CASTING.pptx intoduction power point presentation

CASTING.pptx intoduction power point

1. CASTING FOR ENGINEERS

2. S A N D C A S T I N G

3. INTRODUCTION Sand casting, the most widely used casting process, utilizes expendable sand moulds to form complex metal parts that can be made of nearly any alloy.  The sand casting process involves the use of a furnace, metal, pattern, and sand mould.

4. Casting Process in which molten metal flows by gravity or other force into a mold where it solidifies in the shape of the mold cavity The term casting also applies to the part made in the process Steps in casting seem simple: 1. Melt the metal 2. Pour it into a mold 3. Let it freeze

5. Safety in foundries As in all other manufacturing operations, safety is an important consideration, particularly because of the following factors: dust from sand and other compounds used in casting, thus requiring proper ventilation and safety equipment for the workers fumes from molten metals and lubricants, as well as splashing of the molten metal during the transfer or pouring the presence of fuels for furnace, the control of their pressure, the proper operation of valves, etc. the presence of water and moisture in crucibles, molds, and other locations, since it rapidly converts to steam, creating severe danger of explosion improper handling of fluxes, which are hygroscopic, thus absorbing moisture and creating a danger inspection of crucibles , tools, and other equipment for wear, cracks, etc.

6. Classification of solidification processes

7. Parts Made by Casting Big parts Engine blocks and heads for automotive vehicles, wood burning stoves, machine frames, railway wheels, pipes, bells, pump housings Small parts Dental crowns, jewelry, small statues, frying pans All varieties of metals can be cast - ferrous and nonferrous

8. PRODUCTS OF SANDCASTING

9. PROCESSES Mold cavity with desired shape and size Melting process to provide molten metal Pouring process to introduce the metal into the mold Solidification process controlled to prevent defects Ability to remove the casting from the mold Cleaning, finishing and inspection operations

10. MATERIALS Sand casting is able to make use of almost any alloy. An advantage of sand casting is the ability to cast materials with high melting temperatures, including steel, nickel, and titanium.  The four most common materials that are used in sand casting are shown below, along with their melting temperatures.   Materials Melting temperature Aluminum alloys 1220 °F (660 °C) Brass alloys 1980 °F (1082 °C) Cast iron 1990-2300 °F (1088-1260 °C) Cast steel 2500 °F (1371 °C)

11. The Pattern It is the model from which the final casting is made. It is the image of the casting in every respect, except one, and that is size

12. Types of Patterns Figure : Types of patterns used in sand casting: (a) solid pattern or single (b) split pattern or gated pattern (c) match‑ plate pattern (d) cope and drag pattern

13. solid pattern or single Are single copies of casting. Parting surface is hand formed. Gating systems are cut by hand in mould material

14. split pattern or gated pattern Gating system is part of pattern and eliminates hand cutting the gate

15. match‑ plate pattern Used for large quantities of small castings

16. cope and drag pattern Cope and drag portion of pattern are mounted on separate plates Moulding of medium and large castings on mould machines is facilitated by this type of pattern

17. The Pattern A full‑sized model of the part, slightly enlarged to account for shrinkage and machining allowances in the casting Pattern materials: Wood - common material because it is easy to work, but it warps Metal - more expensive to make, but lasts much longer Plastic - compromise between wood and metal

18. Designing the pattern PATTERN ALLOWANCES For Metallurgical and Mechanical reasons a number of allowances must be on the pattern

19. 1. Shrinkage/ Contraction allowance Allowance made for allowing the contraction during cooling to room temperature There are three stages of volume change between pouring the molten metal and retrieving the casting from the mould:

20. Liquid changes which take place during the fall in temperature from pouring to freezing or solidification. ii. Shrinkage during freezing (solidification). iii. Solid shrinkage, or contraction , in the solidified casting as it cools to atmospheric temperature.

21. Pattern maker's contraction allowance Grey Cast iron 1% Malleable Cast Iron 1.5% Brass 1.3% Gun-metal 1.0-1.6% Phosphor bronze 1.0-1.6% Aluminium alloys (e.g. LM6) 1.3% Zinc and zinc alloys 2.6% Steel 2% Magnesium 1.8%

22. 2. Machining allowance This allowance is added to the pattern over and above the contraction allowance so that when the surface of the casting has been removed by a lathe or other machine tool to give the desired finish, the product will be the correct size. It varies according to the type and size of pattern and the metal being used to produce the casting. Although typical allowances vary between 2 and 6mm, in exceptional cases it can be as high as 25mm.

