Presentation on investment materials!

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3. 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

4. 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

6. 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

7. 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

8. 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)
1. QUARTZ INVESTMENT
2. CRISTOBALITE INVESTMENT

9. GYPSUM BONDED I.M
SUPPLIED 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

10. TYPES OF GYPSUM BONDED I.M
(ADA SPECIFICATION)

11. • USES: USED AS INVESTMENT MATERIAL TO FORM
CAST GOLD ALLOY INLAYS, ONLAYS, CROWN AND
BRIDGE

12. 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)

13. 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

14. 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 PATTERN
1. 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%

15. 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.

16. 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

17. MY LORD, INCREASE ME IN KNOWLEDGE
AAMEEN

18. 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

19. 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

20. EXPANSIONS IN PHOSPHATE BONDED I.M
• SETTING EXPANSION (MORE WHEN COLLOIDAL SILICA IS USED INSTEAD OF
WATER)
• INVERSION EXPANSION AND THERMAL EXPANSION
• HYGROSCOPIC EXPANSION
THERMAL CONTRACTIONS

21. TYPES OF PHOSPHATE BONDED I.M

22. 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

23. 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

24. 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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