gypsum products
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BE SURE TO FOLLOW DIRECTIONS
Be careful not to leave bowls, spatulas or other equipment in the lab…they will probably disappear
Dental Materials Properties and Manipulation. 9th editionPowers & Wataha --can store under 100% humidity for up to 5 days without serious dimensional changes but best to pour as soon as possible.
Gypsum Products
Introduction• A. Gypsum found abundantly in nature = Natural Ore
• B. Dihydrate form of Calcium Sulfate (CaSO4 - 2H2O)
• C. CaSO4 - 2H2O + Heat = Calcium Sulfate Hemihydrate (CaSO4 - 1/2 H2O)
• D. Hemihydrate changes to Dihydrate + 3900 cal/gm
• E. Gypsum Product + Water = Gypsum + HEAT EXOTHERMIC
I. Dental Application• Cast, Dies, Models = Positive Reproduction– Used to evaluate the dentition– Fabrication of Restorations and Prostheses
• Mounting casts on an Articulator• Molds for processing Dental Polymers• Investments for Castings
II. Manufacture (Calcination)
• 3 DIFFERENT FORMS from the Ore:–Dental Plaster–Dental Stone–Improved Dental Stone
A. Dental Plaster• Gypsum + Heat (open kettle at 110/120 C) = Beta CaSO4 -1/2H2O
• Beta Calcium Sulfate Hemihydrate
Characteristics• Irregular Shaped Crystals• Very porous• Random shaped
TYPE I GYPSUM = IMPRESSION PLASTERTYPE II GYPSUM = CAST OR MODEL PLASTER
2 Forms of Plaster:
B. Dental Stone (Cast Stone)
• GYPSUM + Steam & Pressure @ 125 C• Alpha CaSO4 - 1/2 H2O• Alpha Calcium Sulfate Hemihydrate
Characteristics
• More Uniform shaped crystals• More dense than Plaster• Larger crystals• Smoother Particles
TYPE III GYPSUM = Dental Stone
C. Improved Dental Stone
• GYPSUM + 30% Calcium Chloride Solution @ 212 F produces Alpha form
• Alpha CaSO4 - 1/2 H2O• Alpha Calcium Sulfate Hemihydrate
Characteristics
•Very dense crystals•Cubic or rectangular crystals
•Large crystals
TYPE IV GYPSUM = HIGH STRENGTH/LOW EXPANSIONTYPE V = HIGH STRENGTH/HIGH EXPANSION ( CASTING
INVESTMENT )
2 Forms :
Summary• All have the same chemical formula
• All have the same chemical reaction with water
• All have different physical properties
III. Physical Properties
•A. Setting Time:1. Working Time: Time during which the
material can be mixed and poured into the impression
2. Initial Setting Time: usually 7-13 minutes•Material has passed working stage but not completely set (no longer workable)•DETECTED CLINICALLY BY LOSS OF GLOSS
3. Final Setting Time:
•Material is completely set and can be removed from the impression without distortion or fracture•DETECTED AS LOSS OF HEAT•Usually 45 to 60 minutes
4. Setting Time Affected by:
–Temperature of the water: increase temp = faster set–Spatulation: increase speed or time = faster set–Water/powder ratio: »increase water = slower set »increase powder = faster set
5. Control of the Setting Time: • Usually by the Manufacturer: slow/fast
• Accelerators shorten set time: K2SO4
• Retarders lengthen set time: Borax, BLOOD & SALIVA
B. Compressive Strength
– 1. Based on the Water/Powder Ratio:•Plaster = 45 ml/cc to 100 grams powder
•Stone = 30 ml/cc to 100 grams powder•Improved Stone = 19 - 24 ml/cc to 100 grams powder
•More water = less strength–Plaster = 1500 psi–Stone = 4500 psi–Improved Stone = 8000 psi
B. Compressive Strength
– normal water-powder ratio (varies with the particular manufacturer) :
plaster = 37-50ml water / 100 gm (18-31)
stone = 28-32 ml water /100 gm(9-13)improved stone = 19-24 ml/ 100 gm
(0-5)
– 19 cc of water is needed for all 3. Rest is excess water to allow for mixing & handling. This evaporates off.
• 2. Wet Compressive Strength–Strength with some or all of the mixing water remaining
–Example : wet sponge• 3. Dry Compressive Strength –Strength with all excess water out
–Usually 2 times the wet strength–Example: dry sponge
Wet versus DryCompressive Strength
–Plaster : 1500 to 3300 psi
–Stone: 4500 to 8500 psi
–Improved Stone:8000 to 17-18,000 psi
C. Surface Hardness:–Related directly to the compressive strength
–Highest in Improved Stone
D. Abrasion Resistance:–Related directly to the compressive strength
–Highest in Improved Stone
E. Setting Expansion
–All show a Linear Expansion upon setting:•Plaster .2%-.3%•Stone .08%-.1%•Improved Stone .05%-.08%
• They do not contract or shrink!!!
IV. Manipulation• Correct water/powder ratio• Add powder to the water• Mix on a vibrator or vacuum spatulator• Vibrate material into the impression–1. Avoids trapping air = bubbles, voids
–2. Get stronger/denser model = brings excess water to the surface
Dispensing the Material
Can accurately weigh the powder or;
Prepackaged
Note Information on the Packet
USE ONLY FOR CASTSNOT FOR IMPRESSIONS!!!
• No drying
No drying!!Use a vibrator
Add the powder to the water
QuickTime™ and a decompressor
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decompressorare needed to see this picture.
QuickTime™ and a decompressor
are needed to see this picture.
QuickTime™ and a decompressor
are needed to see this picture.
QuickTime™ and a decompressor
are needed to see this picture.QuickTime™ and a
decompressorare needed to see this picture.
BAD TECHNIQUE
POUR A BASE AFTER THE IMPRESSION HAS LOST ITS GLOSS
Next lecture we will discuss the trimming & finishing of the models -do not trim before!!
QuickTime™ and a decompressor
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decompressorare needed to see this picture.
UNTRIMMED MODEL
Rubber molds may be used to form the base
Impression Removal• Remove the impression from the model after set -- preferably no longer than one hour after final set of the gypsum product used
QuickTime™ and a decompressor
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Bite Registration (Baseplate Wax)
Bite Registration with Coprwax
Using the Vacuspat……
A Roggen-Berry Production
Et Voila!!
that’s it---
Cakes Done…
Bon Appetit!!
Happy Model Pouring!!!
THE
BERRY
END
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