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Dental casting alloys and machines Flashcards

1
Q

What is the procedure of dental casting?

A
  1. wax pattern
  2. spruing
  3. investing
  4. burnout
  5. casting
  6. recovery and cleaning of casting
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2
Q

What is an alloy?

A

a metal containing two or more metals, at least one of which is a metal and all of which are mutually soluble in a molten state

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3
Q

What are the biological requirements of dental casing alloys?

A
  • should not cause toxicity, allergy or even irritation
  • should resist the degradation in oral fluids and is achieved by the use of noble alloy and using of base metal alloy with the passive layer (acts as corrosion barrier because it is thin, non porous, adherent & transparent surface oxide layer protecting the underlying metal)
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4
Q

What are the requirements of dental casting alloys interfacially and chemically?

A
  • casting alloys should have low surface energy to reduce the plaque attachment
  • if the alloy is going to be covered with porcelain should be able to form surface oxide layer
  • the alloy should be resistant to both tarnish or corrosion (nobility and passivity)
  • alloy surface should not be affected by the oral environment (show no pitting)
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5
Q

What are the mechanical requirements of dental casting alloys?

A
  • high strength to resist the permanent deformation or even fracture during service
  • ductility where burnishing and marginal closure are needed
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6
Q

What are the working requirements of dental casting alloys?

A
  • ease of casting
  • high density to obtain complete casting with low casting force
  • low melting temperature to use simple melting technique
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7
Q

What is the classification of different casting alloys according to yield strength and hardness?

A
  • type I: soft
  • type II: medium
  • type III: hard
  • Type IV: extra hard
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8
Q

What is the classification of different casting alloys according to nobility?

A
  • high noble metal alloys: contains ≥ 40 wt% Au and ≥ 60 wt% noble metals as gold alloys and low gold containing alloys
  • noble metal alloys: contains ≥ 25 wt% of noble metals as Pd-Ag alloys (silver -palladium)
  • base metal alloys contains < 25 wt% of noble metals as Co-Cr alloys, Ni-Cr alloys and titanium alloys
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9
Q

What is the classification of different casting alloys according to major element?

A
  • gold alloys
  • silver alloys
  • palladium alloys
  • nickel alloys
  • cobalt alloys
  • titanium alloys
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10
Q

What are the characteristics of high noble alloys?

A
  • pure gold is a soft and ductile metal
  • dental casting golds are alloyed commonly with copper, silver, platinum, nickel and zinc
  • alloying gold with these metals not only improves its physical and mechanical properties but also reduces its cost
  • casting shrinkage in gold alloys is less (1.25 to 1.65%) when compared to base metal alloys
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11
Q

What is the function of gold in gold alloys?

A
  • provides tarnish and corrosion resistance
  • provides ductility and malleability
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12
Q

What is the function of copper in gold alloys?

A
  • principal hardener
  • reduces the melting point and density of gold
  • gives the alloy a reddish color
  • in greater amounts, it reduces resistance to tarnish and corrosion of the gold alloy (maximum content should not exceed 16 percent)
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13
Q

What is the function of silver in gold alloys?

A
  • whitens the alloy thus helping to counteract the reddish color of copper
  • increases strength and hardness slightly
  • in large amounts, it reduces tarnish resistance
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14
Q

What is the function of platinum and pallidum in gold alloys?

A
  • increases strength and corrosion resistance
  • increases melting point and has a whitening effect on the alloy
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15
Q

What is the function of zinc in gold alloys?

A
  • it acts as a scavenger for oxygen - without zinc, the silver in the alloy causes absorption of oxygen during melting and later during solidification, the oxygen is rejected producing gas porosities in the casting
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16
Q

What is the function of tin, iron and indium in gold alloys?

A

they help to harden the ceramic gold-palladium alloys, iron being the most effective

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17
Q

What i the heat treatment of hold alloys?

A
  • done in order to alter its mechanical properties
  • a method of controlled temperature handling of metals so as to change the microscopic structure and thus the physical properties
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18
Q

What are the types of heat treatments for gold alloys?

A
  • softening heat treatment (annealing)
  • hardening heat treatment (aging)
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19
Q

What is softening heat treatment (annealing)?

A
  • method: the casting is placed in an electric furnace for 10 minutes at 700°C, and then it is quenched in water
  • the tensile strength, proportional limit, and hardness are reduced by such a treatment, but the ductility is increased
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20
Q

What is hardening heat treatment (aging)?

A
  • method: done by “soaking” or ageing the casting at a specific temperature for a definite time, usually 15 to 30 minutes and then cooled slowly
  • ideally, before age hardening an alloy, it should first be subjected to a softening heat treatment in order to relieve all strain hardening (stresses which occurs during finishing)
  • hardening heat treatment increases the strength, proportional limit, and hardness and reduces ductility of alloys
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21
Q

What are the indications of softening heat treatment?

A
  • before hardening heat treatment
  • increase workability of alloy
  • for structured to be cold worked (shaped or ground)
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22
Q

What is the indication of hardening heat treatment?

A
  • increase strength of cold worked alloys
  • decrease wettability of alloy
23
Q

What is the technique of hardening heat treatment?

