Investment materials and investing techniques


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Investment materials and investing techniques

  1. 1.  first describe by Philbrook D, & Taggart WH Mould/ die for pattern
  2. 2. Sprue attached to patterns
  3. 3. Patterns invested Burnout in furnace Flow of molten metal
  4. 4. Fundamentals of Spruing, Investing & Casting
  5. 5. After casting Finished & polished
  6. 6. Investing: the process of covering or enveloping, wholly or in part, an object such as denture, tooth, wax form, crown, etc. with a suitable investment material before processing, soldering or casting. (GPT 8)  Dental casting investment: A material consisting primarily of an allotrope of silica and a bonding agent. The bonding substance may be gypsum (for use in lower casting temperatures) or phosphates and silica (for use in higher casting temperatures). (GPT 8) 
  7. 7. 1. should be capable of reproducing the shape, size and detail in the wax pattern. 2. easily manipulated. 3. setting time should be less. 4. should maintain the integrity at higher temperatures and should not decompose to give off gases. 5. Possess sufficiently high value of compressive strength at the casting temperature to withstand the stresses set up when the molten metal enters the mould. 6. should expand to compensate for the casting shrinkage
  8. 8. 7. Investment should be porous enough to permit the air or other gases in the mold cavity to escape easily during the casting 8. Investment should produce a smooth surface and fine detail and margins on the casting. 9. should be inexpensive.
  9. 9.  Wax shrinkage- 0.3-0.4%.  Alloy shrinkage- 1.3- 1.6% ( noble alloys), 2.0- 2.3%( Predominantly base metal alloys)
  10. 10.  Investment materials are basically consist of 1. A refractory material 2. A binder material 3. Additives used to improve the handling character.
  11. 11. SILICA (silicon dioxide) is used as refractory material & to regulate the thermal expansion. It is available in four allotropic forms such as Quartz Tridymite Cristobalite Fused quartz Quartz and Cristobalite are used extensively in dental investments
  12. 12. Cristobalite occurs naturally as a rare mineral but is normally manufactured by prolonged heating of the quartz at high temperatures to induce the appropriate slow inversion. Each form of silica exists in two phases. 1. Low temperature phase or alpha phase 2. High temperature phase or Beta phase On heating the change between the two phases is rapid and readily reversible on cooling .this change is known as high – low inversion .
  13. 13. Depending on type of silica used the investment materials are classified as 1. Quartz investments 2. Cristobalite investments
  14. 14. It binds the refractory materials together.  According to the binder used investment materials are of three groups.  1. GYPSUM-BONDED INVESTMENTS 2. PHOSPHATE BONDED INVESTMENTS 3. SILICA BONDED INVESTMENTS
  15. 15. Other chemicals such as sodium chloride, boric acid, graphite, copper powder, are often added in small quantities to modify physical properties.  boric acid and sodium chloride, not only regulate the setting expansion and the setting time, but also prevent most of the shrinkage of gypsum when it is heated above 300 ºC (572° F). 
  16. 16.  They are the mold materials used in the casting of dental gold alloys with liquidus temperatures no more than 1080 ºc.  Used for conventional casting of gold alloys inlays, onlays, crowns and Fpd.
  17. 17. Refractory Crystalline polymorphs of silica (quartz or cristobalite)  55-75%  Silica is added to provide a refractory component during the heating of the investment and to regulate the thermal expansion. 
  18. 18.  BINDER – Alpha hemi hydrate form of gypsum(25-45%) Strength of investment depends on amount of binder present.
  19. 19. contracts
  20. 20. MODIFIER - (4-7%) Used are Reducing agents Modifying chemicals Coloring matter Reducing agents : they reduce any metal oxides formed on the metal by providing a non oxidizing atmosphere in the mold when the alloy enters mold.  Ex– Copper
  21. 21. Modifying chemicals: They regulate setting expansion and thermal expansion and also prevent shrinkage of gypsum when heated above 300 ºc .  They act by reducing the two large contractions of gypsum binder on heating to temperatures above 300 ºc .  Ex– Boric acid Soluble salts of alkali or alkaline earth metals
  22. 22. SETTING TIME According to ANSI/ADA Specification No. 2 for dental inlay casting investment, the setting time should not be shorter than 5 min or longer than 25 min. Usually, the modern inlay investments set initially in 9 to 18 min.  Should allow sufficient time for mixing and investing the pattern. 