23. 3.Taper or draft allowance The first thing to remember in making a pattern is that it must come out of the sand cleanly. There must be no undercuts or awkward corners which will bind on the sand and hold it. No matter how finely made the pattern, or how cleverly designed, if the sand does not release from it, leaving a clean crisp impression, then the pattern is useless.

24. Every pattern must have a draught allowance, or taper. Without this taper the sides of the pattern will bind in the sand and will not come out cleanly. Taper varies from about 1-3 degrees. The greater the taper, the easier it is to draw the pattern out of the Sand.

25. 4. Rapping or Shake Allowance Before withdrawing the pattern it is rapped and thereby the size of the mould cavity increases. Actually by rapping, the external sections move outwards increasing the size and internal sections move inwards decreasing the size. This movement may be insignificant in the case of small and medium size castings, but it is significant in the case of large castings. This allowance is kept negative and hence the pattern is made slightly smaller in dimensions 0.5-1.0 mm.

26. 5. Distortion Allowance This allowance is applied to the castings which have the tendency to distort during cooling due to thermal stresses developed. For example a casting in the form of U shape will contract at the closed end on cooling, while the open end will remain fixed in position. Therefore, to avoid the distortion, the legs of U pattern must converge slightly so that the sides will remain parallel after cooling.

27. 6. Mold wall Movement Allowance Mold wall movement in sand moulds occurs as a result of heat and static pressure on the surface layer of sand at the mold metal interface. In ferrous castings, it is also due to expansion due to graphitisation . This enlargement in the mold cavity depends upon the mold density and mould composition. This effect becomes more pronounced with increase in moisture content and temperature.

28. Pattern Material Decision as to what material to use for specific pattern depends on: Expected production quantity Dimensional accuracy required Size and Shape of casting Moulding processes to be used in foundry

29. Wood pattern Commonly used material due to following Advantages 1 Wood can be easily worked. 2 It is light in weight. 3 It is easily available. 4 It is very cheap. 5 It is easy to join. 6 It is easy to obtain good surface finish. 7 Wooden laminated patterns are strong. 8 It can be easily repaired.

30. Disadvantages 1 It is susceptible to moisture. 2 It tends to warp. 3 It wears out quickly due to sand abrasion. 4 It is weaker than metallic patterns.

31. Metal Pattern Are normally made from Aluminium Alloy, Grey Cast Iron, Steel , Brass or Magnesium Alloy. More expensive to make, but lasts much longer Advantages i ). High Resistance to wear ii). Dimensional stability iii). Good Machinability iv). Ability to provide smooth surface finish v). Mostly used when large number of casting are desired.

32. Plastic Pattern compromise between wood and metal Excellent pattern material due to: i ).Dimensional stability ii). Easily produced with less skill iii) Smooth surface

33. Pattern Material Characteristics

34. FACTORS EFFECTING SELECTION OF PATTERN MATERIAL The following factors must be taken into consideration while selecting pattern materials. Number of castings to be produced. Metal pattern are preferred when castings are required large in number 2. Type of mould material used. 3. Kind of molding process. 4. Method of molding (hand or machine). 5. Degree of dimensional accuracy and surface finish required. 6. Minimum thickness required. 7. Shape, complexity and size of casting. 8. Cost of pattern and chances of repeat orders of the pattern

35. Colour code for patterns Pattern makers use a colour code so that it is clear to the metal caster which surfaces are which. This is as follows: 'As cast' surfaces which are to be left unmachined - red or orange Surfaces which are to be machined – yellow Core prints (see below) for unmachined openings and end prints – black Core prints for machined openings - yellow stripes on black Seats for loose pieces and loose core prints – green or yellow

36. Colour Codes for various alloy casting Cast Iron Red Cast Steel Blue Light Cast Metal Green Malleable Cast Iron Grey Heavy Cast Metal Yellow

37. DESIGN CONSIDERATIONS IN PATTERN MAKING The following considerations should always be kept in mind while designing a pattern. 1. All Abrupt changes in section of the pattern should be avoided as far as possible. 2. Parting line should be selected carefully, so as to allow as small portion of the pattern as far as possible in the cope area 3. The thickness and section of the pattern should be kept as uniform as possible. 4. Sharp corners and edges should be supported by suitable fillets or otherwise rounded of. It will facilitate easy withdrawal of pattern smooth flow of molten metal andensure a sound casting.

