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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/