a) Metal mold casting
1) Gravity or permanent mold casing
2) Die casting:
a) Hot chamber process
b) Cold chamber process
2) Slush casting
3) Pressed or caritas casting
b) Non metallic mold casting
1) Centrifugal casting
a) True centrifugal casting
b) Semi centrifugal casting
c) Centrifuge casting
2) Carbon dioxide molding
3) Invest mold casting or lost wax process
4) Shell molding
5) Plaster molding
6) Mercast process
c) Continuous casting
a) Reciprocating mold
b) Draw casting
c) Stationary mold
d) Direct sheet casting
Gravity die or permanent mold casting
1) The process is known as gravity die casting in England and as permanent mold casting in USA.
2) Molten metal is poured into the mold under gravity only; no external pressure is applied to force the liquid metal into the mold cavity. However the liquid metal is solidifies under pressure of metal in the riser.
Application:
a) Carburetor bodies
b) Hydraulic break cylinders
c) Refrigeration castings
d) Washing machines gears and gear covers
e) Connecting rods and automotive pistons
f) Oil pump bodies
g) Typewriter segments
h) Aircraft and missile castings
Pressures die casting
1) Unlike gravity die casting, in pressure die casting molten metal is forced into the permanent mold (die) cavity under pressure. The pressure is generally obtained by compressed air or hydraulically.
2) The pressure varies from 70 to 5000 kg/cm2 and is maintained while the casting is solidified.
3) In USA this process is called simply “die casting’.
Two general types of molten metal injection mechanism for die casting machines are:
a) Hot chamber
1) Goose neck or air injection type
2) Submerged plunger type
b) Cold chamber
Hot chamber die casting
1) Hot chamber die casting machine is the oldest of die casting machines and is simplest to operate.
2) Depending upon its size, hot chamber die casting machines can produce about 60 or more castings of up to 20 kg each per hour and several hundred castings per hour for single impression castings weighing a few grams.
3) In hot chamber die casting machines the melting point constitutes an integral part of the process.
4) The molten metal posses normal amount of superheat and therefore less pressure is needed to force the liquid metal into the die.
Goose neck or injection type:
1) The cast iron gooseneck is so pivoted that it can be dipped beneath the surface of the molten metal to receive the same when needed. The molten metal fills the cylinder portion and cured passageway of the gooseneck.
2) Gooseneck is then raised and connected to the air line which supplies (air) pressure to force the molten metal into the closed die.
3) Air pressure required for injecting metal into the die is of order 30 to 40 kg / cm2.
4) After casting has solidified, the gooseneck is again dipped beneath the molten metal to receive molten metal again for the next cycle.
Application:
Can produce castings of low melting point metal such as zinc, tin and lead.
Submerged plunger type:
1) Submerged plunger type of hot chamber die casting machine has an injection cylinder which is partly submerged in the pot containing molten metal.
2) A plunger fits the cylinder. The plunger when it is up position, it clears the port in the cylinder and through the port molten metal fills the cylinder.
3) As the plunger moves down, the port gets closed and molten metal is forced through the nozzle (at tip of the gooseneck) into the die. Pressure exerted on the metal is of order of 140 to 200 kg/cm2.
4) When metal has solidified, die is opened and the casting is ejected.
5) The die is then once again closed and locked. The plunger is drawn to the up position and the next cycle thus tends to start.
Application:
Is commonly used to produce zinc, tin, lead and some magnesium castings.
Cold chamber die casting:
1) Melting point is not an integral part of cold chamber die casting machines. Molten metal is brought and poured into the die casting machines with the help of ladle.
2) Molten metal which is poured into the cold chamber die casting machine is at low temperature as compared to that of poured into the hot chamber die casting machine.
3) For this reason, a cold chamber die casting process has to make use of pressure much higher (of order of 200 to 2000 kg/cm2) than those applied in hot chamber process.
4) High pressure tends to increase the fluidity of molten metal possessing relatively low temperature.
5) Lower temperature of molten metal accompanied with higher injection pressure will produce casting of dense structure, sustained dimensional accuracy and free from blow holes.
Applications:
For making castings in aluminum, brass and magnesium.