38. 5. Surfaces of the casting which are specifically required to be perfectly sound and clean should be so designed that they will be molded in the drag because the possible defects due to loose sand and inclusions will occur in the cope. 6. As far as possible, full cores should be used instead of cemented half cores for reducing cost and for accuracy. 7. For mass production, the use of several patterns in a mould with common riser is to be preferred. 8. The pattern should have very good surface finish as it directly affects the corresponding finish of the casting. 9. Shape and size of the casting and that of the core should be carefully considered to decide the size and location of the core prints. 10. Proper material should always be selected for the pattern after carefully analyzing the factors responsible for their selection.

39. 11. Try to employ full cores always instead of jointed half cores as far as possible. This will reduce cost and ensure greater dimensional accuracy. 12. The use of offset parting, instead of cores as for as possible should be encouraged to the great extent. 13. For large scale production of small castings, the use of gated or match- plate patterns should be preferred wherever the existing facilities permit. 14. If gates, runners and risers are required to be attached with the pattern, they should be properly located and their sudden variation in dimensions should be avoided. 15. Wherever there is a sharp corner, a fillet should be provided, and the corners may be rounded up for easy withdrawal of patterns as well as easy flow of molten metal in the mould.

40. MOULDING A Mould/Mold is a cavity in which molten metal is poured during casting Greater output of foundry industry consist of casting made in refractory mould mainly sand mould Mold is a container with cavity whose geometry determines part shape

41. CHARACTERISTICS OF SAND FLOWABILITY    The ability to pack tightly around the pattern. 2. PLASTIC DEFORMATION Have the ability to deform slightly without cracking so that the pattern can be withdrawn. 3. GREEN STRENGTH Have the ability to support its own weight when stripped from the pattern, and also withstand pressure of molten REFRACTORINESS The ability of mould sand to withstand high temperature without fusing.

42. 5. PERMEABILITY    This allows the gases and steam to escape from the mold during casting. 6. THERMAL STABILTY  Ability to resist damage, such as cracking, from the heat of the molten metal. 7. REUSABILITY  Ability of the sand to be reused for future sand molds. COLLAPSIBILITY Readiness with which moulding sand will breakdown in knock on and cleaning operation FINENESS Ability to prevent metal penetration and production of smooth surface.

43. 10. STRENGTH ‑ Ability of mold to maintain shape and resist erosion caused by the flow of molten metal. Depends on grain shape, adhesive quality of binders 11 . COHESIVENESS : ability of the sand to retain a given shape after the pattern is removed. 12.SURFACE FINISH: The size and shape of the sand particles defines the best surface finish achievable, with finer particles producing a better finish. However, as the particles become finer (and surface finish improves) the permeability becomes worse.

44. Advantages:   1. Sand casting is the least expensive of all casting processes .   2 . Sand has almost no upper limit on part weight and minimum part weight ranges from0.075- 0.1 kg.   3. Casting process can be performed on any metal that can be heated to the liquid state.   4. Sand casting is an economical process for creating rough metal parts.

45. Limitations:   Sand casting may require a longer lead time for production at high output rates.   2. Poor dimensional accuracy and surface finish.   3. Need skill labors.   4. It can’t be used at mass production.  

46. MOLDING SAND The general sources of receiving molding sands are the beds of sea, rivers, lakes, granulular elements of rocks, and deserts Molding sands are of two types namely: 1. Natural molding sands 2. Synthetic molding sands

47. Natural molding sands Are those sands in which refractoring grains are associated in with clay needed for moulding But its higher clay content reduces refractoriness and permiability Its moulding property is developed by adding water alone (contain sufficient binder) It is obtained from river banks or dug from pits

48. synthetic molding sands Are prepared artificially using basic sand molding constituents silica sand in 88-92%, binder 6-12%, water or moisture content 3-6% and other additives in proper proportion by weight with perfect mixing and mulling in suitable equipments.

