Model plaster


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Applied Dental Material
Second Year

Published in: Health & Medicine

Model plaster

  1. 1. Models & Die Materials Applied Dental Materials 3rd year
  2. 2. Gypsum Products Used extensively in dentistry to make dental models. Obtained from natural gypsum rock (dihydrate form of Calcium sulfate CaSO4 . 2H2O) CaSO4 . 1/2H2O + 1 1/2 H2O CaSO4 . 2H2O + 3900 cal/g mol
  3. 3. Dental Models Three-dimensional reproductions of the teeth and the surrounding soft tissue of a patient’s maxillary and mandibular arches. Also referred to as study casts.
  4. 4. Use of Dental Models Diagnosis for a fixed or removable prosthetic. Diagnosis of orthodontic treatment. Visual presentation of dental treatment. Making of custom trays. Making of orthodontic appliances. photo-gallery/pauls-work-007.jpg
  5. 5. Ideal Requirements Should have: - Sufficient strength to prevent accidental breakage. - Sufficient hardness to avoid surface damage during use. Should produce fine details and sharp margins of the impression.
  6. 6. Should show little dimensional change on setting and remain dimensionally stable. Should be compatible with the impression materials. (No interaction between the impression surface and the model surface) Should have good color contrast with the other materials being used. Should be cheap and easily used.
  7. 7. Types CaSO4 . 2H2O Plaster fluffy, porous &least dense Hydrocal higher density & more crystalline Dansite desist of raw material
  8. 8. Manufactures Plaster Hydrocal plaster II High-strength low-expansion DS IV. Densite Low- to moderate strength III High-strength high- expansion DS V.
  9. 9. Model plaster: commonly called plaster of Paris, is used primarily for pouring preliminary impressions and the making of diagnostic models. Dental stone: for use as a working model when a more durable diagnostic cast is required. Examples are in the making of custom trays and orthodontic appliances.
  10. 10. High‑strength stone: also known as densite or improved dental stone. Its strength, hardness, and dimensional accuracy make it ideal to create the dies used in the production of crowns, bridges, and indirect restorations.
  11. 11. Plaster Mineral Dehydration gypsum Hydrocal Densite open kettle 110 - 120°C under pressure water vapor 125 °C boiled in 30% CaCl Sol. Model Plaster Dental stone High strength dental stone Chemicals can be added to modify their handling properties.
  12. 12. Chemical Reaction Gypsum Plaster D Stone high-strength DS Mix water 37-50 28-32 19-24 Required water 18.6 18.6 18.6 The chemical reaction takes place the amount of water needed. Excess water 18-31 9-13 0-5 determine Water thick mix, difficult to handle, trap air bubbles but the set gypsum usually strong. Water Thinner mix, porous easily handle, but the set gypsum is inferior.
  13. 13. W/P Ratio for DS and High Strength DS Crystal shapes and concentration play a major role in W/P ratio needed to obtain a workable consistency.
  14. 14. Mechanism of Setting Different solubility between CaSO4 . 2H2O & CaSO4 . 1/2H2O. CaSO4 . 1/2H2O + H2O Supersaturation Nucleation process ( needle-like crystals) Bonding between contacting crystals Final cohesion.
  15. 15. Dissolution Water Ca++ SO -- CaSO4. 2 H2O Nuclei of crystalization Precipitation
  16. 16. Effect of Spatulation Definite effect on the setting time and setting expansion. amount of spatulation setting time
  17. 17. Effect of Temperature Temp. of the mixing water as well as the temp. of the environment affect the setting time in one of two ways: 1. Change in the solubility of CaSO4 . 2H2O & CaSO4 . 1/2H2O. - solubility ratio setting time. slow the reaction
  18. 18. Temperature (°C) CaSO4 . 1/2H2O CaSO4 . 2H2O 20 0.90 0.200 25 0.80 0.202 30 0.71 0.209 40 0.61 0.210 50 0.50 0.205 100 0.17 0.170
