technique used in casting
lost-wax technique
accuracy of casting
20 microns
basic steps of the casting proces
waxing, spruing, investing, burnout, casting
waxing is what type of technique:
indirect - its done on the model
the sprue forms what:
the channel in the investment for the molten alloy to travel to the rastoration
diameter and length of the sprue are imprtant why:
so that the molten alloy solidifies last in the sprue (don't want it to solidify in the sprue and closing the channel)
the sprue should be placed where:
on the non-functional cusp and at an angle so that gold can flow to all portions of the pattern
the name of the power device used for mixing the investment material
vac-u-spat
the vac-u-spat generates a what: and purpose
a vacuum, draws the air out of the mix so that you are mixing in a vacuum - if you have any bubbles or voids the investment material it will ruin your cast because molten metal will get in
investment: what
a stonelike material able to withstand high temps and forces of burnout and casting
types of investment used for gold alloys
thermal and hygroscopic casting investment - gypsum bonded (meaning the binder is gypsum)
investment for casting high melting point alloys is what bonded
phosphate or silica
3 components (and their %) in gypsum bonded investments:
1) binder (calcium sulfate hemihydrate - gypsum) 30-35% 2) refractory component (quartz or cristobalite) 60-65% 3) chemical modifiers - 5%
purpose of the binder (gypsum)
holds the investment together
purpose of the refractory component (quartz or cristobalite)
resists the heat of burnout and casting
purpose of the chemical modifiers
control expansion and prevent gypsum shrinkage on heating
2 examples of chemical modifiers
boric acid, sodium chloride
casting alloys shrink how much when cooled:
1-2.5% (significant amount)- must be compensated for or it will end up being too small
thermal expansion: what happens
the silica refractory crystals in the investments expand when heated due to changes in lattic arrangements
thermal expansion is regulated by: (2)
type of silica refractory used and temp of burnout
for thermal expansion investments the burnout oven is set to:
649 celcius (1200F)
hygroscopic expansion: what
casting ring is immersed in water bath for 30 minutes allowing the investment to set hygroscpically (the water causes the gypsum crystals to grow and expand)
for the hygroscpic investments the burnout oven is set only to:
482C or 900F
no expansion occurs if you don't have a:
liner (asbestos)
phosphate -bonded investments: used to make what
high melting point alloys (palladium and base metal alloys) and ceramic-metal restorations
phosphate bonded investments: casting temps are what
greater than 700C (1292F)
silica-bonded investments used for:
base metal alloys, fabrication of partial dentures
silica bonded investments: temperature range:
1090-1180C or 1994-2156F - very high!
2 types of investments for all ceramic restorations
1) cast glass technique (dicor) and 2) refractory die technique (when the die is refracotry because the model is burned out)
what type of machine is used for casting
a centrifigue machine
the alloy is heated how
with a blowtorch and is heated in a ceramic crucible
when the alloy is molten the casting ring is:
taken from the oven and crucible ring assembly is spun rapidly (centrifuged) for less than 1 second
if the casting ring is cooled slowly it:
produces a harder and stronger restoration
if the casting ring is cooled in cool water (quenching): it
produces a softer restoration
pickling: what
the freshly cast restoration is covered with dark surface oxides - these oxides are removed by placing in hot sulfuric acid bath
pickling is only for:
high gold alloys - NOT base metal alloys
what is the difference between a refracotry die and a regular die stone
refractory stone is made to harden when heated
what is the effect of the quartz/cristobalite in alpha--> beta conversion
they provide thermal expansion of the investment when its crossing from an alpha to a beta state
inlay vs. onlay
inlay is completely within the tooth, onlay has an occlusal component that covers the whole occlusal surface
3 general desirable properties of dental alloys
1) cost (cheaper than using pure gold) 2) biocompatability 3) corrosion and tarnish resistance (gold and the noble metals help the alloys to be tarnish resistant)
Low Stress Inlays: (Example - class V): most important property of the alloy
high ductility - because we can burnish it forcing elongation of the metal and sealing the margins
high stress inlays, onlays crowns: most important properties of the alloy
high stiffness and yield strength (ductility still important buy not as important as these)
denture clasps: qualities needed in alloy
balance of stiffness and yield strength with ductility
cast post is made from what type of alloy
type 3 gold alloy
relationship of density and ease of casting
higher density = easier to cast (rushes into the mold space va centripical motion easier)
density of gold alloys vs. base metal
gold alloys have higher density - easier to cast than base metal alloys
casting shrinkage (cooling contraction): the temperature transitions with cast goes through
from the temp of the molten alloy to its solidifying (freexing temp), then from its solidifcation temp to room temp
relationship of melting temp with shrinkage
higher melting point = higher shrinkage (cooling contraction)
