UNIT-2 MCT - Special casting processes.pptx

UNIT-2 MCT - Special casting processes.pptx

1. Manufacturing Processes Course code: 18PC1ME09 Year & Sem : III B.Tech-Sem1 Academic year: 2020_21 1 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET Unit I- Castings (Special casting processes)

2. Special Casting Processes - Centrifugal, Die &Investment casting - (MT-1, PN Rao , Ch. 6) Special casting processes are more suitable and economical in many applications compared to conventional casting methods. Following are some of the special casting processes: Centrifugal casting Die casting Investment casting 1.0. Centrifugal Casting : (MT-1, PN Rao , Ch. 6.7) In this process the mould is rotated rapidly about its central axis as the metal is poured into it. Because of centrifugal force, a continuous pressure acts on the metal as it solidifies. The slag, oxides and other inclusions , being lighter, gets separated from the metal and segregates towards the center There are three types of centrifugal casting processes: True centrifugal casting Semi centrifugal casting and Centrifuging   2 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

3.   1.1. True Centrifugal Casting: This is normally used for making hollow pipes, tubes, and hollow bushes etc, which are axis-symmetric with a concentric hole . Core is not required for making the concentric holes because of the centrifugal force The axis of rotation is either horizontal, vertical or any angle in between Horizontal axis is used normally for Very long pipes Vertical axis is used for short pieces 3 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET Special Casting Processes – Centrifugal Casting

4. 4 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

5. Method/ Process: (with sand mould) Moulding flask is rammed properly with sand to conform to the outer contour of the pipe . Dry sand cores are placed in the ends for the details like spigot ends or flanged ends. The flask is dynamically balanced to reduce the occurrence of undesirable vibrations during the casting process. The finished flask is mounted in between rollers and the mould is rotated slowly. The molten metal of required quantity is poured into the mould through the movable pouring basin. (The amount of metal poured is based on the thickness of the pipe to be cast) The mould is rotated at the operational speed till the metal solidifies to form the requisite tubing. The mould is then removed and replace by a new mould in the machine. Method/ Process: (with metal mould): Metal moulds can also be used for large scale production A water jacket is provided around the mould for cooling. The machine is mounted on wheels with the pouring ladle with a long spout extended till the end of the pipe to be made. To start, the mould is rotated with the metal delivered at the extreme end of the pipe. The casting machine is moved down the track allowing the metal to be filled all through the length of the pipe The machine is rotated continuously till the pipe is completely solidified. The pipe is extracted from the mould and the cycle is repeated. 5 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET Special Casting Processes – Centrifugal Casting

6. Advantages and Limitations of True Centrifugal Casting: Advantages: Mechanical properties are better than other processes because of inclusions such as slag or oxides get segregated towards the center and can be easily removed by machining. Porosity is eliminated due to the pressure acting on the metal during solidification, giving rise to dense metal. Proper directional solidification can be obtained starting from the mould surface to the center, upto certain thickness of objects No cores are required for making concentric holes in the case of true centrifugal casting. There is no need for gates and runners , hence casting yield increases (almost 100%)   Limitations: Only axi -symmetric jobs with concentric holes are suitable for true centrifugal casting. Equipment is expensive , hence suitable for large scale production 6 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET Special Casting Processes – Centrifugal Casting

7. 1.2. Semi Centrifugal Casting: Used for more complicated jobs than those possible in true centrifugal castings , but of axis-symmetric in nature. It is not necessary to have a central hole (hollow), which requires use of a core. The moulds of sand or metal are rotated about a vertical axis and metal enters the mould through the central pouring basin For large production rates, the moulds can be stacked one over the other , with a common feeding from the central pouring basin. The rotating speeds used are not as high as in the true centrifugal casting. 7 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET Special Casting Processes – Centrifugal Casting

8. 8 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

9. 1.3. Centrifuging: This process is similar to semi centrifugal casting . Used for higher metal pressures during solidification, when casting shapes are not axi -symmetric . Suitable for small jobs of any shape . A number of such small jobs are joined together by means of radial runners , with a central sprue on a revolving table (ref fig. 6.17) Jobs are uniformly placed on the table around the periphery to balance their masses 9 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET Special Casting Processes – Centrifugal Casting

10. 10 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

11. 2.0. Die Casting : (MT-1, PN Rao , Ch. 6.4) Die casting is done by injecting molten metal into a metallic die at high pressures . The metal is forced in with pressure, hence called pressure die casting . The mould is a reusable metallic die , as used in permanent mould casting. Due to high pressure used, narrow sections, complex shapes and fine surface details can be produced. 11 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET Special Casting Processes – Centrifugal Casting

