CDT Dental Materials Science 3 — Questions and Answers
Question 1: Which investment material is used for casting high-fusing noble and base metal alloys due to its ability to withstand higher burnout temperatures?
- Gypsum-bonded investment
- Phosphate-bonded investment (Correct answer)
- Silica-bonded investment
- Plaster-bonded investment
Correct answer: Phosphate-bonded investment
Phosphate-bonded investments tolerate burnout temperatures above 700°C, necessary for high-fusing metal alloys used in crowns and bridges.
Question 2: The Shore A hardness scale is most commonly used to measure the hardness of:
- Dental ceramics
- Cast metal alloys
- Elastomeric impression materials (Correct answer)
- Acrylic denture bases
Correct answer: Elastomeric impression materials
Shore A hardness is appropriate for soft, flexible materials like elastomeric impression materials (silicones, polyethers).
Question 3: When a zirconia coping is sandblasted prior to veneering, the primary purpose is to:
- Reduce coping thickness
- Improve surface wettability and mechanical bonding for porcelain (Correct answer)
- Remove residual sintering contaminants only
- Alter the translucency of zirconia
Correct answer: Improve surface wettability and mechanical bonding for porcelain
Sandblasting creates surface roughness on the zirconia coping that improves micromechanical retention and wettability for the veneering porcelain.
Question 4: Which phenomenon explains why dental porcelain fired in a vacuum produces fewer surface bubbles and higher translucency than air-fired porcelain?
- Vacuum reduces the firing temperature required
- Air is evacuated so gas bubbles cannot be trapped within the material (Correct answer)
- Vacuum increases the coefficient of thermal expansion
- Vacuum firing changes porcelain's chemical composition
Correct answer: Air is evacuated so gas bubbles cannot be trapped within the material
Vacuum firing removes air from the furnace chamber, preventing gas entrapment within molten porcelain and resulting in a denser, more translucent restoration.
Question 5: In the lost-wax casting process, what is the purpose of the sprue former?
- To create the external shape of the casting
- To form a channel through which molten metal flows into the mold (Correct answer)
- To support the investment during burnout
- To regulate the temperature of the casting ring
Correct answer: To form a channel through which molten metal flows into the mold
The sprue former creates the channel (sprue) that guides molten metal from the crucible into the mold space vacated by the wax pattern.
Question 6: Which property of resin cements is most critical for preventing post-operative sensitivity in all-ceramic restorations?
- High film thickness
- Low polymerization shrinkage (Correct answer)
- Opacity for shade masking
- Rapid initial set time
Correct answer: Low polymerization shrinkage
Low polymerization shrinkage minimizes stress at the tooth-cement interface, reducing microleakage and the resultant post-operative sensitivity.
Question 7: The term 'creep' in dental materials refers to:
- Sudden fracture under impact loading
- Slow plastic deformation under a constant applied stress over time (Correct answer)
- Rapid elastic recovery after load removal
- Surface discoloration from oxidation
Correct answer: Slow plastic deformation under a constant applied stress over time
Creep is the time-dependent plastic deformation that occurs in a material subjected to a constant stress, relevant to amalgam marginal breakdown.
CDT Dental Materials Science 3
Which investment material is used for casting high-fusing noble and base metal alloys due to its ability to withstand higher burnout temperatures?