  23. 23. SETTING EXPANSION  1. normal setting expansion: the expansion takes place when the material sets in air  2. hygroscopic setting expansion: when setting occurs under water.  3. thermal expansion: when it is affected by heat Purpose: to enlarge the mold to compensate for the casting shrinkage of the gold alloy
  24. 24. ADA sp no 2 for type 1 investment permits a maximum setting expansion in air of 0.6% setting expansion of modern investments is 0.4%which can be regulated by accelerators and retarders .  It is enlarging the mould containing the wax pattern and it depends on the thermal expansion of pattern caused by heat of reaction.  High gypsum content- high expansion. 
  25. 25. Other variables – lower w/p ratio, drier mix of investment- high NSE.  Thin wall pattern- more expansion than thick walls.  Soft wax- more NSE.  Wax softer than type II inlay wax- NSE causes serious distortion of the pattern. 
  26. 26. Greater in magnitude than NSE.  the gypsum product is allowed to set under or in contact with water for mould expansion. 
  27. 27. The hygroscopic setting expansion may be 6 or more times greater than the normal setting expansion of a dental investment  The increased amount of expansion is because the water helps the outward growth of crystals  The investment should be immersed in water before the initial set is complete.  ADA sp no 2 for such type 2 investments require minimum setting expansion in water of 1.2% and maximum 2.2%. 
  28. 28. Composition: more finer silica particles -more HSE. α hemihydrate greater HSE than β hemihydrate.  W:P ratio: less water, more powder in mix.  Spatulation: more mixing time- more HSE.  Time of immersion: immerse in water before initial set.  Confinement: less opposing force from walls of casting ring (wet cellulose), immersion in water bath at 37.7°C- expansion of wax pattern.  Water: more immersion water.  Shelf life: fresher investment. 
  29. 29. The thermal expansion is directly related to the amount and type of silica present.  Contraction of gypsum is balanced- quartz 75%  More Expansion in the presence of cristobalite and initial contraction of gypsum is eliminated.  Type 1 investments should have thermal expansion of not less than1% and not greater than 1.6%.  Type 1I investments should have thermal expansion of between 0% to 0.6% at 500°C. 
  30. 30.  SETTING CONTRACTION: When an investment is cooled from 700°C, it contracts. On reheating it expand again but not recommended as investment may crack on heating.
  31. 31.  COMPRESSIVE STRENGTH: According to ADA sp no 2 the compressive strength should not be less than 2.5MPA to withstand force of molten metal.  FINENESS: Surface roughness of the casting and setting time depend upon the fineness of the material. Fine silica- more hygroscopic expansion.
  32. 32.  POROSITY: The material should be porous to allow escape of air from mold space while casting. Lower content of hemihydrate- more water gaugingmore porous.
  33. 33. It is a gypsum bonded material mixed with colloidal silica  Setting expansion is 0.9%  Thermal expansion is 0.6% when it is heated to 677 ºc  As it is a gypsum bonded material it is not recommended for high fusing alloys.  Divestment phosphate is a phosphate bonded investment used as a divestment for high fusing alloys. 
  34. 34. Advantages      Easier to manipulate. Adequate strength. Adequate porosity. Less expensive. Amount of dimensional change easier to control. Disadvantages    Thermally unstable at high temp. Powder is hygroscopic. Improper wax burnout & high casting force may produce cracks in investment leading to fins.
  35. 35. Most palladium and base metal alloys used for partial dentures and porcelain fused to metal restorations have high melting temperatures. They should be cast a mold temperature higher than 700 ºc. To withstand these high temperatures ,molds require different types of binders such as phosphate compounds. at
  36. 36.  They are used in construction of high melting . temperature dental alloys  Soldering and porcelain veneering
  37. 37. Type 1  For casting of inlays crowns and other restorations especially for alloys like gold, platinum ,palladium cobalt chromium and nickel chromium Type 2  For casting of removable partial dentures
  38. 38. Refractory materials – (concentration of approximately 80%)silica in quartz , cristobalite or a mixture of two . Purpose  To provide high temperature thermal shock resistance  To provide high thermal expansion.
  39. 39. Binder (<20%)  Magnesium oxide (acid) and a phosphate (base)  Originally phosphoric acid was used but mono ammonium phosphate has replaced it as it can be incorporated in powder form
  40. 40. Modifiers :  Carbon is often added -clean casting.  Facilitates easy divesting of casting and mold.  Generally added when casting alloy is gold.  When silver palladium or base metal alloys are invested with the investment containing carbon, it embrittles the alloys even though the investment is heated to the temperature that burn out the carbon.  Palladium reacts with carbon even at above temp 1500°C, so carbon free phosphate bonded invst used for higher temp.
