Construction of Dental

Restoration1. Dental waxes

2. Casting investment

Dental restoration is a replacement for the lost natural hard or soft dental tissues.

1. Resin (Polymer) - based restorations

Composite fillingsDenture acrylics

Metal-ceramic All-ceramic

2. Ceramic - based restorations

Gold alloysAmalgam alloys

3. Metal - based restorations

Construction of metallic restorations

or .. metal copings

A. Direct Technique

e.g. direct amalgam filling

B. Indirect Technique (Laboratory-made)

1. Swaging (cold working)

Hammering the metal over a die

2. Casting (lost wax technique)

Pouring of molten metal into a refractory

mold

3. Electroforming

Precipitation of metal ions over a master die

Casting metallic restorationusing lost wax technique

Materials necessary for constructing indirect metallic restorations

using the lost wax technique

1. Dental waxes .. (to construct wax pattern)

2. Casting investment .. (to construct the mold)

3. Metallic alloys .. (restorative material)

Dental waxes

Dental waxes are hydrocarbons in nature (contain O2 , H, and C) and have many applications in Dentistry

Ideal requirements of pattern waxes

Should be colored to be easy distinguished either from the die material or the tooth structure.

Should be cohesive and not adhesive

Should soften without flaking.

Should soften at reasonable temperature and harden at mouth temperature

Should have adequate flow to record the details of the prepared tooth

Should burn-out completely with no residue.

1 .Blue inlay pattern wax

Uses To construct wax patterns for cast inlays, onlays ,

crowns and bridges

1. Blue inlay pattern wax

Presentation forms Usually presents as Blue or green sticks or rods Recently, presents in different colors and formats

(blocks or in Tins)

2 .Casting wax

UsesConstructing wax patterns of metallic frameworks

either for removable partial or complete dentures.

2. Casting wax

Available in many shapes and colors to fit different applications

Presentation forms Preformed meshes

and clasps Sheets with different

thickness Bars with round, half

round and half bear cross section.

wires.

3 .Base plate wax

Uses1. Temporary base plates and wax rims (bite blocks) 2. waxing the dentures up.

3 .Base plate wax

Presentation forms Pink or red sheets Preformed shapes are currently presented

Other types of Dental waxes

Processing waxes: Used for laboratory issues1. Sticky wax.. to stick 2 portions of broken restoration or casts before repair.

2. Utility wax .. to position artificial teeth to their sheets.

3. Boxing wax .. to box an impression to facilitate its pouring in stone.

Impression waxes: No longer be used1. Bite registration wax .. to check high points of dental restorations.

2. Corrective wax .. to make functional 2ry impressions.

Stop, give me a break!!!!

Casting Investment

Casting investment is composed of silica particles (SiO2) bonded together with different binding agents

Requirements of ideal investment material

Should be stable (not decompose) on heating (Refractory)

Should show a kind of expansion to compensate for metal shrinkage

Withstand the impact of molten metal Able to wet the wax surfaces registering its details Should be porous to permit escape of gases Not adhere to the surface of cast metal Easy breakdown after casting Easily manipulated, not expensive , not irritant and no

release of toxic products

Types of investment materials

Relative to the type of silica particles used Quartz-based investment Cristobalite- based investment

Cristobalite-based investment provide higher rates of thermal expansion (displasive transformation) than quartz-based investment does (when heated from 0- 600oC)

Relative to the type of binder used Gypsum-bonded investment .. Uses CaSo4. 2H2O binder Phosphate-bonded investment .. Uses magnesium ammonium

phosphate Silica-bonded investment .. Uses silica gel binder

In both types Tridymite could present as impurity

1 .Gypsum-bonded Investment

Uses:

For casting metal alloys that melt below1000oC (i.e. gold alloys only) because the investment decompose if heated above 700oc.

Presentation:

Presented as powder to be mixed with regular tap water

After mixing, the calcium sulfate hemihydrates dissolves in water and changes into calcium sulfate dihydrates in

the form of inter-meshed crystals having the silica particles in between..

