investing and casting
TRANSCRIPT
Investing and Casting
Prof. Hesham Ibrahim OthmanProfessor and head of Crown and Bridge Dept. Al-Azhar University
The lost wax cast technique has been use to convert wax pattern to cast metal, the process consists of:1- Surrounding the wax pattern with a mold of heat resistance investment material2- Eliminating the wax by heating3- Introducing molten metal into the mold through a channel called sprue
Prerequisites 1- Finishing of wax pattern 2- Carefully evaluation of pattern for smoothness, finish, and contour under magnification, and any residual flash (wax thatextends beyond the preparation margin) is removed. 3- A sprue is attached to the pattern. 4- The pattern is then removed from the die and attached to a crucible former5- The wax pattern must be invested immediately because any delay leads to distortion of the pattern as a result of stress relief of the wax.
Sprue Former• It is that passage leading from the crucible
to the mold cavity• Sprue design will vary depending on the
type of the restoration being casted, the alloy used, and the casting machine.
Basic requirements of sprue former
1-The sprue must allow molten wax to escape from the mold2- the sprue must allow molten metal/alloy to flow into the mold with as little turbulence as possible3-The alloy within the sprue must remain molten longer than the alloy that has filled the mold
Different Types of sprue
According to the material of the sprue 1-Wax preferred because they melt at the same rate of the wax
pattern 2-Plastic they may be used when casting fixed partial dentures because of
the added rigidity 3-Metallic a- solid b- hollowed metallic sprue former must be made from non-rusting materials
to avoid contamination of the interface area between the wax and investment
Special care should be taken during removal of metallic sprue to avoid cheeping of the investment into the sprue channel which can lead to incomplete casting
Design of the sprue former
Sprue former DiameterSize:For centrifugal casting machine The sprue should be thicker than the thickest portion
of the wax pattern• 2.6 mm (10 gauge)for molar and metal ceramic
restorations• 2 mm (12 gauge)for premolar and partial coverageFor air pressure casting machine• narrow sprue at the area of attachment to wax
pattern is used for forcing of metal to mold cavity
length of the sprue former
• The length :should not be more that 6 mm. and not less than 2 mm.
• Gypsum distance between ring and pattern is 6-8 mm.
• Phosphate bonded distance is 3-4 mm.
• The sprue should be placed in the center of the ring (hottest zone)
If the pattern is too close to the end of the ring, themolten alloy may blast through the investment during casting; if it is too far, gases may not escape rapidly enough to permit complete filling of the mold with alloy.
Location and angulations• The sprue former is
attached to the largest non functional cusp with angle 45 degree
• In anterior teeth it is found mid incisal
• Point of attachment : should be well flared and avoid any constriction to allow good flow of molten metal
Number and shape of the sprue former
• 1- single may be : a-direct b-indirect ( zigzag)
2- double a-direct ( v and y shape) b-indirect ( stable shape)
• 3- multiple spruing 1- direct
2- Indirect
Accessory sprue (Venting)Small auxiliary sprue or vent have been recommended to improve casting of thin patterns. Their action may help gases escape during casting
Crucible formerConstitute the base of the
casting ring • rubber • metal • plastic
Crucible former
Crucible base
Casting ring and liners • The casting ring serves as a container for the investment
while it sets and restricts the setting expansion of the mold.• Aim of liners:1- act as cushion 2- venting3-Facliltate removal of investment after finishing of casting 4- Share in hygroscopic expansion when it is placed wetDry liner may absorb water from the investment which cause
undesired increase of the total setting expansion
Ringless Investment TechniqueWith the use of higher strength, phosphate-bonded investments, the ringless technique has become quite popular.The method entails the use of a paper or plastic casting ring and is designed to allow unrestricted expansion.This can be useful with higher melting alloys that shrink more because of a larger cooling trajectory.
