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EXTRUSION PROCESS. In extrusion, the material is compressed in a chamber and the deformed material is forced to flow through the die . The die opening corresponds to the cross section of required product. Cylindrical bars, hollow tubes as also shapes of irregular c.s are made.
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EXTRUSION PROCESS • In extrusion, the material is compressed in a chamber and the deformed material is forced to flow through the die. • The die opening corresponds to the cross section of required product. • Cylindrical bars, hollow tubes as also shapes of irregular c.s are made.
Extrusion process equipment consists of a cylinder/ container into which the heated metal billet is loaded. • On one end of container,dieplate with necessary opening is fixed. • From other end, plunger/ ram compress the metal billet against the container walls & die plate, thus forcing it to flow through die opening, acquiring shape of opening. • Dummy block which is steel disc about 40 mm thick, is kept btn billet & ram to protect it from heat & pressure.
Pressures applied may range from 35-1000 Mpa. • Pressures for given metal depends on extrusion temperature, reduction in area & extrusion speed. • Materials that can be extrudes are aluminum, copper, steel, magnesium, and plastics, polymers, ceramics, concrete.
Typical products made by extrusion are railingsfor sliding doors, tubing's having various cross sections. • Extruded parts are cut into desired lengths which then become discrete parts such as brackets, gears, coat hangers. • its ability to create very complex cross-sections and work materials that are brittle, because the material only encounters compressive and shear stresses.
The extrusion process is often likened to the squeezing of tooth paste from a tube, as is illustrated :
Extrusion Processes Hot extrusion: • Keeping the processing temperature to above the materials re-crystalline temperature to keep material from work hardening. • Most are done on horizontal hydraulic presses that range from 230 to 11,000 metric tons. • Pressures range from 30 to 700 MPa (4,400 to 100,000 psi) therefore lubrication is required, which can be oil or graphite for lower temperature extrusions, or glass powder for higher temperature extrusions.
Cold extrusion: • Done at room temperature or near room temperature. • Ductile metals like lead, tin, aluminum, copper, zirconium, titanium, molybdenum, beryllium, vanadium, niobium, and steel. • The advantages of this over hot extrusion are improved mechanical properties, high production rate, closer tolerances, good surface finish. • Tools & automobile parts, motor cycles, collapsible tubes, fire extinguisher cases, shock absorber cylinders and gear blanks.
TYPESOFEXTRUSION PROCESSES • Direct extrusion (or forward extrusion), • Indirect extrusion (or backward extrusion), • Impact extrusion, and • Side extrusion.
It is most widely used method, in which starting ram material used is a heated billet. • It is heated to its heated temperature and fed into its forging temperature and fed into machine chamber/ container & pressed with hydraulically operated ram through die. • Flow of metal in forward extrusion is same as that of ram direction.
There is a reusable dummy block between the ram and the billet to keep them separated. • major disadvantage is force required to extrude the billet is greater than that needed in the indirect extrusion process because of the frictional forces introduced by the need for the billet to travel the entire length of the container. • To reduce this friction lubricants are used. • Ex: Oils with graphite powder at low temp.
Advantages: • Complex Integral Shapes • Cost-Effective • Design Flexibility • Simple fastening and assembly • Easy fabrication • Low Tooling Costs • Good Machinability • Precision Tolerances • Shorter Lead Times • Versatility in Joining • Virtually Seamless
In order to completely overcome the friction, backward extrusion process is used. • Ram/ plunger used is hollow, as it presses the billet against back wall of die chamber/ container the metal flows through die. • As the billet does not move inside the chamber there is no friction between them. • Less force is required in this process .
ADVANTAGES: • A 25 to 30% reduction of friction, which allows for extruding larger billets, increasing speed, and an increased ability to extrude smaller cross-sections • There is less of a tendency for extrusions to crack because there is no heat formed from friction • The container liner will last longer due to less wear • The billet is used more uniformly so extrusion defects and coarse grained peripherals zones are less likely.
DEFECTS IN EXTRUSION • Depending on material condition & on process variables, extruded parts can develop several types of defects that can significantly affect their strength, quality. • Principal extrusion defects are; • Surface cracking (c) • Pipe (b) • Internal cracking (a)
Surface Cracking: • If extrusion temp, friction/ speed is too high, surface temperatures rise significantly, and this condition may cause surface cracking & tearing.(fir-tree cracking, speed cracking) • These occur specially in Al, Mg, Zinc alloys. • These defects can be avoided by lowering the billet temperature & extrusion speed.
Pipe: • This type of metal flow pattern tends to draw surface oxides, impurities towards centre of billet which is called as pipe defect/ tail pipe/ fishtailing. • These are obtained because of high friction, maximum temperatures at outside of billet. • Defects as much as one third of length of extruded product, to be cut off as scrap.
Internal / Centre/ Chevron cracking: • The centre of the extruded product can develop cracks called center-burst, arrow head fracture. • These are attributed to a state of hydrostatic tensile stress at the centre line in the deformation zone in the die.
It is another way to extrude aluminum profiles. • In this process, the chamber is first filled with a fluid which transmits the pressure to the billet. • This billet is then extruded using the die. There is no question of friction along the container -walls. • the main difference is that in case of Hydrostatic extrusion there is no direct contact between the container and the billet.
It is usually carried at room temperature, using vegetable oils as the fluid. • For elevated temperatures waxes, polymers, glass are used. • Brittle materials like gray c.i are extruded successfully by this method, because hydrostatic pressure increases the ductility of material.
Advantages • Owing to the pressurized fluid, the lubrication is very effective • Extruded product has good surface finish along with dimensional accuracy • As friction is almost absent, it is quite possible to use dies that has very low semi-cone angle. • Minimal friction
Some typical shapes produced are angles, rods, T-profiles or dumbbells. • Other shapes to be produced can single-hollow sections like tubes.
In backward cold extrusion called Impact extrusion, the set up consists of die, punch as shown in fig. • The slug for making the component is kept on die & punch strikes the slug against the die. • Metal is then extruded through the gap between punch & die opposite to movement of punch. • The height of side walls is controlled by amount of metal in slug.
This process is mainly used for making short lengths of hollow shapes like collapsible tubes for housing plates, liquids, spray cans. • Diameter of parts made can approach 150 mm. • For making discrete parts. • performed at higher speeds and shorter strokes than conventional extrusion.
Manufacturing of toothpaste tube Applications: Tooth paste tubes, soft drink cans & shell cases.
In side extrusion the movement of the material is in a direction perpendicular to that of ram motion. • Since the metal flows in sideward direction the force required is very high& hence mostly used in the case of non ferrous metals and highly plastic materials like lead. • Presence of cable produces a seamless tube.
