3. Spruing, Investing and Casting Porcelain Fused-to-Metal Alloys Flashcards
Alloys for Metal-Ceramic restorations
• Noble alloys are heavily based on ____ or palladium
◦ You will find noble alloys that have very little gold, maybe 2%, but they are
still noble because they have a high content of ____
gold
palladium
Lost wax casting sequence
• First we have a stone die and a wax pattern crown on top
• The next step is to attach a ____ pin to the crown
◦ We will talk later about what compositions you have for the sprue pin
• The sprue pin is attached to a sprue base
• Then, you attach a ____ ring to the sprue base
• You mix your investment and then pour it into ring
• Next, heat in oven
◦ Temp and time depend on what kind of casting you are doing
• After you heat investment, wax flows out and it is hollow
• Next step, put in casting machine
• Then you break out ____, cut off the sprue pin, polish, and deliver to pt
• Chart shows dimensional changes that occurred during the process
◦ Left shows when there is shrinkage, right is expansion
◦ The initial impression has a little ____
◦ There is some setting ____ when the die is poured
◦ Waxing: when it hardens, there is a little ____
‣ There is also some ____ that occurs in the early stages of heating
◦ All investment materials have a little setting ____
◦ Burnout = ____
◦ Whenever you solidify the molten alloy, there will be significant ____
◦ We are not looking for zero change, we are looking for a little bit of ____ (without this, it was fit very tightly)
sprue
copper
investment
shrinkage
expansion
shrinkage
expansion
expansion
expansion
shrinkage
expansion
Considerations for Accurate Castings
Casting Formula wax shrinkage + solidification shrinkage = die stone \_\_\_\_ + wax \_\_\_\_ + investment setting \_\_\_\_ + hygroscopic \_\_\_\_ + thermal \_\_\_\_ of investment
Tolerance of ± 0.05%
expansion expansion expansion expansion expansions
Casting/solidification shrinkage Gold/Noble metal alloys 1.4 - 1.6% Base metal alloys 2.0 - 2.5% Shrinkage of wax pattern ~0.3-0.8%
• Know that base metal alloys shrink more than ____ alloys
◦ Base metal alloys are cast at much ____ temperatures
gold/noble
higher
Compensation for Solidification Shrinkage I
Master die - Dimensional changes
Setting Expansion
0.1% Type ____ gypsum
0.3% Type ____ gypsum Wax pattern ____
- Setting expansion from die stone ranges from 0.1% to 0.3% depending on type of gypsum used
- There is also a slight wax expansion when we begin to heat the wax
IV
V
expansions
Compensation for Solidification Shrinkage II
Investment Expansion
Setting and hygroscopic expansion
Setting expansion - ____%
Hygroscopic - ____%
Thermal expansion of investment
High heat Thermal Expansion ____%
Low heat Thermal Expansion ____% (hygroscopic + wax expansion)
Use of die spacer
• Hygroscopic expansion
◦ If we mix our investment and pour into casting ring, we can increase amount of expansion any immersing whole thing in water (this is
____ expansion of investment)
◦ Much more expansion than just letting it sit on the bench
• There will be thermal expansion of investment when we start to heat it
• There are different types of investment that accomplish their expansion primarily by thermal expansion
• The manufacturer is using these properties to balance the expansions
depending on the type of alloy being cast
◦ Instructions must be followed exactly
• The die spacer is painted on by technician before wax-up ◦ This is supposed to provide space for ____
- 4
- 2-2.2
1-1.6
0-0.6
hygroscopic
cement
Considerations for Accurate Castings
Adequate ____ for solidification shrinkage & wax pattern shrinkage
Adherence to basic principles of ____
Appropriate ____ materials
• You must use the right type of investment for the casting you’re doing
◦ If you are casting gold inlays, you use ____ bonded investment ◦ But when you are casting PFM alloys, they are cast at much ____
temperatures than gold alloys
‣ Different investment is needed
compensation casting investment gypsum higher
Techniques for Casting \_\_\_\_ Investing \_\_\_\_ & \_\_\_\_ Materials
spruing
casting
investment
The Invested Wax Pattern
• First, we need to have an understanding of the invested wax pattern
◦ This is the wax pattern for an ____ or ____
• Within the casting ring would be the investment material
• We used to line the casting ring with asbestos (lol), we don’t do that anymore
• Notice that the wax pattern is below the top of the ____ by about 6mm
◦ The ____ is also below the top of the ring
inlay
onlay
casting ring
ring liner
SPRUING
Purpose
To provide a ____ in the set investment for wax ____, and for the molten ____ to fill the mold space.