49. Kind of Sand Mold Green-sand molds - mixture of sand, clay, and water; “Green" means mold contains moisture at time of pouring Dry-sand mold - organic binders rather than clay  And mold is baked to improve strength  Skin-dried mold - drying mold cavity surface of a green-sand mold to a depth of 10 to 25 mm, using torches or heating lamps

50. Binders Used with Foundry Sands  Sand is held together by a mixture of water and bonding clay  Typical mix: 90% sand, 7% clay and 3% water  Other bonding agents also used in sand molds:  Organic resins ( e.g , phenolic resins)  Inorganic binders ( e.g , sodium silicate and phosphate)  Additives are sometimes combined with the mixture to increase strength and/or permeability EMU -

51. BINDER 1. Cereal binder It develops green strength, baked strength and collapsibility in core. The amount of these binders used varies from 0.2 to 2.2% by weight in the core sand. 2. Protein binder It is generally used to increase collapsibility property of core.

52. BINDER CONT. 3. Thermo setting resin It is gaining popularity nowadays because it imparts high strength, collapsibility to core sand and it also evolve minimum amount of mold and core gases which may produce defects in the casting. The most common binders under this group are phenol formaldehyde and urea formaldehyde.

53. BINDER CONT. 4. Sulphite binder Sulphite binder is also sometimes used in core but along with certain amount of clay. 5. Dextrin It is commonly added in core sand for increasing collapsibility and baked strength of core

54. BINDER CONT. 6. Pitch It is widely used to increase the hot strength of the core. 7. Molasses It is generally used as a secondary binder to increase the hardness on baking. It is used in the form of molasses liquid and is sprayed on the cores before baking.

55. BINDER CONT. 8. Core oil It is in liquid state when it is mixed with the core sand but forms a coherent solid film holding the sand grains together when it is baked. Although, the core drying with certain core oils occurs at room temperature but this can be expedited by increasing the temperature. That is why the cores are made with core oils and are usually baked.

56. Sand Casting Mold Terms Mold consists of two halves: - Cope = upper half of mold - Drag = bottom half Mold halves are contained in a box, called a flask The two halves separate at the parting line

57. CONT Flask The box containing the mold Cope The top half of any part of a 2-part mold Drag The bottom half of any part of a 2-part mold Core A shape inserted into the mold to form internal cavities Core Print A region used to support the core

58. Open Molds and Closed Molds Two forms of mold : (a) open mold, simply a container in the shape of the desired part; and (b) closed mold, in which the mold geometry is more complex and requires a gating system (passageway) leading into the cavity.

59. Forming the Mold Cavity Cavity is inverse of final shape with shrinkage allowance Pattern is model of final shape with shrinkage allowance Core provides internal features of the part. It is placed inside the mold cavity with Wet sand is made by adding binder in the sand Mold cavity is formed by packing sand around a pattern

60. CONT. When the pattern is removed, the remaining cavity of the packed sand has desired shape of cast part The pattern is usually oversized to allow for shrinkage of metal during solidification and cooling

61. Gating System It is channel through which molten metal flows into cavity from outside of mold  Consists of a down- sprue , through which metal enters a runner leading to the main cavity  At the top of down- sprue , a pouring cup is often used to minimize splash and turbulence as the metal flows into down- sprue

62. GATING SYSTEM COMPRISES OF: POURING CUP/BASIN  This is where the metal is poured into the mold. SPRUE   The vertical channel from the top of the mold to the gating and riser system. Also, a generic term used to cover all gates, runners and risers.

63. GATE SYSTEM CONT. RUNNER  The portion of the gate assembly that connects the sprue to the casting innate or riser. GATE  The end of the runner in a mold where molten metal enters the mold cavity.

64. GATE SYSTEM CONT. RISER A reservoir of molten metal provided to compensate for the contraction of the metal as it solidifies. Indicates the mould cavity is full Allows gases to escape from mould cavity Acts as reservoir of molten metal to compensate during shrinkage MOLD CAVITY  The impression in a mold produced by the removal of the pattern. When filled with molten metal it forms a casting.

