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
