L11: Dental investment and casting procedure

Question 1

what is investing ?

Answer 1

process of covering an object like dentures or teeth with an investment material before curing , casting or soldering

Question 2

what is an investment material?

Answer 2

refractory mold material used for embedding the wax pattern and obtaining a mold for casting / material used to enclose a pattern of a dental restoration for casting of moduling

Question 3

gypsum bonded investment material composition :

Answer 3

powder conatains alpha-hemihydrate of gypsum as binder and polymorphs of silica as fillers

Question 4

setting reaction :

Answer 4

calcium sulphate hemihydrate is mixed with water to from calcium sulphate dihydrate –> contributes in binding silica particles

Question 5

types of gypsum bonded investment materials
;

Answer 5

type 1: casting inlays , onlays and crowns or other fixed prostheses

type 2: removable partial denture frameworks

Question 6

expansion of gypsum bonded investment materials :

Answer 6

during setting , hygroscopic expansion and thermal expansion

Question 7

setting expansion :

Answer 7

result of normal gypsum crystal growth in air

Question 8

hygroscopic expansion :

Answer 8

maximum expansion by immersing investment filled ring in bath water at 38c . water replaces the water used by hydration process

Question 9

thermal expansion :

Answer 9

when investment is heated in burnout event

done either to eliminate wax or prevent molten alloy from cooling too quickly

Question 10

which technique combines wax pattern and thermal expansion of mold ?

Answer 10

low temp. burnout technique (wax pattern expansion )

Question 11

composition of silica bonded investment material :

Answer 11

powdered quartz or cristobalite bonded together with silica gel

silica gel liquid if formed by mixing ehtyl-silicate and dilute hydrochloric acid and spirit

Question 12

temperature silica bonded material is heated at :

Answer 12

1090-1180c

Question 13

silica bonded investment agents are less popular because :

Answer 13

they involve more complex and time consuming procedures

Question 14

composition of phosphate bonded investment material :

Answer 14

colloidal silica liquid and powder containing silica 80%

Question 15

setting reaction of phosphate bonded investment material :

Answer 15

occurs between colloidal silica solution and powder ( phosphate and MgO) to form magnesium ammonium-phosphate

Question 16

burn-out temperature of phosphate bonded investing material :

Answer 16

750-1030c

Question 17

total expansion required for alloy to produce metal ceramic crowns in phosphate bonded investing material :

Answer 17

2%

Question 18

during the casting process , alloy is melted by:

Answer 18

electrical induction heating and forced into mold by centrifugal force

Question 19

which process is used to make metallic dental restorations such as inlays , onlays , crowns , bridges , and cast partial dentures :

Answer 19

casting

Question 20

whats the function of a sprue?

Answer 20

allow molten alloy to reach mold after dewaxing

Question 21

gypsum bonded investment burn-out temperature:

Answer 21

450-700c

Question 22

after casting —> cooling , breaking investment material , sand blasting with high aluminum oxide particles to remove and clean casting

Question 23

after being retrieved , cast appears ___

Answer 23

dark and tarnished due to oxidation

Question 24

what is pickling :

Answer 24

process to clean casting which consists of heating the discolored casting in an acid (30-50% HCL)

after pickling , spurce is removed for final polishing and finishing

Question 25

what is finning ?

Answer 25

if investment is heated up too rapidly in furnace , causing it to crack. molten alloy will flow into the cracks forming “fins”

Question 26

what is porosity :

Answer 26

surface pitting

gaseous perosity : due to incorporation of gasses into molten alloy and can be reduced by vaccum casting