CDT Dental Materials & Laboratory Techniques 3 — Questions and Answers
Question 1: Which property describes a material's ability to return to its original shape after the deforming stress is removed?
- Plasticity
- Elasticity (Correct answer)
- Ductility
- Malleability
Correct answer: Elasticity
Elasticity is the property that allows a material to recover its original dimensions once the applied stress is released below the elastic limit.
Question 2: In lost-wax casting, the spruing technique affects which of the following?
- Shade selection of the final restoration
- Metal flow direction and potential porosity sites (Correct answer)
- Investment expansion coefficient
- Alloy hardness after solidification
Correct answer: Metal flow direction and potential porosity sites
Sprue placement determines the path of metal entry, and improper design can cause turbulence, back-pressure porosity, or incomplete fill.
Question 3: Polyvinyl siloxane (PVS) impression materials set via which reaction?
- Acid-base condensation releasing ethanol
- Condensation releasing hydrogen gas
- Addition polymerization with no by-products (Correct answer)
- Free-radical chain polymerization with heat
Correct answer: Addition polymerization with no by-products
PVS materials cure by addition polymerization between vinyl and hydride groups, producing no volatile by-products and exhibiting excellent dimensional stability.
Question 4: What is the purpose of degassing a noble metal alloy before casting?
- To increase the liquidus temperature
- To remove dissolved gases that cause porosity (Correct answer)
- To improve alloy color for esthetics
- To reduce the alloy's coefficient of thermal expansion
Correct answer: To remove dissolved gases that cause porosity
Degassing melts the alloy briefly before casting to allow trapped gases to escape, reducing subsurface gas porosity in the final casting.
Question 5: The Shore A durometer is used to measure hardness of which category of dental materials?
- Gypsum stone models
- Flexible silicone and rubber-based materials (Correct answer)
- Cast metal alloys
- Fired dental porcelain
Correct answer: Flexible silicone and rubber-based materials
The Shore A scale measures the hardness of soft, flexible materials like silicones, rubbers, and soft denture liners.
Question 6: A feldspathic porcelain crown appears too translucent after firing. The technician should:
- Add more opaque porcelain over the fired surface
- Increase the dentin buildup and re-fire (Correct answer)
- Apply a tinted glaze to mask the translucency
- Reduce the firing temperature on the next piece
Correct answer: Increase the dentin buildup and re-fire
Excess translucency usually means insufficient dentin mass; adding more dentin porcelain provides greater body opacity before glaze firing.
Question 7: Which mixing technique is recommended for zinc phosphate cement used as a luting agent to extend working time?
- Rapid vigorous spatulation on a warm glass slab
- Slow incremental powder addition on a cool glass slab (Correct answer)
- Mixing powder and liquid simultaneously in a rubber bowl
- Ultrasonic mixing to homogenize the mix quickly
Correct answer: Slow incremental powder addition on a cool glass slab
A cool (refrigerated) glass slab and slow incremental powder addition dissipate the exothermic heat of reaction, extending working time.
CDT Dental Materials & Laboratory Techniques 3
Which property describes a material's ability to return to its original shape after the deforming stress is removed?