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Investment materials

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Investment materials

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Investment materials

  1. 1. PRESENTED BY Dr EKTA GARG MDS 1st YEAR DEPARTMENT OF CONSERVATIVE DENTISTRY & ENDODONTICS
  2. 2.  An investment can be described as a material which is suitable for forming a mold into which a metal or alloy is appropriately cast.  The procedure for forming the mold is described as
  3. 3.  Easily Manipulated – not only should it be possible to mix & manipulate the mass readily & to paint the wax pattern easily, but the investment should also harden within a short time.  Sufficient strength at room temperature – To permit ease in handling & provide enough strength at higher temperature to withstand the impact force of the molten metal.  Stability at higher temperature – Investment must not decompose to give off gases that could damage the surface of the alloy.  Sufficient Expansion – Enough to compensate for shrinkage of the wax pattern & metal that takes place during the casting procedure.
  4. 4.  Porosity – Porous enough to permit the air & other gases in the mold cavity to escape easily during the casting procedure.  Smooth surface – Fine details & margins on the casting. Ease of Divestment – The investment should break away readily from the surface of the metal & should not react chemically with it.  Inexpensive  Not be bio hazardous  Reasonable setting time  Long shelf life
  5. 5. REFRACTORY MATERIAL BINDER OTHER CHEMICALS
  6. 6.  Refractory material: Usually a form of silicon dioxide, such as quartz, tridymite, cristobalite or a mixture of these.  Binder material: Common binder used for dental casting gold alloy is α calcium sulphate hemihydrate, phosphates and ethyl silicate.  Other Chemicals: Such as sodium chloride, boric acid potassium sulphate, graphite, copper powder or magnesium oxide.
  7. 7. • Phosphate bonded • Silicate bonded High temperature casting investment • Gypsum bonded Low temperature casting investment CLASSIFICATION 1. Based on processing temperature :-
  8. 8. 2. Based on type of Binder Used Gypsum bonded investment  Type I  Type II  Type III Phosphate bonded investments Ethyl silicate Bonded investments
  9. 9. 3. BASED ON TYPE OF REFRACTORY USED a.) Silica - quartz cristobalite b.) Magnesium oxide c.) Zirconia based investments
  10. 10. The Gypsum based materials represent the type traditionally used for conventional casting of gold alloy inlays, onlays, crowns, & fixed partial dentures (FPDs).
  11. 11. ADA specification No. 2 for casting investments for dental gold alloys encompasses three types of investments.  Type I: Employed for the casting of inlays or crowns when the alloy casting shrinkage compensation is accomplished principally by thermal expansion of the investment.  Type II: Investments are also used for the casting of inlays or crowns, but the major mode of compensation is by the hygroscopic expansion of the investment.  Type III: Used in the fabrication of partial dentures with gold alloys.
  12. 12. Quartz or Cristobalite – Allotropic forms of silica [ 55 – 75 % ] alpha– hemihydrate – provides strength and rigidity serves as a binder [ 25 – 35 %] Chemical modifiers – 5 %
  13. 13.  The α-hemihydrate form of gypsum is generally the binder for investments used in casting gold containing alloys with melting ranges below 1000°C.  When this material is heated to the temperature required for complete dehydration and sufficiently high to ensure complete castings, it shrinks considerably and frequently fractures.
  14. 14. Dimensional Change of three forms of gypsum when heated
  15. 15.  Silica (SiO2) is added to provide a refractory component during the heating of the investment and to regulate the thermal expansion.  During the heating, the investment is expected to expand thermally to compensate partially or totally for the casting shrinkage of the gold alloy.  If proper forms of silica are employed in the investment, the contraction of gypsum during heating can be eliminated and changed to an expansion.
  16. 16. Thermal expansion of four forms of silica Quartz 575°c Cristobalite b/w 200°c & 270°c Tridymite two inversions occur at 117°c & 163°c
  17. 17. Cristobalite Tridymite Fused Quartz
  18. 18.  Modifying agents, coloring matter, & reducing agents, such as carbon & powdered copper.  Reducing agents provide a nonoxidizing atmosphere in the mold when gold alloy is cast.  Some of the added modifiers, such as boric acid, and Na chloride, not only regulate the setting expansion and the setting time, but they also prevent most of the shrinkage of gypsum when it is heated above 300°C.
  19. 19.  According to ADA specification No. 2 for dental inlay casting investment, the setting time should not be shorter than 5 minutes nor longer than 25 minutes.  Usually, the modern inlay investments set initially in 9 to 18 minutes.  Sufficient time should be allowed for mixing and investing the pattern before the investment sets.