Centrifugal castings:
1) The essential feature of the centrifugal casting is introduction of liquid metal into a rotating mold. Centrifugal force plays a major role in shaping and feeding of the casting.
2) Centrifugal force produced owing to mold rotation is large compared with normal hydrostatic force.
3) The centrifugal force is utilized in two ways:
a) Centrifugal force is utilized to distribute metal over the outer surface of a mold. Hollow cylinders and other annular shapes are formed in this way.
b) Centrifugal force tends the poured metal and the freezing metal to fly outward, away from the axis of rotation, and this tendency creates high pressure on the metal or casting while it is freezing thereby producing casting of high metallurgical quality.
Centrifugal casting methods.
1) True centrifugal casting
2) Semi-centrifugal casting
3) Centrifuge casting
1) True centrifugal casting:
a) True centrifugal casting is of straight uniform inner diameter and is produced by spinning the mold about its own axis, either vertically or horizontally.
b) They have more or less symmetrical configuration (round, square, hexagonal) on their outer contour and do not need any centre core.
c) A cylindrical mold is made to rotate on its own axis at a speed such that the the metal being poured is thrown to the outer surface of the mold cavity. The metal solidifies in the form of a hollow cylinder. The cylinder wall thickness is controlled by the amount of liquid metal poured.
d) Casting cools and solidifies from outside towards the axis of rotation, thereby providing conditions which set up directional solidification to produce casting free from shrinkage.
e) True centrifugal casting may be produced in metal or sand lined molds, depending largely upon the quantity desired.
a) The de lavoud process:
- Metal molds prove to be economical when larger quantities of castings are required to be produced.
- De lavoud process makes use of metal molds.
- The de lavoud casting machine contains an accurately machined metal molds (die), entirely surrounded by cooling water.
- The machine is mounted on wheels and it can be moved lengthwise on a slightly inclined track.
- At the end of track there is a ladle containing proper quantity of liquid metal which flows through a long pouring spout initially inserted to the far extremity of the mold.
- As pouring proceeds the rotating molds i.e. the casting machine is moved slowly down the track so that the metal is laid progressively along the length of the mold wall following a helical path, control being achieved by synchronizing the rate of pouring mold travel and speed of mold rotation.
- After completion of the pouring the m/c will be at the lower end of it’s track with mold rotating continuously till the pipe has solidified.
- The pipe after his solidified is extracted from the metal mold by inserting pipe puller which expands as it is pulled.
b) Moore casting system:-
- Moore sand span casting system for small production of large cast iron pipes employs rammed and dried sand lining in conjunction with end pouring.
- the mold rotates; it does not move length-wise rather its one end can be raised up or lowered to facilitate progressive filling of the mold with liquid metal.
- Initially one end of the mold is raised so that it becomes inclined. As the pouring starts and continues, the end is gradually lowered till the mold is horizontal, when they pouring stops. St this stage the speed of mold rotation increased and maintained till the casting has solidified. Eventually the mold rotation is stopped and the casting extracted from the mold.
Applications:
a) Bearing for electrical motors and industrial machinery.
b) Cast iron pipes, alloy steel pipes and tubings.
c) Liners for IC engines.
d) Rings, short or long pots and other annular components.
2) Semi-centrifugal casting;
a) Whereas a simple pipe is the result of true centrifugal castings technique. Gear blanks, sheaves and wheels are produced with the help of true centrifugal casting methods.
b) Like true centrifugal castings, semi centrifugal castings also utilize rotation of the mold about its own axis.
c) Unlike true centrifugal castings, a (sand) core is used to form the central cavity (as in the hub of wheel), permitting internal shapes that could not otherwise be formed, thereby partly overcoming some of the limitations of the true centrifugal castings.
d) Semi-centrifugal castings are normally made in vertical machines. The mold cavity is arranged within the mold so that its central axis is vertical and concentric with the axis of rotation.
e) Spinning speed need not be as high as those for true centrifugal castings. A linear speed of the order of 180-200 mpm at the outside edge of the casting is generally recommended.
f) Directional solidification can be obtained by proper gating of the casting, padding and selective chilling.
g) Casting shapes, more complicated than those suitable for true centrifugal castings can be made on semi-centrifugal casting machines.
h) A number of molds stack together; one over the other can be fed by a common central sprue in order to produce more than one casting at a time.
