1. PRODUCTION OF -- [CaSO 4 ] 1 / 2 H 2 O Calcining process – Gypsum is ground & subjected to temperatures of 110°C to 130°C to drive off part of the water of crystallization that is the amount of water needed to convert gypsum to its hemihydrate form. It is done in a vat or a kiln open to air. Dry calcination & wet calcination. 1DR.R.koohkan

2. The dihydrate form of calcium sulfate, called gypsum, usually appears white to milky yellowish and is found in a compact mass in nature. DR.R.koohkan2

3. As the temperature is raised the remaining water is removed, and products are formed as indicated. 110 0 –130 0 130 0 –200 0 200 0 –1000 0 CaSO 4 +2H 2 O [CaSO 4 ] 1 / 2 H 2 O CaSO 4 CaSO 4 GYPSUM CALCIUM SULFATE HEMIHYDRATE HEXAGONAL ANHYDRATE ORTHOROMBIC ANHYDRATE 3 Densite Mineral gypsum Dehydration by heat or other means Plasters formulation Model plaster Lab plaster Dental stone High-strength dental stone Hydrocal DR.R.koohkan

4. IT IS OF THE FOLLOWING TYPES : Impression plaster TYPE I Model plaster. TYPE II Dental stone. TYPE III Dental Stone, high strength. TYPE IV Dental stone, high strength & TYPE V High expansion. IT IS OF THE FOLLOWING TYPES : Impression plaster TYPE I Model plaster. TYPE II Dental stone. TYPE III Dental Stone, high strength. TYPE IV Dental stone, high strength & TYPE V High expansion. 4DR.R.koohkan

5. Most gypsum products are obtained from natural gypsum rock. The reaction is exothermic, CaSO4 · ½ H2O + 1 ½ H2O →CaSO4 · 2H2O + 3900 cal/g mol Plaster of Paris Water Gypsum Three types of base raw materials are derived from partial dehydration of gypsum rock, depending on the nature of the dehydration process.: Plasters: are fluffy, porous, and least dense, whereas hydrocal variety : has a higher density and is more crystalline. Densite : is the densest of the raw materials DR.R.koohkan5

6. ISO Classification of dental gypsum products  Type 1 : Impression plaster.  Type 2 : Dental plaster – model.  Type 3 : Dental stone – die.  Type 4 : Dental stone – die, high strength, low expansion. Type 5 : Dental stone – die, high strength, high expansion. Although these types have identical chemical formulas of calcium sulfate hemihydrate, CaSO4 · ½H2O,they possess different physical properties,  Type 4 contains extra salts to reduce its setting expansion DR.R.koohkan6

7. the main difference being the manner of driving off part of the water of the calcium sulfate dihydrate. Plasters : gypsum mineral is heated in an open kettle at a temperature of about 110° to 120° C β-calcium sulfate hemihydrate. (irregular shape and porous) Hydrocal : If gypsum is dehydrated under pressure and in the presence of water vapor at about 125° C α-calcium sulfate hemihydrate. more uniform in shape and denser than the particles of plaster low- to moderate-strength dental stones.. Densite: Boiling gypsum rock in a 30% calcium chloride solution, is washed away with hot water(100° C) is ground fineness. Types 4 and 5 high-strength dental stones are A high-density DR.R.koohkan7

8. α – hemimhydrate 1.Type III, IV &V 2.Produced by wet calcination. 3.Requires less water for mixing. 4.Better packing ability. 5.Low surface free energy. 6.Crystal habit of hexagonal calcium sulfate. 7.High apparent density. β – hemihydrate 1.Type I & II 2.Produced by dry calcination. 3.Requires more water form mixing. 4.Less packing ability. 5.High surface free energy. 6.Crystal habit that of hemihydrate. 7.Low apparent density. 8DR.R.koohkan

9. Gypsum Mixing Water (mL/100 g of powder) Required Water (mL/100 g of powder) Excess Water ( mL/100 g of powder) Model plaster 37-50 18.6 18-31 Dental stone 28-32 18.6 9-13 High-strength dental stone 19-24 18.6 0-5 *Water-powder ratio varies with each product.  When the set material is dried, the excess water evaporates and leaves porosity in the structure, weakening it.  This difference in the physical shape and nature of the crystals makes it possible to obtain the same consistency with less excess water with dental stone than with model plaster. DR.R.koohkan9

