Expendable molds: a separate mold for each pour, rather low production rates

Transcription

1 MULTIPLE-USE-MOLD CASTING PROCESSES

2 13.1 INTRODUCTION Expendable molds: a separate mold for each pour, rather low production rates Multiple-use-molds overcome many of these limitations Molds, generally made from metal Restricted to lower-melting-point nonferrous metals Part size is often limited Dies or molds can be rather costly 2

3 13.2 PERMANENT-MOLD MOLD CASTING Also called gravity die casting a reusable mold: gray cast iron, alloy cast iron, steel, bronze, graphite Molds are usually made in segments Preheating, refractory coating, mold assembling Heat from previous cast is usually sufficient to maintain mold temperature, the process is immediately repeated 3

4 13.2 PERMANENT-MOLD MOLD CASTING 4

5 13.2 PERMANENT-MOLD MOLD CASTING Numerous advantages: 1) Near-net shapes (little finish machining) 2) Reusable mold: good surface finish, dimensional accuracy 3) Directional solidification 4) Faster cooling rates: a fine grain structure, reduced porosity, & high-strength products Cores: sand or plaster or metal 5

6 13.2 PERMANENT-MOLD MOLD CASTING On the negative side: 1. Limited to lower-melting-point alloys 2. High mold costs: low production runs 3. useful life of mold: set by molten metal erosion or thermal fatigue products of steel or cast iron: mold life is extremely short 6

7 13.2 PERMANENT-MOLD MOLD CASTING actual mold life will depend on: 1. Alloy being cast: The higher the melting point, the shorter the mold life. 2. Mold material: Gray cast iron has the best resistance to thermal fatigue and machines easily 3. Pouring temperature: Higher pouring temperatures reduce mold life 7

8 13.2 PERMANENT-MOLD MOLD CASTING 4. Mold temperature: too low temperature: misruns & large temperature differences in the mold too high temperature: excessive cycle times & mold erosion 5. Mold configuration: Differences in section sizes: temperature differences within the molds 8

9 13.2 PERMANENT-MOLD MOLD CASTING Permanent molds: mold cavity, pouring basin, sprue, runners, risers, gates, core supports, & some form of ejection system Molds are usually heated at the beginning of a run, continuous operation maintains the mold at a fairly uniform elevated temperature: t minimized thermal fatigue facilitates metal flow controls the cooling rate of molten metal 9

10 13.2 PERMANENT-MOLD MOLD CASTING Mold temperature rises while casting, a mold-cooling delay before the cycle is repeated Refractory washes or graphite coatings to mold walls: control or direct cooling prevent casting from sticking prolong mold life (minimize thermal shock & fatigue) molds are not permeable, special provision for venting, slight cracks between mold halves or very small vent holes 10

11 13.2 PERMANENT-MOLD MOLD CASTING 11

12 SLUSH CASTING A variant of permanent-mold casting for hollow castings Hot metal in the metal mold, allowed to cool until a shell of desired thickness has formed Mold is inverted, remaining liquid is poured out, a hollow shape with good surface detail but variable wall thickness Applications: ornamental objects (candlesticks), statuary from the low-melting-temperature metals. 12

13 LOW-PRESSURE AND VACUUM PERMANENT-MOLD MOLD CASTING Gravity pouring: the oldest, simplest, most traditional form of permanent-mold casting Vacuum permanent-mold casting: Mold is turned upside down Positioned above a sealed airtight chamber (contains a crucible of molten metal) Small pressure difference Molten metal flows upward into the die cavity 13

14 LOW-PRESSURE AND VACUUM PERMANENT-MOLD MOLD CASTING 14

15 LOW-PRESSURE AND VACUUM PERMANENT-MOLD MOLD CASTING Low pressure permanent-mold casting: A low-pressure gas (3 to 15 psi) Introduced into a sealed chamber Driving molten metal up through a refractory fill tube into the gating g system or cavity of a metal mold Metal is exceptionally clean, since it flows from the center of the melt and is fed directly into the mold, never passing through the atmosphere Better product quality, nonturbulent mold filling, minimize gas porosity and dross formation. 15

16 DIE CASTING More specifically: pressure die casting Molten metal is forced into metal molds under pressures (several tens of Mpa), held under high pressure during solidification Combination of metal molds (dies) & high pressure Fine sections and excellent detail can be achieved long mold life Nonferrous metals and alloys: Zn, Cu, Mg, Al, -based alloys 16

17 DIE CASTING Advantages: High production rates Good strength Intricate shapes Excellent dimensional precision & surface quality Complete elimination of subsequent machining Parts can be made up to 10 kg & as large as 600 mm. 17

18 DIE CASTING Die temperatures: 150 to 250 C below the solidus temperature of the metal: promote rapid freezing. Cast iron cannot withstand the high casting pressures; dies are usually made from hardened hot-work tool steels; quite expensive Dies may be simple or complex 18

19 DIE CASTING 19

20 DIE CASTING Basic types of die-casting machines: 1- Hot-chamber (gooseneck): A gooseneck chamber Partially submerged in molten metal The plunger raised, molten metal flows through an open port and fills the chamber. A mechanical plunger then forces the metal up through the gooseneck, runners, gates, and into the die 20

21 DIE CASTING 21

22 DIE CASTING 22

23 DIE CASTING Hot-chamber advantages and drawbacks: Fast cycling times because of: Application of water-cooled dies Injection & melting from the same chamber Cannot be used for higher-melting-point metals Is not suitable for aluminum; molten aluminum tends to pick up some iron during contact with casting equipment Primary use with zinc-, tin-, and lead-based alloys. 23