3) Centrifuge casting:
a) Parts not symmetrical about any axis of rotation may be cast in a group of molds arranged in a circle to balanced each other. The axis of mold and that of rotation do not coincide with each other. The set up is revolved the centre of the circle induce pressure on the metal in the molds.
b) Casting shapes imposes no special limitation in this process and an almost unlimited variety of smaller shapes can be cast.
c) Mold cavities are fed by a central sprue under the action of centrifugal forces.
d) When castings in multiple layers one above the others are produced in one mold, the method is called stack molding. It is used for producing valve bodies, valve bonnets, plug, yokes, pillow blocks and a wide variety of other industrial castings in large quantities.
4) Carbon dioxide molding:
a) The highly flowable mixture of pure dry silica sand sodium silicate binder is rammed or blown into the mold or core-box.
b) Carbon dioxide gas at a pressure of about 1.5 kg/cm2 is diffused through the mixture (of sand and sodium silicate) to initiate hardening reaction which takes from a few seconds to few minutes depending upon the size of core or mold.
c) Passage of carbon dioxide through the sand containing sodium silicate produces carbonic acid in the aqueous solution, this causes a rise in the ratio of SiO2 – Na2O and the formation of colloidal silica gel, which hardens and forms a bond between the sand grains. This reaction is represented by the following equation:
Na2SiO3(Sodium Silicate) + CO2 → NaCO3 + SiO2(Silica Gel)
d) This reaction proceeds rapidly in the early stages and the compression strength of a given sand mixture reaches its maximum at some critical amount of gases passed. If gassing is continued beyond its critical point the strength of the bond is impaired
Application:
a. CO2 process has been widely used for making larger cores and in jobbing foundries for heavy ferrous castings
b. The process is particularly useful for heavy or thick walled steel castings
c. Besides steel, CO2 process is also employed for producing cores for iron, aluminum and copper base alloy casting
- Investment Mold Casting:
Refractory slurry is cast around a pattern formed from wax, plastic or frozen mercury; when slurry hardens pattern is melted out and mold is baked. When poured metal solidifies, this is broken away to take out the casting.
Application:
a. Hollow turbine blades
b. Points for sewing machines, jacks, rifles, beer barrels and burner noxxles
c. Impeller and other pump and valve components in stainless steel and non-ferrous alloys, wave guides, die-inserts and parts of gun mechanisms.
d. Milling cutter and other type of tools.
e. In dentistry and surgical implants
f. For making jeweler and art castings
g. Parts of gas turbine used in locomotive propulsion.
- Shell Molding:
a) A metal pattern having the profile of the required casting is heated to 1800C – 2500C in an oven maintained at 3000C – 4000C
b) Pattern after being heated is taken out of the oven and sprayed with a solution of a lubricating agent or relax agent containing silicon. It is necessary to prevent the shell from sticking to the metal pattern.
c) Metal pattern (made up of iron or steel) is then turned faced down and clamped over the open end of dumpbox the dump box contains sand resin mixture i.e. 4 to 6 kg of phenol-formaldehyde resin to each 100kg of sand
d) The dump box is inverted so that dry sand resin mixture falls on the face of hot metal pattern.
The resin sand mixture in contact with the pattern gets heated up, the resin softens and fuses to form a soft and uniform shell of about 6mm thickness on the surface of the pattern.
Heat, first causes resin to become sticky, then additional heat cures or hardens it.
e) As the dump box is turned to its original position, excess sand resin mixture falls back into the dump box leaving the shell adhering closely to the pattern.
f) The pattern along with the shell adhering to sticks passed directly into an oven for 1 to 2 minutes when the resin in the resin sand mixture cures and the shell acquires rigidity.
g) The shell is then stripped from the pattern plate with the help of the ejection pins which are integral part of the metal pattern.
h) After the shell so obtained have cooled, two mating shells are securely fastened together to form a complete mold.
i) Shell mold may be poured either by keeping the parting surface vertical or horizontal.
Application:
1) Automotive rocker arms and valves.
2) A small hydraulic casting and stainless steel and copper alloys.