gold alloy vs. base metal alloy melting points
gold alloys tend to have lower melting points and therefore lower shrinkage
the 4 truly noble metals (have the highest degree of nobility)
gold, platinum, iridium, osmium
reactivity of these 4 noble metals
non-reactive under normal conditions, will only react under extreme conditions (temp, pressure, presence of corrosive gases such as chlorine and flourine)
silver is not noble, it is:
PRECIOUS
methods of describing gold content: carat
parts of gold in 24 parts of alloy (ex) 12 carat = 50% gold)
methods of describing gold content: fineness
parts of gold in 1000 parts of alloy
noble vs precious metal/alloy
precious = it has silver in it, gold and platinum, etc are precious but should say noble metal)
type 1-4 gold alloys: hardness
type 1: soft 60-90, type 2: medium 90-120, type 3: hard 120-150, type 4: extra hard
type 4: quenched
softened > 150
type 4: hardened
>220
as you go from type 1 to type 4 alloy: the amount of gold:
is DECREASING, and the amount of SILVER/COPPER/etc is increasing
ductility as you go from type 1 to type 4 gold alloy:
goes down (so the more gold, the moer ductile, but as gold decreases and silver/copper increases the ductility decreases)
gold alloys: role of gold (2)
solid solution matrix, corrosion resistance
gold alloys: role of silver (2)
strengthens, counteracts the reddish of copper
gold alloys: role of copper (2)
strengthens (types III, IV), reduces meplting temp
must have less than what % copper
16% or it will cause tarnishing
gold alloys: role of platinum (3)
strengthens, raises melting temp, some corrosion resistance
gold alloys: role of palladium (3)
strengthens, raises melting temp, prevents tarnshing effect of silver
palladium vs. platinum cost
palladium is cheaper and does the same thing, so its now used more often
gold alloys: role of zinc (2)
oxygen scavenger during casting, improves castability by keeping a low melting point and making the molten metal very fluid and easy to cast
gold alloys: role of grain refiners
make the grains very small and finer --> increases the strength because creates more grain boundaries for dislocations to move
strengthening mechanisms: method to soften the alloy
quenching
strengthening mechanisms: example of heat treatment solution to harden
order-disorder reactions
for types 3 and 4 alloys: what is the copper level and what are the 2 effects of this
>11% (but less than 16%) - this increases yield strength and hardness, but reduces ductility
self hardning alloys: what
hardens with slow bench cooling, don't need to undergo any sort of heat treatment, just cool them on the bench after casting and hardening will occur
types 1-4 gold alloys: what makes them so easy to cast
they have a small liquuidoud/solidous gap
effect of Pt and Pd on the gap
increases the gap making it harder to cast
type 1-4 gold alloys have a low casting temperature, therfore they have:
a low casting shrinkage
3 other properties of gold alloys:
1) low hardness (easy to grind and polish), excellent corrosion and tarnish resistance, excellent biocompatability
Type 1: applications and properties
soft, easily deformed and high ductility, easily burnished - used for low stress bearing inlays
Type 2: application and properties
harder but similar to type s, used for occlusal surfaces unless in a thin section (can deform easily)
type 3: applications and properties
greater strength and hardness, difficult to burnsih bc high copper and therfore can fracture if overburnish, used for inlays/onlays/full crowns/posts/etc. things that have high stress
type 4: used for (4)
cast post and cores, bridges, partial dentures, clasp arms
type 4: cant be what: and result of this
burnished - therefore cant be used for inlays
medium gold alloys: content
gold 40-60%, Pd and Ag increased to replace gold, copper 15%
medium gold alloys: used to replace
type 3 and 4 golds
low gold alloys: contents
gold 15-20%, silver 40-60%, palladium 40%
low gold: alloys: what turns them gold from their white appearance: and why
indium: because underneath porcelain, the gold color gives a life like appearance
low gold alloys: used as an alternative to what, and for what
alterantive to type 3 golds, used for posts and cores
why low gold alloys have lower mechanical properties than medium gold alloys:
because the amount of silver is higher
palladium silver alloys: palladium prevents what:
tarnish associated with silver
palladium-silver alloys: are similar to what, and used for what
similar to type 3 alloys, used for crowns
palaldium-silver alloys: are not
heat treatable
when palladium-silver alloys are used as porcelain fused metal alloy: what occurs
greening of the procelain: get greenish margins due to the presence of silver
as the silver content is reduced in silver-palladium alloys the properties change from:
class 3 to class 4 (less silver)
silver palladium alloys are contraindicated for use in what:
long span bridges
silver palladium alloys: 4 characteristics
high casting temperature ranges, therefore must be phosphate bonded investment, not burnishable (too hard), tarnishes more than other noble alloy metals
porcelain fused alloys: what is necessary to do for these alloys
you have to heat them to generate the oxides on the surface so that they will bond to the porcelain, which is just a bunch of oxides
2 types of alloys for pfm restorations
noble metal alloys, base metal alloys (nickel chromium and cobalt-chromium bases)