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Questions and Answers
What initiates the polymerization reaction in chemical-cured acrylic denture plastics?
Which is not an application of chemical-cured acrylic denture plastics?
What is a disadvantage of chemical-cured acrylic denture plastics?
What photo initiator is added in light-activated acrylic resin?
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Which technique is most common for denture processing?
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What is the first step in the compression molding technique for denture base construction?
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What is an advantage of light-activated acrylic resin over chemical-cured acrylic denture plastics?
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What is the purpose of the flasking procedure in denture base construction?
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What is applied to the inner surface of the flask before pouring the gypsum investment?
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What is the role of the separating medium applied after the first layer of gypsum investment?
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During dewaxing, what temperature is the water used to immerse the flask?
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What is the purpose of applying separating medium on the dewaxed mold space?
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What is the primary use of boxing wax?
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What is the correct proportion of polymer to monomer during mixing?
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How does utility wax differ from boxing wax?
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What characterizes the sandy stage in the mixing process?
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During which stage does the mix become smooth and dough-like?
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What characteristic does sticky wax have at room temperature?
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Which of the following statements is true about corrective impression wax?
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What happens to the monomer during the rubbery stage of the mixing process?
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What is a key property of boxing wax that aids in its use?
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Which color typically represents utility wax?
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What is a primary characteristic of carding wax?
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What application does sticky wax have during the repair of dentures?
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What classification pertains to alloys containing more than 40 wt% gold and more than 60 wt% noble metals?
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Which type of dental casting alloy is classified as having very high strength and is suitable for inlays subjected to extreme stresses?
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Which of the following properties is NOT a general requirement of dental casting alloys?
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What defines noble metal alloys according to their composition?
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In the classification based on yield strength, which type is considered medium?
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Which type of alloy is most suitable for removable dentures?
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What is the main function of the lost wax casting technique?
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Which classification relates to alloys based on percentage elongation and yield strength?
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What is a primary characteristic of the thermal expansion of dental waxes compared to other dental materials?
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Which of the following statements about the ductility of waxes is true?
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What is a desirable property of dental wax?
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What type of wax is predominantly used for making patterns for inlays, crowns, and bridges?
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Which property of dental wax is significantly affected by temperature changes?
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What is the primary use of Type II dental wax?
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What happens to dental wax patterns when stress is relieved?
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What is the relationship between wax melting range and ductility?
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Study Notes
Distortion of Wax Patterns
- Adapting wax to a shape with non-homogeneous softening results in stress buildup within the material.
- Relief of these stresses leads to distortion of the wax pattern.
Dimensional Change in Dental Waxes
- Dental waxes have a higher coefficient of thermal expansion and contraction compared to other dental materials.
- This means they expand when heated and contract when cooled.
- Temperature changes can significantly affect the accuracy of finished restorations due to the wax's high expansion and contraction rate.
Mechanical Properties of Dental Waxes
- Dental waxes have a lower elastic modulus, proportional limit, and compressive strength compared to other dental materials.
- These properties are temperature-dependent.
Ductility of Dental Waxes
- Ductility increases as the temperature of the wax increases.
- Waxes with lower melting points exhibit greater ductility.
- Mixed waxes with a wide melting range have higher ductility than those with a narrow melting range.
Pattern Waxes - Used for Constructing Metal Castings and Denture Bases
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Inlay Waxes:
- Supplied in blue or green forms.
- Used to prepare wax patterns for inlays, crowns, and bridges.
Desirable Properties of Dental Waxes
- Not sticky to the touch.
- Low thermal expansion.
- Completely burns out without leaving residue.
- Appropriate melting range.
- Soften without flaking.
- Adequate flow to record cavity details.
Types of Pattern Waxes
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Type I - Medium Wax (Direct Technique):
- Used directly in the mouth, softened, and placed into prepared teeth.
- Lower melting range for patient comfort and accurate wax removal.
- Softening point slightly higher than mouth temperature.
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Type II - Soft Wax (Indirect Technique):
- More frequently used, where wax is melted onto a die outside the mouth.
- Supplied in green or black sheets with a smooth glossy surface.
Boxing Wax - Physical Properties and Use
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Easily manipulated.
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Slightly tacky at room temperature for self-adhesion to secure boxed forms.
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Retains shape with minimal flow.
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Use:
- Forms a gypsum model with its base in one step.
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Technique:
- A 1.5-inch wide, red strip of boxing wax is wrapped around an impression to create a form into which gypsum is poured.
Utility Wax - Physical Properties and Uses
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Also called periphery wax.
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Comes in orange or dark red ropes and sheets.
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Physical Properties:
- Adaptable and workable at room temperature.
