Extrusion of Metals: 4 Types | Metallurgy

The following points highlight the four main types of extrusion processes. The types are: 1. Direct Extrusion 2. Indirect Extrusion and Impact Extrusion 3. Hydrostatic Extrusion 4. Continuous Extrusion.

Type # 1. Direct Extrusion:

In direct extrusion the motion of press ram and that of extrude are in the same direction. The billet smeared with lubricant is put in a cylinder, at the other end of which a die having the desired shaped hole is attached. The billet is followed by a pressure plate, which fits the cylinder bore so that the work material does not flow back. 

The material is compressed by press ram through the pressure plate against the die. The material, being constrained by cylinder walls, is forced to flow out of the die hole. For manufacture of commercial long products, horizontal presses of high tonnage are used. Vertical presses can also be used but it poses a problem of space to accommodate the long products. Generally short products are extruded on vertical presses.

In general the reductions in direct extrusion are large and it requires punch pressures more than the yield strength of material for extrusion to take place.

The billet first gets forged to the diameter of the cylinder and with further compression it starts extruding. The press load is highest at the start of extrusion because of high static friction. Also, as the extrusion proceeds and billet length in the cylinder decreases, this decreases the frictional force between billet and cylinder and hence the press load also decreases.

The speed of central layers of billet is higher than that of outer layers. Towards the end of the process material just opposite die hole gets pulled into die which makes the end portion of product hollow.

The process should be stopped at this stage. At this stage the work piece is in the form of a thin disc and the material has to flow against the frictional forces on the die surface and pressure plate. Consequently the punch load increases rapidly with further punch motion. Therefore, the process is stopped, press ram is withdrawn, the extruded length is cut off and the die is removed for cleaning. Cylinder is also cleaned and lubricated for the next shot.

A significant part of press capacity is wasted in overcoming the frictional force between the work piece and cylinder wall. And hence it is not economical to use very long billets. The maximum length of the billet for direct extrusion should not be more than 5 times the diameter.

Direct extrusion is utilized to extrude circular as well non-circular solid sections as well as tubular products. The three variants of direct extrusion are shown in Fig. 10.4. The first Fig. 10.4(a) shows solid bar extrusion. The second shows the extrusion of cans with the help of mandrel on the opposite side. The third shows the extrusion of long tubes. For tube extrusion the hole in billet may be punched in the process itself. Drilled blank are used when it is not possible to punch a hole in the extrusion die.

Direct extrusions are generally carried out on horizontal hydraulic presses. The process is of intermittent nature. After each extrusion the cylinder and dies have to be cleaned and lubricated which takes time and press remains idle during the time. The valuable time of press is wasted. Therefore, presses with multiple cylinders and dies have been developed.

After the process is over and the ram is withdrawn clear of press, the block carrying the cylinders is rotated to bring cleaned cylinder in position.

Similarly the die block is rotated to bring in cleaned or new die. During the time the next two extrusions take place the used dies and cylinders are cleaned and lubricated and made ready for rotation into extrusion position. Automation in loading of billet and pressure plate further reduces the idle time of press.

Type # 2. Indirect Extrusion and Impact Extrusion:

Figures 10.6(a, b) show the schematic diagrams of indirect extrusion. The work piece or billet smeared with lubricant is placed in the cylinder which is closed at the other end. The punch of the desired shape is pressed on to the billet. The material of billet is forced to flow into the annular space between punch and the container in case of extrusion of a tubular component as shown in Fig. 10.6(a).

If the punch dimensions are much smaller than the billet dimensions it would become a case of simple indentation. The distinction between extrusion and indentation is that in case of indentation the outer diameter of the work piece (billet) may not be in contact with the cylinder wall while in extrusion the billet gets compressed to cylinder diameter. For indirect extrusion of a solid bar the punch has a hole and the material is extruded into the punch hole in Fig. 10.6 (b).

In indirect extrusion the material near the contacting face of the punch undergoes plastic deformation while the work material away from the punch remains stationary. Therefore, there is no frictional force from the stationary part of the billet side to oppose the ram force.

Only the friction between extrude and punch can cause additional load on press. Figure 10.7 shows the ram load v/s ram travel for indirect extrusion. The billet may get forged to cylinder diameter first and with further movement of punch the extrusion starts.

