Final Casting distortions and causes matching

Question 1

air bubbles

Answer 1

• small nodules on surface, normally ground off
• from air entrapment on surface of the wax pattern during investing
• vacuum investing and a wetting agent that air dries can alleviate this problem

Question 2

water films

Answer 2

• investment becomes separated from surface of pattern

* water films cause veins or ridges***

Question 3

veins and ridge caused by?

Answer 3

due to water films and too high of W/P ratio used for investment

Question 4

too rapid heating

Answer 4

causes cracks, fins result in the final casting (fins=cracks=fast heating)

Question 5

high W/P ratio of investment

Answer 5

higher the W/P ratio the high rougher the casting

Question 6

prolonged heating or heating investment at too high a temp causes?

Answer 6

• disintegration of investment which releasessulfur gases and roughens the sides of mold
• sulfur containing gold alloy demonstrates a black surface, brittle, and doesn’t pickle

Question 7

overheating a alloy causes

Answer 7

surface roughened casting

*bc an overheated alloy attacks the surface of the investment when it impacts the mold

Question 8

foreign bodies cause?

Answer 8

well-defined deficiencies in the surface of the casting

Question 9

localized shrinkage porosity? eliminated how?

Answer 9

incomplete feeding of molten metal during solidification

*eliminated by flaring the point of sprue attachement pattern and redicing the casting temperature by 85 degrees

Question 10

suck-back porosity is a type of?

Answer 10

localized shrinkage porosity

Question 11

microporosity

Answer 11

occurs if solidification rate is too high (especially in fine grained alloys) because the mold or casting temps were too low
**small irregular voids within bulk of casting

Question 12

pinhole porosity

what dissolves oxygen and hydrogen?

Answer 12

due to gases being dissolved in the metal during melting; avoided by adjusting flame and only using reducing zone of flame

• Ag and Cu dissolve oxygen
• Pt and Pd dissolve oxygen and hydrogen

Question 13

gas inclusions?

Answer 13

appear as large spherical voids which are caused by gas entrapment during melting

Question 14

back pressure porosity

Answer 14

“pocket” on the cavity side of the casting

*eliminated by using proper mold and casting temps, proper W/P ratio, position wax 1/4” from rim of cast

Question 15

incomplete casting due to incomplete wax elimination

Answer 15

pores of investment filled with residual combustion products preventing air from being vented from mold space
*ROUND, SHINY MARGINS

Question 16

incomplete casting due to low W/P ratio

Answer 16

the investment is too dense to allow air space to escape

*rounded but DULL margins (incomplete waxing causes shiny margins)

Question 17

blackened surface on a gold alloy caused by?

Answer 17

burning out a gypsum investent in a 730 degree C oven

Question 18

fins on casting caused by?

Answer 18

placing a room temperature, gypsum investment mold directly into a 650 degree C oven (heats too fast)

Question 19

sharply defined irregulat pores cuased by?

Answer 19

careless handling when removing the crucible former and sprue former from the investment mold

Question 20

incomplete csting with round. shiney margins caused by?

Answer 20

incomplete wax elimination

***using minimum burn out times with a low heat, hygroscopic technique

Question 21

nodules (spheres) of alloy on the casting surface

Answer 21

investing without vacuum