channel
removal
metal
Spruing
Techniques
Direct or Indirect
◦ ____ [left] are attached to individual units
◦ ____ [right] are attached to a connector bar
direct
indirect
Spruing
Requirements
Size
Provide \_\_\_\_ Direction
{Minimize \_\_\_\_}
Location
\_\_\_\_
Materials
• Sprue size is important
◦ Want a sprue as thick as the ____ portion of the wax pattern
◦ When molten metal is flowing, there are no restrictions that will stop or
slow it down
• Provide a reservoir - same thing as a ____ used with indirect sprees
◦ Reservoir holds molten metal and allows it to flow and fill space freely
• Direction of flow
◦ Direct flow of metal into mold space minimizes ____
• Casts are done under force, therefore, it is important to make sure wax pattern
is well ____ the top of the casting ring
reservoir
turbulence
length
thickest
connector
turbulence
below
Spruing
Requirements
Custom
____ sprues
Wax
____
prefabricated
plastic
Direct Sprues
Single and small multi- units
Easy to use
Constricted Sprues
____ density alloys)
Flared Sprues
____ density alloys
• If you are casting ____ based alloys (high density), the point where the sprue is
attached must be flared (wider attachment point)
• When you are casting low-density alloys, use a constricted sprue
• We are not sure why/how constricted/flared spues work for low density and
high density alloys respectively
◦ No physical laws that explain why this works
low
high
gold
Direct Sprues
Suck back ____ in a direct sprued casting
• Suck back porosity happens because the metal in the sprue ____ before the metal in the desired casting
◦ A part of the metal is sucked back into the sprue pin
porosity
froze
Indirect Sprues \_\_\_\_ melting/\_\_\_\_ density PFM alloys Exposed button will solidify \_\_\_\_ \_\_\_\_ serves as reservoir \_\_\_\_ wax patterns against distortion \_\_\_\_ flow of metal to all parts of the casting
• Typically used for ____ alloys, can also be used with noble
– base alloys are high melting
• This area is an exposed area, so the metal will solidify very quickly
◦ This is why the use of the runner bar is important for high melting alloys ◦ Indirect sprues help resist ____ more than direct
• FYI: he uses the terms “reservoir,” “runner bar,” and “connector bar” interchangeabl
high low rapidly runner bar stabilizes equalizes base distortion
Indirect Sprues
Place pattern in a ____ position to ensure that all parts of mold fill simultaneously
Facilitate orientation with ____ marker on crucible former
• Important to place pattern vertically in casting
◦ The runner bar is aligned ____ in casting ring, giving a more even
distribution of metal into mold space
vertical
wax
vertically
Flow of Alloy During Casting
Indirectly sprued PFM patterns
Orientation mark aligns connector bar vertically in casting ring
Flow of molten metal in connector raises ____ of surrounding investment
temp
Flow of Alloy During Casting
Heat around connector keeps it molten ____
Orientation dots to position connector bar vertically in casting machine
• Some people describe the location of the runner bar as the heat center
longer
VENTING Indications \_\_\_\_ castings High \_\_\_\_ and Basemetal alloys \_\_\_\_ investments
Purpose
Allow rapid escape of ____ from mold cavity
• There is gas present in space that molten metal is flowing into
◦ If you are using a porous investment (____ bonded is very porous), gas
can disperse through porosity of material ‣ Venting is not needed with ____
◦ Force fed bonded and ____ bonded tend to be dense and they do not have a mechanism to disperse of gases
‣ If you are doing a large casting of high palladium and base metal alloys, and you are using ____ investments, you must provide another mechanism to dispose gases from the cavity
large
Pd
dense
gases
gypsum
gyupsum
silica
dense
Vents
Provide ____ for outflow of gases from dense investment
Investment porosity
Location of wax pattern
channels
LINERS
Purpose
Provide room for investment ____
Materials
____
organic cellulose material (paper)
____ fiber (Aluminum silicate) ceramic/cellulose combination
Reason for liner: acts as a cushion allowing for expansion
