Gypsum Products
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Transcript of Gypsum Products
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
are needed to see this picture.QuickTime™ and a
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
are needed to see this picture.QuickTime™ and a
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
are needed to see this picture.
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