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Indirect Exam 2 > Gypsum Products > Flashcards

Gypsum Products Flashcards

1
Q

Gypsum Classifications:

A

type I= impression plaster
type II= model plaster
Type III= dental stone
type IV= die stone or high strength stone
type V= die stone with high expansion
important that chemically all forms are identical just differ in shape and porosity

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2
Q

Gypsum is made up of:

A
  • dehydrate form of calcium sulfate (CaSo4 x 2H2O)
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3
Q

Gypsum equation:

A
  • through heating, dehydrate form loses water of crystallization and is converted to hemihydrate
  • CaSo4 x 2 H2O + heat –> CaSo4 x 1/2H2O + 1.5 H2O
  • done via pressure
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4
Q

Setting Reaction of Gypsum:

A
  • opposite of heating
  • hemihydrate (plaster, stone, destine) is mixed with water, reaction converts hemihydrate to dehydrate
  • CaSo4 x 1/2H2O + 1.5 H2O –> CaSo4 x 2 H2O + heat
  • exothermic reaction
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5
Q

How to mix hemihydrate:

A
  • use EXCESS water
  • this is so the excess water wets the powder so that particles can react
  • plaster= requires MORE water (50ml/100g)
  • die stone= requires LEAST water (24ml)
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6
Q

How is the excess water lost?

A
  • evaporation
  • but make sure you still measure it out or it will take longer to set and cause porosities in the material
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7
Q

What is working time?

A
  • Time during which material can be mixed/poured into impression
  • detected by loss of gloss (excess water has been absorbed)
  • 8-16 minutes from start of mix
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8
Q

What is final setting time?

A
  • time when you can separate the cast from impression without distortion/fracture
  • 45-60 minutes
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9
Q

What is compressive strength after one hour?

A
  • measure of wet strength
  • excess water not completely lost
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10
Q

What is dry strength?

A
  • when cast is completely dry
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11
Q

What makes die stone the strongest?

A
  • mixed with least amount of water, densest, shrinks/expands less than others
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12
Q

The more water you have to put into material….

A

the weaker

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13
Q

The greater the surface hardness….

A

the greater the resistance to abrasion

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14
Q

Expansion ranks:

A

plaster (expands most)< dental stone < die stone (expands least)

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15
Q

What is hygroscopic expansion?

A

if gypsum material immersed in water during setting, the setting expansion will increase

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16
Q

What does increasing the rate of the reaction do? And what is added to do this?

A
  • reduces the setting time
    done via:
  • accelerators (chemicals that increase the rate)
  • include: potassium sulfate, calcium sulfate dihydrate crystals (slurry water)
  • increasing temperate to body temperature (37 degrees)
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17
Q

What does decreasing the rate of reaction do? And what is added to do this?

A
  • prolongs the setting time
    done via:
  • retarders
  • include: blood, saliva, alginate, borax
  • increasing above body temperature (> 37 degrees)
    boiling eater will stop reaction
18
Q

What does high relative humidity do to products?

A
  • can initiate setting reaction
  • can prevent normal setting
  • reduce strength and hardness
    now, store powders in airtight containers/pre-measured packets
19
Q

What will an increased water/powder ratio do?

A
  • thinner mix, longer setting time, weaker, less expansion
20
Q

What will a decreased water/powder ratio do?

A
  • thicker, shorter setting time, stronger, increased expansion
21
Q

The longer the mixing time, the —- the setting time and —- the expansion

A

the shorter the setting time and greater the expansion.

22
Q

How long do you mix gypsum products?

A

1 minute with spatulate
- vibrate bowl for a few seconds to remove air bubbles

23
Q

Desirable properties of gypsum involved investment:

A
  • adequate expansion
  • easy to handle
  • produce smooth surface/fine detail
  • stable at high temps
  • porous
  • high heat strength
24
Q

Components of investment:

A
  • refractory filler (expand under heat):
    form of silicon dioxide specifically Cristobalite (60-65%)
  • binder: calcium sulfate hemihydrate (30-35%)
  • modifiers: 5%
24
Q

What are reducing agents?

A
  • prevent oxidation/films on casts so don’t have to polish as much
25
Q

Most common type of investment:

A

II: inlay; hygroscopic

26
Q

What types of particle size of refractory do we want

A
  • fine for smooth surface on casting
  • uniform less dense packing, better venting
27
Q

What type of expansion do we need to make up for the shrinkage of the gold?

A
  • hygroscopic expansion
    allows extra water to allow gypsum crystals to grow (water added after it has set) using the ring liner to provide lateral expansion of investment
  • leave it for 30 minutes inn bath
28
Q

Hygroscopic expansion decreases with —- time between mixing and immersion into water bath

A

expansion decreases with increasing time

29
Q

How much expansion so we need?

A

1.5%
(actual expansion with gypsum and hygroscopic expansion is 0.6% so thats why we need to put it in a furnace)

30
Q

What is the importance of thermal expansion?

A
  • nearly all materials expand when heated in furnace
  • Cristobalite expands more at a lower temperature
31
Q

What is inversion in regards to thermal expansion?

A

change in form upon heating that is associated with expansion

32
Q

T/F: reheating investment will expand to previous levels

A

False!
It will expand again but not to the level before

33
Q

How long do you have to wait before setting investment into oven?

A
  • 45-60 minutes after the start of the mix
  • 30 minutes in water bath, 15 -30 minutes out
34
Q

Increasing spatulation in investment will —- expansion

A

increase

35
Q

What is the effect of water bath temp on hygroscopic expansion of investment?

A

little…
- will increase expansion of wax

36
Q

What are important casting problems?

A

Improper solidification of metal:
- not heating gold up high enough)
- solidify/freeze 1st: alloy farthest away from the sprue
- if sprue freezes first= suck back porosity occurs (near sprue attachment)

37
Q

What could be the cause of round margins?

A
  • inadequate casting pressure
  • underheating of alloy
  • inadequate burnout of wax (not in furnace long enough)
38
Q

What could cause spherical voids?

A
  • dissolved gases in alloy
  • too high temp or prolonged heating (opposite of round margins)
39
Q

What could cause surface roughness?

A
  • occluded gases
  • breakdown of investment
  • casting force too high
  • excess water in investment evaporated during burnout
40
Q

Cause of suck-back porosity:

A
  • solidified in sprue before getting in crown

Indirect Exam 2 (7 decks)

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