10. SETTING REACTION PROPOSED THEORIES : 1- Colloidal theory – hemihydrate + water --> colloidal state through sol -gel mechanism. In sol state hemihydrate particles are converted to dihydrate and as the measured amount of water is consumed the mass converts to a solid gel. 10DR.R.koohkan

11. 2- Hydration theory: suggests that rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material. 3-Dissolution – precipitation theory: (most widely accepted theory) the setting reaction of water with calcium sulfate - hemihydrate to from calcium sulfate dihydrate is caused by the difference in solubility between these two components. based on dissolution of plaster and instant recrystallization of gypsum, followed by interlocking of the crystals to form the set solid. 11DR.R.koohkan

12.  Hemihydrate is 4 times more soluble in water than is the dihydrate near room temp (20°C). Thus the setting reaction can be understood as follows:-- hemihydrate + water  Suspension is formed that is fluid and workable.  Hemihydrate dissolves until it forms a saturated solution.  This saturated solution supersaturated in dihydrate, precipitates out dihydrate. 12DR.R.koohkan

13. .  Thus solution is no longer saturated with hemihydrate, so it continues to dissolve.  Dissolution of the hemihydrate and precipitation of dihydrate proceeds as new crystals form or further growth occurs on the present crystals. In practice about 0.2% to 0.4% linear expansion is obtained. 13DR.R.koohkan

14.  The ratio of the water to the hemihydrate powder is usually expressed as the W/P ratio  W/P ratio  setting time, strength, setting expansion.  example : if 100g is mixed with 60 ml of water, the W/P ration is 0.6. 14DR.R.koohkan

15.  The fact that the contraction of gypsum is not visible does not invalidate its existence, and when the volumetric contraction is measured by a dilatometer, it is determined to be about 7%. DR.R.koohkan15

16.  The mixing process, called spatulation, has a definite effect on the setting time and setting expansion  spatulation (either speed of spatulation or time or both) setting time  Because an increased amount of spatulation causes more nuclei centers (dihydrate) to be formed, the conversion of calcium sulfate hemihydrate to dihydrate is accelerated. DR.R.koohkan16

17. I.The first effect of increasing temperature is achange in the relative solubilities of calcium sulfatehemihydrate and calcium sulfate dihydrate, which alters the rate of the reaction. II.In general, as the temperature increases, the mobility of the calcium and sulfate ions the rate of the reaction,the setting time  the temperature is raised over37° C, the rate of the reaction decreases, and the setting time is lengthened. 17 20° C30° COver 37° C100° C The ratio of the solubilities of calcium sulfate dihydrate and calcium sulfate hemihydrate 4.53.4 1 the mobility of the calcium and sulfate ionsincrease the rate of the reactionincreasedecreasesno reaction DR.R.koohkan

18.  The initial result is acceleration of setting. Produce dihydrate, providing more nuclei for crystallization.  The final result is further contamination by moisture can reduce the amount of hemihydrate remaining to form gypsum retardation of setting will occur setting time DR.R.koohkan18

19.  Colloidal systems (agar, alginate) retard the setting of gypsum  by being adsorbed on the hemihydrate and dihydrate nucleation sites retards the setting  it is more effective on dihydrate nucleation  potassium sulfate are added to improve the surface quality of the set CaSO4 · 2H2O against agar or alginate. DR.R.koohkan19

20.  Liquids with low pH, such as saliva, retard the setting reaction  Liquids with high pH accelerate setting. DR.R.koohkan20

21. The important properties of gypsum products include  quality  fluidity at pouring time  Setting time  linear setting expansion  compressive strength  hardness  abrasion resistance  Reproduction of detail DR.R.koohkan21

22.  The time required for the reaction to be completed is called the final setting time.  The initial setting time is defined by:  The loss of gloss from the surface  Increase temperature of the mass  INITIAL GILLMORE TEST FOR INITIAL SET  The final setting time is defined as the time at which the material can be separated from the impression DR.R.koohkan22

23. The smaller needle is most frequently used for cements but it is sometimes used for gypsum products. The mixture is spread out, and the needle is lowered onto the surface. The time at which it no longer leaves an impression is called the initial set, noted as “Initial Gillmore”. It takes place at approx 13 mins. 23DR.R.koohkan

