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Questions and Answers
What is the maximum temperature Gypsum bonded investment can withstand?
What is the maximum temperature Gypsum bonded investment can withstand?
Which type of investment is suitable for Metal Ceramic Framework?
Which type of investment is suitable for Metal Ceramic Framework?
What impact does a full-width ring liner have on gypsum-bonded investments?
What impact does a full-width ring liner have on gypsum-bonded investments?
Which of the following conditions contributes to reducing surface roughness in phosphate-bonded investments?
Which of the following conditions contributes to reducing surface roughness in phosphate-bonded investments?
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Which factor leads to easier removal of castings from gypsum-bonded investments?
Which factor leads to easier removal of castings from gypsum-bonded investments?
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What is a key characteristic of phosphate bonded investment regarding its performance at high temperatures?
What is a key characteristic of phosphate bonded investment regarding its performance at high temperatures?
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What effect does prolonged spatulation have on gypsum-bonded investments?
What effect does prolonged spatulation have on gypsum-bonded investments?
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Which characteristic is true for gypsum bonded investments compared to phosphate bonded investments?
Which characteristic is true for gypsum bonded investments compared to phosphate bonded investments?
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What is the primary reason prefabricated plastic and wax sprues are preferred over metal sprues?
What is the primary reason prefabricated plastic and wax sprues are preferred over metal sprues?
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Which prescriptive diameter is recommended for molar metal-ceramic patterns?
Which prescriptive diameter is recommended for molar metal-ceramic patterns?
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What is the consequence of having insufficient smoothed attachment of the sprue to the wax pattern?
What is the consequence of having insufficient smoothed attachment of the sprue to the wax pattern?
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What is one of the functions of vents during the casting process?
What is one of the functions of vents during the casting process?
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In which area should the sprue typically be attached to the wax pattern to avoid inaccuracies?
In which area should the sprue typically be attached to the wax pattern to avoid inaccuracies?
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Why is excessive wetting agent potentially problematic in the casting process?
Why is excessive wetting agent potentially problematic in the casting process?
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The ringless investment technique primarily allows for what benefit?
The ringless investment technique primarily allows for what benefit?
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What occurs when the sprue attachment is widened during casting?
What occurs when the sprue attachment is widened during casting?
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What is one of the aims of wax elimination?
What is one of the aims of wax elimination?
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What is the typical burn out temperature for gypsum-bonded investment when using gold alloy?
What is the typical burn out temperature for gypsum-bonded investment when using gold alloy?
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What should be done before aligning the orifice of the crucible with the sprue during casting?
What should be done before aligning the orifice of the crucible with the sprue during casting?
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What defect may appear as rough casting during evaluation?
What defect may appear as rough casting during evaluation?
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How long should the temperature be held during the burnout process?
How long should the temperature be held during the burnout process?
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What may result from inadequate or excessive expansion during casting?
What may result from inadequate or excessive expansion during casting?
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What is a common appearance of inclusion porosity?
What is a common appearance of inclusion porosity?
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Which process is necessary to prevent cracking of investments during burnout?
Which process is necessary to prevent cracking of investments during burnout?
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Study Notes
Dental Casting Investment
- The lost wax casting technique has been used since ancient times to convert wax patterns into metal castings.
- Prefabricated plastic and wax sprues are preferred over metal sprues because the material is eliminated during the heating cycle.
- Sprue requirements:
- Allow molten wax to escape from the mold.
- Enable molten metal flow into the mold with minimal turbulence.
- Sprue types:
- Wax
- Plastic
- Metal
- Sprue diameter (refer to specific slides for recommended diameters for different applications, e.g., 2.5mm for molar metal-ceramic).
- Sprue location: Attach the sprue to the bulkiest, non-critical part of the pattern. Avoid attachments that are restricted or have neckings which can cause casting porosity. Widening attachments can result in shrink spot porosity.
- Venting: Vents help gases escape during casting and act as a heat sink.
Investment Materials
- Several materials are used to fabricate the dental cast mold. Common materials include refractory materials such as silica and binders.
- Investment classification by binder:
- Gypsum Bonded
- Phosphate Bonded
- Silica Bonded
- Gypsum bonded investment:
- Not stable at high temperatures (not exceed 650°C).
- Difficult to control sitting expansion.
- Used for gold alloys (types II, III, and IV).
- Long working time.
- More porous
- Smooth casting
- Easily removed from investment
- Phosphate bonded investment:
- Stable at high temperatures (1400°C).
- Easy to control sitting expansion.
- Used for metal-ceramic frameworks.
- Short working time.
- Less porous
- Rough casting
- Difficult to remove from investment
- Factors that increase gypsum-bonded investment expansion: Use of a full-width ring liner, prolonged spatulation, storage at 100% humidity, lower water/powder ratio, use of a dry liner, and use of two ring liners.
- Factors that increase phosphate-bonded investment expansion: Heat from setting reaction softens the wax and allows free setting expansion, increased strength of the material at high temps restricts shrinkage of the alloy as it cools, powder mixed with colloidal silica reduces surface roughness of the castings, and also increases expansion.
Investing Technique & Wax Elimination
- Investing technique involves molding.
- Wax elimination aims to completely remove the wax, eliminating carbon vapors from the mould, and allowing for adequate thermal expansion. This process prepares the investment for casting.
- Burnout temperature:
- Gypsum-bonded: 650-687°C (for gold alloys)
- Phosphate-bonded: up to 834°C (for base metal alloys)
- Gradual temperature increase is critical to prevent cracking; the temperature is held for half an hour.
Casting Technique
- Preheating the crucible
- Making the alloy molten
- Removing casting ring from the furnace and placing it in the cradle.
- Tongs to slide the crucible platform into contact with the casting ring.
- Crucibles orifice aligns with sprue.
- Heating continues for few seconds to complete melting allowing casting to proceed.
- Casting arm is pulled until the pin drops.
- Centrifugal force caries melt into mold cavity.
Causes of Cast Failure
- Evaluation: The casting is never fitted on the die until the inside surface has been carefully evaluated under magnification.
- Defects in the casting: Investing and casting require careful attention for properly fitting castings. Potential problems include rough castings, nodules, multiple nodules, nodules on occlusal surfaces, fins, incomplete castings, shiny defects, suck-back porosity, inclusion porosity, and marginal discrepancies (inadequate or excessive expansion).
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Description
Explore the intricacies of the lost wax casting technique used in dental applications. This quiz covers essential topics such as sprue types, optimal sprue diameters, and the importance of venting in mold fabrication. Test your knowledge on the materials and methods that create accurate dental casts.
