DMS-investment materials Flashcards
What are investment materials?
These are the materials used to make inlays/onlays/crowns/bridges
(the investment material surrounds the alloy while it is cast to ensure the alloy dimensions are sustained.
How do we select the most suitable investment material?
By using the melting point of the alloy.
What are the ideal requirements for an investment material?
It should :
expand to compensate for the cooling shrinkage of the alloy.
be porous (to allow the escape of trapped gas during casting.
Be strong (high green strength & when withstanding forces at casting temperature)
Have a smooth surface for easy finishing.
Chemically stable
Easily handled and removed from the cast
Relatively inexpensive (as it is destroyed after use)
What investment type is used for acrylic dentures?
Dental stone or plaster.
What investment type is used for gold casting alloys?
Gypsum bonded materials.
What investment type is used for base metals or cast ceramics?
Phosphate bonded materials.
What investment material is used for base metal alloys?
Silica bonded materials.
What are the components of investment materials and what are their functions.
Binder- to form a coherent solid mass (e.g. gypsum, phosphate, silica)
Refractory- to withstand high temperatures and cause expansion.
(above 573*C silica changes form causing maximum volume expansion.
What does a gypsum bonded investment consist of?
Silica
Calcium sulphate hemihydrate
Reducing agent for oxides
Boric acid
NaCl
What does a phosphate bonded investment material consist of?
A powder of silica, magnesium oxide and ammonium phosphate.
A liquid of water Or Colloidal silica.
Discuss the properties of gypsum investment materials?
the silica undergoes thermal and inversion expansion.
The gypsum undergoes
- hydroscopic expansion
- contraction above 320*C (due to water loss and the presence of sodium chloride and boric acid)
Discuss the properties of phosphate bonded investment materials.
It has a high green strength
It is porous
Chemically stable.
What is green strength?
Strength when handled at room temperature.
gReen- R for room.
What happens when gypsum bonded investment material is heated at above 700*C?
and how do we prevent this?
Carbon monoxide is released from the residue.
This is prevented by heat soaking (leaving the material at a high temperature to ensure that the reaction finishes and gives the gases time to escape)
What happens when gypsum bonded investment is heated above 1200*C.
The gypsum is not chemically stable. Sulphur trixoide is produced that creates voids in the cast alloy.
What happens when you heat phosphate bonded investment at 300*C?
Water and ammonia are released.
What effect do hotter temperatures have on phosphate bonded investment materials .
Silico-phosphates are produced which increases the strength of the investment material.
What is Hydroscopic expansion?
This causes a change in the volume of the investment material, the papillary forces pull the water molecules into the gaps between calcium sulphate, forcing the calcium sulphate particles apart.
This results in a change in volume of the investment material.
What increases the hydroscopic expansion of an investment material?
This is increased by:
- A lower powder/water ratio
- Increased silica content
- Higher water temperature
- Longer immersion time.
Why do we not use silica investment materials?
Because they are not porous meaning that it will capture the gases normally released during casting. (the gases can’t leave) This causes a weak and porous alloy.
