construction of dental restoration Dental waxes Casting investment dental material


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construction of dental restoration Dental waxes Casting investment dental material

  1. 1. Construction of Dental Restoration 1. Dental waxes 2. Casting investment
  2. 2. Dental restoration is a replacement for the lost natural hard or soft dental tissues.
  3. 3. 1. Resin (Polymer) - based restorations Composite fillingsDenture acrylics
  4. 4. Metal-ceramic All-ceramic 2. Ceramic - based restorations
  5. 5. Gold alloysAmalgam alloys 3. Metal - based restorations
  6. 6. Construction of metallic restorations or .. metal copings A. Direct Technique e.g. direct amalgam filling B. Indirect Technique (Laboratory-made) 1. Swaging (cold working) Hammering the metal over a die 2. Casting (lost wax technique) Pouring of molten metal into a refractory mold 3. Electroforming Precipitation of metal ions over a master die
  7. 7. Casting metallic restoration using lost wax technique
  8. 8. Materials necessary for constructing indirect metallic restorations using the lost wax technique 1. Dental waxes .. (to construct wax pattern) 2. Casting investment .. (to construct the mold) 3. Metallic alloys .. (restorative material)
  9. 9. Dental waxes Dental waxes are hydrocarbons in nature (contain O2 , H, and C) and have many applications in Dentistry
  10. 10. Ideal requirements of pattern waxes  Should be colored to be easy distinguished either from the die material or the tooth structure.  Should be cohesive and not adhesive  Should soften without flaking.  Should soften at reasonable temperature and harden at mouth temperature  Should have adequate flow to record the details of the prepared tooth  Should burn-out completely with no residue.
  11. 11. 1. Blue inlay pattern wax Uses To construct wax patterns for cast inlays, onlays , crowns and bridges
  12. 12. 1. Blue inlay pattern wax Presentation forms  Usually presents as Blue or green sticks or rods  Recently, presents in different colors and formats (blocks or in Tins)
  13. 13. 2. Casting wax Uses Constructing wax patterns of metallic frameworks either for removable partial or complete dentures.
  14. 14. 2. Casting wax Available in many shapes and colors to fit different applications Presentation forms  Preformed meshes and clasps  Sheets with different thickness  Bars with round, half round and half bear cross section.  wires.
  15. 15. 3. Base plate wax Uses 1. Temporary base plates and wax rims (bite blocks) 2. waxing the dentures up.
  16. 16. 3. Base plate wax Presentation forms  Pink or red sheets  Preformed shapes are currently presented
  17. 17. Other types of Dental waxes Processing waxes: Used for laboratory issues 1. Sticky wax.. to stick 2 portions of broken restoration or casts before repair. 2. Utility wax .. to position artificial teeth to their sheets. 3. Boxing wax .. to box an impression to facilitate its pouring in stone. Impression waxes: No longer be used 1. Bite registration wax .. to check high points of dental restorations. 2. Corrective wax .. to make functional 2ry impressions.
  18. 18. Stop, give me a break !!!!
  19. 19. Casting Investment Casting investment is composed of silica particles (SiO2) bonded together with different binding agents
  20. 20. Requirements of ideal investment material  Should be stable (not decompose) on heating (Refractory)  Should show a kind of expansion to compensate for metal shrinkage  Withstand the impact of molten metal  Able to wet the wax surfaces registering its details  Should be porous to permit escape of gases  Not adhere to the surface of cast metal  Easy breakdown after casting  Easily manipulated, not expensive , not irritant and no release of toxic products
  21. 21. Types of investment materials Relative to the type of silica particles used  Quartz-based investment  Cristobalite- based investment Cristobalite-based investment provide higher rates of thermal expansion (displasive transformation) than quartz-based investment does (when heated from 0- 600oC) Relative to the type of binder used  Gypsum-bonded investment .. Uses CaSo4. 2H2O binder  Phosphate-bonded investment .. Uses magnesium ammonium phosphate  Silica-bonded investment .. Uses silica gel binder In both types Tridymite could present as impurity
  22. 22. 1. Gypsum-bonded Investment Uses: For casting metal alloys that melt below1000oC (i.e. gold alloys only) because the investment decompose if heated above 700oc. Presentation: Presented as powder to be mixed with regular tap water After mixing, the calcium sulfate hemihydrates dissolves in water and changes into calcium sulfate dihydrates in the form of inter-meshed crystals having the silica particles in between..