The extrusion load may be a bit higher at the start due to higher static friction. After the extrusion start, the dynamic friction being low as well as improvement in lubrication may reduce the press load a bit after which the load remains nearly constant. Towards the end of process when a small length of billet is left in cylinder, the frictional forces on the flat surfaces of billet are more effective in hindering the metal flow and hence the ram load rises rapidly with further movement of punch.

Indirect extrusion can be carried out on hydraulic or mechanical presses. Small components like tooth paste tubes, piston pins etc. are generally extruded on mechanical presses as they are faster. For this reason process is also called impact extrusion. Recent developments have made the impact extrusion a popular process for producing cup shaped components in ferrous as well non-ferrous alloys.

Punch is a critical element in indirect extrusion particularly for extrusion of steel alloys. The design evolved out of experience is illustrated in Fig. 10.8. Punch face is slightly conical to ease the metal flow. The angle varies from 160 to 170 degrees. Except for a small land on the periphery of punch face which has dimensions of the bore diameter of the product the remaining punch-stem is smaller in diameter by 0.1 to 0.2 mm.

For many products both the direct and indirect extrusion processes are combined.

In indirect extrusion a component may not be finished in single stroke of press. Several components like gun shells require repeated extrusions for elongation and sizing. One disadvantage of indirect extrusion is that very large lengths of extrudes cannot be made because of the limited length of gap between press bed and ram.

Defects like cavitation and fish-skin may occur. Cavitation occurs due to failure of metal to flow into the corner of die chamber. The defect may occur when bottom thickness falls below a certain limit. Fish-skin is the circumferential tearing at intervals on the inner and outer surfaces of the extrude. In direct extrusion surface cracks may occur on less ductile alloys. The internal central cracks may also occur in certain process conditions.

Type # 3. Hydrostatic Extrusion:

In case of direct extrusion it was observed that a significant portion of press force is wasted in overcoming frictional force between billet and cylinder wall. Elimination of this frictional force is one of the inspirations in the development of hydrostatic extrusion process. The second important advantage is that slightly brittle materials may also be successfully extruded by this process.

In normal extrusion processes, these metals develop surface cracks on extruded products. If the ductility of an alloy is very low, it may be extruded in a pressure chamber or with a die in two steps. The first step gives heavy reduction while in second step a light reduction is given to build pressure on the exit of first step. The idea is to increase the hydrostatic pressure in the work piece material in order to increase its ductility.

Figure 10.12 shows the schematic view of hydrostatic extrusion. In this process the ram force is transmitted to the billet through a liquid which is generally oil which also lubricates the dies and reduces the friction on the interface between billet and die.

Advantages of Hydrostatic Extrusion:

The general advantages of the process are as follows:

(i) The billet does not touch the cylinder wall and thus there is no friction at cylinder wall as is the case in direct extrusion. Extrusion forces are lower.

(ii) Full press capacity is used for extrusion. There is no wastage of press capacity in overcoming cylinder frictional load.

(iii) Metal-die interface friction is reduced due to pressurized lubrication and hence more complex shapes may be extruded.

(iv) As compared to direct extrusion greater reductions are possible with the same press capacity.

Disadvantages of Hydrostatic Extrusion:

(i) One of the problems is the leakage of oil under high pressure. This can be prevented successfully with the help of O rings, however, it takes quite some time in preparing the set up for extrusion and this is one of the reasons that the process does not find favor with industry.

(ii) The exit speed of extrude is not under control. Because the compressed oil has a storage of elastic energy, once the extrusion starts the dies get better lubricated resulting in lowering of extrusion force and hence releasing part of the energy stored in oil. The billet rushes out with high speed along with the oil, leading to a sudden drop of press load. This gives a shock to the machinery.

Recent developments have partly solved the above problems. Figure 10.13(a) shows a set up in which there is no seal on the punch side but there is very small clearance between punch and cylinder. Punch length in contact with the cylinder is so designed that a high pressure gets developed even with small leakage through the clearance. But this pressure is not enough for extrusion to take place.

The punch must touch and press the billet for it to extrude. Thus the billet will move with the speed of the punch. At the same time the direct force of punch on the billet is not sufficient to compress it to cylinder diameter. So the billet does not touch the cylinder wall and hence there in no frictional force due to it, also, no seals are used on punch side, which reduces set up time.

