Dental Materials Science
What this domain tests
Material selection, handling variables and the physical consequences of getting them wrong - the domain where a 5 mL water error becomes a failed die.
Core rules
- Gypsum. Type I impression plaster, II model plaster, III dental stone, IV low-expansion die stone, V high-expansion die stone. Raising the water/powder ratio lengthens setting time, lowers compressive strength and reduces setting expansion; the manufacturer's ratio is not negotiable.
- Typical setting expansion: plaster ~0.2–0.3%, stone ~0.1–0.15%, die stone ~0.08–0.10%, high-expansion stone 0.2%+.
- Elastomers. Addition silicone (PVS) is accurate and dimensionally stable but hydrophobic; polyether is hydrophilic and stiff, useful for multiple preparations; alginate is an irreversible hydrocolloid and loses accuracy if pouring is delayed more than about 15 minutes.
- Acrylics. Heat-cured PMMA for denture bases; autopolymerizing resin for repairs. Processing shrinkage of roughly 0.4–0.5% linearly is offset by packing technique and remounting.
- Alloys. High noble: ≥60% noble with ≥40% gold. Noble: ≥25% noble. Base-metal Ni-Cr and Co-Cr are stiffer and harder to solder; beryllium-bearing alloys require dust control.
- Ceramics. Match the veneering ceramic's coefficient of thermal expansion to the framework - a mismatch shows up as chipping at the cervical margin.
Worked example
A die stone is specified at 22 mL water per 100 g powder. A technician adds 5 mL to slow the set. Setting time lengthens, but expansion and compressive strength fall, so the die is soft and slightly undersized; crowns built on it will seat high. Correct action: use the specified ratio and the supplied liquid to control setting time.
Traps that cost marks
- Using water/powder adjustment as a setting-time control.
- Pouring alginate hours after the impression was taken.
- Quoting volumetric shrinkage as if it were linear.