The position of the pattern in the casting ringalso affects expansion. For consistent results, a single crown should be centered in the ring, equidistant fromits walls. When fixed prostheses are cast as one piece, accuracy is better if the pattern is placed near the center of a large or special oval ring, rather than if a portion ofa multiunit wax pattern is only partially centered and partially near the edge of a smaller ring
Investments
Gypsum and phosphate bonded investments are used in fixed prosthodontics
Requirement of the investment material
1- Produce a detailed form of the wax pattern2- High strength to resist casting pressure3- Controllable expansion to compensate for the casting shrinkage 4- Good porosity to allow venting5- Stable at high temperature6- Produce a smooth casting7- Do not chemically react with the cast alloy8- Easy removed after casting
Types of investment materialsGypsum bonded Gypsum bonded investment investment
Phosphate bonded Phosphate bonded investmentinvestment
1- binder is gypsum 1- binder is gypsum ((calcium sulphate calcium sulphate hemihydrates )hemihydrates ) 2- Used to cast alloys that 2- Used to cast alloys that have melting temp. up-to have melting temp. up-to 10801080°C°C 3- low amount of expansion3- low amount of expansion4- used with gold alloys type 4- used with gold alloys type I, type II, type III, and type IV I, type II, type III, and type IV gold alloy gold alloy
1-binder is magnesium oxide 1-binder is magnesium oxide and ammonium phosphate and ammonium phosphate binderbinder 2- Used to cast alloys with 2- Used to cast alloys with high melting temp. up-to high melting temp. up-to 14001400°C°C 3- high amount of expansion3- high amount of expansion4-used with silver palladium, 4-used with silver palladium, gold platinum, nickel gold platinum, nickel chromium alloychromium alloy
Investment expansionTypes of investment expansion 1- Normal setting expansion 2- Hygroscopic expansion 3- Thermal expansionFactor increasing the expansion of the investment1-use of one or two dry ring liner2- use of excess wet liner 3-increase spatulation rate4-submerging the ring in water bath for one hourQ :why we need expansion?A : To compensate shrinkage that had been developed during metal solidification after casting
Investing Technique
For success of casting procedures, all of the following armamentarium must be used
Vacuum mixer
Investing Techniques1- Brush Technique2- Vacuum TechniqueArmamentarium Vacuum mixer- bowel- vibrator- spatula- brush- surfactant- casting ring- liner- investment powder& liquid Brush technique1- The pattern is first painted with surface tension reducer2- Surface must be wet completely3- Mix powder and liquid in bowel are mixed manually first, then transferred to vacuum mixer4- Attach the vacuum hose to the bowel, evacuate air, mechanically spatulated5- Coat the pattern with investment pushing the material a head of the brush from one point and gently vibrate6- Place the ring over the crucible base with vibration pour the investment down the side till fill the ring
7- when the investment reach the level of the pattern, tilt the ring several times to minimize the entrapment of air
8- allow the investment to set
Vacuum technique1- Hand spatulate the mix2- Attach the ring to the mixing bowel3- Attach the vacuum hose and mix4- Invert the bowel and under vibration fill
the ring5- Remove the vacuum hose before
shutting off the mixer6- Remove the ring and crucible former
from the bowel7- Immediately clean the bowel and mixing
blade under running water
Burn out of wax
Step by step procedures1- Allow the investment to set for one hours2- Remove the rubber crucible former3- Reexamine the ring for residual particles4- Place the ring with the sprue face down in the furnace on a ribbed tray, to allow easy flow of the wax5- bring the furnace to 200°C for 30 minutes.6- invert the ring and increase the heat to the final burnout temp. 650°C for 45 minutes, to allow complete escape of all gasses7- visually inspect before casting
Accelerated casting Technique Conventional casting techniques require 1 : 2 hours (for setting of investment and wax elimination)An accelerated casting procedure that reduces this time to 30 to 40 minutes has been proposed.Initially suggested as a way to make cast post-and-core restorations in a one-visit procedure,The procedure has been found to produce castings with accuracy and surface roughness similar to those produced by traditional methods.The technique entails the use of a phosphate-bonded investmentthat is given approximately 15 minutes for bench setand a 15-minute wax elimination by placing the ringin a furnace preheated to 815°C.
Casting process
Melting the alloy
• b-Electricity (induction)
Fluxing • Fluxes used with gold casting only• Importance of flux : 1-the primary purpose of flux is to prevent
oxidation of the alloy. 2-increase fluidity of the metal 3- reduce the melting temperature of the alloy
• Examples of flux a- Borax powder with boric acid powder b- Charcoal powder
• Casting machine is given three clockwise turns (four if using metal ceramic alloy) and lock in position by pin.
• The cradle and counterbalanced weight should be checked• The crucible for the alloy is placed and preheated• The alloy is heated in the reducing part of the flame until it is ready for
casting • Gold alloy are ready for casting when it become mirror like shiny surface,
while base metal alloy are ready for casting when the sharp edges of the ingot become round over
• The casting ring is positioned over the cradle with keeping the alloy with the reducing flame then release the arm of casting machine to produce casting
• The machine allowed to spin until it has slow enough to stop by hand, then remove the casting ring with a tongs
• After the red glow has disappeared from the button, the casting ring is plunged under running cold water into a large rubber mixing bowel
peal the investment by blunt instrument till reach to the ring liner
Steady push the investment by finger , at the same time hold the ring by the other hand carefully
The mold is broken open investment is removed from the casting. Care must be takento avoid damaging the margin
Remove all investment
Sand blast machine
Use of 50 µm aluminum oxide particles for cleaning of casting
Ultrasonic cleaner
“Pickle” casting
Remove sprue leaving a little excess for finishing
