Gypsum and Wax Products (Ch. 16) (1)

Chapter 16: Gypsum and Wax Products

Objectives

  • Differentiate positive and negative reproduction.

  • Differentiate diagnostic casts, working casts, and dies.

  • Describe the chemical and physical nature of gypsum products.

  • Explain the manufacturing process for gypsum products and effects on their physical characteristics.

  • Compare gypsum properties: strength, dimensional accuracy, solubility, and reproduction of detail.

  • List ADA-recognized gypsum products and their uses.

  • Explain initial/final set of gypsum, factors affecting setting time, expansion, and strength.

  • Explain mixing/handling gypsum products to create diagnostic casts.

  • Identify common components of dental waxes.

  • Compare properties and clinical significance of waxes.

  • Discuss three classifications of waxes.

  • Prepare model plaster or stone for pouring.

  • Pour maxillary and mandibular anatomic casts.

  • Trim maxillary and mandibular diagnostic casts.

  • Obtain bite registration with bite registration or utility wax.

Uses of Gypsum Products

  • Gypsum: Widely found mineral suitable for making dental replicas.

    • Replicas:

      • Diagnostic casts (study models)

      • Working casts (working models)

      • Dies

    • Note: Accuracy of replicas highly depends on impression accuracy.

Properties of Gypsum Products

  • Chemical Properties:

    • Gypsum: Dihydrate of calcium sulfate.

    • Ground to powder, mixed with water creates a flowable mixture and undergoes an exothermic chemical reaction, reverting to solid mass.

Desirable Qualities of Gypsum Products

  • Casts and dies must be:

    • Accurate

    • Hard

    • Dimensionally stable

  • Working casts and dies must have:

    • Excellent detail reproduction

    • Strength and abrasion resistance

    • Minimal solubility

  • Practical considerations include cost, ease of use, and safety.

Production of Gypsum Products

  • Model Plaster: Porous, irregular shape.

  • Dental Stone: Heated under pressure, steam gives uniform shape and reduced porosity.

  • Die Stone: Further heat/pressure yields uniform and dense product.

Properties of Gypsum Products

  • Physical Properties:

    • Strength related to water amount used.

    • Dimensional accuracy and control of setting expansion:

      • Plaster has highest expansion; stones lower.

    • Detail reproduction inversely related to porosity and contamination.

    • Solubility linked to porosity; plaster more soluble than stone.

Classification of Gypsum Products

  • Type I (Impression Plaster): Rarely used.

  • Type II (Model Plaster): Common for diagnostic casts; durable but weaker.

  • Type III (Dental Stone): For dentures, orthodontic models, and casts.

  • Type IV (High Strength/Low Expansion Dental Stone): Used for crowns/bridges.

  • Type V (High Strength/High Expansion Dental Stone): Suitable for base metals due to casting shrinkage.

Manipulation of Gypsum Products

  • Material Selection

  • Proportioning: Water-to-powder ratio is critical.

  • Mixing: Spatulation for 1 minute, working time starts after mixing.

  • Setting Times:

    • Initial set after loss of gloss (Happens within minutes)

    • Final set: 45 minutes to 1 hour, cool to touch.

  • Control Factors:

    • Water-to-powder ratio

    • Temperature

    • Accelerators/retarders

    • Clean equipment promptly.

Fabricating and Trimming Diagnostic/Working Casts

  • Casts consist of two portions:

    • Anatomic Portion: Replicates hard and soft tissue structures.

    • Art Portion: Aids handling and articulating casts.

  • Methods:

    • Double-pour: Two mixes for anatomic/base.

    • Single-step: One mix.

    • Boxing method: Wax walls around the impression.

    • Caution: Avoid material on impression tray sides to prevent rim lock.

Manipulation of Gypsum Products (Continued)

  • Storage: Gypsum products absorb moisture; store in airtight containers.

  • Cleanup: Clean equipment immediately post-manipulation.

  • Infection Control: Disinfect impressions.

  • Separating Casts: Gently ease a knife under tray to separate; avoid forcing apart to prevent damage.

Trimming Models

  • Study Models: Produce symmetric models with a proper base ratio (Base: 1/3, Anatomic: 2/3).

  • For whitening trays, mouthguards: Trim without a base; symmetry not essential.

Dental Waxes

  • Composition: Mixture of natural/synthetic materials.

    • Remove excess residue to avoid irregularities.

  • Thermal Expansion: Waxes expand when heated and contract upon cooling; follow guidelines for heating.

Classification of Waxes

  1. Pattern Waxes: Inlay, casting, baseplate wax for dentures.

  2. Processing Waxes: Boxing, utility, sticky waxes, for procedures.

  3. Impression Waxes: Corrective impression and bite registration.

Baseplate Wax

  • Pink sheets used to contour dentures and prepare for teeth setting; should not distort at oral temperatures.

Pattern Waxes

  • Inlay Wax: For metal cast restorations using the lost wax technique.

  • Casting Wax: For frameworks of dentures; properties akin to inlay wax.

Processing Waxes

  • Boxing Wax: Forms a box around impressions during casting.

  • Utility Wax: Flexible ropes for patient comfort and stability during impression taking.

Impression and Bite Registration Wax

  • Impression Wax: Rarely used but still relevant for certain impressions; tends to distort.

  • Bite Registration Wax: Used for anatomical model articulation; distortion-prone but some newer materials have replaced it.

Manipulation of Waxes

  • Softening Techniques:

    • Evenly with dry heat, warm water bath, or flame.

  • Storage: Keep at or slightly below room temperature to avoid distortion.

Summary Questions

  1. Identify the three gypsum replicas.

  2. What are desirable qualities of gypsum products?

  3. What results from the chemical reaction of gypsum products?

  4. Contrast the three gypsum products.

  5. Which material is utilized for crowns or bridges?

  6. Describe changes during initial setting time.

  7. Why is material not applied to tray sides?

  8. How should models for whitening trays or night guards be trimmed?

  9. What wax aids patient comfort?

References

  • Malloy, D. C., & Malloy, D. K. K. (2015). Alginate impression (A) and gypsum stone cast (B). Retrieved November 5, 2021, from https://pocketdentistry.com/12-replicating-materials-impression-and-casting/.