spruing and investment wax

Question 1

explain the wax-lost technique used to cast dental crowns

Answer 1

attach sprue wax; surround it by heat-resistant silica mold around casting ring; ; wax burn-out; casting of molten metal

Question 2

what is the investment material

Answer 2

heat-resistant silica mold

Question 3

what is a sprue

Answer 3

piece of prefabricated plastic or wax that attaches wax patterns to crucible former in casting ring

Question 4

what is the lost-head reservoire for

Answer 4

preventing cavities and porosities due to thermal contraction of molten metal during solidification

Question 5

where should you attach the sprue

Answer 5

bulkiest part, away from margins and occlusal contacts, largest non-functional cusp

Question 6

why do we use a liner in the casting ring

Answer 6

• metal casting ring does not allow investment expansion
• stress would be directed to center and cause distortion of wax pattern
• compressible piece of fiber prevents this distortion by allow expansion of the mold

Question 7

what else can be used instead of a liner

Answer 7

silicon casting rings

Question 8

why is silicon good to use

Answer 8

• no resistance to expansion
• poor heat conductors (leads to high temp and stronger expansion so more precise)
• sleeve ensures uniform expansion pressure, more precision, reliable results
• easily cleaned and durable

Question 9

what are the types of investment material expansions

Answer 9

1. normal setting expansion: normal recrystallization and grain growth
2. hygroscopic setting expansion: expansion of investment while immerse in water
3. thermal expansion: expansion while heated

Question 10

hygroscopic expansion

Answer 10

• submerged in water at 100 deg F
• water in bath replaces water used by hydration process
• space between growing crystals is maintained
• crystals grow longer and expand the mold

Question 11

hygroscopic expansion vs normal setting expansion

Answer 11

hygroscopic has more 4-5 times greater magnitude of expansion

Question 12

investment materials are silica based so they ___ when heated

Answer 12

expand

Question 13

why cant we use gypsum for investment

Answer 13

-sulfate breaks down and gets released, causing contamination of casting alloy

Question 14

why are investment materials called refractory materials

Answer 14

they are heat-resistant

Question 15

types of investment materials

Answer 15

• gypsum-bonded: gold
• phosphate-bonded: palladium
• silica-bonded: base metal