Basic Manufacturing Process - 5.2

Q.6    What are rolling defects? Explain in brief.                                         (AKTU - 2001, 02, 03, 04)
Ans.    Some significant rolling defects are summarized below :

1. Surface Defects: These defects result from impurities and inclusions in the material, scale, dirt, rust, roll marks and other factors related to pre-treatments and working of the metal.
2. Structured Defects : These are the defects which distort or affect the integrity of the rolled objects.
3. Edge Cracks : These cracks occur in plates and slabs due to either low ductility of material or uneven deformation specially at the edges.
4. Waviness : It is caused by bending of rolls. Since the edges elongate more than the centre and are restrained from expanding freely, they tend to buckle.
5. Alligatering : It results from non-homogeneous deformation of the material during rolling or from defects in the original cast ingot. The job splits along a horizontal plane on exit side from the rolls.

Q.7    Define the operation of Drawing.                                                                  (AKTU - 2008-09)
Ans.    Drawing: -

        Drawing is a metalworking process which uses tensile forces to stretch metal. It is broken up into two types: sheet metal drawing and wire, bar, and tube drawing. The specific definition for sheet metal drawing is that it involves plastic deformation over a curved axis. For wire, bar, and, tube drawing the starting stock is drawn through a die to reduce its diameter and increase its length. Drawing is usually done at room temperature, thus classified a cold working process, however it may be performed at elevated temperatures to hot work large wires, rods or hollow sections in order to reduce forces.
    Wire drawing is a metalworking process used to reduce the diameter of a wire by pulling the wire through a single, or series of, drawing die(s). There are many applications for wire drawing, including electrical wiring, cables, tension-loaded structural components, springs, paper clips, spokes for wheels, and stringed musical instruments. Although similar in process, drawing is different from extrusion, because in drawing the wire is pulled, rather than pushed, through the die. Drawing is usually performed at room temperature, thus classified a cold working process, but it may be performed at elevated temperatures for large wires to reduce forces. Wires can also be drawn into different shapes, although this is much more difficult than diameter reductions. More recently drawing has been used with molten glass to produce high quality optical fibers.
Tube Drawing: -
    To produce tubes, three methods are generally employed
(i)     Tube sinking        (ii) Tube drawing with plug       (iii) Tube drawing with moving mandrel.
    In tube drawing operation, the diameter of the tube is reduced to the desired shape. Tube sinking is employed when the outside diameter of a tube has to be reduced.
(i) Tube Sinking: -
    It is not generally preferred because no support is provided on the inner surface of the tube, consequently wall thickness may increase slightly.
(ii) Tube Drawing With a Plug: -
    Due to proper support provided both at inner and outer surfaces of the tube, better dimensional accuracy are obtained. Plug used in this operation may be cylindrical or conical shape and may be either fixed or floating type.
(iii) Tube Drawing With a Moving Mandrel: -
    In this methods, a moving mandrel is used. Friction is minimized because of a moving mandrel but mandrel has to be removed by rolling therefore there is a slight increase in the tube diameter.

Q.8    What is extrusion? Differentiate between forward and backward extrusion process with suitable sketches.                                                                                                     (AKTU - 2012 - 13)
Related Questions -
Q.    Define with neat sketch, the basic working principle of extrusion. Describe its applications in industry. Name the four extruded products.                                                              (AKTU - 2008-09)
Q.    What do you understand by Extrusion process ? What are the items made by this process ? Explain in detail giving suitable examples.                                                               (AKTU - 2010-11)
Ans.    Extrusion: -

        Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section. The two main advantages of this process over other manufacturing processes is its ability to create very complex cross-sections and work materials that are brittle, because the material only encounters compressive and shear stresses. It also forms finished parts with an excellent surface finish.
    Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). The extrusion process can be done with the material hot or cold.
Commonly extruded materials include metals, polymers, ceramics, concrete and foodstuffs.
Process: -
    The process begins by heating the stock material. It is then loaded into the container in the press. A dummy block is placed behind it where the ram then presses on the material to push it out of the die. Afterward the extrusion is stretched in order to straighten it. If better properties are required then it may be heat treated or cold worked.
    The extrusion ratio is defined as the starting cross-sectional area divided by the cross-sectional area of the final extrusion. One of the main advantages of the extrusion process is that its ratio can be very large while still producing quality parts.
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.
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, and fast extrusion speeds if the material is subject to hot shortness.
    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.
Types of Extrusion: -
    Extrusion can be classified as described below :
(i) Direct (or Forward) Extrusion: -
    This method employs a power operated ram and a container into which the metal is kept for confinement. A dummy block is used between ram and hot metal. Because of ram pressure, metal first fills the cylindrical shape and then is forced out through the die opening until a small amount is left in the container. Finally it is sawed off next to the die and butt end is removed.    
(ii) Indirect (or Backward) Extrusion: -
    This process is similar to direct extrusion except that the extended metal is forced out through a hollow ram. No friction works between the metal billet and the container walls since the billet does not move the container.
    In comparison of direct extrusion, less force is required in indirect extrusion but the equipment used in complicated in order to accommodate the passage of extruded shape through the centre of hollow ram.
(iii) Tube Extrusion: -
It makes use of a mandrel to shape the inside of the tube. Hot billet is placed in the container and the die containing the mandrel is forced through the billet. Then the ram moves forward and extrudes the metal through the die.
(iv) Impact Extrusion: -
    This method is chiefly used for making small workpieces from ductile materials. The material is placed in position into a blind die and a ram with clearance forced into the die making the metal to flow plastically around the punch. With the help of this process, collapsible medicine tube and tooth paste tubes are produced. This method is restricted to soft metals as lead, aluminium, copper, tin, etc.
(v) Hydrostatic Extrusion: -
    In this process, the container is filled with a fluid which transmits pressure to the billet which is then extruded through the die. No friction acts along the walls of the container. Due to pressurised fluid, lubrication is very effective and as a result extruded product has good surface finish and dimensional accuracy.
Applications: -
    This process is generally used to produce automotive trim, window frame members, cans, railings, tubes, structural parts, L sections, I beam sections, T-sections, etc.

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