A. Dimensional changes:

1. Setting expansion: as a result of gypsum setting2. Hygroscopic setting expansion: as a result of adding water after initial

setting (6 min from the start of mix) …. water could be gained through the following approaches;

a. Water adding using hypodermic syringeb. Immersion in warm waterc. Contact with wet ring liner

3. Thermal expansion: as a result of silica displasive transformation4. Heating (evaporation of water) and cooling (after casting) contraction

Important characters of Gypsum-bonded Investment

Setting Vs Hygroscopic expansion

Step 1. Setting expansion

Step 2. Hygroscopic expansion

Outward

thrusting

Adding water after 6 min

Extra expansion 3 times more than the normal setting expansion

Crystal growth

B. Interfacial properties:• Gypsum-bonded investment usually gives castings with little surface

roughness.

Important characters of Gypsum-bonded Investment

(cont’d)

C. Chemical properties:

• The set investment material is porous permitting the escape of gasses through out the mold…… This fact looks advantageous in preventing the back-pressure porosity in the cast metal

• The material could not be heated above 700oC ……. to avoid the disintegration of gypsum binder

Phosphate-bonded Investment

Uses:

For casting metal-ceramic alloys and some base metal alloys (Can be heated above 1000oc).

Presentation:

Presented as powder (containing ammonium diacid phosphate and magnesium oxide) to be mixed with either water or colloidal silica

solution

After mixing, the following reaction occurs to form crystals of magnesium ammonium phosphate which bind the silica particles together giving the

investment its room temperature strength.MgO + NH4 H2Po4 ---------------> Mg NH4 Po4 + H2O

Important characters of phosphate-bonded Investment

A. Dimensional changes:

1. Setting expansionThis expansion is greater with the investment mixed with colloidal silica.

2. Thermal expansion As a result of silica displasive transformation. It is greater in investments mixed with colloidal silica.

3. Drying shrinkage in water-mixed investment, a drying shrinkage will occur at 200-400oC and this is completely eliminated if the colloidal silica is used.

B. Interfacial properties PBI gives rougher cast surface than the GBI. increasing the silica solution ratio enhances the surface smoothness.

C. Chemical propertiesThe set PBI shows the same degree of porosity as that of GBI.PBI could be heated up to 1100oC

D. Strength properties Set PBI shows a higher strength at room temperature than that of GBI. Set PBI shows a higher strength at high temperature than that of GBI.

Important characters of phosphate-bonded Investment (cont’d)

Silica-bonded Investment

Uses:

For casting high fusing base metal alloys that usually used for constructing partial denture frameworks.

Presentation:Presented as powder (containing silica and MgO) and two bottles of liquids (1. diluted silicate solution, 2. diluted HCl).

Equal volume of each solution should be mixed at first to form silicic acid sol. Powder is then mixed with the sol that will change into silicic acid gel (room temp binder) in

presence of MgO. That gel will dry and change into silica on heating

Important characters of Silica-bonded Investment

A. Dimensional changes1. Setting contraction Due to the transformation of silicic acid sol into silicic acid gel.

2. Green contraction On heating, the investment mass dries and loses alcohol and water.

3. Thermal expansion Due to displasive transformation of silica particles.Because of higher silica content, the amount of thermal expansion is fair enough to compensate for the shrinkage of wax pattern, solidification of metal as well as the setting and green contraction of the investment.

B. Interfacial propertiesSBI produces the roughest cast in comparison to other types of investment.

C. Strength propertiesSBI has higher strength than both GBI and PBI…

D. Chemical properties:SBI has a complicated and time consuming manipulative procedure. SBI could be heated up to 1300oC, and it is used to cast base metal alloys with very high melting temperatures (1300-1500oC) accordingly.

Important characters of silica-bonded Investment (cont’d)

Gases comes out

Metal gets in

Air vent

InvestmentMold cavity

Sprue channel

Silica particles of the set material are closely packed together, producing a negligible degree of porosity. Lack of porosity may lead to pack-

pressure porosity in the cast metal. Air spaces or vents must be left in the investment to allow the escape of gases.

Thank you