Dental Gypsum Flashcards
Gypsum Products
One of the most widely used dental lab materials, used to make a positive replica of the tissues.
Study model/diagnostic cast
used to plan treatment and as a record of treatment progress
Working cast
- a positive replica of the teeth and/or the associated supporting structures
- used for fabricating a prosthesis
Die
- working replica of a single tooth
- used for fabricating wax patterns for cast metal restorations
- can be removed from the cast assembly, to help visualize and contour the proximal surface of the wax pattern
Req of materials:
- Mechanical Properties
- Abrasion resistance
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- Compatibility w/ impression materials
- Color Contrast
- Economy
- High strength to prevent breakage during handling and flasking procedures
- surface should not be easily damaged during wax pattern fabrication
- ability to reproduce fine details and sharp margins
- Dimensional stability and accuracy on setting and should continue to remain stable
- Should not stick to or react w/ the impression material. Should be easily separated from the impression material.
- To easily distinguish the margins of the wax pattern or porcelain build up
-Can be used w/ minimum equipment
Plaster bowl
Plaster spatula
Biocompatibility
Non toxic
Chemical nature
- Gypsum products are made from gypsum rock which is a mineral found in various parts of the world
- has a commercial importance as a source of plaster of Paris
3 forms of gypsum materials
- Model plaster
- Dental stone
- Dental stone of high strength
*all forms obtained from DIHYDRATE form of CALCIUM SULFATE
- Chemical formula
* Chemical reaction
CaSO4 *2H2O
- on heating, during manufacturing, loses 1.5 moles of water of crystallization.
- and converted to the hemi hydrate form of Calcium Sulfate
- When calcium sulfate hemihydrate is mixed with water, it forms CALCIUM SULFATE DIHYDRATE
- It is an exothermic reaction, amount of heat evolved is equal to the heat used for calcinations The crystals are said to form around the nuclei of crystalization
-Setting time
is the beginning of mixing until the material sets
-Initial setting time
-during this time, the crystals just start forming and can be detected by the following:
-Initial setting time
Loss of gloss
when gypsum is mixed w/ water, the excess water is taken up to form dihydrate crystals which results to loss of gloss.
-occurs 9 min after mixing
-Initial setting time
Needle penetration test
- Gillmore penetrometer
- Vicat penetometer
- Penetrometer
- is of a particular diameter which is suspended and penetrated into a gypsum in a bowl just below needle
- the time is noted until needle no longer penetrates into gypsum product
- 2 needles for initial and final setting time
- 1 needle for testing initial setting time
Final setting time
-as more crystals form the material until it finally sets into a solid mass
Within the initial and final setting time:
- Mixing time:
- Working time:
-time of addition of powder to water until it is completely mixed, (1 min)
After mixing, the material forms a workable mass and this then poured into the mold (3 min)
what are the Factors that control setting time
- By Manufacturer
- by Operator
- Solubility of hemihydrate can be altered
- mixing 1% terra alba-increasing nuclei of crystallization thereby increasing setting
- adding accelerators or retarders
- fineness of powder particles-(the finer powder, the faster it sets)
Manufacturer
- WATER/POWDER ratio (more water, more powder should be added)
- MIXING increasing manipulation rate increases setting
- Temperature above 50 deg C, retardation
Operator
- What slows the setting time of gypsum?
- What are examples?
-Retarders
- sodium chloride: 3.4%^
- Sodium sulfate: 3.4%^
- acetaes, borates, citrates, tartarates
- inorganic salts like ferric sulfate, chromic sulfate, aluminum sulfate
- Borax: 1-2%
- Colloids: glue, gelatin, agar, and coagulated blood
What makes setting time faster?
Accelerators
- Sodium sulfate
- potassium sulfate
- sodium chloride
- terra alba
- Modified plaster
- Chemically B calcium sulfate hemihydrate
- used for making final wash impressions for complete dentures
Type I: Impression Plaster
- used to make casts chemically it is B calcium sulfate hemihydrate
- has less strength than other gypsum
- more porous in nature due to irregular crystal shape
Type II: Model Plaster
- used to make master casts for fabrication of final prosthesis
- chemically its alpha calcium sulfate hemihydrate
- has higher abrasion resistance and strength
- Less porous due to regular crystal from and greater number of crystals
Type III: Dental stone
- Used to make dies
- has greater abrasion resistance
- have higher strength than stone or plaster
Type IV: Die stone
-have greater setting expansion in addition to increased strenghth, to compensate for casting shrinkage of base metal alloys
Type V: High strength and expansion type
What are types of impression trays?
- stock tray
- individual tray
- rim locked tray
- partial trays
what are requirments of an ideal impression material?
- should have an adequate taste and odor
- should not contain any toxic and irritating ingredients
- should posses satisfactory consistency and texture
- readily disinfected w/o loss of accuracy
- can be electroplated
- should be compatible w/ die and cast material
- should exhibit dimensional stability
- elastic properties
- economical, easy to use w/ minimum equipment
- adequate setting characteristics that meet clinical requirments
- high deg of repro detail
- adequate strength so not to tear while making and removing impression from mouth
- readily wets oral tissues