65. GATE SYSTEM CONT. COPE Upper or top most section of a flask, mold or pattern. PARTING LINE A line on a pattern or casting corresponding to the separation between the parts of a mold. DRAG Lower or bottom section of a flask, mold or pattern.

66. Steps in Sand Casting 1. Pour the molten metal into sand mold CAVITY 2. Allow time for metal to solidify 3 . Break up the mold to remove casting 4 . Clean and inspect casting Separate gating and riser system 5. Heat treatment of casting is sometimes required to improve metallurgical properties

67. Steps in Sand Casting

68. Steps in Sand Casting

69. Pouring the Molten Metal For this step to be successful, metal must flow into all regions of the mold, most importantly the main cavity, before solidifying Factors that determine success Pouring temperature Pouring rate Turbulence Pouring temperature should be sufficiently high in order to prevent the molten metal to start solidifying on its way to the cavity

70. Pouring the Molten Metal For this step to be successful, metal must flow into all regions of the mold, most importantly the main cavity, before solidifying Factors that determine success Pouring temperature Pouring rate Turbulence Pouring temperature should be sufficiently high in order to prevent the molten metal to start solidifying on its way to the cavity

71. Engineering Analysis of Pouring 1. v: velocity of liquid g:981cm/sec.sec; h: 2. v1: velocity at section of area A1; v2: velocity 3. V: volume of mold cavity metal at base of sprue in cm/sec; height of sprue in cm at section of area A2

72. Calculation of Pouring Parameters: Example

73. Buoyancy Force during Pouring One of the hazards during pouring is that buoyancy of molten will displace the core with the force: Fb = Wm- Wc ( Archimedes principle) Wm: Weight of molten metal displaced; Wc : Weight of core ** In order to avoid the effect of Fb , chaplets are used to hold the core in cavity of mold.

74. (a) Core held in place in the mold cavity by chaplets (b) possible chaplet design, (c) casting with internal cavity.

75. Fluidity A measure of the capability of the metal to flow into and fill the mold before freezing. • Fluidity is the inverse of viscosity (resistance to flow) Factors affecting fluidity are: - Pouring temperature relative to melting point - Metal composition - Viscosity of the liquid metal - Heat transfer to surrounding

76. Solidification of Metals It is the transformation of molten metal back into solid state Solidification differs depending on whether the metal is - A pure element or - An alloy - A Eutectic alloy

77. Shrinkage in Solidification and Cooling Shrinkage occurs in 3 steps: a. while cooling of metal in liquid form ( liquid contraction ); b. during phase transformation from liquid to solid ( solidification shrinkage ); c. while solidified metal is cooled down to room temperature ( solidthermal contraction ).

78. ( 2) reduction in height and formation of shrinkage cavity caused by solidification shrinkage; (3) further reduction in height and diameter due to thermal contraction during cooling of solid metal (dimensional reductions are exaggerated for clarity).

79. Directional shrinkage

80. General Defects: Misrun A casting that has solidified before completely filling mold cavity Reasons: Fluidity of molten metal is insufficient Pouring temperature is too low Pouring is done too slowly Cross section of mold cavity is too thin Mold design is not in accordance with Chvorinov’s rule: V/A at the section closer to the gating system should be higher than that far from gating system

81. Cold Shut Two portions of metal flow together but there is a lack of fusion due to premature (early) freezing Reasons: Same as for misrun

82. Cold Shot Metal splashes during pouring and solid globules form and become entrapped in casting Gating system should be improved to avoid splashing

83. Shrinkage Cavity Depression in surface or internal void caused by solidification shrinkage Proper riser design can solve this issue

84. Hot Tearing Hot tearing/cracking in casting occurs when the molten metal is not allowed to contract by an underlying mold during cooling/ solidification. The collapsibility (ability to give way and allow molten metal to shrink during solidification) of mold should be improved

85. Sand Blow Balloon‑ shaped gas cavity caused by release of mold gases during pouring Low permeability of mold, poor venting, high moisture content in sand are major reasons

