NEET MDS Lessons
Dental Materials
The Sprue :
Its a channel through which molten alloy can reach the mold in an invested ring after the wax has been eliminated. Role of a Sprue: Create a channel to allow the molten wax to escape from the mold. Enable the molten alloy to flow into the mold which was previously occupied by the wax pattern.
FUNCTIONS OF SPRUE
1 . Forms a mount for the wax pattern .
2 . Creates a channel for elimination of wax .
3 .Forms a channel for entry of molten metal
4 . Provides a reservoir of molten metal to compensate for the alloy shrinkage .
SELECTION OF SPRUE
Sprue former gauge selection is often empirical, is yet based on the following five general principles:
1. Select the gauge sprue former with a diameter that is approximately the same size as the thickest area of the wax pattern. If the pattern is small, the sprue former must also be small because a large sprue former attached to a thin delicate pattern could cause distortion. However if the sprue former diameter is too small this area will solidify before the casting itself and localized shrinkage porosity may result.
2. If possible the sprue former should be attached to the portion of the pattern with the largest cross-sectional area. It is best for the molten alloy to flow from the thick section to the surrounding thin areas. This design minimizes the risk of turbulence.
3. The length of the sprue former should be long enough to properly position the pattern in the casting ring within 6mm of the trailing end and yet short enough so the molten alloy does not solidify before it fills the mold.
4. The type of sprue former selected influences the burnout technique used. It is advisable to use a two-stage burnout technique whenever plastic sprue formers or patterns are involved to ensure complete carbon elimination, because plastic sprues soften at temperatures above the melting point of the inlay waxes.
5. Patterns may be sprued directly or indirectly. For direct sprueing the sprue former provides the direct connection between the pattern area and the sprue base or crucible former area. With indirect spruing a connector or reservoir bar is positioned between the pattern and the crucible former. It is common to use indirect spruing for multiple stage units and fixed partial dentures.
Cement Bases
Applications
• Thermal insulation below a restoration
• Mechanical protection where there is inadequate dentin to support amalgam condensation pressures
Types
• Zinc phosphate cement bases
• Polycarboxylate cement bases
• Glass ionomer cement bases (self-curing and light-curing)
Components
o Reactive powder (chemically basic)
o Reactive liquid (chemically acidic)
Reaction
o Acid-base reaction that forms salts or cross linked matrix
o Reaction may be exothermic
Manipulation-consistency for basing includes more powders, which improves all of the cement properties
Properties
Physical-excellent thermal and electrical insulation
Chemical-much more resistant to dissolution than cement liners
Polycarboxylate and glass ionomer cements are mechanically and chemically adhesive to tooth structure
Solubility of all cement bases is lower than cement liners if they are mixed at higher powder- to-liquid ratios
Mechanical- much higher compressive strengths (12,000 to 30,000 psi)
Light-cured hybrid glass ionomer cements are the strongest
Zinc oxide-eugenol cements are the weakest
Biologic (see section on luting cements for details)
Zinc oxide-eugenol cements are obtundent to the pulp
Polycarboxylate and glass ionomer cements are kind to the pulp
SELECTION OF SPRUE
1 . DIAMETER :
It should be approximately the same size of the thickest portion of the wax pattern .
Too small sprue diameter suck back porosity results .
2 . SPRUE FORMER ATTACHMENT :
Sprue should be attached to the thickest portion of the wax pattern .
It should be Flared for high density alloys & Restricted for low density alloys .
3 . SPRUE FORMER POSITION
Based on the
1. Individual judgement .
2. Shape & form of the wax pattern .
Patterns may be sprued directly or indirectly .
Indirect method is commonly used
Gypsum Products
|
Characteristics |
Plaster |
Stone |
Diestone |
|
Chemical Name |
Beta-Calcium Sulfate hemihydrate |
Alpha-Calcium sulfate hemihydrate |
Alpha-Calcium sulfate hemihydrate |
|
Formula |
CaSO4 – ½ H2O |
CaSO4 – ½ H2O |
CaSO4 – ½ H2O |
|
Uses |
Plaster Models ,Impression Plasters |
Cast Stone, Investment |
Improved Stone, diestone |
|
Water(W) Reaction Water Extra Water Total water Powder (P) W/P Ratio |
18ml 32ml 50ml 100g 0.50 |
18ml 12ml 30ml 100g 0.30 |
18ml 6ml 24ml 100g 0.24 |
Impression Material
|
Materials |
Type |
Reaction |
Composition |
Manipulation |
Initial setting time
|
|
Plaster |
Rigid |
Chemical |
Calcuim sulfate hemihydrate, water |
Mix P/L in bowl |
3-5 min
|
|
Compound |
Rigid |
Physical |
Resins, wax, stearic acid, and fillers |
Soften by heating
|
Variable (sets on cooling) |
|
Zinc oxide-eugonel |
Rigid |
Chemical |
Zinc oxide powder, oils, eugenol, and resin |
Mix pastes on pad
|
3-5 min
|
|
Agar-agar |
Flexible |
Physical |
12-15% agar, borax, potassium sulfate, and 85% water |
Mix P/L in bowl
|
Variable (sets on cooling)
|
|
alginate |
Flexible |
Chemical |
Sodium alginate, calcium sulfate, retarders, and 85% water |
Mix P/L in bowl
|
4-5 min
|
|
Polysulfide |
Flexible |
Chemical |
Low MW mercaptan polymer, fillers, lead dioxide, copper hydroxide, or peroxides |
Mix pastes on pad
|
5-7 min
|
|
Silicone |
Flexible |
Chemical |
Hydroxyl functional dimethyl siloxane, fillers, tin octoate, and orthoethyl silicate |
Mix pastes on pad
|
4.5 min
|
|
Polyether |
Flexible |
Chemical |
Aromatic sulfonic acid ester and polyether with ethylene imine groups |
Mix pastes on pad
|
2-4 min
|
|
Polyvinyl siloxane |
Flexible |
Chemical |
Vinyl silicone, filler, chloroplatinic acid, low MW silicone, and filler |
Mix putty or use two-component mixing gun
|
4-5 min
|
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
Investment Materials
Investment is mold-making material
Applications
a. Mold-making materials for casting alloys
b. Mold-making materials for denture production
Classification
a. Gypsum-bonded investments (based on gypsum products for matrix)
b. Phosphate-bonded investments
c. Silicate-bonded investments
Components
a. Liquid-water or other reactant starts formation of matrix binder by reacting with reactant powder
b. Powder-reactant powder, filler, or modifiers
Manipulation
a. P/L mixed and placed in container around wax pattern
b. After setting, the investment is heated to eliminate the wax pattern in preparation for casting