Dental Education and Materials Overview

Vision: Shaping the future of Dentistry through excellence in dental education, innovative research, patient-centered care and health promotion internationally.
Mission: Providing learner-focused dental education, cutting-edge scientific research, patient-centered care, community engagement, all within a positive organizational culture.
Core Values: Leadership, Excellence, Innovation, Collaboration, Respect, Integrity.

Cast: A positive replica of teeth and/or associated supporting soft and hard tissues.
Die: A model of a single tooth, prepared from an impression.
Model & Die Materials Requirements:
- High strength to resist breakage during use.
- Hard to use.
- Ability to reproduce fine details of the impression.
- Little dimensional change on setting, and remain dimensionally stable during storage.
- Compatibility with impression materials (no interaction between the impression and model/die materials).
- Good color contrast.
- Ease of use.
- Cheap.

Gypsum Products: Pure gypsum products used in dentistry are classified as plaster, dental stone, and high-strength stone.
Chemical Composition: All gypsum products are chemically the same – calcium sulfate hemihydrate (CaSO₄·½H₂O).
Differences: They differ in manufacturing method, particle size, W/P ratio, and physical properties.
Manufacturing Methods:
- Plaster: Open vessel at 120°C
- Dental stone: Autoclave under steam pressure at 120°C
- High-strength stone: Boiling in 30% CaCl₂ solution

Reverse of Manufacturing: The setting reaction is the reverse of the manufacturing process.
Reaction: Calcium sulfate hemihydrate + Water →Calcium sulfate dihydrate + Heat

Dissolution: Some hemihydrate dissolves in water, forming Ca²⁺ and SO₄²⁻ ions.
Nucleation: These ions then form dihydrate nuclei for crystallization.
Crystal Growth: More dihydrate precipitates around these nuclei, leading to crystal growth until all hemihydrate is transformed into dihydrate.
Solubility Difference: Solubility of hemihydrate in water is higher than that of the formed dihydrate.
Super Saturation: Concentration of dihydrate increases rapidly, to create a super-saturated solution.

Theoretical Ratio: 100 g of hemihydrate requires 18.6 ml of water for complete hydration.
Practical Ratio: (Actual ratios varies for plaster, stone, improved stone.)
Importance: Excess water leads to porosity in the final product.

Definition: Time elapsed from mixing until hardening.
Divisions:
- Mixing time: Duration from powder addition to water until a homogenous mix.
- Working time (3 minutes): Time available for mixing and use before initial setting occurs.
- Initial setting time (12 minutes): Period beginning mixing until partial setting; material is rigid but not hard.
- Final setting time (Several hours): Complete setting occurs, resulting in hard and strong model/die.

Loss of Gloss: To determine proper working time; indicates partial setting.
Temperature Rise: Reaction is exothermic; implies setting.
Penetration Tests: Resistance of set gypsum to penetration by needles (Vicat & Gillmore needles) to measure setting time.

Operator Factors:
- W/P ratio: High W/P ratio results in less nuclei of crystallization and slow growth rate (retardation).
- Mixing time and rate: More nuclei of crystallization and accelerated crystal growth.
- Temperature:
  - 20-50°C: Accelerates setting.
  - Above 50°C: Retards setting.
  - 100°C: No reaction.
Manufacturer Factors:
- Powder Fineness: Smaller particles increase wetting, dissolution rate, nuclei and crystal growth rate acceleration.
- Impurities: Terra Alba as nucleating agents increases number of nuclei and accelerate crystal growth rate.
- Chemicals (Retarders and Accelerators): Retarders (Borax, blood, saliva) coat hemihydrate and reduce solubility or coat growing crystals inhibiting crystal growth rate (retardation). Accelerators (2% K₂SO₄) accelerate dissolution, nuclei, and crystallization growth.

Dimensional Changes: Volumetric contraction (7%) during setting, but setting expansion is observed which is due to outward thrusting of growing crystals.
Porosity: Set gypsum is porous due to greater external volume than crystalline volume. Excess water remains in the pores.
Strength: Different strengths are noted:
- Wet Strength (green strength) : Strength of gypsum during or immediately after hydration.
- Dry Strength: Strength after water is removed.
Factors affecting strength:
- W/P Ratio: Higher ratio results in more excess water leaving more pores and a weaker product.
- Mixing Time & Rate: Overmixing leads to fracture and decreasing strength.
- Chemicals: Chemicals impact crystal shape and decrease strength.
- Gypsum Product Type: Improved stone > Dental stone > Plaster.

Surface Hardness: Typically low, making surfaces susceptible to scratches.
Resin (Polymer): Applying resin increases surface hardness and abrasion resistance.

Accuracy: Gypsum products do not accurately reproduce surface details.
Porosity: Porous microscopic surface of the set gypsum and air bubbles formed at the interface between the impression and gypsum casts.

The impression should be washed before pouring the model to avoid any contaminants or foreign materials interfering with the reproducibility and strength of the model.