  23. 23. A. Dimensional changes: 1. Setting expansion: as a result of gypsum setting 2. Hygroscopic setting expansion: as a result of adding water after initial setting (6 min from the start of mix) …. water could be gained through the following approaches; a. Water adding using hypodermic syringe b. Immersion in warm water c. Contact with wet ring liner 3. Thermal expansion: as a result of silica displasive transformation 4. Heating (evaporation of water) and cooling (after casting) contraction Important characters of Gypsum- bonded Investment
  24. 24. Setting Vs Hygroscopic expansion Step 1. Setting expansion Step 2. Hygroscopic expansion Outward thrusting Adding water after 6 min Extra expansion 3 times more than the normal setting expansion Crystal growth
  25. 25. B. Interfacial properties: • Gypsum-bonded investment usually gives castings with little surface roughness. Important characters of Gypsum- bonded Investment (cont’d) C. Chemical properties: • The set investment material is porous permitting the escape of gasses through out the mold…… This fact looks advantageous in preventing the back-pressure porosity in the cast metal • The material could not be heated above 700oC ……. to avoid the disintegration of gypsum binder
  26. 26. Phosphate-bonded Investment Uses: For casting metal-ceramic alloys and some base metal alloys (Can be heated above 1000oc). Presentation: Presented as powder (containing ammonium diacid phosphate and magnesium oxide) to be mixed with either water or colloidal silica solution After mixing, the following reaction occurs to form crystals of magnesium ammonium phosphate which bind the silica particles together giving the investment its room temperature strength. MgO + NH4 H2Po4 ---------------> Mg NH4 Po4 + H2O
  27. 27. Important characters of phosphate- bonded Investment A. Dimensional changes: 1. Setting expansion This expansion is greater with the investment mixed with colloidal silica. 2. Thermal expansion As a result of silica displasive transformation. It is greater in investments mixed with colloidal silica. 3. Drying shrinkage in water-mixed investment, a drying shrinkage will occur at 200-400oC and this is completely eliminated if the colloidal silica is used.
  28. 28. B. Interfacial properties  PBI gives rougher cast surface than the GBI.  increasing the silica solution ratio enhances the surface smoothness. C. Chemical properties The set PBI shows the same degree of porosity as that of GBI. PBI could be heated up to 1100oC D. Strength properties Set PBI shows a higher strength at room temperature than that of GBI. Set PBI shows a higher strength at high temperature than that of GBI. Important characters of phosphate- bonded Investment (cont’d)
  29. 29. Silica-bonded Investment Uses: For casting high fusing base metal alloys that usually used for constructing partial denture frameworks. Presentation: Presented as powder (containing silica and MgO) and two bottles of liquids (1. diluted silicate solution, 2. diluted HCl). Equal volume of each solution should be mixed at first to form silicic acid sol. Powder is then mixed with the sol that will change into silicic acid gel (room temp binder) in presence of MgO. That gel will dry and change into silica on heating
  30. 30. Important characters of Silica- bonded Investment A. Dimensional changes 1. Setting contraction Due to the transformation of silicic acid sol into silicic acid gel. 2. Green contraction On heating, the investment mass dries and loses alcohol and water. 3. Thermal expansion Due to displasive transformation of silica particles. Because of higher silica content, the amount of thermal expansion is fair enough to compensate for the shrinkage of wax pattern, solidification of metal as well as the setting and green contraction of the investment.
  31. 31. B. Interfacial properties SBI produces the roughest cast in comparison to other types of investment. C. Strength properties SBI has higher strength than both GBI and PBI… D. Chemical properties: SBI has a complicated and time consuming manipulative procedure. SBI could be heated up to 1300oC, and it is used to cast base metal alloys with very high melting temperatures (1300-1500oC) accordingly. Important characters of silica-bonded Investment (cont’d)
  32. 32. Gases comes out Metal gets in Air vent Investment Mold cavity Sprue channel Silica particles of the set material are closely packed together, producing a negligible degree of porosity. Lack of porosity may lead to pack- pressure porosity in the cast metal. Air spaces or vents must be left in the investment to allow the escape of gases.
  33. 33. Thank you