• If liner is soaked in water, it provides water with ____ expansion
expansion
asbestos
silica alumina
hygroscopic
LINERS
Keep asbestos substitutes short
____ mm short of ring
to secure investment
to produce a more uniform expansion
Ring less casting
• Liner is kept short of top of ring
◦ This locks investment into the ring
• If too long, investment can be propelled out of the ring when heating starts
◦ ____ protocol
• Keeping it short and filling it with water produces a more uniform expansion of the investment
• Ring less casting
◦ Shape a piece of wax to look like a ring, place on sprue former and pour
investment into it
‣ Wax is removed when investment is set
‣ This removes any ____ to the expansion of the investment
• Can increase investment ____ dramatically
3.0
safety
restriction
expansion
Investing
Apply ____ reducing agent
Weigh Wax pattern to determine weight of ____ needed
• Next step, we are going to invest the whole thing
◦ First, we apply a surface tension reducing agent
‣ A surfactant, allows investment to flow and helps prevent air ____ from forming
‣ ____ water can be used
surface tension
alloy
bubbles
soap
INVESTING
Gypsum bonded investments: ____oC (1300oF)
Investments used \_\_\_\_ and silicate investments Mix under \_\_\_\_ to eliminate bubbles \_\_\_\_ investments preferred Dense High strength Smooth surface Vents
• We will now talk about materials used for investing
• Once you exceed 700 with gypsum, it begins to break down
◦ If it breaks down, the metal casting will get messed up
◦ We do not use gypsum bonded investment for PFM alloys
• Phosphate and silicate are the best investments to use
• Must mix under vacuum to eliminate air bubbles
• [reads rest of slide]
700
phosphate
vacuum
thicker
Burnout of the Investment Mold
Mold ____ out during storage (store investment in a ____)
slow heat from a ____ furnace
Rapid heat materials
ensure complete elimination of all ____ by long enough burn out
• We have coated our wax pattern with surface tension reducing agent
• Before the investment has completely set, you want to put it in oven
◦ Rapid heat materials can be heated rapidly without ____
• Very important to follow the manufacturers instructions because you want to
make sure there is no residual wax in the investment
◦ Heat it for long enough to get rid of all ____ during the burnout process
dry humidor cold wax fracturing wax
Investment Burn-out
Phosphate Investment
Heat soak ____oC for 1 hour
Excessive heat soaking
____ of investment - ____ cast surface
- The investment is a ceramic material
- If you over-heat, it will sinter (fuse together) and form a very ____ surface
700-760
sintering
rough
Crucibles
____ -alumina
Quartz
____ coded for different alloys
• Gold pellets are heated in a crucible
• Molten metal flows through hole in crucible and into the casting ring
• Materials used for crucibles: ____, quartz
◦ Do not cross ____! Don’t used the same crucible with different alloys
zirconia
color
zirconia-alumina
contaminate
MELTING
Melting/Casting Temperature \_\_\_\_oC higher Ceramic alloys around \_\_\_\_oC Phosphate and silicate investments Evaporation of \_\_\_\_ melting constituents during melting Reusing buttons
• Alloys melt over a ____ of temperatures
• Want to heat alloy to above the solidus temperature
◦ Typically 100-200 degrees higher than liquids temperature to be sure everything is melted
• There is a metal button cast at the sprue base
◦ This has been heated to the same temp as the ____
◦ For ____ metal alloys, you cannot reuse this button because the
composition has changed too much
◦ For ____ metal alloys, they can be reused
100-200
1300
low range casting alloy base noble
Heat Sources for Melting Alloys
____ Melting ____ Heating
____ Melting
Torch
Electrical
Arc
Torch Melting
Gas/air Blow pipe for melting alloys
Zones of melting flame
Gas (propane)/oxygen (\_\_\_\_0oC) Liable to \_\_\_\_ \_\_\_\_ of alloy Excessive oxidation of \_\_\_\_ metals
• The mixing zone is very ____
◦ Oxygen and gas mixing together
• Next zone is the combustion zone, looks ____
• Next zone is reducing zone, ____
◦ This is the ____ you want to put on the metal to get the most amount of heat
