Investing Flashcards
What are dental casting investments composed of?
- refractory material: Silica (responsible for the thermal expansion of the mold during wax elimination)
- binder: gypsum or magnesium oxide and an ammonium phosphate compound
- Modifiers: accelerators, retarders and reducing agents e.g. (carbon)
What are the ideal properties of investment materials?
- able to control expansion
- produce smooth castings
- accurate surface reproduction
- chemical stability at high casting temperatures
- adequate strength to resist casting forces
- sufficient porosity to allow gas escape
- easy recovery of the casting
What are the types of investment materials?
- gypsum-bonded
- phosphate-bonded
- silicate-bonded
What is the binder of gypsum bonded investments?
gypsum (calcium sulfate hemi hydrate)
What is the refractory material of gypsum bonded materials?
cristobalite or quartz
What are the characteristics of gypsum bonded investments?
- not chemically stable at high temperatures above 1000°(1080°)
- more porous
- long working time
- mixed with distilled water
- produce smooth surface casting
- used for casting of conventional type II,III, and IV gold alloys
What is the binder of phosphate bonded investment?
magnesium oxide and ammonium phosphate compound
What i the refractory material of phosphate bonded investment?
silica
What are the characteristics of phosphate bonded investments?
- mixed with colloidal silica
- higher strength
- higher expansion due to the special liquid
- stable at high temperatures; suitable for metal ceramic alloys
- carbon-containing materials (gray in color) are used for high gold or palladium alloys
- carbon-free investments are used for base metal alloys because carbon if found affect final alloy composition
- lower porosity
- rougher surface
- shorter working time
What are the types of expansions that occur for investmnt materials?
- setting expansion
- hygroscopic expansion
- thermal epansion
What is setting expansion?
occurs during material setting as a result of crystal growth
What is hygroscopic expansion?
- extension of setting expansion by adding water to the setting investment by wet liner and submerging the ring in a water bath at 37°C for 1 hour immediately after investing
What is thermal expansion?
- solid-state phase transformations of silica
- more controllable
How to increase expansion?
- use of two ring liners
- prolonged spatulation
- storage in 100% humidity
- hygroscopic technique
- lower water-powder ratio in gypsum-bonded and increase special liquid in phosphate-bonded
What are the types of investing techniques?
- single investing
- double investing
