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55 terms

Dental Casting Alloys (I)

Test #1
What are the steps in dental casting?
Tooth prep>impression of prep>master die prep>wax pattern fabrication>investment of pattern>burnout>casting>pickling, finishing, and polishing.
What was the 1984 method of classifying casting alloys?
By nobility
What is the definition of a high noble (HN) alloy?
Contains > or = 40% wt gold and > or = 60 wt% noble metals.
What is the definition of a noble alloy?
Contains > or = 25 wt% noble metals, but < HN
Predominantly base (PB) alloy?
Contains < 25 wt% noble metals.
What are the noble metals?
Au, Pt, Pd, Ir, Os, Rh, Ru, and Ag (Ag isn't in dentistry because in corrodes in the mouth).
How are dental casting alloys typified now?
By use and properties.
What is a Type I casting alloy?
Soft, low strength, used in low stress bearing situations like small direct inlays. easily burnished.
What is a type II casting alloy?
Medium hardness and strength, used in moderate stress bearing applications (3/4 crowns, pontics, some full crowns, inlays subject to moderate stress).
What is a type III casting alloy?
Hard, high strength, heat treatable, used in high stress-bearing applications: short-span fixed partial dentures, thin 3/4 crowns, pontics, full crowns, abutments, onlays, inlays.
Type IV casting alloy?
Extra hard, extra high strength, heat treatable, low elongation, used in very high stress-bearing applications.
What is the trends for classification of casting metals for prostheses and partial dentures?
High Nobles have Au
Nobles have Ag for all metal and Pd for M-C, and no Partial deture frameworks.
PB's hare CP, Ti, Ni, Co, Cu.
What is a karat?
Number of parts of pure gold in 24 parts of an alloy.
What is fineness?
Number of parts of gold in a thousand parts of an alloy.
How is metal content in an alloy described now?
percentage of composition.
What does gold do for HN alloys?
Resistance to tarnish and corrosion, ductility, high specific gravity, and characteristic color.
What does platinum do for HN alloys?
Hardens alloy, raises melting point, increases corrosion resistance.
What are the constituents of an HN alloy?
Sn, Zn, Cu, Ag, Pd, Pt, Au
What does palladium do for HN alloys?
reduces corrosion of silver-containing alloys, imparts white color, less expensive than platinum, improves strength.
What does silver do for HN alloys?
Hardens, whitens, reduces cost, corrodes easily.
What does Zn do for an HN alloy?
oxygen scavenger, improves castability of alloy.
What does Copper do for a HN alloy?
Principal hardener because it distorts gold structure causing increased dislocation pile ups and hence INCREASED hardness and strength, LOWERS corrosion resistance, imparts reddish color, responsible for age-hardening.
What does Ir and Ru traces do for a HN alloy?
Grain size refinement
What does Mo and W do for a HN alloy?
Decrease thermal expansion coefficient.
Fe, In, and Sn in HN alloy?
Increase potential of metal surface to form oxides.
The hardness, tensile stress, and strength __________ from type I to type IV HN alloys.
The ductility and corrosion resistance ____________ From type I to type IV HN alloys.
What are the two types of casting shrinkage?
1.Solidification shrinkage
2. "Thermal contraction" of solid metal to room temperature. Typically 1.5% in type III's. Must be compensated for.
Casting shrinkage decreases with gold content, while solidification shrinkage
What properties of the cast are reduced and increased by softening heat treatment?
Reduced: tensile strength, proportional limit, and hardness
Increased: Ductility
What is another term for hardening heat treatment?
Age hardening
What is the result of hardening heat treatment?
Increased tensile strength, proportional limit, and hardness.
Reduced ductility.
Is hardening treatment indicated for small structures such as inlays?
no, but it is for partials, bridges, saddles, etc.
What kind of block do you use when melting pickled HN alloy scrap for reuse?
How much new alloy do you add when planning to reuse HN alloys?
An equal amount.
Should different types or brands of alloys be mixed in the reuse step?
What are the three steps in reuse of HN alloys?
1. Melt clean picked scrap on a charcoal block.
2. Add equal amount of new alloy to make a casting, as this restores burnt-out elements
3. When melting alloys, different types or brands of alloys should not be mixed.
Ag-Pd alloys with higher Ag content have properties similar to type ____ gold alloys
Ag-Pd-Cu alloys with lower Ag content have properties similar to type ____ gold alloys but are more susceptible to tarnish and corrosion due to their copper content
What are high palladium noble metal alloys almost exclusively used for?
Crown and bridge
high palladium noble metal alloys offer a ___________________ between high noble gold and base metal alloys because they have density midway between that of base metal and high gold alloys and cost between half to a third of the price of gold alloys.
What are the percentage differences and property differences between Ag-Pd alloys and Pd-Ag alloys?
Ag-Pd: 60-72 wt% Ag and >= 25 wt%.
Pd-Ag: 53-61 wt% Pd and 28-40 wt% Ag. Traces of tin or indium are added to increase hardness and promote porcelain bonding by oxide formation. If Pd is replaced by silver, lower MP, increased coefficient of thermal contraction, and increased porcelain discoloration.
What does greening mean?
Refers to the greenish-yellow tint resulting from the escape of silver vapor from surface during porcelain firing followed by the diffusion of the ion into the porcelain causing reduction of the silver on the porcelain to form colloidal metallic silver.
What factors accentuate greening?
Ag content of the alloy, Na content of the porcelain, number of firing cycles, use of a vacuum during firing, and lighter shade porcelain.
What helps stop greening by Pd-Ag alloys?
gold metal conditioners, ceramic coating agents, and non-greening porcelains.
What are economical alternaitives to high gold alloys?
Pd-Ag alloys.
What is the composition of Pd-Cu alloys?
74-80% wt% Pd and 9-15 wt% Cu and sometimes include 2 wt% Au.
What is the esthetic issue with palladium-copper alloys?
Copper content may cause porcelain discoloration by formation of a dark oxide layer, which can be masked by the application of opaque porcelain.
Are the Pd-Cu alloys easy to adjust and burnish?
No, they have high yield strengths and hardness and low elongation values.
What else are Pd-Cu alloys susceptible to?
Creep deformation (SAG) at high porcelain firing temps due to their low melting range. Very technique sensitive!
Pd-Co alloy composition?
78-88 wt% Pd and 4-10 wt% Co.
What are the most sag resistant of all noble metal alloys?
Do palladium-cobalt alloys form dark oxides?
Yes, but do not require metal cotaing agents because the can be masked by the application of opaque porcelain.
Pd-Ga-Au-Ag alloy composition?
75 Pd, 6 Au, 6 Ga, 6 In, 5-8 Ag.
Do they cause greening?