86. Pin Holes Formation of many small gas cavities at or slightly below surface of casting Caused by release of gas during pouring of molten metal. To avoid, improve permeability & venting in mold

87. Penetration When fluidity of liquid metal is high, it may penetrate into sand mold or core, causing casting surface to consist of a mixture of sand grains and metal Harder packing of sand helps to alleviate this problem Reduce pouring temp if possible Use better sand binders

88. Mold Shift A step in cast product at parting line caused by sidewise relative displacement of cope and drag It is caused by buoyancy force of molten metal. Cope an drag must be aligned accurately and fastened. Use match plate patterns

89. Core Shift Similar to core mold but it is core that is displaced and the displacement is usually vertical. It is caused by buoyancy force of molten metal. Core must be fastened with chaplet

90. Sand Wash An irregularity in the casting surface caused by erosion of sand mold during pouring. Turbulence in metal flow during pouring should be controlled. Also, very high pouring temperature cause erosion of mold.

91. Scabs Scabs are rough areas on the surface of casting due to un-necessary deposit of sand and metal It is caused by portions of the mold surface flaking off during solidification and becoming embedded in the casting surface Improve mold strength by reducing grain size and changing binders

92. Mold Crack Occurs when the strength of mold is not sufficient to withstand high temperatures Improve mold strength by reducing grain size and changing binders

93. Other Casting Processes Expendable mold processes – uses an expendable mold which must be destroyed to remove casting Mold materials : sand, plaster, and similar materials, plus binders Advantage : more complex shapes possible Disadvantage : production rates often limited by time to make mold rather than casting itself Permanent mold processes – uses a permanent mold which can be used over and over to produce many castings Made of metal (or, less commonly, a ceramic refractory material) Advantage : higher production rates Disadvantage : geometries limited by need to open mold

94. Expendable Mold Casting Processes – Shell mold – Vacuum mold – Expanded polystrene mold – Investment casting – Plaster mold and ceramic mold

95. Permanent Mold Casting Processes – Basic permanent mold – Variations of permanent mold – Die casting – Centrifugal casting

96. Shell Molding Casting process in which the cavity (& gating system) is a thin shell of sand held together by thermosetting resin binder

97. Steps in shell-molding: (1) a match-plate or cope-and-drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin.

98. Steps in shell-molding: (2) box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell; (3) box is repositioned so that loose uncured particles drop away;

99. Steps in shell-molding: (4) sand shell is heated in oven for several minutes to complete curing; (5) shell mold is stripped from the pattern;

100. Steps in shell-molding: (6) two halves of the shell mold are assembled, supported by sand or metal shot in a box, and pouring is accomplished; (7) the finished casting with sprue removed.

101. Advantages and Disadvantages Advantages of shell molding: Smoother cavity surface permits easier flow of molten metal and better surface finish Good dimensional accuracy - machining often not required Mold collapsibility minimizes cracks in casting Can be mechanized for mass production Disadvantages: More expensive metal pattern Difficult to justify for small quantities

102. Vacuum Molding

103. Advantages and Disadvantages Advantages of vacuum molding: Easy recovery of the sand, since no binders Sand does not require mechanical reconditioning done when binders are used Since no water is mixed with sand, moisture‑ related defects are absent Disadvantages: Slow process Not readily adaptable to mechanization

104. Expanded Polystyrene Process or lost‑ foam process Uses a mold of sand packed around a polystyrene foam pattern which vaporizes when molten metal is poured into mold Other names : lost‑ foam process, lost pattern process, evaporative‑ foam process, and full‑ mold process Polystyrene foam pattern includes sprue, risers, gating system, and internal cores (if needed) Mold does not have to be opened into cope and drag sections

105. Expanded polystyrene casting process: (1) pattern of polystyrene is coated with refractory compound;

106. Expanded polystyrene casting process: (2) foam pattern is placed in mold box, and sand is compacted around the pattern;

107. Expanded polystyrene casting process: (3) molten metal is poured into the portion of the pattern that forms the pouring cup and sprue. As the metal enters the mold, the polystyrene foam is vaporized ahead of the advancing liquid, thus the resulting mold cavity is filled