- Adhesive and tacky at room temperature.
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Uses:
- Adapting the periphery of the impression for patient comfort.
Sticky Wax - Uses
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Comes in yellow or orange sticks.
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Uses:
- Temporary alignment of fractured denture pieces or casts during repair.
- Fixing soldered components (e.g., bridge parts) during soldering.
Base Plate Wax - Physical Properties
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Hard and brittle at room temperature.
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Becomes soft and sticky when heated under a flame.
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Free from tackiness at room temperature.
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Use:
- Forming a base for dentures.
Carding Wax - Use
- Use for preparing the base for carding teeth.
Impression Waxes - Used To Obtain Impressions of Oral Structures
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High flow at mouth temperature.
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Corrective Impression Wax:
- Wax veneer over original impression materials for edentulous impressions.
- Flows at mouth temperature to record soft tissue details in a functional state.
- Gradually replaced by elastomeric impression materials.
Chemical-Cured Acrylic Denture Plastics (Self-Curing, Cold-Curing, or Autopolymerizing Resins)
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Liquid contains a chemical activator (tertiary amine) that initiates polymerization at room temperature.
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Reacts with peroxide initiator to produce free radicals, starting the polymerization reaction.
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Uses:
- Denture bases.
- Repair of broken dentures.
- Special trays.
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Advantages:
- Denture base fabrication in a short time.
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Disadvantages:
- Lower mechanical properties (stiffness).
- Higher porosity.
- Color instability due to tertiary amine.
- Unreacted monomer can act as a plasticizer, decreasing strength and irritating soft tissues.
Light-Activated Acrylic Resin
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Composition: UDMA (Urethane dimethacrylate) and PMMA, with a photoinitiator (Comphro-quinon).
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Polymerization occurs when exposed to blue visible light.
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Properties:
- Adapted to the working cast and cured for 5 minutes in a light-curing unit.
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Applications:
- Denture bases.
- Repair of broken dentures.
- Special trays.
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Advantages:
- No MMA monomer present, reducing allergic reactions.
- Lower polymerization rate, leading to better denture base fit.
- Short processing time, no need for flasking.
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Disadvantages:
- Lower mechanical and physical properties.
Denture Processing Techniques
- Compression Molding - Most common technique.
- Injection Molding.
Denture Base Construction Using Compression Molding
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1. Flasking Procedure:
- Investing the waxed denture in a flask to create a mold for the acrylic resin denture base.
- Inner flask surface is coated with Vaseline, and the cast base is painted with a separating medium to prevent plaster adhesion.
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2. Wax Elimination:
- Dewaxing removes the wax pattern, creating a mold space for acrylic.
- The flask is immersed in boiling water (100° C) for 5 minutes.
- Separating medium is applied to the dewaxed mold space before packing the acrylic resin.
- The separating medium prevents water transfer from gypsum to resin, minimizes monomer transfer to plaster, and facilitates flask separation.
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3. Proportioning and Mixing:
- Polymer to monomer ratio of 3:1.
- Powder and liquid are mixed with a stainless steel spatula and kept in a sealed glass jar during initial reaction stages to prevent monomer evaporation.
- Mixing produces a plastic mass that goes through five stages:
- Sandy stage: Polymer settles into the monomer, forming a grainy, incoherent mass.
- Stringy or sticky stage: The mixture becomes stringy and adhesive when touched or pulled apart.
- Dough stage: Mix becomes smooth and doughy, no longer tacky and does not adhere to the mixing jar.
- Rubbery stage: Monomer evaporates, and the mass becomes cohesive and rubber-like.
- Final stage: The mix is ready for packing.
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4. Packing:
- The acrylic mass is packed into the mold space, ensuring complete filling and minimal voids.
- Excess material is removed.
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5. Curing:
- The packed flask is heated to cure the acrylic resin.
- Curing time and temperature depend on the specific acrylic resin used.
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6. Cooling & De-Flasking:
- The flask is cooled slowly to prevent thermal stresses and distortion.
- The flask is then opened, and the cured denture base is removed.
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7. Finishing & Polishing:
- The denture base is trimmed, shaped, and polished to achieve a smooth, aesthetically pleasing finish.
General Requirements of Casting Alloys
- Must not tarnish or corrode in the mouth.
- Must be sufficiently strong for the intended use.
- Must be biocompatible (non-toxic and non-allergenic).
- Must be easy to melt, cast, cut, and grind (easy to fabricate).
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Description
This quiz covers the mechanical properties, thermal behaviors, and distortion effects of dental waxes. It highlights how temperature changes influence their dimensions and stress buildup within the material. Perfect for dental students and professionals seeking to deepen their understanding of dental wax applications.
