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Questions and Answers
What is the primary use of Type I Impression Plaster?
What is the primary use of Type I Impression Plaster?
- Creating master casts
- Mounting and flasking models
- Formulating diagnostic casts
- Making impressions in edentulous ridges (correct)
Which type of dental stone is characterized as the weakest and least expensive?
Which type of dental stone is characterized as the weakest and least expensive?
- Type V High Expansion Improved Dental Stone
- Type III Dental Stone
- Type II Dental Plaster (correct)
- Type IV Improved Dental Stone
Which type of gypsum product is used for creating diagnostic casts?
Which type of gypsum product is used for creating diagnostic casts?
- Type III Dental Stone (correct)
- Type I Impression Plaster
- Type IV Improved Dental Stone
- Type II Dental Plaster
What property distinguishes Type IV Improved Dental Stone from other types?
What property distinguishes Type IV Improved Dental Stone from other types?
Which factor has the most significant effect on the setting time of gypsum products?
Which factor has the most significant effect on the setting time of gypsum products?
What effect does increased spatulation have on the setting time of dental stone?
What effect does increased spatulation have on the setting time of dental stone?
Which type of dental stone is best suited for making master casts and dies?
Which type of dental stone is best suited for making master casts and dies?
Which property do all gypsum products share?
Which property do all gypsum products share?
What happens to the solubility ratios of calcium sulfate hemihydrate and calcium sulfate dihydrate as temperature increases?
What happens to the solubility ratios of calcium sulfate hemihydrate and calcium sulfate dihydrate as temperature increases?
How does an increase in temperature above 37° C affect the setting time of gypsum?
How does an increase in temperature above 37° C affect the setting time of gypsum?
What is the effect of moisture contamination on the setting of gypsum products?
What is the effect of moisture contamination on the setting of gypsum products?
How does the pH of a liquid affect the setting of gypsum products?
How does the pH of a liquid affect the setting of gypsum products?
What defines the final setting time of gypsum materials?
What defines the final setting time of gypsum materials?
What effect does increasing the water-powder ratio have on setting time?
What effect does increasing the water-powder ratio have on setting time?
What consequence occurs if the rate of the reaction is too fast?
What consequence occurs if the rate of the reaction is too fast?
What occurs at 100° C regarding the solubilities of dihydrate and hemihydrate?
What occurs at 100° C regarding the solubilities of dihydrate and hemihydrate?
What is the primary reason for rinsing an impression in cold water before preparing a gypsum model?
What is the primary reason for rinsing an impression in cold water before preparing a gypsum model?
What happens if free water accumulates in the impression before preparing the gypsum model?
What happens if free water accumulates in the impression before preparing the gypsum model?
Which of the following is a recommended storage method for alginate impressions when not immediately prepared?
Which of the following is a recommended storage method for alginate impressions when not immediately prepared?
What effect does exudate from syneresis have on the alginate impression?
What effect does exudate from syneresis have on the alginate impression?
Which of the following elastomeric impression materials was introduced first?
Which of the following elastomeric impression materials was introduced first?
Which disinfectant solution is commonly used for immersing impressions?
Which disinfectant solution is commonly used for immersing impressions?
For maximum accuracy with gypsums, when should the model material be poured into the alginate impression?
For maximum accuracy with gypsums, when should the model material be poured into the alginate impression?
Which consistency options are available for addition silicones used in impressions?
Which consistency options are available for addition silicones used in impressions?
What is the recommended waiting time before pouring gypsum models and dies?
What is the recommended waiting time before pouring gypsum models and dies?
Which type of gloves do not adversely affect the setting of addition silicone impressions?
Which type of gloves do not adversely affect the setting of addition silicone impressions?
What effect does residual monomer in acrylic provisional restorations have on addition silicone materials?
What effect does residual monomer in acrylic provisional restorations have on addition silicone materials?
What component is essential in the composition of polyether impression materials?
What component is essential in the composition of polyether impression materials?
What phenomenon describes the decrease in viscosity of impression materials under applied shear force?
What phenomenon describes the decrease in viscosity of impression materials under applied shear force?
What role does a microcrystalline triglyceride network play in polyether impression materials?
What role does a microcrystalline triglyceride network play in polyether impression materials?
What should be done to latex gloves to potentially minimize their effect on addition silicone impressions?
What should be done to latex gloves to potentially minimize their effect on addition silicone impressions?
Which body consistency categories are polyethers supplied in?
Which body consistency categories are polyethers supplied in?
What type of setting expansion occurs when the investment is placed in contact with water?
What type of setting expansion occurs when the investment is placed in contact with water?
Which factor primarily influences the hygroscopic expansion of the investment material?
Which factor primarily influences the hygroscopic expansion of the investment material?
What happens to the strength of the investment when the silica/stone ratio is increased?
