Gypsum Products Flashcards
Gypsum Classifications:
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
Gypsum is made up of:
- dehydrate form of calcium sulfate (CaSo4 x 2H2O)
Gypsum equation:
- 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
Setting Reaction of Gypsum:
- 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
How to mix hemihydrate:
- 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)
How is the excess water lost?
- evaporation
- but make sure you still measure it out or it will take longer to set and cause porosities in the material
What is working time?
- 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
What is final setting time?
- time when you can separate the cast from impression without distortion/fracture
- 45-60 minutes
What is compressive strength after one hour?
- measure of wet strength
- excess water not completely lost
What is dry strength?
- when cast is completely dry
What makes die stone the strongest?
- mixed with least amount of water, densest, shrinks/expands less than others
The more water you have to put into material….
the weaker
The greater the surface hardness….
the greater the resistance to abrasion
Expansion ranks:
plaster (expands most)< dental stone < die stone (expands least)
What is hygroscopic expansion?
if gypsum material immersed in water during setting, the setting expansion will increase
What does increasing the rate of the reaction do? And what is added to do this?
- 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)
What does decreasing the rate of reaction do? And what is added to do this?
- prolongs the setting time
done via: - retarders
- include: blood, saliva, alginate, borax
- increasing above body temperature (> 37 degrees)
boiling eater will stop reaction
What does high relative humidity do to products?
- can initiate setting reaction
- can prevent normal setting
- reduce strength and hardness
now, store powders in airtight containers/pre-measured packets
What will an increased water/powder ratio do?
- thinner mix, longer setting time, weaker, less expansion
What will a decreased water/powder ratio do?
- thicker, shorter setting time, stronger, increased expansion
The longer the mixing time, the —- the setting time and —- the expansion
the shorter the setting time and greater the expansion.
How long do you mix gypsum products?
1 minute with spatulate
- vibrate bowl for a few seconds to remove air bubbles
Desirable properties of gypsum involved investment:
- adequate expansion
- easy to handle
- produce smooth surface/fine detail
- stable at high temps
- porous
- high heat strength
Components of investment:
- refractory filler (expand under heat):
form of silicon dioxide specifically Cristobalite (60-65%) - binder: calcium sulfate hemihydrate (30-35%)
- modifiers: 5%
What are reducing agents?
- prevent oxidation/films on casts so don’t have to polish as much
Most common type of investment:
II: inlay; hygroscopic
What types of particle size of refractory do we want
- fine for smooth surface on casting
- uniform less dense packing, better venting
What type of expansion do we need to make up for the shrinkage of the gold?
- 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
Hygroscopic expansion decreases with —- time between mixing and immersion into water bath
expansion decreases with increasing time
How much expansion so we need?
1.5%
(actual expansion with gypsum and hygroscopic expansion is 0.6% so thats why we need to put it in a furnace)
What is the importance of thermal expansion?
- nearly all materials expand when heated in furnace
- Cristobalite expands more at a lower temperature
What is inversion in regards to thermal expansion?
change in form upon heating that is associated with expansion
T/F: reheating investment will expand to previous levels
False!
It will expand again but not to the level before
How long do you have to wait before setting investment into oven?
- 45-60 minutes after the start of the mix
- 30 minutes in water bath, 15 -30 minutes out
Increasing spatulation in investment will —- expansion
increase
What is the effect of water bath temp on hygroscopic expansion of investment?
little…
- will increase expansion of wax
What are important casting problems?
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)
What could be the cause of round margins?
- inadequate casting pressure
- underheating of alloy
- inadequate burnout of wax (not in furnace long enough)
What could cause spherical voids?
- dissolved gases in alloy
- too high temp or prolonged heating (opposite of round margins)
What could cause surface roughness?
- occluded gases
- breakdown of investment
- casting force too high
- excess water in investment evaporated during burnout
Cause of suck-back porosity:
- solidified in sprue before getting in crown
