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DH220 Dental Materials
Lecture #9
Prof. Lamanna RDH, MS
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GYPSUM:Plaster and Stone
Bird & Robinson p. 804 806, 812 - 814
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I. Use to produce a positive reproduction ofthe oral structures
A. Plaster 1. 1 purpose is to fabricate study models for tx planning- diagnostic aid view mouth from all angles- serves as a permanent record of the ptsocclusion & alignment of their teeth prior to tx- visual aid during case presentation & pt ed
2. Mounting casts to articulator - reproducesthe hinge motion of the jaw
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B. Stone- produces working casts (replica of quad orarch) & to form dies (replica of a preparedtooth) to fabricate cast restoratives (inlays,onlays, crowns, bridges, RPDs, & full
dentures)
cast
die
cast & die
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II. Composition & Types
Class of material ceramic due to physical properties Chemically identical; difference is physical form (shape of powder
particles) of the compound calcium sulfate dihydrate.
Calcination process: Plaster gypsum in rock form, ground & heatedto remove water
- Powder particles porous & rough beta-hemihydrate.
Stone - gypsum in rock form, ground & heatedunder steam pressure to remove water.
- Powder particles smooth & dense alpha-hemihydrate.
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Modifiers added to regulate setting time & expansion Potassium sulfate accelerator Borax retarder & reduce setting expansion
Pigments buffs & pastels added to distinguish between
plaster (white) & stone (buff & pastels)
Composition.cont
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Types ADA classification system 2 plasters & 3 stones - variance in properties & handling Strength increases from Type I V by lowering W/P ratio 5 Types
TYPE I: Impression plaster rigid, edentulous pts only
TYPE II: Model plaster construct study models (Tx plan)
TYPE III: Dental stone casts for denture fabrication (RPD, RFD)plus:
TYPE IV: Die stone casts for in/onlays, crowns, bridge fabrication High strength, hardness & low expansion
TYPE V: High strength, high expansion stone newest edition;accommodates the greater shrinkage of newer alloys
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III. Strength & Hardness
Directly related to amount of water added to powder. More water = weaker mix Plaster = more water = weaker material Stone = less water = stronger material
As material dries, entrapped water evap,leaves voids = porous, weak model/cast
Inadequate strength results in fracture or distortion Reaches maximum strength after 24 hrs
How much water??
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IV. Handling & Mixing Setting reaction chemical - results in exothermic
reaction What is that?? A. Water controls strength, rigidity, & hardness
1. Formation of gypsum crystals cluster formation;center of each cluster: nuclei of crystallization
intermeshing of crystals gives final strength & rigidity
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2. Water to Powder (W/P) ratio- too much water added increase in setting time,
fewer crystals reduced strength
- not enough water reduces setting time, less workingtime, crumbly mixture of reduced strength
3. Water temperature (68 - 72 F)- temp accelerates set- temp prolongs set
Crystals unable toaccurately intermesh.
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B. Armamentarium1. Powder (gypsum) & water; measuring vial
* plaster = ___ ml / 100 g (W/P)* dental stone = ___ ml / 100 g (W/P)* die stone = ___ ml / 100 g (W/P)
2. rubber bowl3. spatula wide & narrow blade4. scale measure powder 5. mechanical vibrator
removes air bubbles
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Prepare the impression1. Disinfection of impression
________ __________ _______________
2. Inspect impression for standing moisture
- surface should be dry of moisture- gently air dry with A/W syringe- pooled water will result in voids in cast/model
Manipulation of Gypsum Products
Agents used: ____________, _______________, _______________
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Manipulation of Gypsum Products
Measure powder & water
Add powder to water (over 20 to 30 seconds)
Spatulation(1 minute at 2 revolutions
per second)
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Vibrate(approx 30 sec)
Pour impression(sets in 45 60 minutes)
Working time = ~5 min- mixing & pouring
Setting time:
Initial set = 7 10 min loss of gloss or flow
Final set = 45 60 min after exothermic reaction (feels warm)
-- wet strength (feels wet & cool to touch)
24 hours = dry strength
Disinfect cast(if nec.)
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V. Construction of cast/model - chapter 281 2
3 4
5 Or rubber base formers
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VI. Separation of cast/model from impression chpt 28 Timing
Working time mixing & pouring = 5 mins Initial set = 7 - 10 min Final set - Wet strength = 45 min 1 hr after exothermic
reaction feels cool to touch Dry strength = 24 hours
** WHY remove the impression immediately after exo reaction???- Syneresis dehydrated imp matl will shrink & harden making
it very difficult to separate from model/cast.
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VIII. Model Trimming creating symmetrical form Alginate in interproximal undercuts
Blebs or nibs of gypsum on occ surface
Model trimmer
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Test #3 Review
Impression Materials & Gypsum
Chapters 8 & 9* Give EDPuzzle a try *
1. Use
2. Desirable qualities & special characteristics
3. Definitions & terminology
4. Flow charts, Common Difficulties handout,Impression grid
5. Factors effecting the final end-product
http://www.edpuzzle.com/http://www.edpuzzle.com/