24.  The Vicat apparatus:  is commonly used to measure the initial setting time of gypsum products. It consists of a rod weighing 300g with a needle of 1-mm diameter. DR.R.koohkan24

25. The setting time can control by:  changing the temperature of the mix water a  changing the degree of spatulation.  Water-powder(W/P) ratio  The easiest and most reliable way to change the setting time is to add different chemicals. The setting time is usually shortened for power mixing compared with hand mixing. DR.R.koohkan25

26. ACCELERATORS: Sodium Sulphate - 3-4 % Potassium Sulphate - 2-3 % Sodium Chloride - 2 % Gypsum - < 20% Potassium sulfate “ROCHELLE potassium sodium tartrate SALT” The acceleration caused by an additive depends on the amount and rate of solubility of the hemihydrate versus the same effect on the dihydrate. 26DR.R.koohkan

27. RETARDERS: Act by forming an adsorbed layer on the hemihydrate and on gypsum crystals to reduce its solubility.(organic materials) forms a layer of calcium salt that is less soluble than is the sulfate salt.(salts) ORGANIC MATERIALS: Glue, gelatin and some gums. SALTS: Borax 1-2% (sodium tetraborate decahydrate) sodium chloride ~ 20% 27DR.R.koohkan

28. Manipulative VariableSetting TimeConsistencySetting Expansion Compressive Strength Increase water/powder ratio Increase Decrease Increase rate of spatulationDecrease IncreaseNo effect Increase temperature of mixing water from 23° to 30° C Decrease IncreaseNo effect DR.R.koohkan28

29.  Gypsum products may be formulated with chemicals that modify their handling characteristics and properties. 29  sodium citrate is a dependable retarder.  2% aqueous solution of borax prolong the setting time (a few hours) DR.R.koohkan

30. More voids were observed in casts made from the stones with the higher viscosities. DR.R.koohkan30

31.  The compressive strength is inversely related to the W/P ratio of the mix.  The more water, the lower compressive strength.  The excess water is uniformly distributed in the mix and contributes to the volume but not the strength of the material  the most porous the weakest stone  Porous Model plaster>Dental stone > High-strength stone  strength Model plaster<Dental stone <. High-strength stone DR.R.koohkan31

32.  The dry compressive strength is usually about twice that of the wet strength. 32 Compressive strength (MPa) 40 30 50 60 20 0 2 4 6 8 10 Weight loss (%) Effect of loss of excess water on Compressive strength of dental stone. DR.R.koohkan

33.  The surface hardness of un modified gypsum materials is related in a general way to their compressive strength.  Increased surface hardness does not necessarily mean improved abrasion resistance because hardness is only one of many factors that can affect wear resistance DR.R.koohkan33

34.  ANSI/ADA specification No. 25 requires that:  types 1 and 2 reproduce a groove 75 mm in width  types 3, 4, and 5 reproduce a groove 50 mm in width  Gypsum dies do not reproduce surface detail as well as electroformed or epoxy dies  gypsum does not wet some impression material (silicon)  use of vibration during the pouring of a cast  Use surfactants in silicone impression materials  Rinsing the impression DR.R.koohkan34

35.  Gypsum product shows linear expansion during the setting due to outward thrust of crystals that is change from hemihydrate to dihydrate.  Low – 0.06 % High – 0.5 % [CaSO4]₂ H₂O + 3H₂O 2 CaSO 4 2H₂O Molecular mass 290.284 54.048 344.322 Density(g/cm³) 2.75 0.997 2.32 Equivalent volume 105.556 54.211 148.405 Total volume 159.767 148.405 35DR.R.koohkan

36. SETTING EXPANSION CONTD..  Net change in volume is : (148.405 – 159.767) * 100 = -7.11% 159.767  CRYSTALLIZATION MECHANISM: crystals grow  outward thrust or stress develops  expansion of the entire mass. Practically the product is greater in external volume but less in crystalline volume. 36DR.R.koohkan

37. ) Hygroscopic expansion: In one technique the investment is immersed in water after setting has begun. A greatly increased setting expansion occurs. So less thermal expansion is required. Increased hygroscopic expansion is obtained in the following cases: 1) When a lower water/powder ratio is used 2) For an investment material of greater silica content. 3) If water of higher temperature is used. 4) For longer immersion in water DR.R.koohkan37

