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extrusion

Extrusion is a deformation process used to produce long, straight metal products such as bars, solid and hollow sections, tubes , wires , and strips.

Extrusion is done by squeezing metal in a closed cavity through a tool, known as a die using either a mechanical or hydraulic press.

Cross sections of varying complexity can be extruded , depending on the material and the dies used.

There are 4 characteristic differences among the various methods of extrusion and the presses used :

(a) The movement of extrusion relative to the stem (direct and indirect processes)

(b) The position of the press axis (horizontal or vertical presses)

(c) Type of derive (hydraulic or mechanical presses)

(d) Method of load application (conventional or hydrostatic extrusion)

Direct Extrusion

Direct extrusion is also known as forward extrusion. Direct extrusion is one of the most critical as well as common method for making extruded profiles. Here the position of the die is stationary, while the metal gets pushed through the orifice. Here the billet is placed properly in the container and pushed along the die by using ram pressure.

Billet-to-Billet Extrusion

Billet-to-Billet extrusion is a special method for alloys that easily weld toghether at the extrusion temperature.Using this process, continuous lenghts can be produced by discontinuous extrusion.

Lead and aluminum alloys_ Al99.5, AlMn, AlMgSi0.5, AlMgSil; i.e., the low-alloy types_ that easily weld toghether during extrusion are suitable.

Indirect Extrusion Indirect Extrusion is also

termed as Backward/ Reverse extrusion. Here the die is not stationary and moves with respect to the container. Therefore the difficulty of direct extrusion in terms of the frictional force between the container walls and the billet is eliminated. In case of indirect extrusion, the die placed at the front end of the hollow stem moves in relation to the container.

Indirect Extrusion

However one limitation is the difficulty on holding the extruded part once it exits the die. The basic advantage enjoyed by the indirect extrusion process other than the absence of frictional forces include a more uniform metal flow, less power requirement and better section uniformity along the length.

Indirect Extrusion – Hollow Cross Section

Comparing Friction Components of Direct and Indirect Extrusion Friction components are completely dependent

on the type of extrusion , Direct or indirect. Using the popular flat-face dies. The figures show the friction-force components in indirect and direct extrusion. It is revealed from the flow pattern that in case of indirect extrusion, a dead-metal zone exists that has a much higher zone semi angle as denoted by in comparison to that in direct extrusion process. For a same size extrusion, i > d.. i is the dead metal zone semi angle of indirect extrusion and d is the dead metal zone semi angle of direct extrusion.

Comparing Friction Components of Direct and Indirect Extrusion

DIRECT EXTRUSION INDIRECT EXTRUSION

Hot Extrusion

Hot extrusion is done at an elevated temperature to keep the material from work hardening and to make it easier to push the material through the die. Most hot extrusions are done on horizontal hydraulic presses that range from 250 to 12,000 tons. Pressures range from 30 to 700 MPa (4,400 to 102,000 psi), therefore lubrication is required, which can be oil or graphite for lower temperature extrusions, or glass powder for higher temperature extrusions. The biggest disadvantage of this process is its cost for machinery and its upkeep.

Hot extrusion temperature for various metals[1]

Material Temperature [°C (°F)]

Magnesium 350-450 (650-850)

Aluminium 350-500 (650-900)

Copper 600-1100 (1200-2000)

Steel 1200-1300 (2200-2400)

Titanium 700-1200 (1300-2100)

Nickel 1000-1200 (1900-2200)

Refractory alloys up to 2000 (4000)

Cold Extrusion Cold extrusion is done at room temperature

or near room temperature. The advantages of this over hot extrusion are the lack of oxidation, higher strength due to cold working , closer tolerances, good surface finish.

Materials that are commonly cold extruded include: lead, tin, aluminum, copper, zirconium, titanium, molybdenum, beryllium, vanadium, niobium, and steel.

Examples of products produced by this process are: collapsible tubes, fire extinguisher cases, shock absorber cylinders, automotive pistons, and gear blanks.