Another approach is to reduce the stored energy in fluid by reducing its volume in Fig. 10.13(b). This can be achieved by reducing the clearance between cylinder and billet to as low as 0.3 mm. The clearance between the billet end and pressure plate is also reduced. In some cases the pressure plate may be allowed to touch the billet as well so that there is a controlled movement of billet during extrusion. This process is also called thick-film hydrostatic extrusion.

Also there is no sudden shock to the machinery at the end of the extrusion. Setting up is also easier and quicker if the scheme of (Fig. 10.13a) is also incorporated in this, then seal is required only on the die side. In this case the complete billet does not rush out and the dies will have to be cleaned of the residual billet after each extrusion.

Applications of Hydrostatic Extrusion:

Hydrostatic extrusion may be used to reduce the diameter of metal wires such as copper and aluminum, for cladding, for extrusion of brittle materials and for extrusion of tube etc.

In extrusion of brittle materials there is danger of development of cracks on the surface of extrude. Development of cracks may be eliminated by extruding into a pressure chamber or by redesigning the die in two steps, so that a small reduction is given in the second step. Thus a compressive stress in induced at the exit of the first extrusion die which prevents development of cracks.

Helical Extrusion:

The process is also called ‘hydrospin’. It is a variant of hydrostatic extrusion. The billet is pressed by hydrostatic pressure against a conical mandrel which converts it into a tubular form which is pressed on to rotating die co-axial with mandrel and having an abutment.

The abutment has a hole of the shape of the desired extrude. Because of rotation of die and abutment the work piece material flows out through the hole in the abutment. Thus the tube formed in the first part of process is completely extruded through the hole in the abutment into wire or strip of desired shape.

The advantage of the process is that very high area reductions of the order of 15000: 1 may be realized. The process may be used for manufacture of wires of copper and aluminium directly from billets of 100 to 150 mm diameter.

Type # 4. Continuous Extrusion:

Several attempts have been made using many different ideas to make extrusion process continuous but most of them have failed. Only one such attempt has been successful.

The process uses the concept of moving cylinder, so that the friction between the billet and cylinder wall can create enough pressure in the billet that it gets extruded. The equipment consists of a disc or pulley having a deep rectangular groove on its periphery. The pulley is rotated by a motor through a gear box. A stationary shoe is mounted on the pulley. The shoe has a small tongue which fits into the groove in the pulley.

The groove and the tongue together make a hollow rectangular circular cavity whose three sides are moving and one side, i.e. tongue is stationary. On the forward side of the shoe (at the front end of the cylindrical cavity) an extrusion die is fixed. The die completely blocks the rectangular groove except for a hole of desired shape in it. The billet which may be round or rectangular is only as wide as the groove and fits snugly into it.

The billet is pushed into the cavity at its other end. The friction between the billet and the groove walls pulls the billet into the groove and presses it against the die. Since tongue is stationary the frictional stress on it is in the opposite direction. Nevertheless, there is net forward force because of three walls of the groove. The length of the shoe can be calculated to be sufficiently long so that the pressure build up near the die is sufficient for extrusion to take place.

The process has been developed to extrude aluminum, aluminum-magnesium-silicon alloy, aluminum silicon alloys and copper. The process can be conveniently used to manufacture solder wire. The process requires high torque so hydraulic motors are best suited for it. However, a small experimental machine developed at IIT Delhi uses electric motor with high reduction gear box. Lead wires have been extruded successfully on this machine.

The maximum possible reduction may be increased by making the groove surface rough and the tongue surface smooth and polished. Encouraging results have been obtained by plating the tongue surface by nickel and chromium.

For calculation of the groove length required, let us divide the entire grip length into two parts. In the first part, the grip width on any wall is small. At the end of the first portion the entire billet is in plastic condition and is upset to fill the groove. The following notation is used.

A = Area of groove cross section = w²

b = Average width of contact between billet and groove wall on the length l₁.

l₁ = Length of first part of groove.

l₂ = Length of second part of groove.

p_e = Extrusion pressure.

µ₁ = Co-efficient of friction between billet and wall of groove.

µ₂ = Co-efficient of friction between billet and tongue.

The length of the first part l₁ is given by