casting errors Flashcards
What is casting?
the process of melting the metal alloy and forcing it quickly in a mold cavity
What can be used to melt alloys?
- blow torch
electricity
What are the types of blow torch?
- simple gas air torch (pr and semi-pr)
- gas oxygen torch (multi-orifice, base metal alloy)
- blowpipe flame
What are the zones of blowpipe flame?
- mixed zone: colourless and pure gas
- consumption zone: mixture of gas and air
- reducing zone: the hottest zone (light blue)
- oxidizing zone: the outer zone (red)
What isthe use o reducing zone?
melt the alloy as it is the hottest and prevents the formation of oxides
What are the types of electricity melting?
- conventional heating muffle
- induction current
What is a flux?
reducing agent that is added to prevent oxide formation; increase fluidity of alloy; and reduce the melting temp of the alloy.
What is flux used for?
precious alloys; borax is most commonly used
What are casting machines?
- air pressure technique
- vapor pressure technique
- centrifugal casting technique
- vacuum casting technique
Wha is done for recovery of castings for precious alloys?
- the ring is quenched when the red glow of the button disappears
- this helps disintegrate the investment
- pickling in warm HCl solution to remove black oxide layer
Wha is done for recovery of castings for base metals?
- ring cooled to room temp, then broken manually
- sandblasting with Al2O3 particles under pressure
How is finishing and polishing done?
- using uncontaminating stones and disc the sprue and button are removed
- polishing is done using rubber wheels, with the help of iron rouge
- proper finishing and polishing decreases plaque accumulation and decreases tarnish and corrosion
What might cause no casting?
- sprue blockage: investment fragments/metal sprue not removed
- molten metal not directed to mold
- premature metal solidification: cold ring/incomplete heating of alloy/too wide sprue which allows alloy to enter and solidify
- low casting pressure
- fracture of the investment at the base (too thin layer)
What causes frayed margins?
- dropped investment
- increases W/P ratio
- too rapid/over heating of investment
- pattern placed too close to base (easily cracked)
- fins can be removed, remake if at margin
What are frayed margins (fins)?
these represent cracks in the investment that has been filled by the molten alloy
What are air bubbles or nodules?
- consist of trapped air between the wax pattern and investment
- will prevent seating of restoration
What are the causes of bubbles and air nodules?
- trapped air during investing
- inadequate vacuum during investment mixing
- lack of debubblizer (surfactant)
- can be removed if not at critical areas
What causes rough surface on casting?
- rough wax pattern
- excessive wetting agent or investing before agent is dry
- investment material has large grain size
What are the causes of incomplete casting?
- inadequate amount of alloy
- inadequate heating of alloy
- inadequate casting pressure
- incomplete wax burnout
- too long sprue
- cold ring
Where can incomplete casting occur and what causes it?
- margins
- rough: foreign body
- convex: insuffiecent casting pressure/cold ring
- concave: trapped air from less venting/ incomplete wax burnout
What causes perforated castings and where is ti found?
- at sprue: insufficient alloy
- at thin areas (veneering areas) isufficient accessory sprues
What causses back pressure porosity?
- low casting pressure (not enough to push air)
- not enough venting
- not enough porosity of investment
short sprue
What causes gas inclusion porosity?
- blow torch (not using reducing zone)
- over heating of alloy (boiling)
- vapors from overheating of investment
What causes shrink spot porosity?
shrinkage of a portion of the metal as it solidifies from the molten state without flow of additional metal from the surrounding areas
Wha causes suck back pressure porosity?
occurs when the metal in the sprue cools before the metal in the mold
What is the cause of subsurface porosity?
- dangerous as it only appears during the finishing stage
- due to cold mold where the superficial metal cools and compensates from the inside part
How to prevent oversized and undersized casting?
- to prevent this, attention must be placed during the whole casting process, from choosing the correct investment material; p/w ratio; heating of investment etc
- undersized casts can be stripped from the fitting surface
What causes discolored castings?
- occurs when there is decomposition of the gypsum bonded investment, releasing S which combines with Cu and Ag of gold alloy
- this compound resists pickling and remake is the only solution