  20. 20.  The purpose of setting expansion is to aid in enlarging the mold to compensate partially for the casting shrinkage of the gold.  ADA specification No. 2 for Type I investment permits a maximum setting expansion ‘in air’ of only 0.6%.  The setting expansion of such modern investment is approximately 0.4%. It can be regulated by retarders and accelerators.  A mixture of silica and gypsum hemihydrate results in setting expansion greater than that of the gypsum products when it is used alone.
  21. 21.  Setting expansion is influenced by the exothermic heat transferred to the pattern.  Variables other than the exothermic heat of reaction also influence the effective setting expansion. • As the investment sets, it essentially gains sufficient strength to produce a dimensional change in the wax pattern & mold cavity as setting expansion occurs. • Also, the softer the wax, the greater the effective setting expansion, because the softer wax is more readily moved by the expanding investment.
  22. 22.  One of the methods for expanding the casting mold to compensate for the casting shrinkage of the gold alloy.  Occurs when the gypsum product is allowed to set when placed in contact with water.  Greater in magnitude than normal setting expansion  ADA specification No. 2 for Type II investments requires a minimum setting expansion in water of 1.2% while the maximum allowed is 2.2%.
  23. 23.  Factors influencing hygroscopic expansion- a. Effect of composition- The magnitude of setting expansion of a dental investment is generally proportional to the silica content of the investment. • Finer the particle size of silica greater the expansion. • α-hemihydrate will produce a greater expansion than β-hemihydrate.
  24. 24. b. Effect of water:powder ratio: The highest the W:P ratio of the original investment water mixture, the less the hygroscopic setting expansion. c. Effect of spatulation: With most investments, as the mixing time is reduced, the hygroscopic expansion is decreased.
  25. 25. d. Shelf life of the investment: Older the investment, the lower its hygroscopic expansion. e. Effect of time of immersion: The greatest amount of hygroscopic setting expansion is observed if the immersion takes place before the initial set.
  26. 26. f. Effect of the amount of water added:  The magnitude of hygroscopic expansion is in direct proportion to the amount of water added during the setting period until a maximum expansion occurs, no further expansion is evident regardless of any amount of water added.  Expansion can be detected when water is poured into a vessel containing only small, smooth quartz particles. The water is drawn between the particles by capillary action and thus causes the particle to separate, creating an expansion.
  27. 27.  The effect is not permanent after the water is evaporated, unless a binder is present.  The greater the amount of the silica or the inert filler, the more easily the added water can diffuse through the setting material and the greater is the expansion.
  28. 28.  The thermal expansion of a gypsum bonded investment is directly related to the amount of silica present and to the type of silica employed. A considerable amount of quartz is necessary to counterbalance the contraction of gypsum during heating.  The contraction of the gypsum is entirely balanced when the quartz content is increased to 75%.  The investments containing cristobalite expand earlier and to a greater extent than those containing quartz.
  29. 29. The desirable magnitude of the thermal expansion of a dental investment depends on its use. If hygroscopic expansion is to be used to compensate for the contraction of the gold alloy, as for the Type II investment. ADA specification No. 2 requires that the thermal expansion be between 0% and 0.6% at 500°C.
  30. 30. However, for Type I investment, which rely principally on thermal expansion for compensation, the thermal expansion must be not less than 1% nor greater than 1.6%. Another desirable feature of an inlay investment is that its maximum thermal expansion be attained at a temperature not higher than 700°C. Thus when a thermal expansion technique is employed, the maximum mold temperature for casting of gold alloy should be less than 700°C.
  31. 31.  Factors affecting thermal expansion- a. Effect of Water:Powder ratio The magnitude of thermal expansion is related to the amount of solids present. Therefore it is apparent that the more water that is used in mixing the investments, the less is the thermal expansion that is achieved during subsequent heating.
  32. 32. b. Effect of chemical modifiers: The addition of small amounts of sodium, potassium, or lithium chlorides to the investment eliminates the contraction caused by the gypsum and increases the expansion without the presence of an excessive amount of silica.
  33. 33. c. Thermal contraction/cooling of investment: When the investment is cooled from 700˚c, its contraction follows the expansion curve during the inversion of the beta- quartz or beta crytobalite to its stable alpha form at room temperature. On cooling – investment shows overall contraction as compared to its dimensions before heating . On reheating – does not expand to its previous level – also it can cause cracks & affect quality of casting .
  34. 34. d. Strength: According to ADA specification No. 2, the compressive strength for an inlay investment should not be less than 2.4Mpa tested 2 hours after setting. Heating the investment to 700°C may increase or decrease the strength as much as 65%, depending on the composition. The greatest reduction in strength on heating is found in investments containing sodium chloride.