  19. 19. 2. Change in the Ion mobility: - Temp. sol. & mobility of Ca++ & SO4-reaction rate setting time. - at 100°C rate of reaction decreased and the the setting time is lengthened.
  20. 20. Effect of Humidity When humidity reaches 70% and above: the moist convert CaSO4 . 1/2H2O into CaSO4 . 2H2O which will act as extra centers of nucleation, thus accelerating the reaction. Further contamination CaSO4 . 1/2H2O amount of available of retarding the reaction.
  21. 21. Effect of Colloidal Systems & PH Agar and alginate retard the reaction (potassium sulfate). High pH accelerate the reaction, while low pH retard the reaction.
  22. 22. Properties Setting time: the time required for the reaction to be completed (final setting time). Final setting time: when the material can be removed from the impression without distortion. Initial setting time: the time required to reach a certain arbitrary stage of firmness in their setting process. working time.
  23. 23. Control of Setting Time Addition of accelerators or retarders. Temp. of water. Degree of spatulation. W/P ratio.
  24. 24. Viscosity more voids are observed in cast made of stones with higher viscosity. Material Viscosity (cp) high strength DS A 21,000 B 29,000 C 50,000 D 54,000 E 101,000 Impression Plaster 23,000
  25. 25. Compressive Strength Water Compressive strength. Wet strength: with some or all the excess water present in the specimen. Dry strength: with all the excess water drive out. Dry C.S is almost twice the wet C.S. 8.8% excess water is in the hardened stone.
  26. 26. 0.50 11 9 31 0.30 20.5 0.50 10.5 0.24 High-strength Dental Stone 12.5 0.27 Dental Stone Compressive Strength (MPa) 0.55 Model Plaster W/P Ratio (ml/g) 0.45 Material 38 0.30 21.5 0.50 10.5
  27. 27. Surface Hardness and Abrasion Resistance After setting occur the C.S remain constant tell all the excess water evaporate from the surface, after which it increase. Hardness increase at higher rate than C.S. Attempts have been made to increase the hardness of gypsum products. Impregnating with epoxy or MMA, oils, colloidal silica.
  28. 28. Production of Details ADA specification requires type I & II reproduce a groove 75 µm in width, type III, IV & V reproduce a groove 50 µm in width. Gypsum Vs Epoxy. Elastomeric impressions. Contaminations.
  29. 29. Setting Expansion The % of setting expansion varies from one type to another. Plaster 0.2% to 0.3%. Low to moderate-strength DS 0.15% to 0.25%. High-strength DS 0.08% to 0.10%. High-strength high expansion DS 0.10% to 0.20%.
  30. 30. almost 75% of the expansion observed during the first 24 hrs occurs at the first hour of setting. Machine mixing. W/P ratio. chemicals. hygroscopic expansion.
  31. 31. Manipulation The powder is added to the water and allowed to settle for 30 sec. Mixing can be done by hand spatulation or by mechanical spatulation. Pouring requires care to avoid trapping air. Once poured it has to be allowed to set for 45 to 60 min.
  32. 32. Other Die Materials Silico Phosphate Cement: Hard but shrink on setting. Amalgam: Amalgam reproduces fine details but has some disadvantages: Should be packed against rigid impression materials. Needs a long time to reach maximum strength. Needs a separating medium. Has high thermal conductivity so it cools the wax pattern rapidly, which may lead to pattern distortion.
  33. 33. Acrylic resin and other polymeric materials: They have high abrasion resistance and they are harder than gypsum but their shrinkage during polymerization leads to inaccuracy. Epoxy die: Epoxy die is very toxic and it shrinks during polymerization.
  34. 34. Metal sprayed die: Bismuth tin metal sprayed directly on impression to form metal shell then the gypsum is directly poured in it. Electroplated die: Silver plated die is used with rubber impression materials but is not used with impression compound. Copper plated die is used with impression compounds.
  35. 35. QUESTIONS?