12. Special Casting Proce sses - Die Casting The die consists of two parts - Stationary half or cover die , fixed to the die casting machine Moving half or ejectors die which is moved out for the extraction of the casting. Operations/ Process: The cycle starts with the two parts of the die apart. The lubricant is sprayed on the die cavity manually or by auto lubrication system, to avoid sticking of the casting to the die. Metal is injected into the die , of required amount Cooling channels are provided for quick extraction of heat of the molten metal, and maintain proper die temperature. The die is opened and casting is ejected, after the casting is solidified under pressure. Ejector pins are provided in the die for easy removal of the casting from the die. ------------------------------------------------------------------------------------------------------------------------------------------------------- Die Casting is generally of two types: Hot chamber die casting Cold chamber die casting 12 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

13. Special Casting Proce sses - Die Casting 2.1. Hot-chamber Diecasting Process: Machine construction & operation: A goose neck , made of grey alloy or ductile iron , is used for pumping the liquid metal into the die cavity. The goose neck is submerged in the holding furnace with the molten metal. A hydraulically operated plunger made of alloy cast iron , moves up in the goose neck to uncover the entry port, for the entry of liquid metal into the gooseneck. The plunger then develops pressure for forcing the metal into the die cavity. A nozzle at the end of the gooseneck is kept in close contact with the sprue located in the cover die. 13 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

14. Special Casting Proce sses - Die Casting Operation sequence: 14 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

15. Special Casting Proce sses - Die Casting Operation sequence of the Hot chamber diecasting : The cycle starts with the closing of the die , when plunger is in the highest position, in the gooseneck. Plunger moves down to force the metal in the gooseneck to be injected into the die cavity. Metal is held at the same pressure till the metal is solidified. The die is opened, and any core, if present is retracted. The plunger moves back, returning the unused liquid metal to furnace through the gooseneck. The casting is ejected from the die. Application : The hot chamber process is used mostly for low melting temperature alloys - zinc, lead, and tin 15 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

16. Special Casting Proce sses - Die Casting 2.2. Cold-chamber Process: Due to high melting temperatures, the materials like aluminium , brass etc, it is difficult to be used in hot-chamber process. In the cold-chamber process , the molten metal is poured with a ladle into the shot chamber for every shot (this process reduces the contact time between the liquid metal and the shot chamber) Operating sequence of Cold-chamber die-casting process: The operation sequence is similar to hot chamber process. 16 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

17. Special Casting Proce sses - Die Casting Operating sequence of Cold-chamber die-casting process: Lubricating die, closing the die and pouring molten metal with a ladle . Operation starts with spraying of the lubricant in the die cavity and closing the die. Molten metal is poured into the shot chamber with a ladle, either manually or with auto ladle (Robotic device is one, which automatically scoops the metal from the furnace and pours it into the shot chamber at an exact instance of time as required). The volume of metal can be controlled with the auto ladle and improves the quality of casting. The plunger forces the metal into the die cavity and maintains the pressure till it solidifies. The die is opened The casting is ejected . Simultaneously the plunger returns to its original position .   Disadvantage of the cold-chamber process: Process takes longer cycle time compared to hot-chamber process. The metal will lose the super heat while pouring it into the machine with ladle, and may cause defects such as cold shuts . 17 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

18. Special Casting Proce sses - Die Casting Construction of a Cold-chamber machine: 18 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

19. Special Casting Proce sses - Die Casting T he Die casting die consists of two parts- a Cover die and an Ejector die - The cover die consists of the sprue (also called biscuit ), runner and gates and is connected with the shot chamber . The ejector pins moves through the moving die to eject the castings from the ejector die. The ejector pins leaves a mark on the casting, hence should be placed properly T he cores are metallic and of two types- Fixed cores and moving cores Fixed cores : are fixed to the die halves, and are parallel to the die movement Moving cores : are not parallel with the die movement, hence to be removed before the casting is ejected from the die. Water circulation : The cycle times being very low, the dies are provided with water cooling, to maintain uniform temperature in the die and water is circulated through the hot regions of the die Overflows: sometimes overflows are provided in the parting plane for the first metal (usually cold die), to provide heat input to the die initially (being costly, this is to be avoided for small components) Ejector pins may be placed in the overflows Flash : some excess metal may be forced into the parting plane is termed as flash. - Flashes or overflows should be removed by trimming, either manually or using with dies which are similar to the sheet metal dies 19 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