  41. 41. It is available as two component systems  1- It is a Powder which contains refractory materials and binders and modifiers  2- Aqueous solution stabilized with colloidal silica Colloidal silica suspension facilitate greater expansion of the investment which can compensate the greater casting shrinkage of alloys used in MC & newer gold alloys.
  42. 42. Expansion can be varied by the proportions of silica and water. More silica and less water – more expansion.  When mixed with water- initial shrinkage is seen same as that of GBI. Completely eliminated with colloidal silica. Liquid can be used as full strength or diluted with water to provide some degree of control over setting or thermal expansion. But if more liquid is used it caused dense nonporous investment which can effect casting. 
  43. 43. Early thermal expansion is associated withdecomposition of binder, magnesium ammonium phosphate and evolution of ammonia gas.  Some shrinkage is masked by expansion of refractory filler ( crystobalite) 
  44. 44. 1 Compressive strength  Type 1- 2.5 mpa  Type 2- 3 mpa 2 Thermal expansion  0.8% when 50:50 mixture of liquid and water
  45. 45. Temperature Warmer the mix faster it sets The setting reaction liberate the heat and accelerates rate of setting  Mixing time increased mixing time and mixing efficiency result in faster set. 
  46. 46. 1.They have high fired strength. This make them handle without breaking before they are placed in a furnace for the wax burn out process and strong enough to with stand the impact and the pressure of centrifugally cast molten alloy 2.They also provide high setting and thermal expansion enough to compensate cast metal prosthesis or porcelain veneers during cooling 3. They can withstand temp more than 900°C
  47. 47.  These are used for high fusing base metal alloy
  48. 48. Refractory material – Silica Binder –Silica gel or ethyl silicate Modifier – Magnesium oxide (strengthen the gel) Ammonium chloride - accelerator
  49. 49. Silica gel: pH of sodium silicate lowered by addition of an acid or acid salt.  Colloidal silica converted to gel by adding ammonium chloride. 
  50. 50. Ethyl silicate: colloidal polysilicic acid is formed by hydrolizing ethyl silicate in presence of Hcl, ethyl alcohol & water. It is then mixed with quartz or cristobalite and magnesium oxide ( alkaline). Coherent gel of polysilicic acid formed, dried at temp 168°C, loses alcohol & water to form concentrated hard gel . Volumeric contraction during drying is green shrinkage.  Amines added to ethyl silicate- hydrolysis and gelation occurs simultaneously. 
  51. 51. It is supplied as a powder and liquid or two liquids If supplied as a powder and liquid  Powder consists of refractory particles of silicas and glasses along with the calcined magnesium oxide and some other refractory oxides in minor amounts  Liquid contains stabilized alcohol solution of silica gel
  52. 52. If supplied as 2 liquids  One is ethyl silicate Certain types of amines are added for hydrolysis and gelation to occur simultaneously  Other is acidified solution of denatured alcohol
  53. 53. The powder is added to hydrolyzed ethyl silicate liquid, mixed quickly and vibrated into a mold , that has an extra collar to increase the height. The mold is placed on a vibrator that has a tamping action This allows the heavier particles to settle while the excess liquid and some of the finer particles rise to the top . The top of the mold is prone to cracking due to greater drying shrinkage from evaporation of the ethyl alcohol
  54. 54. In about 30 minutes the accelerator in the powder hardens the settled part, and the excess is poured off (to avoid crack formation). The liquid powder ratio in settled part is greatly reduced and the setting shrinkage is reduced to 0.1%
  55. 55. Can be used for higher temp castings ( 1090°C1180°C) and compactible with higher fusing alloys.  Care should be taken while handling and burnout as inflammable alcohol is given off. 
  56. 56. Investment can be mixed in 2 ways 1. Hand mix: The liquid is added to a clean, dry mixing bowl, and the powder is gradually added to the liquid, using the care and caution to minimize air entrapment. Mixing is formed gently until all the powder has been wet. 2. Vacuum mixing: this is a type of mechanical mixing done under vacuum created by a vacuum mix machine.
  57. 57. Patterns invested Burnout in furnace Flow of molten metal