38. 38DR.R.koohkan

39.  If during the setting process, the gypsum materials are immersed in water, the setting expansion increases.  Mechanical mixing decreases setting expansion. 39 Properties of a High-Strength Dental Stone Mixed by Hand and by a Power-Driven Mixer with Vacuum DR.R.koohkan

40. 40 Flexible rubber mixing bowl and metal spatula with a stiff blade. (Courtesy of Whip Mix Corporation,Louisville, KY.) A Vibrator is designed to promote the release of bubbles in the gypsum mix and to facilitate pouring of the impression. (Courtesy of Whip Mix Corporation,Louisville, KY.) Power-driven mechanical spatulator with a vacuum attachment. (Courtesy of Whip Mix Corporation,Louisville, KY.) DR.R.koohkan

41. Properties Required of an Investment : 1.Easily manipulated: 2.Sufficient strength at room temperature: 3.Stability at higher temperatures: 4.Sufficient expansion: 5.Beneficial casting temperatures: 6.Porosity: 7.Smooth surface: 8.Ease of divestment: 9.Inexpensive. DR.R.koohkan41

42. Composition  Investment is a mixture of three distinct types of materials: 1.Refractory material 2.Binder material 3.Other chemicals DR.R.koohkan42

43.  During the heating, the investment is expected to expand thermally to compensate partially or totally for the casting shrinkage of the gold alloy. Such as:  Quartz  Tridymite  Cristobalite  or a mixture of these. 43 Microstructure of the surface of a set cristobalite investment. The large, irregular particles are,silica, and the rodlike particles are cristobalite (~3000) Courtesy oR Earnshdw.) DR.R.koohkan

44.  The common binder used for dental casting:  α-calcium sulfate hemihydrate (gold alloy)  Phosphate, (for high-temperature casting )  ethyl silicate, (for high-temperature casting) DR.R.koohkan44

45. To produce the desirable properties required of an investment. such as :  Sodium chloride  Boric acid  Potassium sulfate  Graphite  Copper powder  Magnesium oxide  small amounts of chlorides or boric acid enhance the thermal expansion of investments bonded by calcium sulfate. 45DR.R.koohkan

46.  The final product’s properties are influenced by both the ingredients present in the investment and the manner in which the mass is manipulated and used in making the mold.  The investment may contain 25% to 45% of the calcium sulfate hemihydrate. The remainder consists of silica allotropes and controlling chemicals. DR.R.koohkan46

47. Dimensional change of Three form of gypsum When heated The calciumsulfate portion of the investment decomposes into  embrittle the casting metal. 47 Temperature ("C) (Courtesy of K. Neiman,Whip-Mix (‘corporation, Louisville, I<Y.) at temperatures over 700° C sulfur dioxide sulfur trioxide Tending to DR.R.koohkan

48.  Effect of Temperature on Silicon Dioxide Refractories  Effect of Temperature on Calcium Sulfate Binders DR.R.koohkan48

49.  The percentage of expansion varies from one type to another  cristobalite and quartz each exist in two polymorphic forms  β-Form.Stable at a higher temperature  α-Form Stable at a Lower temperature. stable at room  Tridymite has three stable polymorphic forms. DR.R.koohkan49

50. DR.R.koohkan50 573°c C Quartz: At 573° C also shows a break in the expansion curve,. cristobalite, the expansion is uniform up to about 200° C. 0.5% to 1.2%, above 250° C it again becomes more uniform. Expansion increases Tridymite : Shows a similar break at a much lower temperature 105° and 160° C

51. Displacive transition temperatures. A displacive change involves expansion or contraction in the volume of the mass without breaking any bonds. ( cristobalite 220° C quartz573° C, tridymite105° and 160° C ) changing α-form β-form at all three forms of silica expand. The amount of expansion is highest for cristobalite and lowest for tridymite. DR.R.koohkan51 to

52. DR.R.koohkan52 Reconstructive transition during which bonds are broken and a new crystal structure is formed. The quartz can be converted to cristobalite and tridymite by being heated through a B-quartz B-tridymite B-cristobalite Fused silica 870° C 160° C Middle tridymite 105° C α-tridymite 1475° C 220° C 1700° C α-cristobalite Fused silica α-quartz 573° C Displacive transition temperatures

53.  Dehydration of the dihydrate and a phase change of the calcium sulfate anhydrite cause a contraction. DR.R.koohkan53 up to about 200° C between 200° -700° C. Remains unchanged Registers varying degrees of expansion, anhydrous calcium sulfate Thermal expansion Investment contracts slightly 105° C then Depending on the silica and composition of the investment

