2-Hot
Extrusion
New Definition
(Manufacturing Method)
(i)Direct
Hot Extrusion (Forward extrusion)
This is the most widely used method. A hot
billet is placed in the container and the forced through the die with the help
of pressure by a hydraulic driven ram. The extruded metal comes out of the die
opening. In this process, the flow of metal through the die is in the same
direction as the movement of the ram. The length of the extruded part will
depend on the sin of the billet and corss-section of die. Direct extrusion is
shown in Fig. 2.36
(ii)
Indirect Extrusion
For this
type of extrusion, the ram used is hollow and the die is mounted over the bore
of the rain. In this process, the billet remains stationary, while the die is
pushed into the billet by hollow ram. The metal flows in the direction opposite
to the movement of the ram (Fig. 2.37). Indirect extrusion does not require as
much force as direct extrusion because no force is required to move the billet
inside the chamber.
(iii)
Backward Extrusion
This is another indirect extrusion method used in
manufacturing hollon sections as shown in Fig 2.38. in direct and indirect
extrusion methods the ram is of the same diameter as the bore of the container,
where as in backward extrusion the ram is smaller in diameter than the
container. In this process, the metal is extruded through the gap bemeen the
ram as the container.
Advantages
(I) It is a very fast process.
(ii) Materials and shapes that are difficult by rolling can easily extruded.
(iii) Close tolerances can be achieved.
(iv) The mechanical properties of the products are superior than obtained by rolling process.
(v) Products of complex shapes can be easily extruded.
(I) It is a very fast process.
(ii) Materials and shapes that are difficult by rolling can easily extruded.
(iii) Close tolerances can be achieved.
(iv) The mechanical properties of the products are superior than obtained by rolling process.
(v) Products of complex shapes can be easily extruded.
There is no previous definition.
3- Shell Molding (Shell Moulding)
New Definition (Manufacturing Method) (Better)
It is a modification of the sand mould
process. In this process, the mould is made up of mixture of dried silica sand
and phenolic resin. formed into a thin half- mould shells which are clamped
together for pouring metal.
The sand is
first mixed with either urea or phenol formaldehyde resin in a miller. Metal
pattern is heated to 205 to 230°C in an oven and sprayed with silicon grease
and kept on the top of the dump box. The dump box contains sand mixed with
thermoplastic resin. The box is inverted, causing the sand mix to fall on the
hot pattern. The resin melts and flow in between the grains of sand, acting as
a bond. After 30 seconds, a hard layer of sand is formed over the pattern. Then
the dump box is inverted back to its original position. The pattern with a thin
shell is cured for two minutes at 315°C. The shell is finally removed from the
pattern by ejector pins. The two shells are clamped together to form the mould
and placed in the flask with backing sand.
Advantages
1.High
dimensional accuracy and good surface finish.
2. The chances of blow holes or pockets are reduced since the shells are
highly permeable.
3. Thin wall sections can be produced.
4. Shells can be stored for longtime.
Disadvantages
I. The metal patterns are costly than wood pattern.
2. Resin is an expensive binder.
3. Specialized equipments are to
be used.
Applications
Cylinders of IC engines, automobile
transmission parts, chain scat brackets, small crank shafts.
(K Varaprasad Rao, Manufacturing Science And Technology - Manufacturing Processess And Machine Tools, pp.31-32)
Previous Definition
It is a process in which, a thin mold is made around a heated metallic pattern plate. The moulding material is a mixture of dry, fine silica sand (clay content should be kept very low), and 3-8% of a thermosetting resin like phenol formaldehyde or ureaformaldehyde. Conventional dry mixing techniques are used for obtaining the moulding mixture. Specially prepared resin coated sands are also used.
When the moulding mixture drops on to the pattern plate, which is heated to a temperature of 35 to 700 F, a shell of about 6 mm thickness is formed. In order to cure the shell completely, it must be heated to 440 to 650 F for about 1-3 minutes. The shell is then released from the pattern plate by ejector pins. To prevent sticking of the baked shell to the pattern plate, a silicone release agent is applied to the latter before the moulding mixture drops on to it.
Shell moulding is suitable for mass production of thin walled, grey cast iron (and aluminum alloy) castings having a maximum weight between 35 and 45 pounds. However, castings weighing up to 1000 pounds can be made by shell moulding on an individual basis. The advantages of shell moulding include good surface finish, fewer restrictions on casting design, besides rendering itself suitable for automation.
(Product Design and Manufacturing, A. K. Chitale,R. C. Gupta, p.90)
Previous Definition
It is a process in which, a thin mold is made around a heated metallic pattern plate. The moulding material is a mixture of dry, fine silica sand (clay content should be kept very low), and 3-8% of a thermosetting resin like phenol formaldehyde or ureaformaldehyde. Conventional dry mixing techniques are used for obtaining the moulding mixture. Specially prepared resin coated sands are also used.
When the moulding mixture drops on to the pattern plate, which is heated to a temperature of 35 to 700 F, a shell of about 6 mm thickness is formed. In order to cure the shell completely, it must be heated to 440 to 650 F for about 1-3 minutes. The shell is then released from the pattern plate by ejector pins. To prevent sticking of the baked shell to the pattern plate, a silicone release agent is applied to the latter before the moulding mixture drops on to it.
Shell moulding is suitable for mass production of thin walled, grey cast iron (and aluminum alloy) castings having a maximum weight between 35 and 45 pounds. However, castings weighing up to 1000 pounds can be made by shell moulding on an individual basis. The advantages of shell moulding include good surface finish, fewer restrictions on casting design, besides rendering itself suitable for automation.
(Product Design and Manufacturing, A. K. Chitale,R. C. Gupta, p.90)
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