What happens to the strength of the investment when the silica/stone ratio is increased?
How does the particle size of silica affect the setting expansion of investments?
How does the particle size of silica affect the setting expansion of investments?
What is the effect of water in the mix on the normal and hygroscopic setting expansions?
What is the effect of water in the mix on the normal and hygroscopic setting expansions?
Why is it important to store investment containers in a dry environment?
Why is it important to store investment containers in a dry environment?
Which casting technique enhances hygroscopic setting expansion?
Which casting technique enhances hygroscopic setting expansion?
What is the main requirement for compensating the casting shrinkage of alloys during the setting of dental investments?
What is the main requirement for compensating the casting shrinkage of alloys during the setting of dental investments?
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Study Notes
Types of Dental Stone
- Type I: Impression Plaster (Plaster of Paris). Contracts on setting. Used for impressions of edentulous ridges for denture fabrication (rarely used now).
- Type II: Dental Plaster. Weakest and least expensive. Used for models and laboratory tasks (mounting, flasking).
- Type III: Dental Stone. Low to moderate strength. Used for diagnostic casts.
- Type IV: Improved dental stone. High strength and low expansion. Used for master casts and dies.
- Type V: High expansion improved dental stone. High strength and high expansion. Used for master casts and dies.
Factors Affecting Setting Time
- Spatulation: Increasing spatulation speed or time shortens setting time.
- Temperature: Setting time is greatly affected by temperature change. Increasing temperature initially slows the reaction, increasing setting time. Subsequently, increasing temperature speeds the reaction, shortening setting time. At 100°C, no reaction occurs, and plaster does not set.
- Humidity: Initial moisture accelerates setting, but further contamination can retard the reaction.
- Colloidal Systems and pH: Colloidal systems (agar, alginate) retard setting. Low pH liquids (saliva) retard the reaction, while high pH liquids accelerate setting.
- Powder/Water Ratio: Higher water/powder (W/P) ratio prolongs setting time.
Gypsum Setting Time & Stages
- Final Setting Time: Time required for the reaction to be completed.
- Working Time: The time the mixed mass can be manipulated before it hardens.
- Initial Setting Time: The time it takes for the material to reach a certain stage of firmness.
Gypsum Dimensional Stability
- Alginate Impressions lose water through evaporation and shrink when exposed to air.
- To ensure accuracy, models should be poured as soon as possible after impression is taken.
- If delayed, impressions should be stored in 100% relative humidity (plastic bag or damp paper towel).
Gypsum Disinfection
- Viruses can be transferred on gypsum models, posing a risk to lab and operating personnel.
- Disinfection can be achieved by spraying or immersing models in 1% sodium hypochlorite or 2% potentiated glutaraldehyde.
Elastomeric Impression Materials
- Polysulfides: First synthetic elastomeric impression material.
- Condensation Silicones:
- Addition Silicones (Polyvinylsiloxanes): Available in various consistencies (extra-low, low, medium, monophase, high, putty).
- Polyethers: Available in low, medium, and high consistencies.
Addition Silicone
- Gloves: Latex gloves can negatively affect setting due to sulfur compounds. Vinyl and nitrile gloves are recommended.
- Residual Monomer: Found in acrylic provisional restorations and composite cores, can inhibit setting.
- Cleaning: 2% chlorhexidine can be used to remove contaminants.
Polyethers
- Viscosity: A characteristic of time after mixing.
- Shear Thinning: The phenomenon where viscosity decreases with increasing outside force or shearing speed. This allows for a single-phase material to be used as both low and medium consistency.
Calcium Sulfate-Bonded Investments
- Composition: A mixture of refractory material, binder material, and other chemicals (chlorides or boric acid enhance thermal expansion).
- Types: Type 1 - for inlay and crown castings. Type 2 - for complete denture and partial removable denture bases.
Setting Expansion of Calcium Sulfate-Bonded Investments
- Setting Expansion: The change in dimension that occurs as the investment sets.
- Thermal Expansion: The change in dimension that occurs when the investment is heated.
- Total Dimensional Change: The sum of setting expansion and thermal expansion. This is crucial for compensation of casting shrinkage.
- Normal Setting Expansion: Expansion when setting in air.
- Hygroscopic Setting Expansion: Greater expansion when setting in contact with water.
Factors Affecting Hygrospcopic Expansion
- Particle Size of Silica: Finer silica produces higher setting and hygroscopic expansion.
- Silica/Binder Ratio: Increased ratio leads to higher hygroscopic expansion but weaker investment.
- Water/Powder Ratio: Thinner mix (higher W/P ratio) leads to lower setting and hygroscopic expansion.
- Spatulation: Similar effect on setting expansion as in gypsum products.
- Age of Investment: Older investment expands less. Container should remain closed, especially in humid environments.
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