54.  When the investment is allowed to cool, the refractory and binder contract according to a thermal contraction curve that is different from the thermal expansion curve of the investment 54 Thermal expansion and contraction curves for calcium sulfate–bonded investment (thermal expansion type). Curve 1 is first heating, curve 2 is cooling curve 3 is reheating. DR.R.koohkan

55. ) Hygroscopic expansion: In one technique the investment is immersed in water after setting has begun. A greatly increased setting expansion occurs. So less thermal expansion is required. Increased hygroscopic expansion is obtained in the following cases: 1) When a lower water/powder ratio is used 2) For an investment material of greater silica content. 3) If water of higher temperature is used. 4) For longer immersion in water DR.R.koohkan55

56. DR.R.koohkan56 Thermal expansion curves for calcium sulfate–bonded investments. A, Hygroscopic type; B,thermal expansion type. For hygroscopic expansion,: the additional water provided must be presented to the investment during setting. The additional water be presented before the observed loss of gloss, which is when the setting reaction is not complete. This allows the additional water to join the remaining mix water and extend the water surface so that the action of surface tension is either delayed or inactive.

57.  Base metal alloys are usually cast into molds at 850° to 110° C. To with stand these high temperatures, the molds require different types of binders, such as silicate and phosphate compounds. ( less than 20% binder, remainder of investment is quartz or another form of silica.) DR.R.koohkan57 melting temperatures Over 700° C under700° Ccalcium sulfate–bonded investments Investment Phosphate- bonded investments Silica-bonded investments cobalt- chromium alloys Gold alloys

58. The particle sizehygroscopic expansion calcium sulfate hemihydratelittle effect silicasignificant effect 58  Finer silica produces higher Setting and hygroscopic expansions. Silica/Binder Ratio: Investments usually contain 65% to 75% silica, 25% to 35% calcium sulfate hemihydrate, 2% to 3% of some additive chemicals to control the different physical properties If the silica/stone ratio is increased, The hygroscopic expansion of the investment The strength of the investment increases, decreases. DR.R.koohkan

59. Manipulative VariableSetting TimeConsistencySetting Expansion Compressive Strength Increase water/powder ratio Increase Decrease Increase rate of spatulationDecrease IncreaseNo effect Increase temperature of mixing water from 23° to 30° C Decrease IncreaseNo effect DR.R.koohkan59

60.  The water bath has a measurable effect on the wax pattern. At higher water-bath temperatures 1-the wax pattern expands, requiring less expansion of the investment to compensate for the total casting shrinkage. 2- soften the wax provides less resistance to the expansion of the investment making the setting expansion more effective. DR.R.koohkan60

61.  Casting techniques involving gypsum bonded investments are often classified as 1- Thermal techniques DR.R.koohkan 61 the invested ring placing into the burnout oven (649° C), directs after setting the invested ring before setting immersing in a water bath after settin g (482° C). the burnout oven Although all gypsum-bonded investments exhibit both thermal and hygroscopic setting expansion, 2 - Hygroscopic techniques.

62. 62  Investments used in the thermal technique usually contain cristobalite has a high thermal expansion.  Investments used in the hygroscopic technique usually contain lower thermal expansions higher hygroscopic setting expansions. quartz or tridymite Temperature ( C) Expansion (%) Thermal expansion of mixed hygroscopic- thermal gold casting investment. DR.R.koohkan

63. 63 Temperature ( C Expansion (%) Setting and hygroscopic expansion of mixed hygroscopic-thermal gold casting investment.

64. 1. A water-soluble phosphate ion. 2. The second component reacts with phosphate ions at room temperature. 3. The third component is a refractory, such as silica. This type of investment consists of three different components. ANSI/ADA specification No. 126 (ISO 9694) for dental phosphate-bonded casting investments Specifies two types of investments for alloys having a solidus temperature above 1080° C: Type 1: For inlays, crowns, and other fixed restorations Type 2: For removable dental prostheses 64DR.R.koohkan

65. 65 an acid-base reaction between acid monoammonium phosphate + Mgo forming a binding medium with filler particles embedded in the matrix. Spatulation continues The water produced by this reaction at room temperature Phases formed at high temperatures DR.R.koohkan