3) Small pipes, camshaft, bushing, valve bodies, spacer, brackets, manifolds, bearing caps, shafts and gears.
6) Plaster mold casting
a) Plaster of Paris (gypsum or CaSO4.1/2 H2O) is used for making plaster molds
b) Prepared plaster slurry is poured over the pattern on which parting agent has already been applied.
c) After the slurry has been poured over the pattern, it develops an initial set in a few minutes and the pattern can be withdrawn from the mold.
d) Mold halves are placed in an oven held at 400 to 800 f until all free and combined moisture gets removed. It is necessary because molds of (metal casting) plaster posses very low permeability. Heating in the oven may require 20 or more hours
e) While the mold halves are still hot from the drying oven, they are quickly assembled, plaster cores are set if required.
f) Assembled mold is poured with molten metal immediately so as to minimize the absorption of the moisture from the atmosphere.
g) Casting, after they have solidified is best removed from the molds with a high pressure water jet.
Application:
Aircraft parts propeller
Small gears ornaments
Plumbing fixture fitting pressure cast aluminium match plates
Cams core boxes
Handles core dryers and other high grade aluminium castings.
7) Antioch process:
a) It is an alternative plaster technique in which a high permeability is achieved with a more consistent finish than is usual with ordinary plaster mold casting.
b) Antioch process is planted and can be operated only under license.
c) A slurry is prepared by adding 50 parts water to 100 parts of dry ingredients consisting of 50% silicate , Portland cement and magnesium oxide.
d) The slurry is poured around the pattern into separate cope and drag flasks and after the initial set the pattern are withdrawn from the mold. In about 7 minutes, the plaster mold develops a set strength of about 5kg/cm2 in compression.
e) Following air drying for about 6 hours , the molds are assembled and placed in a steam autoclave (the special additional feature of Antioch process) at about 2 atmospheric pressure and this develops more permeable structure in the mold.
f) After being auto-clave , molds are dried in a air for about 12 hours and finally in an oven for 12 to 20 hours at 450 f in order to drive off the free and combined water.
g) At this stage molds have about 25 to 50 AFS permeability and they are ready to poured.
Applications:
Is well suited for intricately shaped castings especially of aluminum, where surface smoothness and dimensional accuracy is primary consideration.
8) Slush casting:
In certain castings, such as statuary works, only the external features of the castings are important(for their esthetic value). Since such castings are not designed for engineering use, uniformly of wall thickness is not an important consideration. A core is not necessary-instead the mold is filled with molten metal and held stationary until a thin skin of solid metal freezes against the mold walls; the mold is then inverted and the unfrozen metal ‘bled’ from the casting. The mold must not be inverted too sharply or air will be excluded as molten metal chokes the cavity; a partial vacuum will be formed and the still weak solid skin of the casting will collapse at its weakest point. Also the casting must be made of a relatively pure metal, as most alloys do not form a strong solid skin.
It is used only for a limited amount of art and decorative works.
9) Continuous castings:
a) Round ingots, slabs, square billets and sheets can be cast by a continuous process directly from molten metals.
b) Continuous casting is accomplished by pouring molten metal into (one end of ) a metal mold open at both ends and by keeping it filled at all times. The metal at the lower end of the mold is cooled so that if solidifies and the solid product thus formed is extracted in a continuous length from the lower end.
Applications:
1) Can produce any shape of uniform cross-section such as rectangular, square, hexagonal, fluted, gear toothed, solid or hollow.
2) Production of blooms, billets, slabs and sheets.
3) Bushing and pump gears.
4) Copper (wire) bar
10) Squeeze casting:
The basic principle of squeeze casting is displacement of an accurately metered dose of liquid aluminum alloy, in a closable , preheated die, where the punch (or copper die) continues to exert a load( between 31-108 Mn/m2) on the metal, throughout solidification. This can be said to take three stages,
a) Delivery of the dose into the open lower die
b) Rapid descent of punch into the metal pool and solidification under load
c) Component ejection and die maintenance.
1 comment:
I appreciate to your awesome blog posting! your blog content is very informative. Now, I have added your blog to my favorites and will look forward for more updates. Great Job, Keep it up.
Post a Comment