Dental Materials: Impression and Casts
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Questions and Answers

What should not come into contact with the skin during the manipulation of unset activators?

  • Clay products
  • Toxic activators (correct)
  • Plastic materials
  • Dental tools

What is the occurrence of shrinkage during the hardening process of materials?

  • 1.0%
  • 0.5%
  • 0.2%
  • 0.1% (correct)

Which impression materials should epoxy resins not be used with?

  • Silicone impression materials
  • Water-containing agar and alginate impression materials (correct)
  • Polyvinyl impression materials
  • Rubber impression materials

What is the mineral gypsum primarily used for?

<p>Source of plaster of Paris (C)</p> Signup and view all the answers

Which gypsum product is the densest among the types derived from partial dehydration of gypsum rock?

<p>Densite (D)</p> Signup and view all the answers

What is a key desirable quality of a cast or die material?

<p>Dimensionally stable under normal conditions (B)</p> Signup and view all the answers

Which impression material can be used with gypsum materials such as plaster or stone?

<p>Alginate hydrocolloid (C)</p> Signup and view all the answers

Which of the following types of casts is stronger and more resistant to abrasion?

<p>Stone casts (C)</p> Signup and view all the answers

What is the purpose of using dental plaster?

<p>To create study casts for record purposes (B)</p> Signup and view all the answers

What is an advantage of epoxy die materials over stone die materials?

<p>More resistant to abrasion (D)</p> Signup and view all the answers

What type of mouth impressions can impression plaster be used for?

<p>Edentulous mouths (B)</p> Signup and view all the answers

Which characteristic of a cast or die material enhances manipulative procedures?

<p>Color (B)</p> Signup and view all the answers

What is a disadvantage of epoxy dies?

<p>Not mentioned in the content (D)</p> Signup and view all the answers

Which type of dental stone is mainly used for making diagnostic casts?

<p>Type III: Dental Stone (D)</p> Signup and view all the answers

What is a primary factor that shortens the setting time of gypsum products?

<p>Increase in spatulation speed (B)</p> Signup and view all the answers

Which type of dental stone is characterized by high strength and low expansion?

<p>Type IV: Improved Dental Stone (C)</p> Signup and view all the answers

What type of gypsum product is rarely used for making impressions in edentulous ridges?

<p>Type I: Impression Plaster (D)</p> Signup and view all the answers

Which type of dental stone is considered the weakest and least expensive?

<p>Type II: Dental Plaster (D)</p> Signup and view all the answers

What effect does temperature have on gypsum setting time?

<p>It affects setting time more than any other property. (D)</p> Signup and view all the answers

Which type of gypsum product is primarily used for making master casts and dies?

<p>Type IV: Improved Dental Stone (C)</p> Signup and view all the answers

What is the primary characteristic of the crystals found in model plaster?

<p>They are irregular in shape and porous in nature. (B)</p> Signup and view all the answers

What effect does increasing the water-to-powder (W/P) ratio have on the compressive strength of gypsum products?

<p>It decreases the compressive strength. (B)</p> Signup and view all the answers

How does the addition of potassium sulfate (K2SO4) affect the setting time of model plaster?

<p>It reduces the setting time from approximately 10 minutes to about 4 minutes. (D)</p> Signup and view all the answers

What is the relationship between the manipulation method and setting expansion in gypsum products?

<p>Mechanical mixing decreases setting expansion. (B)</p> Signup and view all the answers

What is the effect of rinsing an impression before pouring gypsum die material?

<p>It improves the detail reproduction of the gypsum die material. (A)</p> Signup and view all the answers

What chemical is commonly used as a retarder in gypsum products?

<p>Sodium citrate (D)</p> Signup and view all the answers

How does increasing temperature affect the solubility ratios of calcium sulfate hemihydrate and calcium sulfate dihydrate?

<p>The solubility ratios decrease. (A)</p> Signup and view all the answers

What is the effect of temperature on ion mobility?

<p>Ion mobility increases with rising temperature. (A)</p> Signup and view all the answers

What occurs at a temperature of 100° C regarding the solubilities of dihydrate and hemihydrate?

<p>The solubilities of both become equal. (C)</p> Signup and view all the answers

How does humidity initially affect the setting of gypsum products?

<p>It accelerates the setting. (C)</p> Signup and view all the answers

What is the primary characteristic of gypsum materials related to setting time?

<p>Proper setting time is crucial for manipulation. (A)</p> Signup and view all the answers

What happens to the viscosity of the mixed mass as the reaction proceeds?

<p>Viscosity increases and flow becomes difficult. (B)</p> Signup and view all the answers

What effect does a higher water-powder (W/P) ratio have on setting time?

<p>It prolongs the setting time. (A)</p> Signup and view all the answers

Which liquids are known to retard the setting of gypsum products?

<p>Liquids with low pH. (D)</p> Signup and view all the answers

Flashcards

Dental Cast Materials

Materials used to create casts and dies from dental impressions, like plaster, stone, epoxy resin.

Gypsum Materials

Crucial for dental casts; various types (plaster, stone) used for different purposes.

Dental Stone Types

Classified by properties and uses (e.g., Type I-weakest, Type IV-high strength).

Impression Plaster

Type of gypsum used for impressions in edentulous mouths.

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Dental Stone

Gypsum material for creating dies that replicate oral structures.

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Gypsum Setting Time

The time it takes for gypsum to harden after mixing.

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Spatulation

Mixing process that affects gypsum setting time.

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Working Time

Time when a mixed mass is workable.

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Final Setting Time

Material is firm enough to be removed from impression.

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Initial Setting Time

Point at which material starts firming.

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Water-Powder Ratio (W/P)

Ratio of water to powder in gypsum mix, affecting strength and setting time.

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Compressive Strength

Force a material can withstand before collapsing.

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Setting Expansion

Change in volume after gypsum sets.

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Epoxy Die Materials

Stronger, abrasion-resistant materials for dies.

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Epoxy Resin

A paste that needs a liquid activator for hardening.

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Temperature

Affects the setting time and expansion process.

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Humidity

Initial affects setting rate; excessive can delay.

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Contamination

Harmful to detail, e.g., saliva or blood.

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Type I Gypsum

Impression plaster, weakest, contracts on setting.

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Type II Gypsum

Dental plaster, low strength, models and mounting.

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Type III Gypsum

Dental stone, low to moderate strength, diagnostic casts.

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Type IV Gypsum

Improved dental stone, high strength, low expansion, master casts and dies.

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Type V Gypsum

High expansion improved stone, high strength, high expansion for master.

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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.

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