• Need a hot flame when casting PFMs
◦ Typically propane/oxygen mixtures are used
• If you overheat, you change properties of the alloy
1800-2500
overheating
evaporation
base
dark
yellowish
blueish
zone
CASTING
Centrifugal casting Centrifugal force proportional to \_\_\_\_ of casting arm \_\_\_\_ of metal Extra turns for \_\_\_\_ density alloys
No flux for ____ & ____ metals
may alter compositions and affect bond strength
• Centrifugal force is proportional to the ridges of the casting arm
• High density alloys will flow easily into the casting space
• Low density alloys require a faster centrifuge to increase the force
• Flux helps to remove the oxides on the surface of noble/gold based alloys
◦ Flux will contaminate alloy if using silver-palladium and base metal alloys
‣ Reduces bond strength to porcelain
radius mass low Ag-Pd base
Vacuum/pressure Casting
Melt alloy to prescribed \_\_\_\_ Place preheated nvestment over crucible Apply \_\_\_\_ Turn casting chamber 180o, apply \_\_\_\_
• Another method of casting
• Once metal is melted, the chamber is flipped [bottom pic] and ____ pulls the
molten metal into the cast
◦ You then introduce high pressure gas so the mold space is filled
adequately
temperature
vacuum
pressure
gravity
Cleaning PFM Alloys
- sandblast PFM alloys
- gold based alloys are pickled in ____
• Once you have your casting and it is cool, you break it out of the investment
• Gold based alloys cleaned in a pickleing solution
◦ Typically diluted ____ ◦ For ____ based alloys ONLY
acid
sulfuric acid
gold
Casting Defects
Distortion - ____ pattern Fins - cracked ____ Surface roughness - grain
____ of investment
Bubbles - excess ____, air
• Problems that can occur during casting process
• If your sprue wax pattern is not ____ and rigidly attached, you can get
distortion
◦ Any deviation of wax pattern from its position in the investment, you can
get distortion
• Heated too quickly = cracks in investment
◦ You get fins on your casting where mental flowed into cracks
• Over heated ceramic can fuse together and give a rough casting
• Bubbles on surface of casting
wax
investment
size
debubblizer
stable
Casting Defects
Underheating
- ____ casting
____ coating
Prolonged heating ____ of investment
Foreign bodies
Porosity
• Carbon coating can occur with under and over heating depending on type of investment being used
◦ Some investments contain carbon
• Prolonged heating will result in a rough surface
• Must work in clean environment to avoid ____ in the investment
incomplete
carbon
disintegration
foreign bodies
Casting Defects
Adequate Reservoir
No ____ cast
Inadequate reservoir ____ is cast Inadequate ____
• The connector bar was not adequate enough and we have incomplete casting
• Adequate reservoir, there is no button cast, so most of the metal has flowed
into the space to give us a good casting
button
button
casting
Poor Alloy Castability
Incomplete / rounded margins due to poor ____ castability
• If your alloy doesn’t flow well when it is heated, you can have rounded margins
• The base metal alloys contain Beryllium
◦ Beryllium is present to improve ____ of metal and to avoid ____ edges in casting
alloy
flow
rounded
Casting Defects
Nodule: ____ trap during investing
Inadequate ____ during mixing
Prolonged ____
Shrink spot porosity
Long thin sprue bulky ____
____ button
- If you don’t use a vacuum, bubbles can form
- Prolonged vibration during investing can also produce bubbles
- Inadequate sprueing can result in a ____ spot porosity in your casting
air vacuum vibration attachment small shrink
- Impurities a.k.a. foreign bodies. Work in a ____ environment to avoid this
- Fins are formed when there are ____ in the investment
- Blunt margins = poor ____
- Carbon can occur from both excessive ____ and under ____
clean cracks flow heating heating
INVESTMENT MATERIALS FOR METAL-CERAMIC RESTORATIONS
____ Bonded Investments
____ bonded investments
- We will end by mentioning a few properties of the 2 types of investment materials used in PFM alloys
- Silicate is used for much ____ temperature alloys than phosphate bonded
phosphate
silicate
higher
Phosphate bonded investments
