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INVESTMENT MATERIALS

IT IS RECOMMENDED TO FOLLOW MCCABE APPLIED DENTAL MATERIALS ALONG WITH THIS LECTURE PERSENTATION

REFERENCES FOR THIS LECTURE:

RESTORATIVE DENTAL MATERIALS, CRAIG

NOTES ON DENTAL MATERIALS, COMBE’S

DENTAL MATERIALS, PHILLIPS

WHAT IS AN INVESTMENT MATERIAL

•A MATERIAL SUITABLE FOR FORMING A MOLD INTO WHICH A METAL OR ALLOY IS CAST

• WAX PATTERNS (OF APPLIANCES OR RESTORATIONS) ARE INVESTED IN INVESTMENT MATERIAL...

• ASSEMBLY IS HEATED TO REMOVE WAX...

• THE MOLD SPACE FORMED IS THEN OCCUPIED BY THE MATERIAL OF WHICH THE RESTORATION OR APPLIANCE IS TO BE MADE (ACRYLIC OR ALLOY)…

REQUIREMENTS FOR INVESTMENT MATERIALS

• EASILY MANIPULATED

• SUFFICIENT STRENGTH AT ROOM TEMP.

• STABILITY AT HIGH TEMPERATURE

• SUFFICIENT EXPANSION TO COMPENSATE FOR WAX PATTERN SHRINKAGE AND CASTING SHRINKAGE OF METAL

• POROUS ENOUGH TO PERMIT AIR AND GASSES TO ESCAPE MOLD CAVITY

• SMOOTH SURFACE

• EASE OF DIVESTMENT

• SUITABLE SETTING TIME

• INEXPENSIVE

COMPOSITION OF INVESTMENT MATERIALS

1. A REFRACTORY MATERIAL – A MATERIAL THAT CAN WITHSTAND HIGH TEMPERATURES

AND WILL NOT DISINTEGRATE ON HEATING. E.G. QUARTZ, TRIDYMITE, CRYSTOBALITE

2. A BINDER – A MATERIAL THAT WILL SET AND BIND THE REFRACTORY MATERIAL

TOGETHER. E.G. GYPSUM (ALPHA HEMIHYDRATE), PHOSPHATE, ETHYL SILICATE

3. OTHER CONSTITUENTS:

1. SODIUM CHLORIDE

2. BORIC ACID

3. POTASSIUM SULPHATE

4. MAGNESIUM OXIDE

5. GRAPHITE

TYPES (BASED ON BINDER)

1. GYPSUM BONDED I.M

2. SILICA BONDED I.M

3. PHOSPHATE BONDED I.M

TYPES (BASED ON SILICA/REFRACTORY MATERIAL)

4. QUARTZ INVESTMENT

5. CRISTOBALITE INVESTMENT

GYPSUM BONDED I.MSUPPLIED AS POWDERS WHICH ARE MIXED WITH WATER

• COMPOSITION:

Binder Calcium sulphate alpha-hemihydrate

25% - 45%Binds the components together and provide rigidity and strength

Refractory material Silica (quartz and cristobalite)

Regulate thermal and inversion expansion

Other minor components

Powdered graphite/carbon or powdered copper and various modifiers

Control setting time, act as reducing agent

TYPES OF GYPSUM BONDED I.M(ADA SPECIFICATION)

• USES: USED AS INVESTMENT MATERIAL TO FORM

CAST GOLD ALLOY INLAYS, ONLAYS, CROWN AND BRIDGE

GYPSUM (BINDER)

• CALCIUM SULPHATE ALPHA HEMI HYDRATE AS BINDER USED FOR INVESTMENT USED IN CASTING GOLD ALLOYS WITH MELTING RANGES BELOW 1000°C OR 1800°F

• THERMAL EXPANSIONS AND CONTRACTIONS

BETWEEN 200°C AND 400°C – ALL FORMS SHRINK BECAUSE OF DEHYDRATION

BETWEEN 400°C AND 700°C – SLIGHT EXPANSION

AFTER 700°C – LARGE SHRINKAGE BECAUSE OF DECOMPOSITION AND RELEASE OF SULFUR DIOXIDE GAS. (SOME TEXTS SAY ABOVE 1200’C)