Warm Extrusion

Warm extrusion is done above room temperature, but below the recrystallization temperature of the material the temperatures ranges from 800 to 1800 °F (424 to 975 °C). It is usually used to achieve the proper balance of required forces, ductility and final extrusion properties .

Hydrostatic extrusion In the hydrostatic extrusion process the billet is completely

surrounded by a pressurized liquid, except where the billet contacts the die. This process can be done hot, warm, or cold, however the temperature is limited by the stability of the fluid used. The process must be carried out in a sealed cylinder to contain the hydrostatic medium. The fluid can be pressurized two ways:

Constant-rate extrusion: A ram or plunger is used to pressurize the fluid inside the container.

Constant-pressure extrusion: A pump is used, possibly with a pressure intensifier, to pressurize the fluid, which is then pumped to the container.

Products

Components made from aluminium extrusions are some of the commonest parts used in the manufacturing industry and can be seen in many day to day applications, from cars to aircraft and office equipment and etc.

Once extruded, a component can be cut, machined, punched or formed prior to the final surface finishing as required.

Products

wire production :Hot extrusion has many advantages over the other techniques for wire production.

Extruded Angles : Extruded angles are another popular item to be made of the extrusion process.

Here also the most popular item is aluminium followed by plastics and steel.

Products Extruded Pipes of

Plastic : A very popular material for extruding Pipes is the thermoplastic material.

Plastic material are first fed into the hopper and then to a rotating screw. It is here they are heated up and subsequently softened. Consequently the pipe that is now softened up is forcibly passed through a die in continuous stream to form a pipe. Then they are passed through a cooling chamber and cut according to the required length

Products Materials used in the Extruded Plastic Pipes Some of the popular plastic materials used includes: PVC

(The following image show clear extruded PVC pipes.)

Polyethylene HDPE Polypropylene

Products

Extruded Sheets A diverse range of

material is used in the manufacturing of extruded sheets. Ranging from different types of plastic materials to even rubber to metals. In metals aluminium is the most preferred option. Extruded aluminium sheets find application in construction activities.

Products Extruded Aluminium

Tubes Extruded aluminium tubes are a popular choice for application in many of the industries, there is a considerable usage of it in the automobile industry examples include in air cooling systems, car radiators, condensers, air conditioning, heating devices and in tubes for oil as well. Another popular application lies in the building sector where it is used in frames for shades.

Extruded Copper Tubes Extruded Copper tubes have an extensive use in many industries they are common in plumbing systems, as fire sprinkler systems, airconditioning and refrigerator industry etc.

Products

Products

Advantages An important feature of extrusion is that all

three principal stresses are compressive, in contrast to most other deformation processes.

Material Yield Saving Complex extruded shapes close to the finished

product profile can be produced in one operation, which enables very significant savings to be made on raw material usage.

Short Runs The extrusion process is well suited to short

production runs. The tooling costs are low and the production

change over down time between different sections is very short. Therefore either long or short production runs are both practical and economical using the extrusion process.

Advantages Low Tooling Costs The tooling charges for a set of extrusion dies

are very modest when compared to the set up costs for rolling, casting and forging.

Design and Change Flexibility Modifications to die sizes can normally be

achieved rapidly in-house at minimal cost and, offering design flexibility without commitment to large volumes of material.

Quality The extrusion process produces inherently

sound sections with a good surface finish. Extruded sections do not suffer from "rolled in scale".

Advantages Production Lead Times Extrusion production lead times can be

relatively short. Overall production times and costs can be significantly reduced, compared to other methods of manufacture by reducing or even eliminating machining and finishing operations.

Many types of material Complex integral shapes Simple fastening and assembly Easy fabrication Precision tolerances Small quantities, down to 2 metric tons or

even less, can be economically produced

Disadvantages

Design possibilities severely limited because of linear nature of process :

Limited complexity of parts Uniform cross-sectional shape only

The biggest disadvantage of Hot Extrusion is its cost for machinery and its upkeep.

References

Aluminum Extrusion Technology ; Pradip K. Saha ; The Material Information Society

Internet