investment materials

Question 1

how many type of gypsum bonded investment are there and what are each used for

Answer 1

type1: for inlays and crowns, casting shrinkage manily comprensated by thermal expansion

type 2 for inlay and crown, casting shrinkage mainly compensated by hygroscopic expansion

type3 for construction of partial dentures w gold alloy

Question 2

what is gypsum bonded investment use for

Answer 2

alloys that melt <1300deg

Question 3

what is added to protect molten metalfrom SO2 that is realeased during decomposition of investment greater than 700deg

Answer 3

oxalatae decomposes during heating to form CO2 in the mould

Question 4

what is the composition of gypsum bonded investment

Answer 4

1.a-hemihydrate of gypsum
dental stone used bc stronger than plaster
serves as binder to hold other constituents, provides rigidity

2. silica
   provides refractory(high temp strength during heating), regulates thermal expansion, types of silica commonly used are quartz cristobalite
3. reducing agents like carbon and powdered copper
   provide reducing atmosphere in mould when alloy is used
4. modifying agent like boric acid and nacl
   prevent shrinkage of gypsum when heated over 300deg, regulates setting expansion and time
5. colouring agent

Question 5

what is the temp from a-quartz to b-quartz

Answer 5

575 degree

Question 6

what is the temp from a-cirstobalite to B-cristobalite

Answer 6

200-270deg

Question 7

does setting expansion increase with silica?

Answer 7

yes, silica interfere with intermeshing and interlocking of crystals as they form

Question 8

what is the theory of hygroscopic expansion

Answer 8

immersion water replaces water of hydration, prevent confinement of growing crystals by surface tension of excess water

diluent effect of silica particles lead to expansion greater than gypsum alone

Question 9

what is the max expansion permitted by hygroscopic expansion, and how do you achieve that of gypsum bonded invetsment

Answer 9

max expansion is 2.2%, set it under water

Question 10

how does silica present affect thermal expansion of gypsum bonded investment

Answer 10

more silica more thermal expansion. Cristobalite expand to a greater extent than quartz

Question 11

what are some factors affecting thermal expansion of gypsum bonded investment

Answer 11

1. water powder ratio, increase ratio, decrease thermal expansion
2. add chemical modifiers like chlorides of Na/K/Li, eliminates contractionf of gypsum when <700deg, increase thermal expansion without excessive silica

Question 12

how does thermal shrinkage of gypsum bonded investment occur

Answer 12

when investment cool fro 700deg, B invert to a- form, contraction until less than original dimension

Question 13

what are some factors influencing strength of gypsum bonded investment

Answer 13

1. type of gypsum binder: dental stone>dental plaster
2. water powder ratio, increase ratio decrease strength
3. chemical modifiers: allow more gypsum binder to be used can increase strength

Question 14

what are phosphate bonded investment used for

Answer 14

for alloys that melt >1300degree

Question 15

what does mixing phosphate bonded powder in special liquid do

Answer 15

decreases thermal expansion, increases strength. Can expand hygroscopically

Question 16

what causes back pressure porosity

Answer 16

insufficeint porosity in investment, lack venting

sprue too short and narrow

insufficient casting pressure