Applications

a. Dentulous impressions for casts for prosthodontics

b. Dentulous impressions for pedodontic appliances

c. Dentulous impressions for study models for orthodontics

d. Edentulous impressions for casts for denture construction

Gypsum Products

Denture Teeth

Use-complete or partial dentures

Type

a. Porcelain teeth
b. Acrylic resin teeth
c. Abrasion-resistant teeth (microfilled composite)

Structure and properties

1. Porcelain teeth (high-fusing porcelain)
Only bonded into denture base mechanically. Harder than natural teeth or other restorations and abrades those surfaces. Good aesthetics.Used when patients have good ridge support and sufficient room between the arches

2. Acrylic resin teeth (PMMA  [polymethyl methacrylate])

Bonded pseudochemically into the denture base. Soft and easily worn by abrasive foods . Good initial aesthetics
Used with patients with poor ridges and in cases where they oppose natural teeth

3. Abrasion-resistant teeth (microfilled resins)
Bonded pseudochemically into the denture base.Better abrasion resistance then  acrylic resin teeth
 

I . Procedure for single casting :

A 2.5 mm sprue former is recommended
for molar crowns 2.0 mm for premolars & partial coverage crowns .

II . Procedure for multiple casting :

Each unit is joined to a runner bar .

A single sprue feeds the runner bar

4 . SPRUE FORMER DIRECTION
Sprue Should be directed away from the delicate parts of the pattern
It should not be at right angles to a flat surface .(leads to turbulance  porosity .)
Ideal angulation is 45 degrees .

5 . SPRUE FORMER LENGTH

Depends on the length of casting ring .. Length of the Sprue former should be such that it keeps the wax pattern about 6 to 8 mm away from the casting ring. Sprue former should be no longer than 2 cm. The pattern should be placed as close to the centre of the ring as possible.

Significance

Short Sprue Length:

The gases cannot be adequately vented to permit the molten alloy to fill the ring completelyleading to Back Pressure Porosity.

Long Sprue Length:

Fracture of investment, as mold will not withstand the impact force of the entering molten alloy.

Top of wax should be adjusted for :

6 mm for gypsum bonded investments .

3 -4 mm for phosphate bonded investments .
TYPES OF SPRUES

I . - Wax . II . Solid

- Plastic . Hollow
- Metal .

Properties of Acrylic Resins.

• They have a low thermal conductivity. These resins are not easily washed out by the acids of the oral cavity (low solubility). Acrylic resins are also resilient, which allows them to be used in stress-bearing areas.
• Acrylic resins exhibit a moderate shrinkage of from 3 to 8 percent. This shrinkage and low marginal strength can lead to marginal leakage. Acrylic resins have a low resistance to wear. Acrylic resins cannot be used over a zinc oxide and eugenol-type base because eugenol interferes with the acrylic curing process.
• Mixing. Insufficient mixing will cause an uneven color or streaks in the mixture. Overmixing will cause the material to harden before it can be placed
• Poor distortion resistance at higher temperatures, therefore dentures should not be cleaned in hot water
• Good resistance to color change
• Absorbs water and must be kept hydrated  (stored in water when not in mouth) to prevent dehydration cycling and changes in dimensions
• Not resistant to strong oxidizing agents
• Low strength; however, flexible, with good fatigue resistance
• Poor scratch resistance; clean tissue-bearing surfaces of denture with soft brush and do not use abrasive cleaners

Manipulation

Mixture of powder and liquid is painted onto working cast to create shape for acrylic appliance à  After curing of mixture, the shape and fit are adjusted by grinding with burrs and stones with a slow-speed handpiece .Acrylic dust is irritating to epithelial tissues of nasopharynx and skin and may produce allergic dermatitis or other reactions.  Grinding may heat polymer to temperatures that depolymerize and release monomer vapor. which may be an irritant