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What should not come into contact with the skin during the manipulation of unset activators?
What is the occurrence of shrinkage during the hardening process of materials?
Which impression materials should epoxy resins not be used with?
What is the mineral gypsum primarily used for?
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Which gypsum product is the densest among the types derived from partial dehydration of gypsum rock?
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What is a key desirable quality of a cast or die material?
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Which impression material can be used with gypsum materials such as plaster or stone?
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Which of the following types of casts is stronger and more resistant to abrasion?
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What is the purpose of using dental plaster?
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What is an advantage of epoxy die materials over stone die materials?
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What type of mouth impressions can impression plaster be used for?
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Which characteristic of a cast or die material enhances manipulative procedures?
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What is a disadvantage of epoxy dies?
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Which type of dental stone is mainly used for making diagnostic casts?
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What is a primary factor that shortens the setting time of gypsum products?
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Which type of dental stone is characterized by high strength and low expansion?
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What type of gypsum product is rarely used for making impressions in edentulous ridges?
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Which type of dental stone is considered the weakest and least expensive?
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What effect does temperature have on gypsum setting time?
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Which type of gypsum product is primarily used for making master casts and dies?
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What is the primary characteristic of the crystals found in model plaster?
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What effect does increasing the water-to-powder (W/P) ratio have on the compressive strength of gypsum products?
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How does the addition of potassium sulfate (K2SO4) affect the setting time of model plaster?
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What is the relationship between the manipulation method and setting expansion in gypsum products?
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What is the effect of rinsing an impression before pouring gypsum die material?
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What chemical is commonly used as a retarder in gypsum products?
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How does increasing temperature affect the solubility ratios of calcium sulfate hemihydrate and calcium sulfate dihydrate?
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What is the effect of temperature on ion mobility?
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What occurs at a temperature of 100° C regarding the solubilities of dihydrate and hemihydrate?
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How does humidity initially affect the setting of gypsum products?
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What is the primary characteristic of gypsum materials related to setting time?
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What happens to the viscosity of the mixed mass as the reaction proceeds?
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What effect does a higher water-powder (W/P) ratio have on setting time?
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Which liquids are known to retard the setting of gypsum products?
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Study Notes
Impression Materials and Casts
- The text mentions materials used to make casts and dies from dental impressions, including plaster, stone, epoxy resin, and refractory materials.
- The choice of material depends on factors like the type of impression and the specific purpose of the die or cast.
Gypsum Materials
- Gypsum is crucial for creating casts and dies used with various impression materials.
- Stone casts offer greater strength and abrasion resistance, making them suitable for restorations or appliances.
- Plaster is often used for study casts that are intended for recording purposes only.
- Different gypsum products are used in various applications, including:
- Impression plaster for creating impressions in edentulous mouths or mounting casts
- Dental stone for forming dies that duplicate oral anatomy
- Gypsum as a binder for silica in casting investments, soldering investments, and investments for nickel-chromium alloys
- Mold material for processing complete dentures
Types of Dental Stones
- Dental stones are categorized based on their properties and uses:
- Type I: Impression Plaster (weakest, contraction on setting) - Rarely used now.
- Type II: Dental Plaster (least expensive, low strength) - Used for models and laboratory purposes like mounting and flasking.
- Type III: Dental Stone (low-moderate strength) - Used for diagnostic casts.
- Type IV: Improved Dental Stone (high strength, low expansion) - Used for master casts and dies.
- Type V: High Expansion Improved Dental Stone (high strength, high expansion) - Suitable for master casts and dies.
Factors Affecting Gypsum Setting Time
- The spatulation (mixing) process influences the setting time and expansion of gypsum materials. Increased speed and duration of spatulation shorten the setting time.
- Temperature plays a significant role. The solubility of calcium sulfate hemihydrate and dihydrate changes with temperature, which affects the reaction rate and setting time.
- Humidity initially accelerates the setting process but can later retard it if the impression becomes excessively contaminated with moisture.
- Colloidal systems like agar and alginate, as well as liquids with low pH (e.g., saliva), can retard setting. Liquids with high pH accelerate setting.
- Increasing the water-powder (W/P) ratio in the mix prolongs the setting time.
Setting Time Definitions
- Working time refers to the period during which the mixed mass is still fluid enough to be manipulated.
- Final setting time is when the material can be safely removed from the impression without distortion or fracture.
- Initial setting time is the point at which the material reaches a specific level of firmness.
Other Physical Properties of Gypsum
- The compressive strength of gypsum products is inversely proportional to the W/P ratio: more water leads to lower strength.
- Setting expansion can be modified through manipulation and adding chemicals.
- Contamination with saliva or blood can affect detail reproduction; rinsing the impression is crucial.
Epoxy Die Materials
- Epoxy resins are offered as a paste that requires the addition of a liquid activator (amine) for hardening.
- Epoxy dies are more abrasion-resistant and stronger than high-strength stone dies.
- Epoxy materials are limited in use because:
- Activators are toxic and should not come into contact with skin.
- Shrinkage occurs during hardening (up to 0.1%).
- Epoxy materials cannot be used with water-containing impression materials (e.g., agar and alginate).
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Description
This quiz covers the selection and properties of impression materials and casts used in dentistry, focusing on gypsum and other materials. Learn about their specific applications, strengths, and how they influence the dental restoration process.