A
  1. softening heat treatment (relief all sresses and start at sidordered structure)
  2. heating alloy at 700 degrees celcius = bench cooling
  3. heating alloy at 450 degrees celcius = bench cooling to form 450 degrees to 250 = quenching
  4. maintain alloy between 350-450 degrees for 15 minutes = quenching
24
Q

What are silver-pallidum (noble metal alloys)?

A
  • introduced as a cheaper alternative to gold alloys
  • predominantly silver in composition
  • the properties of the silver-palladium alloys are similar to gold alloys but they have lower ductility and corrosion resistance
  • they also have a significantly lower density than gold alloy which may affect its castability
25
Q

What are base metal alloys?

A
  • extensively used
  • were introduced as a cheaper alternative to the very expensive noble metal ceramic alloys
  • very strong , hard and half the density of the gold alloys making them much lighter
  • twice as stiff as the gold ceramic alloys. which means that we can make thinner, lighter castings or use it in long span bridges (gold alloys require a minimum thickness of at least 0.3 to 0.5 mm, whereas base metal alloys copings can be reduced to 0.3 mm some even claim 0.1 mm)
  • porcelain bonding: these alloys form an adequate oxide layer which is essential for successful porcelain bonding. These alloys are highly resistant to tarnish and corrosion. This is due to the property known as passivation
26
Q

What is passivation?

A

property by which a resistant oxide layer forms on the surface of chrome containing alloys. This oxide layer protects the alloy from further oxidation and corrosion

27
Q

What are metal ceramic alloys?

A

alloys that are compatible with porcelain and capable of bonding to it

28
Q

What are the requirements of alloys for porcelain bonding?

A
  • its melting temperature should be higher than porcelain firing temperature
  • should be able to resist creep or sag at these temperatures
  • its CTE should be compatible with that of porcelain
  • should be able to bond porcelain
  • should have high stiffness (modulus of elasticity) as any flexing of metal framework might cause porcelain to fracture
  • should no stain or discolor porcelain
29
Q

Why is a heat source needed for casting?

A

melt alloy

30
Q

Why is a casting force needed for casting?

A

force molten alloy into mold

31
Q

What is the casting force?

A

surface tension of alloy + resistance offered by gas in the mold

32
Q

What is the heat source of casting?

A
  • torch fo flame (for low temperature metals)
  • electricity (for high temperature metals)
33
Q

What are the characteristics of torch flame?

A
  • for low temperature metals
  • mixture of natural/artificial gas, oxygen/tank gas –oxyacetylene
    -less faster than electric heating but more faster than resistance heating
34
Q

What are examples of torch flames?

A
  • gas air
  • gas oxygen
  • air acetylene
  • oxygen acetylene
  • hydrogen oxygen
35
Q

What are the types of torch tips?

A
  • multi-orifice
  • single-orifice
36
Q

What are the zones of blow torch flame?

A
  • zone 1: colourless zone (non combustion zone)
  • combustion zone
  • reducing zone
  • oxidising zone
37
Q

What is the mixing zone?

A
  • dark in color
  • no heat present
38
Q

What is combution zone?

A
  • green in color
  • zone of partial combustion and has oxidising nature so shouldbe kept away from molten metal
39
Q

What is reducing zone?

A
  • dim blue in colour
  • hottest part pf flame
  • used for fusion of casting alloy
40
Q

What is oxidising zone?

A
  • has lower temperature than reducing zone
  • oxidises the metal so should never be used for melting alloys
41
Q

What are the types of electricity heat?

A
  • arc melting
  • electrical induction melting
42
Q

What is arc melting?

A
  • is produced between 2 electrodes
  • the temperature within the arc exceeds 4000˚c, the alloy melts very quickly
  • disadvantage: alloy can become overheated
43
Q

What is electrical induction melting?

A
  • the electric induction furnace is a transformer in which an alternating current flows through the primary winding coil and generates a variable magnetic field in the location of the alloy to be melted in a crucible
  • it is more commonly used for melting base metal alloys not been used for noble alloy casting
44
Q

What are the types of casting force?

A
  • centrifugal force
  • ## vaccuum force
45
Q

What is centrifugal force?

A
  • makes use of centrifugal force to thrust the liquid metal into the mold
  • the method involves pouring molten metal into a cylindrical mold spinning about its axis of symmetry
  • the mold is kept rotating till the metal has solidified
46
Q

What is vaccum force?

A
  • used for titanium and its alloys
  • temperature at 1668 degrees
  • centrifugal force is not sufficient so vacuum is used
  • melting is done under electric arc and casting is done under argon gas pressure
  • vaccum is applied through the base beneath the casting ring and the molten alloy can be drawn into the mold by negative pressure
47
Q

What is divesting?

A

refers to removal of casting from the investment mold

48
Q

What is sandblasting?

A

casting is held in a sandblasting machine to clean the remaining investment from its surface

49
Q

What is done during cleaning of casting?

A

after the casting has solidified, the ring is removed and quenched in water

50
Q

What is pickling?

A

heating a discolored casting in an acid (used for cleaning gold casting)

51
Q

What should be masked in castings during pickling?

A

dark/tarnished appearance of adherent oxide

52
Q

What solution is used for pickling?

A
  • (50%) dil HCL, 2. (50%) dil sulphuric acid
  • Others: ultrasonic device
53
Q
A

crown (6 decks)

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