CDT Dental Materials & Laboratory Techniques 2 — Questions and Answers
Question 1: Which type of investment material is used for casting high-fusing alloys such as cobalt-chromium?
- Gypsum-bonded investment
- Phosphate-bonded investment (Correct answer)
- Ethyl silicate-bonded investment
- Silica-bonded investment
Correct answer: Phosphate-bonded investment
Phosphate-bonded investments withstand the high casting temperatures required for cobalt-chromium and other base metal alloys.
Question 2: What is the primary purpose of using a separating medium on a stone cast before applying wax?
- To strengthen the cast surface
- To prevent wax from bonding to the stone (Correct answer)
- To improve wax flow characteristics
- To disinfect the cast
Correct answer: To prevent wax from bonding to the stone
Separating mediums such as tinfoil substitute create a barrier so wax can be removed cleanly without sticking to the stone model.
Question 3: The term 'liner' in a porcelain fused-to-metal restoration refers to:
- A thin opaque ceramic layer applied over the metal coping (Correct answer)
- The cast metal framework before porcelain application
- A glaze fired last on the restoration
- An adhesive used to bond porcelain to metal
Correct answer: A thin opaque ceramic layer applied over the metal coping
The opaque liner masks the metal color and establishes the chemical bond between the metal coping and overlying dentin/enamel porcelain.
Question 4: Which hardener is commonly added to acrylic resin to improve its cross-link density and reduce porosity?
- Benzoyl peroxide alone
- Ethylene glycol dimethacrylate (Correct answer)
- Methyl methacrylate monomer only
- Zinc oxide
Correct answer: Ethylene glycol dimethacrylate
Cross-linking agents like ethylene glycol dimethacrylate (EGDMA) form bridges between polymer chains, increasing strength and reducing residual monomer.
Question 5: When trimming a diagnostic cast, the posterior heel of a maxillary cast should be:
- Parallel to the occlusal plane
- Parallel to the mid-palatal raphe (Correct answer)
- Angled 20 degrees toward the base
- Rounded to prevent breakage
Correct answer: Parallel to the mid-palatal raphe
Trimming the posterior wall parallel to the mid-palatal raphe ensures symmetrical articulator mounting and accurate representation of midline.
Question 6: What is the Vickers Hardness Number (VHN) scale primarily used to assess in dental alloys?
- Thermal conductivity
- Resistance to surface indentation (Correct answer)
- Tarnish susceptibility
- Casting shrinkage
Correct answer: Resistance to surface indentation
The Vickers Hardness test measures a material's resistance to permanent deformation when a diamond indenter is pressed into its surface.
Question 7: A technician notices small voids (porosity) throughout a cast metal framework after casting. The most likely cause is:
- Over-heating the investment mold
- Insufficient alloy volume in the crucible (Correct answer)
- Too rapid a quench after casting
- Excessive wax burnout temperature
Correct answer: Insufficient alloy volume in the crucible
Using too little alloy means insufficient metallostatic pressure to fill the mold completely, leading to shrinkage porosity throughout the casting.
CDT Dental Materials & Laboratory Techniques 2
Which type of investment material is used for casting high-fusing alloys such as cobalt-chromium?