model and die materials Flashcards
In dentistry, what is the purpose of replicas of hard and soft tissues
diagnosis and treatment of oral disease
what are the 3 types of repicas
- study models
- casts
- dies
what are study models used for, give an example
they are used for the observation of the patients oral stucture
eg; orthodontists use study models to evaluate the crowding of teeth or progress of correction of that crowding
what are two usages of casts
- they are used to make orthodontic retainers
2. cast of the remaining ridges in edentulous patients are used to make dentures for that patient
what are dies and what are they generally used for
are highly accurate replicase of a single tooth and are used for metal crowns or inlays
What are two factors on which the accuracy of replicas depend on
- the accuracy of the impression
2. materials used to pour the impression
what are 6 desirable qualities required of materials to be used for making models, casts, and dies
- accuracy
- dimensional stability
- reproduction of fine detail
- strength and resistance to abrasion
- ease of adaption to the impression
- color and safety
what do desirable qualities of model materials depend on , give an example.
they depend on dental application
eg: dentist do not require the same dimensional accuracy for a study model as for a die ( die must be more dimensionally accurate)
what are two common materials used to pour impressions and what is more prominently used and why
- gypsum materials
- epoxy materials
Gypsum materials because they can be poured into almost all impression materials used in practice today
what stands as an issue for epoxy materials
model material-impression compatibility, ghey are compatible with most eLASTOMERIC IMPRESSION materials but cannot be used with the alginates or agar impression materials
what are the 5 types of gypsum products
1.impression plaster
2model plaster
3dental stone
4die stone ( high strength-low expansion dental stone)
5high strength-high expansion dental stone )
what are model plasters used for
study models that doe not require abrasion resistance
what is orthodontic plaster
mixture of model plaster and dental stone
what is an IMPRESSION plaster used for
mounting casts on an articulator
what does an impression plaster differ from a model plaster
in impression plasters, setting time has been shortened and dimensional change on setting has been reduced to 0.06%.
what type of gypsum material is used for casts that need abrasion resistance
dental stones
what differs in plasters, dental stones, and high-strength dental stones, chemical or physical nature
all of the mentioned materials differ in their physical forms, their chemical makeup however is the same, they are all made up of the same chemical ; calcium sulfate hemihydrate
name 4 disadvantages of using excess water in the gypsum mix
- increases setting time
- reduces hardness of set gypsum
- reduces strength
- reduces expansion
all from of gypsum materials are manufactured from what
gypsum mineral; calcium sulfate dihydrate
what is the only difference between the model materials and the gypsum
the amount of water in the crystal, during the manufacture of model materials, etc. ,water is driven out of the dihydrate gypsum to form the hemihydrate
what are the three types of hemihydrate
- plaster hemihydrate
2.dental stone ‘’ ‘’
3,high-strength dental stone ‘’ ‘’
how is beta-calcium sulfate hemihydrate produced
also known as plaster hemihydrate, it is produced in an open kettle at about 115 C
how is alpha-calcium sulfate hemihydrate produced
aka dental stone hemihydrate, high pressure steam 125 C
WHAT hemihydrate is produced by boiling the gypsum in a 30 % solution of calcium chloride
high-strength dental stones
the hemihydrate produced in this case is the least porous, most regular and largest of all hemihydrates
high-strength dental stones
to produce a workable mass , which material requires the most excess water and whcih requires the least , consecutively and why
plaster, has small porous crystals
high-strength stone requires the least excess water because of its large dense crystals
Basically, what is the rule when adding excess water to hemihydrates
the larger and more dense the crystal size of the hemihydrate, the less excess water needed to get a workable mass
what chemical when added, reduces setting time from 10 to 4 minutes
an accelerator eg ; potassium sulfate
what chemical when added extends setting time to several hours
A RETARDER ;borax BORING
what is the mechanism of an accelerator
it makes the dihydrate less soluble than the hemihydrate so the rxn is moved towards the dihydrate because the dihyrate assumes its solid form more rapidly
what does this fractton mean
0.45 or 45/100
water_powder ratio meaning 45 ml of water is added to 100 g of powder
which respectively has the lowest and highest water powder ratio of the gypsum materials
lowest: high strength stones
highest: plaster
Gypsum materials are hygroscopic what does that mean
ut means that they absorb water from the air to some extent, so if they are left in an open conatiner the surface of the particles will convert to dihydrate this subsequently will result in an increases setting time
to avoid changes in the rate of setting reaction, how should gypsum materials be stored
in closed container to protect them form humidity
name 5 properties of gypsum materials
- reproduction of detail
- dimensional accuracy
- strength
- hardness and abrasion resistence
- setting time
the total setting time for gypsum materials can be divided into two psrts, nsme them
- initial setting time
2. final setting time
define the initial setting time
the interval between the time the water and powder are mixed and the time that the mix can no longer be poured into an impression
the initial setting time is identical to what
working time of the material, 8-16 min
definethe final setting time
the time at which the conversion of the hemihydrate to the dihydrate is essentially complete
BY DEFINITION, HOW CAN WE KNOW THAT THE MATERIAL HAS REACHED FINAL SET
when gilmore needle cannot penetrate into set mass
to capture the most detail, the die material must fulfill two conditions, name them
- the die material must be chemically compatible with the impression
- and it must also wet the impression material readily
what does wetting mean
the die material must flow into the details of the impression and must not bead up or resist onto the impression material
is gypsum water or oil based
gypsum is water-based
name water-based impression materials
alginates, agars
name oil-based impression materials
polyethers, addition silicone
what happens when gypsum is poured into water-based impressions
good wetting occurs
reduvtion in the number of bubbles and improves rep of details
what happens when gypsum is poured into polyether impressions, how can it be solved
wetting is reduced rsylting in a limited ability of the gypsum to flow into all the details of the impression
this can be solved by adding a surfactant
define a surfactant
a chemical that acts like a soap
define strength
is the indicative of the ability of the material to resist fracture
whichof the gypsum materials have the least compressive strength and why
plasters, beacause of the amount of excess water present
which gypsum material is almost 4 times as strong in compression
high-strength dental stones
a proper manipulation of gypsum materials consists of 4 phases name them
- the measurement and combination of powder and liquid
- spatulation
- pouring ( sets in 45-60 min)
- disinfection if necessary
what is the proper method of comnbination of gypsum powders with water
proper water-powder ratio is critical
water is added first, then powder is sprinkled into the water slowly over a period of 30 sec
do not add powder to water all at once or bubbles will form during the final mix
what is the time needed for the gypsum model to set
45 to 60 min
which is preferred, the disinfection of impressions or models
impressions
why is the disinfection of impressions preferred more than that of models
- easier
- more effective
- limits the inroduction of contaminated items into the labratoy area
epoxy materials set by a chem rxn name it
polymerization
WHAT PRECAUTION must be known before handling an epoxy material
the hardener is toxic and allergenic, and must not come into contact with the skin during the manipulation of the unset material
an edentulous patient who has undergone radiation which led to xerstomia, what material is best for making an impression
a. polusulfide
b. polyether
c. zoe
d. irreversible hydrocolloids( algenates)
Becasue the patient has undergone radiation all hydrophillic impressions cannot be used
so we rule out d. zoe can adhere to tissure and may cause ulcers upon removal so we rule that out too
so it is a)