24 DIE CASTING 2-Cold-chamber machines: Usually for materials that are not suitable for the hot- chamber design: alloys of Al, Mg, Cu, high-aluminum zinc. Operation stages: Metal has been melted in a separate furnace Is transported to die-casting machine A measured quantity is fed into an unheated shot chamber Driven into the die by a hydraulic or mechanical plunger 24

25 DIE CASTING Cold-chamber process has a longer operating cycle compared to hot-chamber machines 25

26 DIE CASTING 26

27 DIE CASTING 27

28 CENTRIFUGAL CASTING Inertial forces of rotation or spinning are used to distribute the molten metal into the mold cavity A dry-sand, graphite, or metal mold is rotated about a horizontal or vertical axis at speeds of 300 to 3000 rpm. As the molten metal is introduced, it is flung to the surface of the mold, where it solidifies into some form of hollow product. The exterior profile is usually round, but hexagons and other symmetrical shapes are also possible. 28

29 CENTRIFUGAL CASTING 29

30 CENTRIFUGAL CASTING 30

31 CENTRIFUGAL CASTING 31

32 CONTINUOUS CASTING Usually employed in solidification of basic shapes, feedstock for deformation processes By producing a special mold, can produce long lengths of complex cross-section product A single mold is all that is required to produce a large number of pieces. High quality, protecting metal from contamination during melting and pouring, a minimum of handling 32

33 CONTINUOUS CASTING 33

34 CONTINUOUS CASTING General Description of the Process 1) Delivery of liquid metal to the casting strand 2) Flow of metal through a distributor (tundish) into the casting mold 3) Formation of the cast section in a water-cooled copper mold 4) Continuous withdrawal of the casting from the mold 5) Further heat removal to solidify the liquid core from the casting by water spraying beyond the mold 6) Cutting to length and removing the cast sections 34

35 CONTINUOUS CASTING 35

36 CONTINUOUS CASTING 36

37 CONTINUOUS CASTING 37

38 CONTINUOUS CASTING 38

39 MELTING FURNACES DIRECT FUEL-FIRED (REVERBERATORY) FURNACES A fuel-fired flame passes directly over the pool of molten metal Heat transfer: radiant heating from the refractory roof and walls and convective heating from the hot gases Capacity is greater than crucible furnaces Operation is still limited to batch melting of nonferrous metals and the holding of cast iron that has been previously melted in a cupola 39

40 MELTING FURNACES 40

41 MELTING FURNACES 41

42 MELTING FURNACES INDIRECT FUEL-FIRED (CRUCIBLE) FURNACES Small batches of nonferrous metal Are essentially crucibles whose outer surface is heated by an external flame Crucibles: clay and graphite, silicon carbide, cast iron, or steel. Stirring action, temperature control, and chemistry control are often poor Furnace size and melting rate are limited Low capital and operating cost. 42

43 MELTING FURNACES 43

44 MELTING FURNACES 44

45 MELTING FURNACES 45

46 MELTING FURNACES 46

47 MELTING FURNACES CUPOLAS A significant amount of gray, nodular, and white cast iron is still melted in cupolas A cupola is a refractory-lined, vertical steel shell into which alternating layers of coke (carbon), iron, limestone or other flux, and possible alloy additions are charged & melted under forced air draft 47

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49 MELTING FURNACES 49

50 MELTING FURNACES Cupolas specifications: 1) are simple and economical 2) have a wide range of capacities 3) can produce cast iron of excellent quality 4) control of temperature and chemistry is somewhat difficult 50

51 MELTING FURNACES ARC FURNACES Preferred method of melting in many foundries because of the: (1) Rapid melting rates (2) Ability to hold the molten metal for any period of time (3) Ease of incorporating pollution control equipment. 51

52 MELTING FURNACES 52

53 MELTING FURNACES Top of the wide, shallow unit is first lifted or swung aside to permit the introduction of charge material. The top is then repositioned, and the electrodes are lowered to create an arc between the electrodes and the metal charge. The path of the heating current is usually through one electrode, across an arc to the metal charge, through the metal charge, and back through another arc to another electrode. 53

54 MELTING FURNACES Fluxing materials are usually added, a protective slag over the pool of molten metal. Metal is covered, can be maintained at a given temperature for long periods of time Arc furnaces can be used to produce high-quality metal of almost any desired composition sizes up to 200 tons (but 25 tons or less are most common) up to 50 tons per hour can be melted generally used with ferrous alloys, especially steel good mixing and homogeneity to the molten bath. 54

55 MELTING FURNACES Noise and level of particle emissions can be rather high Consumption of electrodes, refractories, and power results in high operating costs. 55

56 MELTING FURNACES 56

57 MELTING FURNACES INDUCTION FURNACES Very rapid melting rates Relative ease of controlling pollution Another popular means of melting metal. 57

58 MELTING FURNACES Two basic types: 1- High-frequency (coreless): a crucible surrounded by a water-cooled coil of copper tubing A high-frequency electrical current passes through the coil Creating an alternating magnetic field Varying magnetic field induces secondary electrical currents in the metal being melted Bring about a rapid rate of heating. 58

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60 MELTING FURNACES Coreless induction furnaces, are used for virtually all common alloys Good control of temperature and composition and up to about 65 tons There is no contamination from the heat source, very pure metal 60

61 MELTING FURNACES 2- Low-frequency or channel-type induction furnaces Only a small channel is surrounded by the primary coil A secondary coil is formed by a loop, or channel, of molten metal All the liquid metal is free to circulate through the loop and gain heat. To start, enough molten metal must be placed into the furnace to fill the secondary coil Heating rate is high, and the temperature can be accurately controlled, are preferred as holding furnaces up to about 250 tons 61

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63 MELTING FURNACES 63