Invesring And Casting

Question 1

List steps of investing and casting

Answer 1

1-spruing
2-investing
3-casting

Question 2

Sprue: is ..

Answer 2

a channel through which the wax pattern is eliminated& the molten alloy is Pushed to fill the mould cavity completely

Question 3

Sprue former: is …

Answer 3

a wax , plastic, or metal rod used to produce sprue channel with Desired shape and size.

Question 4

Basic requirement of sprue

Answer 4

1-The sprue must allow complete escapement of molten wax

2-Easy flow of the molten alloy

3-The metal within it must remain molten until complete solidification

Question 5

The metal within it must remain molten until complete solidification of the molten Alloy of the mold cavity, why ?

Answer 5

act as a reservoir to compensate for the shrinkage that Occurs during solidification of the casting

Question 6

Wax (sprue) advantages

Answer 6

it melt at same temperature of wax pattern

Question 7

Plastic (sprue) advantages , disadvantages

Answer 7

Used when spruing long span fixed partial dentures

plastic sprue formers soften at higher temperature than the wax pattern

Question 8

Location of sprue former

Answer 8

Attached to the thickest non-functional cusp on the wax pattern , thin area causes localized shrinkage porosity

Question 9

Diameter of sprue former

Answer 9

2.5mm(10-gauge)s for molar and metal-ceramic patterns

smaller 2mm(12-gauge) for premolars and partial coverage restorations.

sprue former should be thicker than the thickest portion of the wax pattern

Question 10

If the sprue former diameter is too small the result ….

If too large diameter …..

Answer 10

(suck-back) porosity

spread the molten alloy faster before The gases eliminated from the mold cavity (incomplete casting.)

Question 11

using air-pressure casting Machine It is preferred to use ….

Answer 11

narrower diameter sprue formers

Question 12

Angulations of sprue former

Answer 12

The sprue former should be attached at 45 to the surface of the wax pattern and should not attached at right angle

Because the molten alloy when enter to the mold change the direction and lead To development of turbulence which will cause porosity

Question 13

Attachment of sprue former to wax pattern ?

Answer 13

by adding drop of sticky wax

Question 14

Reservoir

Answer 14

It is apiece of wax attached to the sprue former , and placed as close to the wax pattern as possible to compensate the solidification shrinkage

Question 15

Accessory spruing is

Answer 15

vent to allow escape of the air& gases from the mold during casting

Question 16

Length of sprue former

Answer 16

6-8mm should be left from the end of the ring in case of gypsum-bonded

3-4mm in phosphate –bonded investment.

Question 17

highest point of the wax pattern should be at…. Distance from the end of the ring.

Answer 17

6.0mm

Question 18

If the sprue too short it will

Answer 18

increase the thickness of the Investment bridge, so during casting the air inside the mold will not escape readily through the thick bridge and result in back-pressure porosity.

Question 19

If the sprue too long

Answer 19

the investment bridge will be very thin And may fracture due to casting force of the molten alloy And if the sprue too thin and long the alloy may solidify Before complete fill of the mold.

Question 20

Sprue former should be (in shape )

Answer 20

straight as possible to minimize turbulence as the alloy Enter the mold.

Question 21

There are two basic designs of sprue formers;

Answer 21

Direct spruing: in which the sprue former provides direct connection between the wax pattern and crucible former.

Indirect spruing: in which the wax pattern connect indirectly to the crucible former by bar .

Question 22

For Single small size restoration the design is

Answer 22

straight single and direct
Sprue used for casting

Question 23

in spruing anterior teeth for metal-ceramic restoration the sprue is

Answer 23

attached to the thin area of the wax pattern

Question 24

For single large size restorations…….. is recommended

Answer 24

double spruing , Direct double V-shaped sprue former (noble alloy)

Question 25

Indirect double sprue former design for…

Answer 25

large restorations casted With base metal alloy

Question 26

Direct spruing are preferred for…….. Indirect spruing are preferred for……….

Answer 26

air-pressure casting machine casting base metal alloy

Question 27

Crucible former

Answer 27

Base of the casting ring during investing

Question 28

If the ring liner is absent

Answer 28

the investment will expand in inward direction towards the mold cavity leading to distortion of the restoration.

Question 29

Before investing the wax pattern should be sprayed by… which act as ….

Answer 29

wetting agent (Debubblizer) to Act as surface tension reducer

Question 30

Requirements of dental casting investment

Answer 30

1-accuratly wet the wax pattern produce its details 2-withstand high temperature of burn out of wax 3-high strength enough to with-stand casting pressure 4-sufficient porosity to allow air escapement during casting 5-chemical stability at high temperature of casting 6-have total amount of expansion (setting-hygroscopic-thermal) 7-has minimum setting expansion to prevent wax pattern distortion 8-easy recovery of casting

Question 31

GYPSUM BONDED INVESTMENT, used for casting of gold alloys because

Answer 31

A-it is not chemically stable above 650 B-produce expansion suitable for shrinkage of the gold

Question 32

PHOSPHATE BOMDED INVESTMENT , used for casting of high melting alloys e.g. base metal alloys because

Answer 32

1-it is chemically stable up to 1100c 2-it produce thermal expansion higher than gypsum bonded investment

Question 33

gypsum and phosphate bonded investment are available in two forms:

Answer 33

Conventional type ((750c for gypsum-950c for phosphate)) Heat sock type

Question 34

Silicate Bonded Investments used with

Answer 34

Base metal alloys such as chrom-cobalt and steel needs high temperatures for melting 1250-1400 and silica can be heated up to 1200c

Question 35

Investing techniques:

Answer 35

I- Brush technique II-Vacuum technique

Question 36

Brush technique produce better casting because.

Answer 36

it can reduce the air entrapment and the investment wet the pattern very well

Question 37

Reducing zone:

Answer 37

hottest zone which blue in color

Question 38

zone used to melt the alloy

Answer 38

The reducing zone

Question 39

There are two types of blow torch:

Answer 39

1-Gas-air torch , used for melting of gold alloy 2-Gas-oxygen torch , used for melting of base metal and porcelain fusion

Question 40

Casting machines

Answer 40

1-air pressure casting machine 2-vacuum casting machine both of them used with gold alloy 3-centrifugal casting machine used for gold and base metal alloy

Question 41

The crucible may be….. which used for……., or …..used for…….

Answer 41

ceramic , base metal alloy graphite, gold alloy melting

Question 42

finishing can be divided to zones:

Answer 42

Zone-1-internal margin any defect at this area usually need remake of restoration Zone-2-internal surface Zone-3-the sprue Zone-4-proximal contacts Zone-5-occlusal surface Zone-6-axial walls Zone-7-External margins

Question 43

List Casting defects

Answer 43

1-Incomplete casting 2-Suk-back porosity 3-Rough casting 4-Large nodule 5-Multiple nodule 6-fins 7-Marginal discrepancy

Question 44

Inclusion porosity ……. Back-pressure porosity…..

Answer 44

-particles of investment dislodged during casting -due to improper air escapement through the pores of investment During casting due to: 1-lake of pores in investment 2-improper investment bridge thickness (more than 8mm) 3-too short sprue

Question 45

Under-sized restoration = Over sized restoration =

Answer 45

-new restoration should be done * difference in solidification (less/more)