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Dental Materials > Gypsum model and Dies > Flashcards

Gypsum model and Dies Flashcards

1
Q

Models and dies are needed to make … in a …

A
  • indirect devices
  • lab
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2
Q

Define ‘model’

A
  • replica of soft tissues, associated bony tissue and remaining teeth
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3
Q

Define ‘die’

A
  • replica of the prepared tooth/teeth
  • for device manufacture
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4
Q

Requirements of model and die material

A
  • accurate
  • dimensionally stable
  • compatible with impression materials
  • colour contact
  • ease of use
  • cost
  • strength, toughness and stiffness
  • hardness
  • fine detail reproduction
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5
Q

Why do model and die materials need to be accurate?

A
  • minimal dimensional change on setting
  • free of voids on setting
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6
Q

Why do model and die materials need to be dimensionally stable?

A
  • accuracy after storage
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7
Q

Why do model and die materials need to be compatible with impression materials?

A
  • no interaction - adhesion can cause damage
  • no rejection - model must flow into impression
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8
Q

Why do model and die materials need to have colour contrast?

A
  • easy to read
  • aids in device manufacturer
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9
Q

Why do model and die materials need to be strong, tough and stiff?

A
  • resist fracture
  • and deformation
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10
Q

Why do model and die materials need to have hardness?

A
  • resist scratching and abrasion
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11
Q

Why do model and die materials need to have fine detail reproduction?

A
  • retain features recorded in impression
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12
Q

What is gypsum?

A
  • common names for calcium sulphate dihydrate
  • CaSO42H2O
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13
Q

How is gypsum used in dentistry?

A
  • supplied in dry state
  • based on calcium sulphate hemihydrate
  • mixed with water to set
  • 2CaSO42H2O -> (CaSO4)2 + 3H2O
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14
Q

Uses of gypsum in dentistry

A
  • model and die material
  • impression material
  • investment material
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15
Q

Types of hemihydrate

A
  • plaster/Plaster of Paris
  • stone (Kaffir D, Calestone)
  • special stone
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16
Q

Features of plaster gypsum

A
  • heat to 120 degrees in open vessel (calcination)
  • calcined hemihydrate
  • large, irregular, porous
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17
Q

Explain stone hemihydrate

A
  • heat to 120-130 degrees under pressure/steam autoclave
  • autoclaved hemihydrate
  • smaller, regular, dense, less porous
18
Q

Explain special stone hemihydrate

A
  • heat in boiling solution of calcium chloride or magnesium chloride
  • densite
19
Q

Plaster is normally what colour?
Stone is normally what colour?

A
  • plaster is white
  • stone contains coloured dye
20
Q

Setting of hemihydrate

A
  • reacts with water to form 2CaSO42H2O
  • setting occurs through reacting powder with water
  • need the most water for plaster, then stone
21
Q

Explain the reaction of hemihydrate with water

A
  • on mixing with water, some hemihydrate dissolves (low solubility)
  • hydration occurs immediately
  • dihydrate crystallises (very low solubility), comes out of solution and starts to solidify
  • crystals are spherulitic
  • excess water gives porosity
22
Q

What does spherulitic mean?

A

crystals grow from impurities or gypsum

23
Q

Physical changes during gypsum setting

A
  • hardening
  • further hardening by standing/drying out
  • exothermic reaction
  • dimensional change
24
Q

How does gypsum harden?

A
  • initial mix is fluid - can be poured
  • initial set time - loses fluidity, excess can be carved away
  • final set time can begin working on model
25
Q

Why do we let additional gypsum hardening by standing out happen?

A
  • allow to age for safety
26
Q

What does the exothermic setting reaction of gypsum relate to?

A
  • can be monitored
  • correlates with increasing hardness
27
Q

How does gypsum undergo dimensional change on setting?

A
  • setting expansion occurs
  • outward thrust of crystals - correlates with temp change
  • can be used to monitor setting
28
Q

Compare dimensional change in plaster and stone gypsum

A
  • 0.3-0.5% for plaster
  • 0.05-0.2% for stone
29
Q

How to control setting time and expansion?

A
  • added nuclei
  • accelerators/retarders
  • anti-expansion solutions
30
Q

What does added nuclei do to gypsum?

A
  • accelerate setting
  • can be added by manufacture or from debris in mixing bowl
31
Q

Explain accelerators/retarders

A
  • potassium sulphate accelerates setting but can be too fast
  • borax compensates for it
  • reduces setting expansion
  • can be added to the powder
32
Q

Explain role of anti-expansion solutions

A
  • some manufacturers add potassium sulphate and borax to water
  • may be used in place of water
33
Q

Effect of temp on setting of gypsum

A
  • limited effect
  • increased temp leads to increase of solution process
  • leads to decrease in crystallisation
34
Q

Increasing the W/P ratio causes what?
Why?

A
  • slower setting
  • dilution effect
  • decreases concentration of crystallisation nuclei
35
Q

What accelerates setting? Why?

A
  • increasing mixing time
  • mixing breaks up dihydrate crystals and disperses them
  • more nuclei formed
36
Q

Gypsum products are rigid and brittle/ soft and supple

A

rigid and brittle

37
Q

…. of gypsum depends on porosity. Porosity depends on …
These properties improve when?

A
  • strength, toughness and hardness
  • W/P ratio
  • more water means more space between crystals so sotne is always tougher/harder than plaster
  • after drying
38
Q

Applications of gypsum

A
  • working models/dies (use stone)
  • study models/mounting articulators (use plaster)
39
Q

Advantages of gypsum

A
  • inexpensive
  • good colour contrasts
  • can reproduce fine detail
  • good dimensional stability
40
Q

Disadvantages of gypsum

A
  • need good technique (careful with mixing, vibration on pouring, low dimensional change)
  • mechanical properties not ideal (common fractures)
  • some compatibility problems (agar, plaster impressions both water based and adhere to gypsum models - separating sprays available too)

Dental Materials (17 decks)

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