SILICA (REFRACTORY MATERIAL)

• PROVIDE REFRACTORY COMPONENT DURING HEATING OF INVESTMENT AND REGULATE THERMAL EXPANSION

• INVERSION EXPANSION:

WHEN HEATED QUARTZ CHANGES FROM LOW ALPHA FORM TO HIGH BETA FORM AT 575°C AND EXPANSION OCCURS

SIMILARLY, CRISTOBALITE CHANGES FROM LOW ALPHA FORM TO HIGH BETA FORM AT 200°C – 270°C AND EXPANSION OCCURS

OVERALL EXPANSIONS IN GYPSUM BONDED I.M

TO COMPENSATE FOR CASTING SHRINKAGE. MOLTEN ALLOYS SHRINK ON SOLIDIFICATION/COOLING. MOLD MUST BE MADE CORRESPONDINGLY LARGER THAN

ORIGINAL WAX PATTERN1. NORMAL SETTING EXPANSION- OCCURS AS INVESTMENT HARDENS IN AIR

• SILICA PARTICLES INTERFERE WITH THE INTERLOCKING OF CRYSTALS, THE OUTWARD THRUST OF THE CRYSTALS INCREASES THE EXPANSION OF INVESTMENT. MAXIMUM EXPANSION IN AIR 0.6%

• FACTORS EFFECTING: ACCELERATORS AND RETARDERS, MANIPULATIVE CONDITIONS LIKE LOW W/P RATIO

2. HYGROSCOPIC EXPANSION- OCCURS AS INVESTMENT HARDENS WHILE IMMERSED IN WATER

• A CONTINUATION OF NORMAL SETTING EXPANSION B/C IMMERSION IN WATER ENCOURAGES CONTINUED GROWTH OF CRYSTALS. EXPANSION RANGE 1.2-2.2%

• FACTORS EFFECTING: COMPOSITION, W/P RATIO, SPATULATION, TIME OF IMMERSION, LIFE OF INVESTMENT, CONFINEMENT, AMOUNT OF ADDED WATER

3. THERMAL AND INVERSION EXPANSION- OCCURS WHEN INVESTMENT IS HEATED

• THERMAL EXPANSION: 1% - 1.6%

• FACTORS EFFECTING: W/P RATIO, CHEMICAL MODIFIERS

HYGROSCOPIC EXPANSION

1. INVESTMENT MOULD IS PLACED INTO WATER AT THE INITIAL SET STAGE. KNOWN

AS THE WATER IMMERSION TECHNIQUE AND CAN RESULT IN AN

EXPANSION OF FIVE TIMES THE NORMAL SETTING EXPANSION.

2. THE WATER ADDED TECHNIQUE IN WHICH A MEASURED VOLUME OF WATER

IS PLACED ON THE UPPER SURFACE OF THE INVESTMENT MATERIAL WITHIN THE CASTING RING. THIS PRODUCES A MORE READILY CONTROLLED EXPANSION.

3. LINING THE CASTING RING WITH A LAYER OF DAMP ASBESTOS WHICH IS

ABLE TO FEED WATER TO A LARGE SURFACE AREA OF THE INVESTMENT MOULD.

DECOMPOSITION REACTIONS ON HEATING

• GYPSUM-BONDED INVESTMENTS DECOMPOSE ABOVE 1200ºC BY INTERACTION OF SILICA WITH CALCIUM SULPHATE TO LIBERATE SULFUR TRIOXIDE GAS.