Two types
Carbon Containing - ____ of divesting
Carbon contamination of alloy makes alloy ____
____ containing
• 2 types of phosphate bonded ◦ Carbon containing
‣ When heating, the loss of carbon makes it easy to break the casting out of the investment (divesting)
◦ For some alloys, especially base metal alloys, carbon is going to make the alloy brittle
◦ Non carbon containing are typically used for ____ metal alloys
ease
brittle
non-carbon
base
Phosphate Bonded Investments Composition
Binder
Mono ____ 8-12% ____ 8-10%
Refractory
____ + Cristobalite 30-50%
____ only (high temperature ____)
Setting agent/Reaction ____
Binder + setting agent –> ____ strength Setting ____
• The binder here is ammonium phosphate and magnesium oxide mixed together
• There is a refractory, quartz and cristobalite
◦ The role of the refractory is to provide for thermal ____
‣ When we heat our investment, the refractory component of the investment (it is either quartz or cristobalite)
• The setting agent is a colloidal silica that reacts with the ammonium phosphate the magnesium oxide
• The binder and the setting agent react to give us an early green strength and some setting expansion
ammonium phosphate magnesium oxide quartz quartz burnout
colloidal silica
green
expansion
burnout
Setting Expansion
Usual silica sol:water ratio = ____
More silica sol and less water = more
____
Less silica sol and more water = less ____
- If you want to increase the expansion, increase the amount of silica present
- To decrease, decrease amount of silica
3:1
expansion
expansion
Expansion
____ expansion ____ expansion Thermal expansion
Inversion of cristobalite and ____
α —-> ß (200-270oC)
Max burnout temp. - ____oC
• The main way of obtaining our investment expansion is by thermal ____
• Cristobalite and quartz are different forms of ____ (SiO2)
◦ There is a transformation from alpha form to ____ form, creating an increase in ____
setting hygroscopic quartz 900-1000 expansion silica beta expansion
Thermal expansion of quartz and crystobalite
Crystobalite
Max expansion above ____oC
Quartz
Max expansion
above ____oC
• This graph shows the difference in thermal expansion between crystobalite and quartz
◦ Crystobalite reaches maximum expansion at ____ temp than quartz
‣ If you are using a high melting alloy, you will probably have ____
present providing the maximum expansion
• IT IS IMPORTANT TO KNOW THAT CRYSTOBALITE AND QUARTZ ARE
BOTH ____ FORMS OF SILICA (he emphasized this)
400 600 lower quartz allotropic
Mold expansion
• The main method of obtaining expansion is by ____ EXPANSION
thermal
Handling of Phosphate Bonded Investments
Poor surface ____
Greater likelihood of trapping ____
Use ____ or open investing
Allow investment to set in ____ pot
coat surface of wax pattern with ____ before pouring investment into ring
• When we are applying phosphate bonded investments to our wax, we need wettability, important to use ____ reducing agent
◦ Don’t overuse (could cause bubble
wettability
bubbles
vacuum
pressure
investment
surface tension
Silicate bonded investments
Binder
____
Refractory
____ only (Coarse grained)
Setting agent
____- promotes gelation setting reaction- polymerization of ____
• Silicate investment binder is a ethylorthosilicate
• Remember Silicate bonded investments are much ____ temp investments
◦ They only have quartz for thermal expansion
• The setting agent is magnesium oxide
◦ This reacts with ethylorthosilicate to provide a ____
‣ The gel is basically polymerized silicic acid when the reaction is
completed
ethylorthosilicate
quartz
magnesium oxide
silicic acid
higher
gel
Expansion
____ expansion only α Quartz—> ß Quartz
Max burnout temp. ____oC
• The main method of expansion for silicate bonded investment is ____ expansion (He said this many times)
◦ Alpha quartz to beta quartz
thermal
1100-1300
thermal
Electrical Heating
____ heating
____ heating ____ arc
Electrical heating systems
Resistance heating
with
____ casting unit
resistance
induction
electric
centrifugal
Electrical heating systems
Induction Heating
____ cooled induction coil
____ crucible positioned within induction coils
water
vertical
