Invesring And Casting Flashcards
List steps of investing and casting
1-spruing
2-investing
3-casting
Sprue: is ..
a channel through which the wax pattern is eliminated& the molten alloy is Pushed to fill the mould cavity completely
Sprue former: is …
a wax , plastic, or metal rod used to produce sprue channel with Desired shape and size.
Basic requirement of sprue
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
The metal within it must remain molten until complete solidification of the molten Alloy of the mold cavity, why ?
act as a reservoir to compensate for the shrinkage that Occurs during solidification of the casting
Wax (sprue) advantages
it melt at same temperature of wax pattern
Plastic (sprue) advantages , disadvantages
Used when spruing long span fixed partial dentures
plastic sprue formers soften at higher temperature than the wax pattern
Location of sprue former
Attached to the thickest non-functional cusp on the wax pattern , thin area causes localized shrinkage porosity
Diameter of sprue former
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
If the sprue former diameter is too small the result ….
If too large diameter …..
(suck-back) porosity
spread the molten alloy faster before The gases eliminated from the mold cavity (incomplete casting.)
using air-pressure casting Machine It is preferred to use ….
narrower diameter sprue formers
Angulations of sprue former
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
Attachment of sprue former to wax pattern ?
by adding drop of sticky wax
Reservoir
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
Accessory spruing is
vent to allow escape of the air& gases from the mold during casting
Length of sprue former
6-8mm should be left from the end of the ring in case of gypsum-bonded
3-4mm in phosphate –bonded investment.
highest point of the wax pattern should be at…. Distance from the end of the ring.
6.0mm
If the sprue too short it will
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.
If the sprue too long
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.
Sprue former should be (in shape )
straight as possible to minimize turbulence as the alloy Enter the mold.
There are two basic designs of sprue formers;
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 .
For Single small size restoration the design is
straight single and direct
Sprue used for casting
in spruing anterior teeth for metal-ceramic restoration the sprue is
attached to the thin area of the wax pattern
For single large size restorations…….. is recommended
double spruing , Direct double V-shaped sprue former (noble alloy)
Indirect double sprue former design for…
large restorations casted With base metal alloy
Direct spruing are preferred for……..
Indirect spruing are preferred for……….
air-pressure casting machine
casting base metal alloy
Crucible former
Base of the casting ring during investing
If the ring liner is absent
the investment will expand in inward direction towards the mold cavity leading to distortion of the restoration.
Before investing the wax pattern should be sprayed by… which act as ….
wetting agent (Debubblizer) to Act as surface tension reducer
Requirements of dental casting investment
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
GYPSUM BONDED INVESTMENT, used for casting of gold alloys because
A-it is not chemically stable above 650
B-produce expansion suitable for shrinkage of the gold
PHOSPHATE BOMDED INVESTMENT , used for casting of high melting alloys e.g. base metal alloys because
1-it is chemically stable up to 1100c
2-it produce thermal expansion higher than gypsum bonded investment
gypsum and phosphate bonded investment are available in two forms:
Conventional type ((750c for gypsum-950c for phosphate))
Heat sock type
Silicate Bonded Investments used with
Base metal alloys such as chrom-cobalt and steel needs high temperatures for melting 1250-1400 and silica can be heated up to 1200c
Investing techniques:
I- Brush technique II-Vacuum technique
Brush technique produce better casting because.
it can reduce the air entrapment and the investment wet the pattern very well
Reducing zone:
hottest zone which blue in color
zone used to melt the alloy
The reducing zone
There are two types of blow torch:
1-Gas-air torch , used for melting of gold alloy
2-Gas-oxygen torch , used for melting of base metal and porcelain fusion
Casting machines
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
The crucible may be….. which used for……., or …..used for…….
ceramic , base metal alloy
graphite, gold alloy melting
finishing can be divided to zones:
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
List Casting defects
1-Incomplete casting
2-Suk-back porosity
3-Rough casting
4-Large nodule
5-Multiple nodule
6-fins
7-Marginal discrepancy
Inclusion porosity …….
Back-pressure porosity…..
-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
Under-sized restoration =
Over sized restoration =
-new restoration should be done
* difference in solidification (less/more)
