Lecture 13: Stones Flashcards
why do we use dental models and die?
create replicas of the hard and soft tissue used for diagnosis and treatment of oral disease
what is investing?
captures all details of the wax pattern and once the wax is burned away it provides a defined space into which molten alloy or ceramic is cast
what are the model die materials?
- gypsum
- epoxy resin
- 3d printed resins
what are the types of gypsum?
- impression plaster: to mount casts in articulator
- model plaster (type II): weak cast for study models, diagnostics and art
- dental stone (type III): full/partial denture, less water and stronger than type 2, hydrocal
- high strength/low expansion die stone (type IV): densite
- high strength/high expansion dental stone (type V): high expansion to compensate for shrinkage of casting alloys
gypsum is a mineral from where and what is the main component?
the earth, calcium sulfate dihydrate
what is dental stone and what is its composition?
powder product one buys, calcium sulfate hemihydrate
what is the cast/die and whats it composition?
final product, calcium sulfate dihydrate
how do you go from dihydrate to hemihydrate?
calcination
from hemihydrate back to dihydrate what happens?
exothermic reaction
what is the water to powder ratio?
the amount of water added to 100 g of powder
as the w/p ratio increases the ……?
- setting time increases
- strength of gypsum product decreases
- setting expansion increases
- porosity increases
- strength decreases
how does temperature effect setting time?
if it is raised above 37 C the setting time will be delayed, and if the water temp is below 37 C the setting time is faster
how does spatulation effect setting time?
increase in speed and time, shortens setting time
how does humidituy effect setting time?
- gypsum materials are hygroscopic so they absorb water from ambient
- exposed powder will dihydrate
- increase setting time during mixing
what do accelerators do? and what is an example?
- increase the rate of setting and the number of crystal nuclei available for growth
- potassium sulfate or sodium chloride (increases expansion)
what do retarders do? and whats an example?
- decreases the rate of setting, makes hemihydrate slightly less soluble
- borax or sodium citrate
what is initial setting time?
interval between mixing with water and the mix cant be poured into mold
what is final setting time?
finalize the hemi to di-hydrate conversion where it can be removed from the impression
gypsum materials _____ on setting
expand
rank the types of stone worst to best
plaster
dental stone
high strength stone
what are hardening solutions?
harden the gypsum and increase the abrasion resistance
