Models and dies

Question 1

What do we need in a dental modelling material?

Answer 1

cheap
easy to form from an impression 
safe
robust
rigid
will not chemically react with wax, impressions etc
resistance to wear
resistance to heat

Question 2

What does agar react with and how can this be prevented?

Answer 2

Reacts with plaster but Alum solution can be used as a barrier

Question 3

What is Plaster of Paris used for?

Answer 3

Models and dies
Impression material
Moulds for denture construction
Binder in refractory investments for casting

Question 4

What is plaster of paris?

Answer 4

Gypsum - natural mineral that is mined

Question 5

What happens with water is added to plaster of paris?

Answer 5

Becomes regular gypsum again - hardens

Question 6

What is a model and a die?

Answer 6

Model - whole mouth

Die - a tooth (section of the model) that is removable to be worked on

Question 7

What colour is b-hemihydrate?

Answer 7

White powder

Question 8

What colour is a-hemihydrate?

Answer 8

Yellow

Question 9

What can be added to the die-stone mix so it can be used in 3D scanners?

Answer 9

Titanium dioxide

Question 10

What are the stages of the setting reaction?

Answer 10

Hemihydrate dissolves in water

Super saturated solution forms

dihydrate precipitates out

more hemihydrate dissolves

crystal growth - crystal size gives you the properties

Question 11

What happens to the crystals if the setting time is much longer?

Answer 11

Crystals are much further apart so they have to grow further/longer to make contact with other crystals = longer setting time

Question 12

What happens to the crystals during expansion?

Answer 12

Crystals are interlocking with water in the pores

Question 13

What reduces the expansion in plaster and stone compared to uncontrolled?

Answer 13

Accelerators and retarders are added in a balance to reduce setting expansion

Question 14

What are the bonus properties of plaster of paris?

Answer 14

Easy to trim or grind
Can be coloured (appealing shades)
Sticks to itself

Question 15

What can you use to stop plaster bonding to itself?

Answer 15

sodium alginate based solution - painted on to plaster (becomes a skin/barrier) between plaster layers

Question 16

What is the water/powder ratio for plaster and stone?

Answer 16

plaster - 50ml water to 100gm powder

Stone - 20-35ml water to 100gm powder

Question 17

What happens if you over spatulate?

Answer 17

Smashes up crystals as you-re mixing which then start growing new crystals and it snaps

Question 18

What can you use as accelerators?

Answer 18

Potassium sulphate (manufacturer adds in) - articulating solution causes setting in seconds
gypsum - sprinkles on top
sodium chloride - small amounts, causes pink spots

Question 19

What chemicals can be used as retarders (slows down reaction)?

Answer 19

Borax - main thing but tends to decay in the bag
potassium citrate - dissolves plaster
sodium chloride (large amounts)

Question 20

What is the typical composition of a dental plaster?

Answer 20

Powder:
calcium sulphate -hemihydrate (CaSO4)2.H2O
borax (slows the setting rate)
potassium sulphate (reduces expansion)

Liquid:
water

Question 21

What is thixotrophy?

Answer 21

Material becomes thinner when stressed but stays put when no vibrations applied (plaster has this property)

Question 22

What affects the properties of gypsum?

Answer 22

its preparation, composition and mixing