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

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QuickTime™ and a decompressor

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QuickTime™ and a decompressor

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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!!

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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

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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