108. Advantages and Disadvantages Advantages of expanded polystyrene process : Pattern need not be removed from the mold Simplifies and speeds mold‑ making , because two mold halves are not required as in a conventional green‑ sand mold Disadvantages : A new pattern is needed for every casting Economic justification of the process is highly dependent on cost of producing patterns

109. Applications : Mass production of castings for automobile engines Automated and integrated manufacturing systems are used to Mold the polystyrene foam patterns and then Feed them to the downstream casting operation

110. Investment Casting (Lost Wax Process) A pattern made of wax is coated with a refractory material to make mold, after which wax is melted away prior to pouring molten metal " Investment " comes from a less familiar definition of " invest " - "to cover completely," which refers to coating of refractory material around wax pattern It is a precision casting process - capable of producing castings of high accuracy and intricate detail

111. Steps in investment casting: (1) wax patterns are produced, (2) several patterns are attached to a sprue to form a pattern tree

112. Steps in investment casting: (3) the pattern tree is coated with a thin layer of refractory material, (4) the full mold is formed by covering the coated tree with sufficient refractory material to make it rigid

113. Steps in investment casting: (5) the mold is held in an inverted position and heated to melt the wax and permit it to drip out of the cavity, (6) the mold is preheated to a high temperature, the molten metal is poured, and it solidifies

114. Steps in investment casting: (7) the mold is broken away from the finished casting and the parts are separated from the sprue

115. Advantages and Disadvantages Advantages of investment casting : Parts of great complexity and intricacy can be cast Close dimensional control and good surface finish Wax can usually be recovered for reuse Additional machining is not normally required ‑ this is a net shape process Disadvantages Many processing steps are required Relatively expensive process

116. Plaster Mold Casting Similar to sand casting except mold is made of plaster of Paris (gypsum ‑ CaSO 4 ‑2H 2 O ) In mold-making, plaster and water mixture is poured over plastic or metal pattern and allowed to set Wood patterns not generally used due to extended contact with water Plaster mixture readily flows around pattern, capturing its fine details and good surface finish

117. Advantages and Disadvantages Advantages of plaster mold casting : Good accuracy and surface finish Capability to make thin cross‑sections Disadvantages : Mold must be baked to remove moisture, which can cause problems in casting Mold strength is lost if over-baked Plaster molds cannot stand high temperatures, so limited to lower melting point alloys can be casted

118. Ceramic Mold Casting Similar to Plaster Mold Casting except the material of mold is refractory ceramic material instead of plaster. The ceramic mold can withstand temperature of metals having high melting points. Surface quality is same as that in plaster mold casting.

119. Permanent Mold Casting Processes Economic disadvantage of expendable mold casting : a new mold is required for every casting In permanent mold casting, the mold is reused many times The processes include: Basic permanent mold casting Die casting Centrifugal casting

120. The Basic Permanent Mold Process Uses a metal mold constructed of two sections designed for easy, precise opening and closing Molds used for casting lower melting-point alloys (Al, Cu, Brass) are commonly made of steel or cast iron Molds used for casting steel must be made of refractory material , due to the very high pouring temperatures

121. Steps in permanent mold casting: (1) mold is preheated and coated

122. Steps in permanent mold casting: (2) cores (if used) are inserted and mold is closed, (3) molten metal is poured into the mold, where it solidifies.

123. Advantages and Limitations Advantages of permanent mold casting : Good dimensional control and surface finish Very economical for mass production More rapid solidification caused by the cold metal mold results in a finer grain structure, so castings are stronger Limitations : Generally limited to metals of lower melting point Complex part geometries can not be made because of need to open the mold High cost of mold Not suitable for low-volume production

124. Variations of Permanent Mold Casting: a. Slush Casting The basic procedure the same as used in Basic Permanent Mold Casting After partial solidification of metal, the molten metal inside the mold is drained out, leaving the part hollow from inside. Statues, Lamp bases, Pedestals and toys are usually made through this process Metal with low melting point are used: Zinc, Lead and Tin

125. b. Low-pressure Casting The basic process is shown in Fig. In basic permanent and slush casting processes, metal in cavity is poured under gravity. However, in low-pressure casting, the metal is forced into cavity under low pressure (0.1 MPa ) of air.

