Podcast
Questions and Answers
A dental laboratory technician notices that a wax pattern is distorting during the investing process. Which property of wax is most likely the primary cause of this issue?
A dental laboratory technician notices that a wax pattern is distorting during the investing process. Which property of wax is most likely the primary cause of this issue?
- Negligible excess residue after burnout.
- Significant thermal expansion and contraction. (correct)
- Minimal flow even at elevated temperatures.
- High melting range exceeding room temperature.
In the lost wax technique, what is the purpose of investing the wax pattern before burnout?
In the lost wax technique, what is the purpose of investing the wax pattern before burnout?
- To ensure complete removal of all wax residue without any carbon deposits.
- To prevent the molten metal from cooling too rapidly during casting.
- To create a mold that can withstand the high temperatures of casting. (correct)
- To refine the surface detail of the final cast restoration.
Which of the following is NOT a classification of waxes used in dentistry?
Which of the following is NOT a classification of waxes used in dentistry?
- Polishing waxes (correct)
- Impression waxes
- Processing waxes
- Pattern waxes
During the double-pour method of model creation, why is it important that the occlusal plane is parallel to the base?
During the double-pour method of model creation, why is it important that the occlusal plane is parallel to the base?
If the viscosity of a wax increases, how would this affect the flow of a wax material?
If the viscosity of a wax increases, how would this affect the flow of a wax material?
Which of the following gypsum products requires the LEAST amount of excess water during mixing, leading to higher strength?
Which of the following gypsum products requires the LEAST amount of excess water during mixing, leading to higher strength?
A dental assistant needs to create a working cast that requires good strength and abrasion resistance. Which gypsum product is MOST suitable for this purpose?
A dental assistant needs to create a working cast that requires good strength and abrasion resistance. Which gypsum product is MOST suitable for this purpose?
During the calcination process of gypsum, what chemical change occurs?
During the calcination process of gypsum, what chemical change occurs?
How do particle shape and porosity of gypsum affect its strength and abrasion resistance?
How do particle shape and porosity of gypsum affect its strength and abrasion resistance?
A dental laboratory technician needs to create a die with high resistance to heat and stress. Which type of gypsum product should be selected?
A dental laboratory technician needs to create a die with high resistance to heat and stress. Which type of gypsum product should be selected?
Why is the water/powder ratio crucial in determining the strength of a gypsum product?
Why is the water/powder ratio crucial in determining the strength of a gypsum product?
For which dental application is dimensional expansion of a gypsum model MOST detrimental?
For which dental application is dimensional expansion of a gypsum model MOST detrimental?
What qualities affect gypsum material selection? (Select all that apply)
What qualities affect gypsum material selection? (Select all that apply)
A dental lab technician notices that a gypsum model has poor surface detail. Which factor is LEAST likely to contribute to this issue?
A dental lab technician notices that a gypsum model has poor surface detail. Which factor is LEAST likely to contribute to this issue?
A dentist needs a gypsum material with high compressive strength and abrasion resistance for fabricating a die. Which type of dental stone is MOST suitable?
A dentist needs a gypsum material with high compressive strength and abrasion resistance for fabricating a die. Which type of dental stone is MOST suitable?
A dental student is mixing Type III dental stone and accidentally adds too much water. What is the MOST likely consequence of this error?
A dental student is mixing Type III dental stone and accidentally adds too much water. What is the MOST likely consequence of this error?
Which of the following factors would MOST effectively retard the setting time of a gypsum mixture?
Which of the following factors would MOST effectively retard the setting time of a gypsum mixture?
A dental assistant is preparing to pour up an impression using dental stone. To achieve optimal strength and accuracy, what should be the primary focus when mixing the gypsum?
A dental assistant is preparing to pour up an impression using dental stone. To achieve optimal strength and accuracy, what should be the primary focus when mixing the gypsum?
A prosthodontist is fabricating a complete denture and requires a gypsum material for the master cast. Which property of Type III dental stone makes it suitable for this purpose?
A prosthodontist is fabricating a complete denture and requires a gypsum material for the master cast. Which property of Type III dental stone makes it suitable for this purpose?
What is the effect of increased spatulation on the working time and viscosity of a gypsum mix?
What is the effect of increased spatulation on the working time and viscosity of a gypsum mix?
A dental technician observes that a gypsum model is setting too quickly. Besides using chemical retarders, what procedural adjustment could they make to MOST effectively extend the setting time?
A dental technician observes that a gypsum model is setting too quickly. Besides using chemical retarders, what procedural adjustment could they make to MOST effectively extend the setting time?
Flashcards
Anatomic Portion
Anatomic Portion
The portion of a dental model that replicates the teeth and soft tissues.
Double-Pour Method
Double-Pour Method
A method of pouring dental models where the anatomic and art portions are poured in two separate stages.
Pattern Waxes
Pattern Waxes
Waxes used to create a model of a dental restoration, such as inlays, crowns, and bridges.