  35. 35. Other gypsum investment considerations: Fineness:- Finer the investment, the smaller are the surface irregularities on the casting. Porosity:- More gypsum crystals present in the set investment, less porosity. More uniform the particle size, greater is its porosity. Storage:- The investment should be stored in airtight & moisture proof containers.
  36. 36. As suggested by Skinner (1963) “The definite advan type of investment is that there is less chance for c alloy during casting and hence could be the investmen The present trend is towards the use of less expens Alloys, most of which require phosphate investments. The rapid growth in use of metal ceramic restorations and a higher melting point alloys have resulted in an increased use of “phosphate bonded investment.”
  37. 37. Refractory fillers Binders
  38. 38. Filler: silica, in the form of cristobalite, quartz, or a mixture of the two and in the concentration of approximately 80%. • The purpose of this filler is to provide high temperature thermal shock resistance (refractoriness) and a high thermal expansion. The binder: consists of magnesium oxide and a phosphate (Monoammonium phosphate).
  39. 39.  Colloidal silica liquid suspensions are available for use with the phosphate bonded investments in place of water. For base metal alloys, a 33% dilution of the colloidal silica is required.  Carbon is often added to the powder to produce clean castings, and facilitate the ‘divesting’ of the casting from the mold. Water Exhibit less SE Hygroscopic expansion negligible Colloidal silica - Exhibit higher SE – useful as shrinkage more in base metal alloys . Can expand hygroscopically Strength of investment more
  40. 40. Setting Reaction The chemical reaction for the binder system that causes the investment to set and harden is NH4H2PO4 + MgO + 5H2O  NH4 MgPO4 6H2O
  41. 41. MgO+NH4H2PO4+ 5H2O (NH4MgPO4 .6H2O)n Room Temperature MgO NH4H2PO4 Colloidal-type particles H2O Prolonged setting at 25°c or dehydration at 50°c (NH4MgPO4 .6H2O)n Dehydrated at 160°c (NH4MgPO4 .H2O)n Heated from 300-650°c (Mg2P2O7)n Noncrystalline polymeric phase Mg2P2O7 Heated above 1040°c Mg3 (P2O4) 2 On heating the binder undergoes the following thermal reactions :
  42. 42. When phosphate bonded investments are mixed with water they exhibit a shrinkage within essentially the same temperature range as gypsum bonded investments i.e. 200°c-400°c This contraction is practically eliminated when a colloidal silica solution replaces the water. The early thermal shrinkage of phosphate investments is associated with the decomposition of the binder, magnesium ammonium phosphate and is accompanied by the evolution of ammonia, which is readily apparent by its odor. Setting and Thermal Expansion
  43. 43. Influence of liquid concentration on setting & thermal expansion. Thermal expansion of the investment when mixed with water as compared to special liquid.
  44. 44. Phosphate investments are markedly affected by temperature. The warmer the mix, the faster it sets. The setting reaction itself gives off heat, and this further accelerates the rate of setting. The more efficient the mixing better the casting in terms of smoothness and accuracy. The ideal technique is to mix, as long as possible, yet have enough time for investing. Mechanical mixing under vacuum is preferred.
  45. 45. Ethyl Silicate Bonded Investment
  46. 46. HYL SILICATE bonded investments are being us nstruction of the high fusing base metal partial d alloys. e investments are losing popularity because of th mplicated and time consuming procedures involve The silica is used as the binder h may be derived from ethyl silicate or sodium sil
  47. 47. POWDER LIQUID It’s a mixture of  Powder – refractory particles of silica , MgO & other oxides .  For these systems – 2 or more liquids – reacted together – before being mixed with powder .
  48. 48. The REACTION The silica is first formed by the hydrolysis of ethyl silicate in the presence of hydrochloric acid, ethyl alcohol & water. The reaction can be expressed as: Si (OC2H5) + 4H2O HCl Si(OH)4 + 4C2H5OH
  49. 49. Because a polymerized form of ethyl silicate is used, a colloidal sol of polysilicic acids is expected instead of the simpler silicic acid sol shown in the reaction- Stage called - HYDROLYSIS The second stage of reaction – GELATION . Sol – mixed with quartz or cristobalite + small amount of MgO to render the mixture alkaline . Coherent gel of polysilicic acid formed – accompanied by slight ‘setting shrinkage’.
  50. 50. Third stage – DRYING Soft gel – dried to a temp. below 168 °C . During drying – Gel loses alcohol & water – to form hard concentrated gel of silica particles – tightly packed together . Volumetric contraction accompanies drying – “ green shrinkage” .
  51. 51.  Process of ethyl silicate-bonded investment is a little more complicated than that of phosphate type in that care must be exercised during handling & burnout because inflammable alcohol is given off. This type of investment can be heated between 1090°c and 1180°c and is compatible with the higher fusing alloys. Its low setting expansion minimizes distortion.

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