126. Advantages: Clean molten metal from the center of ladle (cup) is introduced into the cavity. Reduced- gas porosity, oxidation defects, improvement in mechanical properties

127. c. Vacuum Permanent-Mold Casting This is a variation of low-pressure permanent casting Instead of rising molten into the cavity through air pressure, vacuum in cavity is created which caused the molten metal to rise in the cavity from metal pool.

128. Die Casting A permanent mold casting process in which molten metal is injected into mold cavity under high pressure Pressure is maintained during solidification, then mold is opened and part is removed Molds in this casting operation are called dies ; hence the name die casting Use of high pressure ( 7-35MPa ) to force metal into die cavity is what distinguishes this from other permanent mold processes

129. Die Casting Machines Designed to hold and accurately close two mold halves and keep them closed while liquid metal is forced into cavity Two main types: Hot‑ chamber machine Cold‑ chamber machine

130. Hot-Chamber Die Casting Metal is melted in a container, and a piston injects liquid metal under high pressure into the die High production rates - 500 parts per hour not uncommon Injection pressure: 7-35MPa Applications limited to low melting‑ point metals that do not chemically attack plunger and other mechanical components Casting metals: zinc, tin, lead, and magnesium

131. Cycle in hot‑ chamber casting: (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification. Because the die material does not have natural permeability (like sand has), vent holes at die cavity needs to be made

132. Cold‑ Chamber Die Casting Molten metal is poured into unheated chamber from external melting container, and a piston injects metal under high pressure (14-140MPa) into die cavity High production but not usually as fast as hot‑ chamber machines because of pouring step Casting metals: aluminum, brass, and magnesium alloys Advantage of cold chamber is that high melting point metals can be casted:

133. Cycle in cold‑ chamber casting: (1) with die closed and ram withdrawn, molten metal is poured into the chamber

134. Cycle in cold‑ chamber casting: (2) ram forces metal to flow into die, maintaining pressure during cooling and solidification.

135. Molds for Die Casting Usually made of tool steel, mold steel, or maraging steel Tungsten and molybdenum (good refractory qualities) are used to make die for casting steel and cast iron Ejector pins are required to remove part from die when it opens Lubricants must be sprayed into cavities to prevent sticking

136. Advantages and Limitations Advantages of die casting : Economical for large production quantities Good accuracy (±0.076mm)and surface finish Thin sections are possible Rapid cooling provides small grain size and good strength to casting Disadvantages: Generally limited to metals with low metal points Part geometry must allow removal from die, so very complex parts can not be casted Flash and metal in vent holes need to be cleaned after ejection of part

137. Centrifugal Casting A family of casting processes in which the mold is rotated at high speed so centrifugal force distributes molten metal to outer regions of die cavity The group includes: True centrifugal casting Semicentrifugal casting Centrifuge casting

138. (a) True Centrifugal Casting Molten metal is poured into a rotating mold to produce a tubular part In some operations, mold rotation commences after pouring rather than before Rotational axes can be either horizontal or vertical Parts: pipes, tubes, bushings, and rings Outside shape of casting can be round, octagonal, hexagonal, etc , but inside shape is (theoretically) perfectly round, due to radially symmetric forces

139. Shrinkage allowance is not considerable factor

140. Rotational Speed of Mold

141. Example Problem: A true centrifugal casting is to be performed horizontally to make copper tube sections: OD =25cm; ID= 22.5cm; GF= 65. Find rotational speed. Solution: OD =D= 25cm= 0.25m; g= 9.81m/s2; GF=65 On solving we get: 681.7 RPM (rev/min)

142. (b) Semicentrifugal Casting Centrifugal force is used to produce solid castings rather than tubular parts Molds are designed with risers at center to supply feed metal Density of metal in final casting is greater in outer sections than at center of rotation

143. Axes of parts and rotational axis does not match exactly Often used on parts in which center of casting is machined away, thus eliminating the portion where quality is lowest Examples: wheels and pulleys G factor keeps from 10-1

144. (c) Centrifuge Casting Mold is designed with part cavities located away from axis of rotation, so that molten metal poured into mold is distributed to these cavities by centrifugal force Used for smaller parts Radial symmetry of part is not required as in other centrifugal casting method