66. The special liquid is a form of silica sol in water phosphate-bonded investments possess higher setting expansion when they are mixed with the silica sol than when mixed with water. ( increases its strength ) DR.R.koohkan66 Effect of silica sol concentration on thermal expansion (solid lines) at 800° C and setting expansion (dotted lines) of two phosphate-bonded investments. A, Thermal expansion type; B, hygroscopic expansion type. Thermal expansion type

67. This type of investment may derive its silica bond from ethyl silicate, an aqueous dispersion of colloidal silica, or from sodium silicate. DR.R.koohkan67

68. Setting reaction: a) Stage 1 : hydrolysis. Ethyl silicate can be hydrolysed to silica acid, with liberation of ethyl alcohol: Si(OC2H5)4 + 4H2O Si(OH)4 + 4C2H5OH ethyl silicate In practical, a polymerised form of ethyl silicate is used, yielding a sol of polysilicate acid. a colloidal solution of silicic acid and ethyl alcohol, 68DR.R.koohkan

69. 69 b) Stage 2: gelation. The sol is mixes with cristobalite or quartz, then gel formation is made to occur under alkaline conditions by adding magnesium oxide. There is a slight shrinkage at this stage. c) Stage 3: drying. on heating, considerable shrinkage occurs and there is a loss of alcohol and water, leaving a mould made of silica particles tightly packed together. As alternative of the above, simultaneous hydrolysis and gel formation can occur, when an amine such as piperidine is incorporated.

70. ANSI/ADA specification No. 126 (ISO 11246) The setting time must not differ by more than 30% from the time stated by the manufacturer. The compressive strength at room temperature shall not be less than 1.5 MPa. The linear thermal expansion must not differ by more than 15% from the time stated by the manufacturer. Brazing Investment When brazing (soldering) the parts of a DR.R.koohkan70

71.  ANSI/ADA specification No. 126 (ISO 11244)  for dental brazing investments defines two types of investment:  Type 1: Gypsum-bonded dental brazing investments  Type 2: Phosphate-bonded dental brazing investments  Soldering investments are designed to have lower setting and thermal expansions than casting investments, a feature that is desirable so the assembled DR.R.koohkan71

72.  INTRODUCTION Casting is the process by which a wax pattern of a restoration is converted to a replicate in dental alloy.  It is used to make dental restorations such as inlays, onlays, crowns, bridges, and removable partial dentures DR.R.koohkan72

73. 1. TOOTH PREPARATION. 2. IMPRESSION. 3. DIE PREPARATION. 4.WAX PATTERN FABRICATION. - There are 4 methods for making wax patterns for a cast restoration. 5. SPRUING. DR.R.koohkan73

74.  Gingival retraction  Retraction cord 74DR.R.koohkan

75. 1.Create a wax pattern of the missing tooth / rim 2.Sprue the wax pattern 3.Invest the wax pattern 4. Eliminate the wax pattern by burning it (inside the furnace or in hot water). This will create a mould. 5. Force molten metal into the mould - casting. 6.Clean the cast. 7.Remove sprue from the cast 8. Finish and polish the casting on the die DR.R.koohkan75

76. 76DR.R.koohkan

77.  The management of these dimensional changes is complex, but can be summarized by the equation:  wax shrinkage + metal shrinkage = wax expansion + setting expansion + hygroscopic expansion + thermal expansion DR.R.koohkan77

78.  the wax will shrink significantly because of the high coefficient of thermal expansion of waxes.  Metal shrinkage occurs when the molten metal solidifies, but this shrinkage is compensated by introducing more metal as the casting solidifies  Cooling shrinkage may reach 2.5% for an alloy that cools from a high solidus temperature (1300" to 1400' C), depending on the coefficient of thermal expansion of the alloy. DR.R.koohkan78

79. DR.R.koohkan79  FUNCTIONS OF SPRUE 1. Forms a mount for the wax pattern. 2. Creates a channel for elimination of wax. 3.Forms a channel for entry of molten metal 4. Provides a reservoir of molten metal to compensate for the alloy shrinkage

80. 1. DIAMETER : It should be approximately the same size of the thickest portion of the wax pattern.Too small sprue diameter suck back porosity results. 2. SPRUE FORMER ATTACHMENT : Sprue should be attached to the thickest portion of the wax pattern.It should be Flared for high density alloys & Restricted for low density alloys. DR.R.koohkan80

