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casting errors Flashcards

1
Q

What is casting?

A

the process of melting the metal alloy and forcing it quickly in a mold cavity

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2
Q

What can be used to melt alloys?

A
  • blow torch
    electricity
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3
Q

What are the types of blow torch?

A
  • simple gas air torch (pr and semi-pr)
  • gas oxygen torch (multi-orifice, base metal alloy)
  • blowpipe flame
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4
Q

What are the zones of blowpipe flame?

A
  • 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)
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5
Q

What isthe use o reducing zone?

A

melt the alloy as it is the hottest and prevents the formation of oxides

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6
Q

What are the types of electricity melting?

A
  • conventional heating muffle
  • induction current
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7
Q

What is a flux?

A

reducing agent that is added to prevent oxide formation; increase fluidity of alloy; and reduce the melting temp of the alloy.

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8
Q

What is flux used for?

A

precious alloys; borax is most commonly used

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9
Q

What are casting machines?

A
  • air pressure technique
  • vapor pressure technique
  • centrifugal casting technique
  • vacuum casting technique
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10
Q

Wha is done for recovery of castings for precious alloys?

A
  • 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
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11
Q

Wha is done for recovery of castings for base metals?

A
  • ring cooled to room temp, then broken manually
  • sandblasting with Al2O3 particles under pressure
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12
Q

How is finishing and polishing done?

A
  • 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
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13
Q

What might cause no casting?

A
  • 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)
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14
Q

What causes frayed margins?

A
  • 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
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15
Q

What are frayed margins (fins)?

A

these represent cracks in the investment that has been filled by the molten alloy

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16
Q

What are air bubbles or nodules?

A
  • consist of trapped air between the wax pattern and investment
  • will prevent seating of restoration
17
Q

What are the causes of bubbles and air nodules?

A
  • trapped air during investing
  • inadequate vacuum during investment mixing
  • lack of debubblizer (surfactant)
  • can be removed if not at critical areas
18
Q

What causes rough surface on casting?

A
  • rough wax pattern
  • excessive wetting agent or investing before agent is dry
  • investment material has large grain size
19
Q

What are the causes of incomplete casting?

A
  • inadequate amount of alloy
  • inadequate heating of alloy
  • inadequate casting pressure
  • incomplete wax burnout
  • too long sprue
  • cold ring
20
Q

Where can incomplete casting occur and what causes it?

A
  • margins
  • rough: foreign body
  • convex: insuffiecent casting pressure/cold ring
  • concave: trapped air from less venting/ incomplete wax burnout
21
Q

What causes perforated castings and where is ti found?

A
  • at sprue: insufficient alloy
  • at thin areas (veneering areas) isufficient accessory sprues
22
Q

What causses back pressure porosity?

A
  • low casting pressure (not enough to push air)
  • not enough venting
  • not enough porosity of investment
    short sprue
23
Q

What causes gas inclusion porosity?

A
  • blow torch (not using reducing zone)
  • over heating of alloy (boiling)
  • vapors from overheating of investment
24
Q

What causes shrink spot porosity?

A

shrinkage of a portion of the metal as it solidifies from the molten state without flow of additional metal from the surrounding areas

25
Q

Wha causes suck back pressure porosity?

A

occurs when the metal in the sprue cools before the metal in the mold

26
Q

What is the cause of subsurface porosity?

A
  • dangerous as it only appears during the finishing stage
  • due to cold mold where the superficial metal cools and compensates from the inside part
27
Q

How to prevent oversized and undersized casting?

A
  • 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
28
Q

What causes discolored castings?

A
  • 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

crown (6 decks)

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