Investment materials
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What are investment materials used for
To produce metal/alloy inlays, crowns, onlays and bridges
How is an investment material used
Poured around a wax pattern to create a mould
What are the types of investment materials
Dental stone/plaster Gypsum bonded materials Phosphate bonded materials Silica bonded materials
What investment is used for acrylic
Dental stone
What material is used for gold casting alloys
Gypsum bonded
When is phohsphate bonded material used
For base metals/cast ceramics
What material is silica bonded investment used for
Base metal alloys
What are the required properites of investment materials
Expand: compensate for cooling shrinkage of alloy Porous: allow escape of trapped gases on casting - back pressure effect Strong: -Room Temp - ease of handling (“Green strength”) -Casting Temp - withstand casting forces Smooth surface: easy finishing Chemically stable: porosity, surface detail Easy removal from cast: technician time Handling: not complicated Relatively inexpensive: - destroyed
What material offers expansion
Silica - withstands high temp
What factors increas the hygroscopic expansion of gypsum
lower powder / water ratio increased silica content higher water temperature longer immersion time
What are the properties of gypsum bonded material
Easy manipulation and controlled setting time Expansion sufficient for gold alloys Smooth surface Adequate strength if powder to water ratio correct Porous
Why is gypsum bonded heat soaked
To allow reactions to complete and gases to escape
At what temp can gypsum reactions occur
above 700 degrees
Above what temp does gypsum bonded chemical stabillity become problmatic
Above 12000 degrees
What is the composition of phasphate bonded material
Powder -silica -magnesium oxide -ammonium phosphate Liquid -water or colloidal silica
How does colloidal silica solution alter properties
Increases strength Gives hygroscopic expansion
What is the heating of phosphate bonded investment
1000-1100 degrees
What are the properties of phosphate bonded
High “green” strength: - don’t need metal casting ring for support Easy to use High Strength Porous Chemically Stable
What are the stages to prepping silica investment
Stage 1: prepare stock solution Stage 2: add powder (quartz or cristobalite) – gelation Stage 3: drying - tightly packed silica particles
What is in the stock sokution of silica
ethylsilicate dilute hydrochloric acid industrial spirit (to mix silicate with water)
What are properties of silica
Strength – sufficient NOT porous - needs vents complicated manipulation contraction at early stages of heating -water and alcohol loss from gel substantial thermal and inversion expansion -lots of silica present
What does porous mean
Trapped gas is released during casting