81. 3. SPRUE FORMER POSITION Shape & form of the wax pattern. Patterns may be sprued directly or indirectly.. Indirect method is commonly used 4.Reservoir prevents localised shrinkage porosity. Reservoir And Its Location Reservoir portion of a Spruing system is a round ball or a bar located 1mm away from the wax pattern.. Round ball reservoir & a bar reservoir also called connector Significance of Reservoirs: DR.R.koohkan81

82. Reservoir should be positioned in the heat centre of the ring. This permits the reservoir to remain molten for longer and enables it to furnish alloy to the pattern until they complete solidification process. 5. SPRUE FORMER DIRECTION Ideal angulation is 45 degrees 6. SPRUE FORMER LENGTH Depends on the length of casting ring 7.TYPES OF SPRUES - Wax. II. Solid - Plastic. Hollow - Metal. DR.R.koohkan82

83. VENTING DR.R.koohkan83  Small auxilliary sprues or vents improve casting of thin patterns Acts as a HEAT SINK.

84. To minimise the irregularities on the investment & the casting a wetting agent can be used. FUNCTIONS OF A WETTING AGENT 1. Reduce contact angle between liquid & wax surface. 2.Remove any oily film left on wax pattern DR.R.koohkan84

85. It serves as a base for the casting ring during investing.Usually convex in shape.May be metal, plastic or rubber DR.R.koohkan85

86.  Most common way to provide investment expansion is by using a liner in the casting ring.Traditionally asbestose was used. Non asbestose ring liner used are :  1) Aluminosilicate ceramic liner. 2) Cellulose paper liner.  a resilient liner is to allow different types of investment expansion facilitate venting during casting procedure. facilitate the removal of the investment block after casting. DR.R.koohkan86

87.  RINGLESS INVESTMENT TECHNIQUE  Useful for high melting alloys  used for phosphate bonded investments  This method uses paper or plastic casting ring  Solid rings do not permit the investment to expand laterally during the setting and hygroscopic expansions of the mold DR.R.koohkan87

88.  the correct water powder ratio of the investment mix,  A required number of spatulation turns,  A proper investing technique are essential to obtain acceptable casting results.  hand investing and vacuum investing.  requires 45 to60 minutes DR.R.koohkan88

89.  Two type of torch tips: Multi-orifice is widely used for metal ceramic alloys. Main advantage is distribution of heat over wide area for uniform heating of the alloy. Single-orifice tip concentrate more heat in one area. DR.R.koohkan89

90. Three fuel sources are used for Casting Torch; 1-Acetylene 2-Natural Gas 3-Propane CASTING CRUCIBLES Four types are available ; 1) Clay. 2) Carbon. 3) Quartz. 4) Zirconia –Alumina. DR.R.koohkan90

91.  It is a device which uses heat source to melt the alloy casting force.  Heat sources can be : 1) Reducing flame of a torch. ( conventional alloys & metal ceramic alloys ) 2) Electricity.(Base metal alloys )  It is a device which uses heat source to melt the alloy casting force.  Heat sources can be : 1) Reducing flame of a torch. ( conventional alloys & metal ceramic alloys ) 2) Electricity.(Base metal alloys ) DR.R.koohkan91

92.  Advantages of electric heating : - heating is evenly controlled. -minimal undesirable changes in the alloy composition - Appropriate for large labs.  Disdvantages : Expensive. DR.R.koohkan92

93. 1) Air pressure. 2) Centrifugal force. 3) Evacuation technique 93 Centrifugal casting machine, DR.R.koohkan

94. 1)Alloy is melted in a separate crucible by a torch flame & is cast into the mold by centrifugal force.(centrifugal C M ) 2)Alloy is melted electrically by a resistance heating or by induction furnace & then cast centrifugally by motor or spring action ( spring wound CM electrical resistance ) 3) Alloy is melted by induction heating cast into mold centrifugally by motor or spring action.(Induction CM ) 4) Alloy is vacuum melted by an argon atmosphere Torch melting / Centrifugal casting machine Electrical resistance /Heated casting machine DR.R.koohkan94

95. DR.R.koohkan95 Electrical resistance Spring-wound casting Induction melting casting machine, water cooling induction coil. Induction melting casting machine, vertical crucible positional within the induction coil

96. DR.R.koohkan96

97. DR.R.koohkan97