casting

what do casting machines require?

1. heat source (casting torch) to melt the alloy

2. casting force to deliver the molten alloy into the mold cavity

what can the heat sources be?

1. a torch (casting torch)

2. electricity (electric melting)

how can casting torch be classified based on the type of gas used?

1. gas air torch

2. gas oxygen torch

3. oxyacetylene torch

what can gas air torch be used for?

conventional noble alloys that have a melting temperature less than 1000°C

what can gas oxygen torch be used for?

metal ceramic alloys with a melting temperature up to 1200°C

what can oxyacetylene torch be used for?

base metal alloys with a melting temperature more than 1200°C

how can casting torch be classified based on shape of torch tip?

1. single orifice tip

2. multi orifice tip

what is a single orifice tip used for?

it provides concentrated heat and is used for gold alloys

what is a multi orifice tip used for?

it provides uniform heating and is used for base metal and metal ceramic alloys

the flame used in casting is composed of how many zones?

1. mixing zone

2. combustion zone

3. reducing zone

4. oxidizing zone

what is the mixing zone?

cool, colorless zone

what is the combustion zone?

greenish-bluish zone that oxidizes the alloy

what is the reducing zone?

the alloy is heated using the reducing zone of the flame because it minimizes surface oxides and it is the hottest zone

what is the oxidizing zone?

produces oxidation of the alloy

how can electric heating occur?

easier and faster in melting the alloy than torch melting, it is done either by:

• convection from a heating muffle that contains an alloy

• generation of an induction current in the alloy by an induction casting machine

what is the main disadvantage of electric melting?

heating is not controlled and can cause volatilization (evaporation) of lower melting point elements, which can change alloy composition. the machines required for it are also expensive and more appropriate for larger dental laboratories, whereas a torch would be the method of choice for smaller laboratories and dental offices

what is used for casting forces in casting machines to fill the mold?

• air pressure

• centrifugal force

what are the types of casting machines?

1. centrifugal casting machines

2. air or gas pressure casting machines

3. vapor pressure casting machine

4. vacuum casting machine

5. induction casting machine

what are centrifugal casting machines used for?

they depend on centrifugal force to force the alloys into the mold cavity and are used to cast large and small castings

what are air or gas pressure casting machines used for?

they depend on air pressure to force the molten alloys into the mold cavity and are used to cast small castings

what are casting crucibles?

a vessel in which the metallic alloy is melted by heat to be cast into the mold, with 3 types being available:

• clay crucible

• carbon crucible

• quartz (zircon-alumina) crucible

what is clay crucible used for?

gold alloy. may break down when exposed to high temperature

what is carbon crucible used for?

gold alloy. can cause carbon contamination for palladium nickel cobalt alloys

what is quartz (zircon-alumina) crucible used for?

high-fusing alloys of any type (noble and base metal)

what is the first step of casting?

it is given a three clockwise turns, four if using metal ceramic alloys, and locked in position with the pin

what is the second step of casting?

the cradle and counterbalance weights are checked for the appropriate size of casting ring

what is the third step of casting?

a crucible for the alloy and the alloy to be melted is placed inside the crucible.

what is the fourth step of casting?

the torch is lit and adjusted, gas-air for regular alloys and gas-oxygen for metal-ceramic

what is the fifth step of casting?

the crucible is preheated particularly in the area that will be in contact with the alloy and the alloy is added. preheating avoids excessive slag formation during casting and also prevents freezing of casting if the crucible is too cool , resulting in an incomplete casting

what is the sixth step of casting?

add sufficient mass of the alloy to sustain adequate casting pressure

• 6g is typically adequate for anterior and premolar castings

• 9g is adequate for molar castings

• 12g is adequate for pontics

what is the seventh step of casting?

the alloy is heated using the reduced part of the flame until the alloys are ready to cast, a little flux can be added to conventional gold alloys, not metal ceramic alloys, to prevent oxygen from contacting the hot metal

what is the flux composed of?

equal proportions of borax and boric acid

when are gold alloys ready to cast?

gold alloys ball up and have a mirror like shiny surface

when are base metal alloys ready to cast?

round over margins

what is the eighth step of casting?

the casting ring is placed in the cradle of the casting machine and kept on the alloy with the reducing flame o

what is the ninth step of casting?

the casting arm is then released to make the casting, and then the molten alloy is forced into the mold cavity

how is recovery of the casting done?

after casting procedure is completed, the casting ring is taken immediately from the casting machine and is thrust or plunged under running cold water into a large mixing rubber bowl, which is a procedure known as quenching

what are the benefits or purposes of quenching?

1. to anneal the alloy to provide better working qualities during finishing

2. the disintegration of the hot investment material when it contacts cold water

how is cleaning of cast restorations done?

after quenching the casting and its removal from the ring, the investment is removed and the sprue is cut. it then undergoes a cleaning procedure according to the type of alloys used

how are base metal alloys cleaned?

it is usually cleaned by carried by sandblasting 50µm alumina particles to remove investment particles and surface oxides

what is a sandblast?

a machine that throws sand and compressed air on the cast restoration to clean it

how are gold metal alloys cleaned?

by pickling, which is when the cast restoration then placed into a pickling solution, which is compromised of 50% hydrochloride acid

what are the advantages of pickling?

1. it is used to remove oxide from the surface of the metal

2. it is used to remove any remnant of investment material

why is finishing and polishing of gold alloy is easier than that of base metal alloys?

stiffer and harder, meaning base metal alloys requires more time to finish and polish

how is smoothening of the casting surface is done by?

1. sand paper disc

2. green stone and pumice

3. rubber wheel stone and pumice