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BDS3 DMS > Investment Materials > Flashcards

Investment Materials Flashcards

1
Q

What are the functions of investment materials in dentistry?

A

to produce metal/alloy inlays, inlays, crowns and bridges

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2
Q

Outline the process by which prosthesis are made with investment materials

A
  • wax pattern made
    • positive replica of prosthesis
  • investment material set around wax
    • negative replica of prosthesis
    • mould
  • wax eliminated
    • boiling water or burning oven
  • molten alloy forced into mould
    • alloy forced in through sprues
    • pressure used to prevent void formation
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3
Q

What happens to investment material on heating?

A
  • experiences large forces
  • expands
  • must capture gases released
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4
Q

What are dental stone and plaster used to cast?

A

acrylic dentures

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5
Q

What are gypsum bonded materials used to cast?

A

gold casting alloys

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6
Q

What are phosphate bonded materials used to cast?

A
  • base metals
  • cast ceramics
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7
Q

What are silica bonded materials used to cast?

A

base metal alloys

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8
Q

What are the requirements of an investment material?

A
  • expansion
    • to compensate for cooling shrinkage
  • porosity
    • allow escape of cases
    • back pressure effect
  • strength
    • green strength (at room temperature)
    • casting forces
  • smooth surface
    • ease of finishing
  • chemically stable
    • porosity
    • surface detail
  • easy removal from cast
    - technician time
  • handling
    - ease of use
  • expense
    • should be relatively inexpensive
    • destroyed after use
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9
Q

What is green strength?

A

the strength of a material at room temperature

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10
Q

What happens is an investment material doesn’t allow gas to escape?

A

voids form in the material, weakening the prosthesis

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11
Q

What are the contraction percentages of gold, Ni/Cr and Co/Cr alloys?

A

Gold - 1.4%
Ni/Cr - 2%
Co/Cr - 2.3%

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12
Q

What are the two components in all investment materials and what are their functions?

A
  • binder
    • gypsum, phosphate, silica
    • forms a coherent solid mass
  • refractory
    • silica (quartz or cristobalite)
    • withstand high temperatures
    • provides expansion
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13
Q

Does quartz or cristobalite have greater thermal expansion?

A

Cristobalite

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14
Q

At what temperature does inversion expansion occur for silica?

A

573 degrees
- quartz changes from alpha to beta
- beta quartz is at maximum volume

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15
Q

What is the composition of gypsum bonded investment material?

A

silica - 60-56%
calcium sulphate hemihydrate - 30-35%
reducing agent for oxide
boric acid, NaCl (inhibit heating shrinkage)

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16
Q

What is the setting reaction for gypsum bonded investment material?

A

calcium sulphate + water = calcium sulphate dihydrate

17
Q

What dimensional changes do silica and gypsum experience?

A
  • silica
    • thermal expansion
    • inversion expansion
  • gypsum
    • setting expansion
      • hygroscopic expansion
      • contraction above 320 degrees
18
Q

what is hygroscopic expansion?

A
  • capillary forces pulling water molecules into the gaps between crystals of calcium sulphate hemihydrate, forcing them apart
    • mechanism not fully understood
    • up to 5x setting expansion
    • variable increase in volume
19
Q

What factors increase hygroscopic expansion?

A
  • lower powder/water ratio
  • increased silica content
  • higher water temperature
  • longer immersion time
20
Q

Why does gypsum contract above 320 degrees?

A
  • water loss
  • significant reduction by sodium chloride and boric acid
21
Q

What are the properties of gypsum bonded investment material?

A
  • expansion
    • sufficient for gold alloys
  • smooth surface
    • fine particles
  • manipulation
    • easy
    • controlled setting time
  • porosity
    • good
  • strength
    • adequate
    • correct powder/liquid ratio
    • correct manipulation
22
Q

What is heat soaking?

A
  • an unwanted reaction in gypsum bonded investment above 700 degrees between calcium sulphate and carbon in the presence of wax or graphite residue in the investment material
    • carbon monoxide/sulphur dioxide produced
    • gases must be able to escape
    • must be held at high temperature
23
Q

Discuss the chemical stability of gypsum bonded investment

A
  • chemically stable below 1200 degrees
  • above 1200 degrees not chemically stable
    • calcium sulphate reacts with silica
    • sulphur trioxide produced
    • porosity in prosthesis
24
Q

What is the composition of phosphate bonded investment?

A
  • powder
    • silica
    • magnesium oxide
    • ammonium phosphate
  • liquid
    • water
      OR
    • colloidal silica
25
Q

Why would colloidal silica solution be used in place of water for phosphate bonded investment?

A
  • increases strength
  • gives hydroscopic expansion
    • 2%
    • compensates for alloy shrinkage
26
Q

What is the phosphate bonded investment setting reaction?

A

ammonium phosphate reacts with magnesium oxide and water to produce magnesium ammonium phosphate

27
Q

What happens to phosphate bonded investment on heating?

A

330 degrees - water and ammonia liberated

> 330 degrees - complex reactions
- with silica-phosphates
- increased strength

28
Q

What are the properties of phosphate bonded investment?

A
  • high green strength
    • do not need metal casting ring for support
  • easy to use
  • high strength
  • porous
  • chemically stable
29
Q

What are the three stages of silica investment?

A
  1. prepare stock solution
  2. add powder (gelation)
  3. drying (tightly packed silica particles)
30
Q

What are the dimensional changes experienced by silica?

A
  • contraction during early heating
    • water and alcohol loss from cell
  • substantial thermal and inversion expansion
    • lots of silica present
31
Q

What are the properties of silica>

A
  • strength
    • strong enough
  • porosity
    • not porous
    • requires specialist equipment to form vents
    • risk of porous alloy
  • usability
    • complicated manipulation

BDS3 DMS (10 decks)

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