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Gypsum Materials

Gypsum Materials. DA 122 Dental Materials. Applications for Gypsum. Used to make a model from an impression Use of model determines type of gypsum used Gypsums vary in Strength Dimensional accuracy Resistance Reproduction detail Water/powder ratio Setting times. Calcination.

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Gypsum Materials

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  1. Gypsum Materials DA 122 Dental Materials

  2. Applications for Gypsum • Used to make a model from an impression • Use of model determines type of gypsum used • Gypsums vary in • Strength • Dimensional accuracy • Resistance • Reproduction detail • Water/powder ratio • Setting times

  3. Calcination • Manufacturing process used for gypsum production • Gypsum is mined rock; then ground into a fine powder • Powder is heated until specific amount of water is driven out of the gypsum = CALCINATION

  4. Types of Gypsum Materials See pages 859-860 in Dental Assisting text

  5. Type I: Impression Plaster original impression material rigid, fractures and breaks easily used for edentulous models Ratio: 60 ml water : 100 gm powder

  6. Type II: Lab Plaster/Model Plaster • Slightly stronger than Type I • Used for: diagnostic casts or study models • Used to fabricate bleach trays, night guards, sports mouth guards • Used in treatment planning, measuring dentition, legal record of, comparison of before and after treatment • Ratio: 50 ml water : 100 gm powder

  7. Type III: Lab Stone • Stronger than plaster • Normally yellow (“buff”) • Used for: study models, working casts, models for partial and full dentures • Ratio: 30 ml water : 100 gm powder

  8. Orthodontic Stone • White • Used for: diagnosis and treatment of orthodontic cases • Ratio: 30 ml of water : 100 gm powder

  9. Type IV: Die Stone • Positive replica of prepared tooth • Strong and resistant to abrasion • Most accuracy and detail • Used for: dies and where strong model is needed in casting metals or making crowns • Ratio: • 24 ml water : 100 grams powder

  10. Type V: High-strength Die Stone • High strength and high expansion • Recently added by the ADA • Strongest accepted gypsum product • Ratio: 18-22 ml water : 100 gms powder

  11. Water : Powder Ratio • Less water • Shortens setting time; dry and crumbly • More water • Thin mixture; weaker product with air bubbles • After mix starts: adding water to thin or powder to thicken =weakens final product • Increasing water temperature = faster set with no affect on quality of final product

  12. Armamentarium • Flexible rubber bowl • Stiff narrow spatula • Water measuring device • Scale • Model vibrator, with protective covering • Room temperature water

  13. Mixing Process (hydration) • Add powder to water; let it dissolve into water • Slowly mix particles with spatula (20 seconds) • Add powder to desired creamy, putty-like thickness (peanut butter) (1 minute) • Material will stick to spatula when lifted from bowl, when it is proper thickness • Vibrate to remove air bubbles (1 minute)

  14. Mixing Techniques Using vibrator to remove air bubbles Checking for proper thickness

  15. Caution re: mixing • DO NOT WHIP: will add air and bubbles • DO NOT OVERMIX: will add weak spots to model

  16. Setting Process (dihydration) • Exothermic reaction (release of heat) drives off water to harden gypsum material • Material will heat up and then cool down

  17. Setting Time (40 – 60 minutes) • Initial Set: • Time between start of spatulation until material loses gloss • Firm and solid enough to handle • Still moist and pliable (cold and wet) • Final Set: • After all heat is driven away • Cool, hard and dry

  18. Factors Affecting Setting Time • Type of gypsum powder • Water and air temperature • Water : powder ratio • Addition of • Retardants (slow down = cold water) • Accelerators (speed up) = warm water OR • SLURRY (mix of watery trimmed plaster residue)

  19. Microbes • Microbes can live within gypsum for 7 days • PPE very important!

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