Processing Waxes
Processing Waxes
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Lost Wax Technique
Lost Wax Technique
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Gypsum Products
Gypsum Products
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Qualities of Gypsum
Qualities of Gypsum
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Calcination
Calcination
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Model Plaster
Model Plaster
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Dental Stone
Dental Stone
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Die Stone
Die Stone
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Porosity and Strength
Porosity and Strength
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Dimensional Accuracy
Dimensional Accuracy
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Porosity vs. Detail
Porosity vs. Detail
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Impression Contamination
Impression Contamination
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Surfactant
Surfactant
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Model Plaster (Type II)
Model Plaster (Type II)
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Dental Stone (Type III)
Dental Stone (Type III)
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High-Strength/Low-Expansion Stone (Type IV)
High-Strength/Low-Expansion Stone (Type IV)
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High-Strength/High-Expansion Stone (Type V)
High-Strength/High-Expansion Stone (Type V)
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Water-to-Powder Ratio
Water-to-Powder Ratio
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Study Notes
- Gypsum and wax products are covered in chapter 16.
Gypsum Products
- It is a naturally occurring mineral that makes replicas of hard and soft tissues.
- Three types of gypsum products exists including; diagnostic casts, working casts, and dies.
- The type used depends on the use of the intended model.
Qualities of Gypsum
- Key attributes include; accuracy, hardness, and dimensional stability
- Other qualities encompass; fine detail, strength, abrasion resistant and solubility.
- Additional qualities are color, cost and safety of the products.
Gypsum Chemical Properties
- Calcination is a process by which dihydrate of calcium sulfate loses H2O when heated, becoming calcium sulfate hemihydrate, grounding down.
- When mixing gypsum products, hemihydrate turns into dihydrate.
- Plaster contains the most excess water, while die stone consist of the least excess water.
Production of Gypsum Products
- Material options include; model plaster, dental stone, and die stone/high strength stone.
- Model plaster (β-hemihydrate) is porous, irregularly shaped, and diagnostic casts
- Dental stone (α-hemihydrate) is shaped under steam, heat, and pressure, less porous, and used for working casts
- Die stone/high strength stone (α-hemihydrate) boiled in 30% calcium chloride to be a product of densite with high resistance to heat and stress.
Gypsum Physical Properties
- Important for material selection
- Factors for strength and abrasion resistance are particle shaped and water needed for the mix.
- Increased porosity decreases strength, while increased water increase strength.
Dimensional Accuracy
- Expansion always occurs but there is no expansion after final set
- Expansion would be most detrimental to the model type.
Reproduction of Detail
- Greater porosity results in less detail.
- Contamination negatively affects surface detail with blood, food, or saliva.
- Good wetting reduces likelihood of voids
- Water-based impression material reacts best with water-based gypsum.
- Surfactant improves wettability
Gypsum Products: Types, Properties, and Uses
| Type | Porosity | Compressive Strength (MPa) | Abrasion Resistance | Setting Expansion | Examples and/or Uses |
|---|---|---|---|---|---|
| Model Plaster (Type II) | High | 8.8 | Low | High | Diagnostic cast |
| Dental Stone (Type III) | Moderate | 20.6 | Moderate | Moderate | Denture/ortho models |
| High-strength/low-expansion stone (Type IV) | Low | 34.3 | High | Low | Die stone; cast restorations |
| High-strength/high-expansion stone (Type V) | Low | 48.0 | High | High | Withstands highest temperatures |
Water-to-Powder Ratios (W/P)
- Increase in water leads to a weaker model.
- Decrease in water lead to poor flow.
- Type II Plaster ratio is 100g to 45-50mL
- Type III Stone water ratio is 100g to 30-32mL
- Type IV High-strength water ratio is 100g to 19-24mL
Setting Time Factors
- Accelerate set time with chemical accelerators like potassium sulfate and set gypsum.
- Increase water temp and increased spatulation to accelerate setup
- Retard set time by using chemical retarders like borax.
- Decreased water temp and decreased spatulation retards set up.
Controlling the Set
| Working time | Viscosity | Strength | |
|---|---|---|---|
| ↑ W/P ratio | Increase | Decrease | Decrease |
| ↓ W/P ratio | Decrease | Increase | Increase |
| ↑ Rate | Decrease | Increase | No effect |
| ↑ Temp H2O | Decrease | Increase | No effect |
| ↓ Temp H2O | Increase | Decrease | No effect |
Model Parts
- A model consists of two parts; an Anatomic portion, for hard and soft tissues, and an Art portion/base.
Pouring Methods
- Double-pour method
- Inverted-pour method
- Boxing method
Model Trimming
- Considerations for trimming are neatness, symmetry, and occlusal plane parallel to base
Properties of Wax
- Melting range has to be higher than environment temp
- Temperature increases and viscosity decreases causing flow.
- Excess residue includes inaccuracies,
- Thermal expansion changes waxes drastically
Classifications of Wax
- Included are pattern waxes such as inlay, casting, & baseplate wax
- Processing waxes include; boxing, utility, & sticky wax
- Impression waxes include; corrective impression & bite registration
Pattern Wax
| Wax: | ADA Specification: | Uses: |
|---|---|---|
| Inlay wax | Type A | Direct patterns in mouth |
| Type B (Type 1) | Indirect patterns on dies | |
| Type C (Type II) | ||
| Casting wax | Constructing metal framework of dentures | |
| Baseplate wax | Type I | Impression in cool climates |
| Type II | Impression in warm climates | |
| Type III |
Lost Wax Technique
- Pour the die
- Wax the die
- Sprue the die
- Attach sprue to sprue base
- Invest wax pattern
- Burn out wax
- Cast restoration
- Final steps
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