Investment materials Flashcards
What are investment materials used for in dentistry?
- To produce metal/alloy inlays, onlays, crowns and bridges
- Technique involves casting molten alloy under pressure by centrifugal force
- Need a mould cavity of required shaped
- Alloy surrounded by investment material
What are the stages of the Lost wax technqiue?
- Wax pattern made e.g. crown/ inlay etc
- Investment material poured around wax pattern and allowed to set (mould)
- Wax is eliminated (by boiling water or burning in oven)
- Molten alloy forced into mould cavity by wax via channels (sprues) prepared in the investment material
Why must gases be allowed to escape during the lost wax technique?
- As alloy is cast gases are produced
- They must escape and be captured by investment material
- Otherwise the alloy will have voids and it will be porous
Why won’t the alloy be the same shape as the mould cavity?
- Upon cooling the alloy contracts and shrinks
- Not same shape upon devestment
What are some investment types?
- Dental stone or plaster
- Gypsum bonded materials
- Phosphate bonded materials
- Silica bonded materials
- Acrylic dentures
- Gold casting alloys
- Base metals/cast ceramics
- Base metal alloys
What are some ideal properties of investment material?
- Must expand to compensate for cooling shrinkage of alloy
- Must be porous to allow escape of trapped gases on casting
- Must be strong at room temp for ease of handling and referred to as Green strength
- Must be strong on casting temp to withstand casting forces
- Must have smooth surface for easy finishing
- Chemically stable
- Easy removal form cast to reduce technician time
- Handling not complicated
- Relatively inexpensive as it is destroyed
What are the typical contractions (by volume) from alloy melting point to room temp of some alloys?
Gold alloys - 1.4%
Ni/Cr alloys - 2%
Co/Cr alloys - 2.3%
What are the two components of investment materials ?
- Binder
- Refractory
What is the refractory component usually made up of and what is it for?
- Silica (quartz or critsobalite)
- Withstands high temps
- Undergoes expansion
What is the binder component usually made up of an what is it for?
- Gypsum or Phosphate or Silica
- Form a coherent mass to provide substance
- Each has different setting reaction which yields different material characteristics and pin points what type of investment material it is
What two forms can quartz exist in?
- Below temp of 570C it exists as alpha-quartz and has squashed crystalline lattice structure
- Above temp of 570C it exists as beta-quart where it explodes to max vol of crystalline lattice structure
What is the composition of Gypsum-bonded investment?
- It is a powder mixed with water
- Silica 60-65%
- Calcium sulphate hemihydrate 30-35%
- Reducing agent for oxides
- Chemicals to inhibit heating shrinkage and control setting time (boric acid, NaCL)
What is the setting reaction of gypsum bonded investment?
Calcium hemihydrate and water produces calcium sulphate di-hydrate
What dimensional changes do gypsum bonded investment material undergo?
- Silica undergoes thermal expansion and inversion expansion
- Gypsum undergoes setting expansion that has two aspects
- Hygroscopic expansion and contraction above 320oC
What is Gypsum Hygroscopic expansion?
- The mechanism is not fully understood
- Water molecules attracted between calcium sulphate hemi-hydrate crystals by capillary forces
- Forces crystals apart
- Can be up to 5X setting expansion
