successfully extruded by this method.

Impact extrusion: 

In this process, which is shown in Figure the punch descends with high velocity and strikes in the centre of the blank which is placed in a die. The material deforms and fills up the annular space between the die and the punch flowing upwards. Before the use of laminated plastic for manufacturing tooth paste, shaving cream tubes etc., these collapsible tubes containing paste were and are still made by this process. Other examples of products made by impact extrusion are light fixtures, automotive parts, and small pressure vessels. Most nonferrous metals can be impact extruded in vertical presses and at production rates as high as two parts per second. This process is actually a backward extrusion process.

Impact extrusion:  In this process, which is shown in Figure the punch descends with high velocity and strikes in the centre of the blank which is placed in a die. The material deforms and fills up the annular space between the die and the punch flowing upwards. Before the use of laminated plastic for manufacturing tooth paste, shaving cream tubes etc., these collapsible tubes containing paste were and are still made by this process. Other examples of products made by impact extrusion are light fixtures, automotive parts, and small pressure vessels. Most nonferrous metals can be impact extruded in vertical presses and at production rates as high as two parts per second. This process is actually a backward extrusion process.
Impact extrusion

Cold extrusion forging: 

This process is depicted in Figure. This is generally similar to the impact extrusion process; but there are two differences: In this process the punch descends slowly. The height of extruded product is short and the side walls are much thicker than the thin walled products produced by the impact extrusion process. In essence, this process is one of backward extrusion.

Cold extrusion forging:  This process is depicted in Figure. This is generally similar to the impact extrusion process; but there are two differences: In this process the punch descends slowly. The height of extruded product is short and the side walls are much thicker than the thin walled products produced by the impact extrusion process. In essence, this process is one of backward extrusion.
Cold extrusion process

EXTRUSION DEFECTS

Sometimes the surface of extruded metal/products develops surface cracks. This is due to heat generated in the extrusion process. These cracks are specially associated with aluminium, magnesium and zinc alloy extrusions. The extruded product can develop internal cracks also. These are variously known as centre burst, centre cracking and arrowhead fracture. There are three principal extrusion defects: surface cracking, pipe, and internal cracking.

Surface Cracking:

If extrusion temperature, friction, or speed is too high, surface temperatures can rise significantly, which may cause surface cracking and tearing. These cracks are intergranular (i.e., along the grain boundaries. and usually are caused by hot shortness . These defects occur especially in aluminum, magnesium, and zinc alloys, although they may also occur in high-temperature alloys. This situation can be avoided by lowering the billet temperature and the extrusion speed. Surface cracking also may occur at lower temperatures, where it has been attributed to periodic sticking of the extruded product along the die land. Because of the similarity in appearance to the surface of a bamboo stem, it is known as a bamboo defect. When the product being extruded temporarily sticks to the die land. The extrusion pressure increases rapidly. Shortly thereafter, the product moves forward again, and the pressure is released. The cycle is repeated continually, producing periodic circumferential cracks on the surface.

Pipe:

The type of metal-flow pattern in extrusion tends to draw surface oxides and impurities toward the center of the billet-much like a funnel. This defect is known as pipe defect, tailpipe, or fis/atailing. As much as one-third of the length of the extruded product may contain this type of defect and thus has to be cut off as scrap. Piping can be minimized by modifying the flow pattern to be more uniform, such as by controlling friction and minimizing temperature gradients. Another method is to machine the billet’s surface prior to extrusion, so that scale and surface impurities are removed. These impurities also can be removed by the chemical etching of the surface oxides prior to extrusion.

Internal Cracking:

The center of the extruded product can develop cracks, called center cracking, center-burst, arrowhead fracture. These cracks are attributed to a state of hydrostatic tensile stress at the centerline in the deformation zone in the die, a situation similar to the necked region in a tensile-test specimen . These cracks also have been observed in tube extrusion and in tube spinning

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