Lecture 13: Stones

Question 1

why do we use dental models and die?

Answer 1

create replicas of the hard and soft tissue used for diagnosis and treatment of oral disease

Question 2

what is investing?

Answer 2

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

Question 3

what are the model die materials?

Answer 3

• gypsum
• epoxy resin
• 3d printed resins

Question 4

what are the types of gypsum?

Answer 4

1. impression plaster: to mount casts in articulator
2. model plaster (type II): weak cast for study models, diagnostics and art
3. dental stone (type III): full/partial denture, less water and stronger than type 2, hydrocal
4. high strength/low expansion die stone (type IV): densite
5. high strength/high expansion dental stone (type V): high expansion to compensate for shrinkage of casting alloys

Question 5

gypsum is a mineral from where and what is the main component?

Answer 5

the earth, calcium sulfate dihydrate

Question 6

what is dental stone and what is its composition?

Answer 6

powder product one buys, calcium sulfate hemihydrate

Question 7

what is the cast/die and whats it composition?

Answer 7

final product, calcium sulfate dihydrate

Question 8

how do you go from dihydrate to hemihydrate?

Answer 8

calcination

Question 9

from hemihydrate back to dihydrate what happens?

Answer 9

exothermic reaction

Question 10

what is the water to powder ratio?

Answer 10

the amount of water added to 100 g of powder

Question 11

as the w/p ratio increases the ……?

Answer 11

• setting time increases
• strength of gypsum product decreases
• setting expansion increases
• porosity increases
• strength decreases

Question 12

how does temperature effect setting time?

Answer 12

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

Question 13

how does spatulation effect setting time?

Answer 13

increase in speed and time, shortens setting time

Question 14

how does humidituy effect setting time?

Answer 14

• gypsum materials are hygroscopic so they absorb water from ambient
• exposed powder will dihydrate
• increase setting time during mixing

Question 15

what do accelerators do? and what is an example?

Answer 15

• increase the rate of setting and the number of crystal nuclei available for growth
• potassium sulfate or sodium chloride (increases expansion)

Question 16

what do retarders do? and whats an example?

Answer 16

• decreases the rate of setting, makes hemihydrate slightly less soluble
• borax or sodium citrate

Question 17

what is initial setting time?

Answer 17

interval between mixing with water and the mix cant be poured into mold

Question 18

what is final setting time?

Answer 18

finalize the hemi to di-hydrate conversion where it can be removed from the impression

Question 19

gypsum materials _____ on setting

Answer 19

expand

Question 20

rank the types of stone worst to best

Answer 20

plaster
dental stone
high strength stone

Question 21

what are hardening solutions?

Answer 21

harden the gypsum and increase the abrasion resistance