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
What factors increase hygroscopic expansion?
- Lower powder/ water ratio
- Increased silica content
- Higher water temp
- Longer immersion time
At what temp will Gypsum contract?
- Above 320oC
- Due to water loss and
- Presence of sodium chloride and boric acid
What properties do Gypsum bonded investment material have?
Expansion - total expansion sufficient for casting gold alloys - 1.4% by volume
Smooth surface - Fine particles give good surface
Manipulation - Easy and setting time is controlled
Porous which is good as it takes in gases released when casting alloys
Strength is adequate if correct powder/liquid ratio and has correct manipulation
What unwanted reaction occurs above 700oC in Gypsum bonded investment material and how is this combatted?
- If any wax residue or graphite in the investment material then reaction between CaCO4 and carbon occurs
- Release carbon monoxide and CaS
- CaS react with CaCO4 to produce SO2 (sulphur dioxide gas)
- Crucial that CO2 and SO2 escape
- Heat soaking used so investment material is held at high temp for some time so gases can gradually escape
What temp is the chemical stability of Gypsum bonded investment material suitable?
- Below 1200oC satisfies requirements
- Above 1200oC causes problems
- CaSO4 reacts with silica to generate sulphur trioxide
- This produces voids in cast alloy and not suitable
- If alloy melts above 1200oC then can’t use Gypsum bonded investment
What is the composition of Phosphate bonded investment material?
Powder
- Silica
- Magnesium oxide
- Ammonium phosphate
Liquid
- Water or colloidal silica
Why is colloidal silica used in Phosphate bonded investment?
- Increases strength
- Gives hygroscopic expansion of 2%
What is the setting reaction of Phosphate bonded investment?
Ammonium phosphate reacts with magnesium oxide and water
- Produces magnesium ammonium phosphate
What temp is phosphate bonded investment heated to and what is released at 330oC?
- Heated to 1000-1100oC
- Water and ammonia liberated at 330oC
- At higher temps other reactions occur with silico-phosphates which generate increased strength
What are the properties of Phosphate bonded investment materials?
- High green strength (don’t need metal casting ring for support)
- Easy to use
- High strength
- Porous
- Chemically stable