20. Special Casting Proce sses - Die Casting Die material : hot working tool steels are used for dies, die inserts and cores Zinc alloys : AISI P20 is used for low volume and H13 for high volume Aluminium and Magnesium alloys : H13 and H11 are used Copper alloys : H21, H20 and H22   Advantages of Die-casting: Fairly complex castings can be made with the use of movable cores , more complex than permanent mould casting Very small thicknesses can be easily filled due to high pressure. High production rates due to automation (typically about 200 pieces per hour ) Good surface finish can be achieved due to use of metallic dies Closer dimensional tolerances (of +0.08 mm ), even for small dimensions can be achieved over sand casting Long life of the dies (over 3,00,000 pieces for Zinc alloys and 1,50,000 for aluminum alloys) Better mechanical properties over sand castings, because of fine grained surface during solidification Die Inserts can be easily replaced Very economical for large scale production 20 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

21. Special Casting Proce sses - Die Casting Limitations of Die-casting: The maximum size is limited (normal sizes are <4 kgs , with a max of 15 kgs ) due to machine/ die size limitations Not suitable for all materials , due to limitations of the die materials. (normally used for Zinc, Aluminium , Magnesium and Copper alloys) The air inside the cavity gets trapped inside the casting The dies and machines are very expensive (hence economical only in large quantities of production) Applications of Die casting: Carburetors, Crank cases, magnetos and other parts of 2 wheelers, Wheels & other decorative items of automobiles Rotors of electrical motors etc. 21 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

22. Special Casting Proce sses - Investment Casting 3.0. Investment Casting : (MT-1, PN Rao , Ch. 6.2) The mould is prepared around an expendable pattern. Also called as “Lost wax casting”. The steps involved are as follows: Preparation of the pattern: Solidification of pattern : Assembly of patterns: Mould/Shell coating: Shell/ Mould completion: Pattern removal: Pouring: 22 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

23. Special Casting Proce sses - Investment Casting The steps involved are as follows: Preparation of the pattern: Solidification of pattern : Assembly of patterns: Mould/Shell coating: Shell/ Mould completion: Pattern removal: Pouring: Shake out Casting separation & finishing 23 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

24. Special Casting Proce sses - Investment Casting Preparation of the pattern : molten wax, used as pattern material is injected under pressure of about 2.5 MPa into a metallic die with the cavity of the casting to be made. Solidification of pattern : The wax when allowed to solidify produces the pattern. Assembly of patterns : a cluster of patters are attached to the gating system by applying heat. Mould/Shell coating : the assembly is dipped in a slurry of fine ceramic materials (suspended in a liquid such as Ethyl Silicate or Sodium Silicate ). The mould is allowed to drain off the excess liquid. Shell/ Mould completion : Dry refractory grains (fused silica or zircon ) are stuccoed on the assembly and shell is cured. (stucco- coating with a finish coating) . The process of ceramic refractory coating and stuccoing is repeated to reach a shell thickness of 6 to 15 mm . (the shell thickness depends on the casting shape and mass, type of ceramic and the binder used) Pattern removal : wax patterns along with runner are removed by heating and draining through the sprue , by inverting the mould. - Wax still remaining in the shell is removed by dissolving with the help of hot vapour of solvents like trichloro ethylene . - The mould/ shell is preheated to a temperature of 100 to 1000 o C , depending on the size, complexity and the metal of the casting. (this is done to remove the traces of wax left over and permit proper filling of the intricate mould section) Pouring : Molten metal is poured into the evacuated mould under gravity, or under slight pressure, depending on the type of the casting 24 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

25. Special Casting Proce sses - Investment Casting Other pattern material: The other pattern materials are Plastics and Mercury in place of wax a) Mercury: in this process “ Mercast ”, mercury is kept under -57 o C where the mercury is frozen . - The mould preparation is to be done under -38 o C . Advantage : it does not expand when changed from solid to liquid state. Disadvantage : Keeping the pattern at a low temperature is difficult. Advantages of Investment Casting : Complex shapes can be made , which are difficult in other methods, as the pattern is withdrawn by melting Very fine details and thin sections can be produced (as the mould is heated before pouring) Very close tolerances and better surface finish can be produced . Castings produced are ready for use with little or no machining (useful for hard to machine materials such as Nimonic alloys- Ni, Cr, Ti and Al) Mechanical properties can be controlled by controlling the grain size, grain orientation and directional solidification. Dimensions across would not vary, since there is no parting line . Limitations of Investment Casting : Limited by the size and mass of the casting (max of 5 kgs ) More expensive because of large manual labour involved in the preparation of pattern and the mould.   Applications: complex machinery parts, blades and other components for turbine engines , jewelry , and dental fixtures etc 25 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

26. Special Casting Proce sses - Investment Casting Limitations of Investment Casting : Limited by the size and mass of the casting (max of 5 kgs ) More expensive because of large manual labour involved in the preparation of pattern and the mould. Applications: complex machinery parts, blades and other components for turbine engines , jewelry , and dental fixtures etc 26 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET

27. 27 Dr.M.S.Sreenivasa Rao,Asst. Professor- MED, VNRVJIET Thank You