DMS - Investment Materials Flashcards
what restorations/appliances are investment materials used for in the construction process? (4)
- Metal/alloy inlays
- Onlays
- Crowns
- Bridges
what investment material do we use for making gold alloys?
gypsum
what investment material do we use for base metal alloys (except gold)? (2)
silica bonded materials
phosphate bonded materials
how do we use investment materials? (1)
The Lost Wax Technique:
describe the lost wax technique. (4)
- Make a wax pattern of (e.g.) a crown = positive replica
- Pour the investment material around the wax patter and allow to set = mould of a negative replica
- Remove the wax with boiling water = leaves a cavity of the desired shaoe surrounded by the investment material
- Pour the molten alloy into the cavity
what types of materials are used as investment materials? (4)
- Dental stone/plaster – acrylic dentures
- Gypsum bonded materials – gold alloy
- Phosphate bonded materials – base metals/cast ceramics
- Silica bonded materials – base metal alloys
what investment material do we use for acrylic dentures?
Dental stone/plaster
what investment material do we use for cast ceramics/base metals?
Phosphate bonded materials
What are the requirements of an investment material? (8)
- When an alloy is being cast pressure must be applied to ensure no voids – since the investment material is surrounding this alloy it must be able to withstand the pressure.
- It must be porous so that it’s able to capture the gases produced and let them escape
- Chemically stable so that it doesn’t degrade
- Must be able to expand via thermal, hygroscopic or inversion means to compensate for the cooling shrinkage of the alloy
(above 4 in bold)
- It must also be strong at room temp when being handled = green temp
- Smooth surface: so that the alloy wont need much finishing.
- Easily removed from the cast
- Inexpensive
what is the shrinkage of a Co/Cr alloy after casting?
2.3%
what is the shrinkage of a Ni/Cr alloy after casting?
2%
what is the shrinkage of a gold alloy after casting?
1.4%
what are the components that make up investment materials? (2)
- Refractory – type of silica i.e. quartz or cristobalite
- Binder – i.e. gypsum, phosphate, silica
- determines what kind of investment material is made
- Forms a coherent mass to provide substance
what components of investment materials determines what kind of investment material is made?
the binder
at what temperature does silica exist in a form called alpha quartz?
< 573 degrees
at what temperature does silica exist in a form called beta quartz and is expanded to its maximum volume?
> 573 degrees
what dimensional changes does silica undergo? (2)
inversion expansion
thermal expansion
what dimensional changes does gypsum undergo?
hygroscopic expansion
contraction expansion
what materials significantly reduce the hygroscopic expansion and contraction expansion of gypsum?
sodium chloride
boric acid
What factors affect hygroscopic expansion in gypsum bonded investment materials? (4)
- Lower powder/water ratio
- Increased silica content
- Using water at a higher temp
- Immersing the powder in water for longer
what are the properties of gypsum bonded investment materials? (5)
- Expands by 1.4% - sufficient for casting gold alloys
- Smooth surface from fine particles
- Easily manipulated and the setting time is controlled
- Sufficiently porous to allow gases to escape when casting the alloy
- Adequate strength when the correct powder to liquid ratio is used.
describe the unwanted reaction occurs at > 700 degrees in gypsum bonded investment materials? what products are produced? (2)
There is a reaction between the calcium sulphide and the carbon to release carbon monoxide
There is also a reaction calcium sulphide and calcium sulphate to produce sulfur dioxide gas.
how do we overcome the gases produced by the unwanted reaction that occurs in gypsum bonded investment materials?
undergo heat soaking (held at high temperatures for some time) to allow the gases to escape and prevents voids forming in the cast alloy
why can gypsum bonded investment materials not be used in alloys which require heating above 1200 degrees?
the calcium sulphate reacts with silica
= sulphur trioxide produced which leaves voids in the cast alloy.
why is colloidal silica used in phosphate bonded investment materials? (2)
- To increase it’s strength
- Achieve 2% hygroscopic expansion to compensate for the alloy shrinkage when cooled to room temp.
what temperature are phosphate bonded investment materials heated to in order to increase their strength?
1000-1100 degrees
What are the properties of phosphate bonded investment materials?
- High strength
- Sufficiently porous
- Chemically stable
- Easy to use and strong whilst being prepared.