CASO4 + SIO2 → CASIO3 + SO3

• ANOTHER REACTION WHICH MAY TAKE PLACE ON HEATING GYPSUM-BONDED INVESTMENTS ABOVE 700ºC IS THAT BETWEEN CALCIUM SULPHATE AND CARBON:

CASO4 + 4C → CAS + 4CO

• THE CARBON MAY BE DERIVED FROM THE RESIDUE LEFT AFTER BURNING OUT OF THE WAX PATTERN OR MAY BE PRESENT AS GRAPHITE IN THE INVESTMENT. FURTHER REACTION CAN OCCUR LIBERATING SULPHUR DIOXIDE:

3CASO4 + CAS → 4CAO + 4SO2

MY LORD, INCREASE ME IN KNOWLEDGE

AAMEEN

PHOSPHATE BONDED I.M• AVAILABLE AS POWDER THAT IS MIXED WITH WATER OR COLLOIDAL SILICA

• COMPOSITION OF POWDER:

• USES:• INVESTMENT FOR BASE METAL AND GOLD CASTING ALLOYS USED TO MAKE COPINGS AND

FRAMEWORKS FOR METAL-CERAMIC PROSTHESIS

• FOR CAST CERAMICS

• REFRACTORY DIE FOR CERAMIC BUILD-UP

Refractory material Silica (quartz or cristobalite or combination)

80%High thermal shock resistance and thermal expansion

Binder Magnesium oxide and Ammonium phosphate.

others carbon Too produce clean casting and ease divesting

SETTING REACTION• ON MIXING THE POWDER WITH WATER, FOLLOWING REACTION TAKES PLACE:

NH4·H2PO4 + MGO + 5H2O → + MG·NH4·PO4·6H2O

• THIS BINDS THE SILICA TOGETHER TO FORM THE SET INVESTMENT MOULD.

• AT A TEMPERATURE OF ABOUT 300ºC AMMONIA AND WATER ARE LIBERATED BY THE REACTION:

2(MG·NH4·PO4·6H2O) → MG2·P2O7 + 2NH3 + 13H2O

EXPANSIONS IN PHOSPHATE BONDED I.M• SETTING EXPANSION (MORE WHEN COLLOIDAL SILICA IS USED INSTEAD OF

WATER)

• INVERSION EXPANSION AND THERMAL EXPANSION

• HYGROSCOPIC EXPANSIONTHERMAL CONTRACTIONS

TYPES OF PHOSPHATE BONDED I.M

SILICA BONDED I.M• COMPOSITION

BINDER: SILICA GEL THAT REVERTS TO SILICA (CRISTOBALITE) ON HEATING

REFRACTORY: QUARTZ OR CRISTOBALITE POWDER

THE BINDER SOLUTION IS GENERALLY PREPARED BY MIXING ETHYL SILICATE WITH A MIXTURE OF DILUTE HCL AND INDUSTRIAL SPIRIT. A SLOW HYDROLYSIS OF ETHYL SILICATE OCCURS PRODUCING A SOL OF SILICIC ACID WITH THE LIBERATION OF ETHYL ALCOHOL AS A BYPRODUCT.

(C2H5O)4SI + 4H2O → SI(OH)4 + 4C2H5OH

THE SILICIC ACID SOL FORMS SILICA GEL ON MIXING WITH QUARTZ OR CRISTOBALITE POWDER UNDER ALKALINE CONDITIONS WHICH SHRINKS ON SETTING.

• USES:

CASTING HIGH FUSING METAL PARTIAL DENTURE ALLOYS

GREEN SHRINKAGE

• SILICIC ACID SOL IS MIXED WITH QUARTZ OR CRISTOBALITE AND A GEL OF POLYSILICIC ACID IS FORMED WHICH SHRINKS ON SETTING.

• THIS GEL IS DRIED AT TEMP BELOW 168’C

• DURING THIS DRYING, GEL LOSES ALCOHOL AND WATER AND FORMS CONC. HARD GEL AND VOLUMETRIC CONTRACTION OCCURS WHICH REDUCES SIZE OF MOLD.

• THIS IS GREEN SHRINKAGE THAT OCCURS IN ADDITION TO SETTING SHRINKAGE

THERMAL EXPANSION IN SILICA BONDED I.M

• CONTINUED HEATING CAUSES CONSIDERABLE EXPANSION DUE TO THE CLOSE PACKED NATURE OF THE SILICA PARTICLES. A MAXIMUM LINEAR EXPANSION OF APPROXIMATELY 1.6% IS REACHED AT A TEMPERATURE OF ABOUT 600ºC.

• NO HYGROSCOPIC OR SETTING EXPANSION TAKES PLACE.

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