Lec 1/19? Investing and Casting Flashcards
Sprue former is attached to:
crucible former
Direct surround investment material in casting:
ring liner, then casting ring
investment material must be:
heat resistant
Benefit of plastic prefabricated sprues:
added rigidity, minimizes distortion
this can be used for multiple castings:
runner bar
sprue allows for:
wax to escape mold, molten metal in
is more or less turbulence desired as molten metal flows into sprue?
less
What provides a reservoir to compensate for shrinkage during solidification of casting alloy?
Metal in sprue must remain molten slightly longer than the alloy that has filled the mold
What is the crucible former usually made of?
rubber
Function of crucible former:
base for the casting ring during investment
Function of casting ring:
container for the investment while it sets and restricts the setting expansion of the mold
Technique designed to allow unrestricted expansion of casting ring:
Ringless casting ring, uses plastic casting ring
Investment to use with ringless technique:
higher strength, phosphate-bonded investment, usefull for high melting alloys that melt more
Function of ring liner:
allow more expansion, slightly compressible
What to do if you require increased setting expansion of the investment material.
Use a 2nd liner
How to increase the hygroscopic expansion of the ring liner:
wet it
TF? Wax has both high CTE and shrinkage.
T
Why remelt, readapt, and resmooth wax margin directly before investing:
wax pattern tends to release ito incorporated strain
Sprue size/gauge recommended for molar and metal ceramic patterns:
2.5mm (10-gauge)
Sprue size/gauge recommended for premolar castings and most partial-coveage restorations:
20.mm (12 gauge)
Where to attach sprue:
bulkiest, non critical part of the pattern, normally the largest non-functional cusp
Cusps for sprue, #30:
MB or ML cusps (?)
attach sprue w this:
sticky wax (goes in sprue as well)
to reinforce joint bw sprue and cusp and allow a smooth flow of molten alloy:
inlay wax
Attachment of sprue and crucible former:
wax pattern and sprue should be centered
TO avoid shrinkage porosity:
position a reservoir of the sprue at the heat center
Min distance to leave bw wax pattern and top of crucuble
6mm
lid of crucible is __mm.
3mm
is the reservoir typically closer to the crown or further from?
closer to
Base of crucible is __mm.
3
Types of investment materials:
gypsum, phosphate, or silica bonded
investment material to use with metal alloys: ADA Type II, III, and IV:
gypsum bonded
investment material to use with metal ceramic framework:
phosphate-bonded
investment material to use with high-melting base metal alloys:
silica bonded
Beauty cast is what type of investment?
gypsum bonded
Spray mounted pattern w:
surfactant, gently blow excess liquid from pattern
Trim this before burnout:
top glazed surface
What to do if mold was allowed to set more than 12h:
rewet prior to burnout, soak 1-3m
Removed this layer after burnout:
dense oxide layer
Trim this once the investment has set and before burnout:
the “skin” (glassy part) at the top of the ring, remvoe rubber crucible former, place in oven
Preheated oven for burnout is using this technique:r
apid technique
Preheat temp for most alloys:
1200’F
How long to heat the soak at final temperature for both rapid and standard techniques?
30m
standard burnout technique:
cold oven, molds in oven at room temp, heat to desired temp at 25-40’F/ m
For a complete casting, the casting force must be:
high enough to overcome the high surface tension of molten alloys, as well as the resistance of the gas within the mold.
Casting force we use:
centrifugal force
Heat sources for casting:
gas and compressed air torch, electricity
steps to use gas and compressed air torch:
light gas, then turn on air flow valve
zone within flame of torch:
unburnt gas –> burning gases (reducing atmosphere, hottest zone) –> burnt gas (oxidizing atmospheree)
Direction and number of turns for casting anything but metal ceramic:
3, clockwise
Direction and number of turns for casting metal ceramic:
4, clockwise
Steps after winding arm and placing pin:
place crucible with preheated allop in machine, pull crucible away from cradle
Function of the plux we sprinkle on alloy surface:
in fluidity, prevent or reduce oxidation (don’t add to metal ceramic alloy)
Place casting ring with molten alloy here:
in cradle
Use tongs to move:
crucible platform into contact w casting ring
How to get the pin to drop and the arm to release:
pull arm forward
Do this after red glow is gone from button
submerge casting ring in cold water, break off investment from the casting and clean w stiff brush
How to clean casting:
ultrasonically
Can cause rough casting:
excess surfactant, improper water/powder ratio, excessive burnout temp
Can cause large nodules:
air trapped during investment procedure
Can cause nodules on occ surface:
excessive vibration
Can cause multiple nodules:
inadequate vacuum during investment, improper brush technique, lack of surfactant
Can lead to fins:
inc water/powder ratio, pattern too near edge of investment, premature heating (mold still wet), too-rapid heating, dropped mold
Can cause incomplete casting:
wax pattern too thin, cool mold, or inadequate heating, not enough metal, insufficient casting force
Can cause suck-back porosity:
improper pattern position too narrow or too long sprue
