this presentation describes the various casting procedures, their history, the various metal alloys that are used for casting. it explains the various casting procedures in details and the casting defects in brief.

1. CASTING
PROCEDURES
Presented by
Dr. Swapnaneel Pradhan
Guided by
Dr. Eshwaran MA

3. Definition
❖ According to GPT-9, Casting is “the action of pouring or injecting a flowable
material into a refractory mould”.
❖ According to S. Mahalaxmi, “Casting is the process by which the wax pattern
of a prepared tooth is fabricated and converted to its metallic replica.”

4. ❖ In dentistry, the casting procedure employs
the “Lost-wax technique”.
❖ According to GPT-9, lost-wax technique is is
defined as “the casting of a metal alloy or
ceramics into a mold produced by
surrounding (investing) an expandable (wax)
pattern with a refractory slurry that sets at
room temperature, after which the pattern is
removed through the use of heat.”
❖ The “lost-wax technique” is explained in
the flow chart.

5. Casting Accuracy
❖ The accuracy of a cast restoration can be as good as 0.05% which
translates to a tolerance of less than 20µ for most dental restoration.
❖ In order to achieve this level of accuracy, each step must be performed
accurately. An error is any step has the potential of rendering the final
restoration clinically unacceptable.
John M Powers, John C Wataha, Yen Yei Chen. Dental Materials Foundations and Applications. Elsevier Inc; 2017.

6. Steps in Casting
Procedure
Dental Materials Foundation and Applications by Powers and Wataha, Pg. no. 156

7. 1. Fabrication of wax pattern
❖ A wax pattern is a detailed model of the final
restoration, including all anatomy, contours,
occlusal functions and proximal contacts.
❖ It can be fabricated directly within the patient’s
oral cavity (direct technique) or a die (indirect
technique).
❖ Wax is used because it is easy to use &
manipulate and can be easily eliminated by
burnout.

8. Direct technique
❖ The wax pattern is directly fabricated intra-
orally on the prepared tooth.
❖ The wax that is used should not be deformed
by the high intra-oral temperature.
❖ Type I hard inlay wax is used for this purpose.
It is softened by carefully waving over a flame
and adapted directly on the prepared tooth.
❖ After this carving is done.

9. Indirect technique
❖ A high accuracy impression of the prepared tooth is
obtained and a die is prepared using type IV die stone.
❖ Surface hardness of the die is increased using surface
hardeners like K2SO4, borax, cellulose-resins etc.
❖ Die spacers like nail paint, resin paint etc. are added to
create suitable cementing space.
❖ A lubricant or oil is then applied over it so that the
prepared wax-pattern can be easily removed.

10. ❖ Wax pattern is then prepared using Type II soft inlay wax.
❖ It is carefully melted using dry heat and placed on the prepared
die, layer by layer, until some excess is formed.
❖ The wax should be uniformly heated, melted and bonded in
increments, so that distortion does not occur due to release of
internal stress.
❖ Carving should be done with very sharp carvers to prevent
pulling away of wax from the margins. Care should be taken not
to abrade the surface of the die.

11. Distortion of wax pattern
❖ Wax has a tendency of getting distorted due to its tendency to flow and
change dimension by release of stress or with change in temperature.
❖ Even when it is supported by a die, it tend to distort if there is considerable
change in ambient temperature. But if removed from the die, the change may
occur in minutes.
❖ Thus it is recommended that minimum time should elapse between
fabrication of wax pattern and investment. It is also recommended to connect
the sprue before removing the wax pattern from the die.

12. 2. Sprue
❖ The sprue is defines as “the channel or hole
through which plastic or metal is poured or
cast into a gate or reservoir and then into a
mold.” -GPT9
❖ Sprue former is defined as “a wax, plastic or
metal pattern used to form the channel or
channels allowing molten metal to flow into a
mold to make a casting.”

13. ❖ Classification of sprue formers:
❖ According to material used
❖ Wax sprue former
❖ Plastic sprue former
❖ Metal sprue former
❖ According to manufacturing
❖ Pre-fabricated
❖ Custom made
❖ According to shape
❖ Solid
❖ Hollow

14. Design
Principles for
Sprue former

15. Sprue former
❖ Material: wax, plastic or hollow metal. Since metal
cannot be eliminated by burnout, it must be coated
with inlay wax before using so that it can be easily
removed.
❖ Diameter: approximately equal to the greatest
thickness of the wax-pattern. For bigger patterns,
sprues of larger diameter may be used. Generally
ranges form 1.6mm (16 gauge) to 2.6mm (10 gauge).
For bigger patterns, spurs of larger diameter may be
used.

16. Sprue former
❖ Length: it should be such that the wax pattern is in the middle of
the casting ring.
❖ There should be enough porosity of the investment such that it
allows the trapped air to escape during casting.
❖ Thickness of the investment beyond the extreme end of the wax-
pattern should be 6mm for gypsum bonded and 3-4mm for
phosphate bonded, because phosphate bonded has lower
porosity.
❖ If the sprue is too long or too thin, pre solidification of metal takes
place in the screw leading to incomplete casting.
Metal sprue former with
reservoir

17. Sprue wax
❖ This type of wax is highly ductile.
❖ It does not leave any residue during wax burnout.
❖ Composition:
Ingredient Functions
1. Paraffin wax Establishes melting point
2. Ceresin wax Improves carving characteristics
3. Bees wax Reduces flow at intra oral temperatures and reduces brittleness.
4. Natural resins Induces suitable flow

18. Sprue former
❖ Sprue attachment criteria:
❖ It is attached at the bulkiest part.
❖ Sprue is oriented at 45 degrees inclined towards the proximal areas to
prevent turbulent flow towards the thinner extremities.
❖ Presence of reservoir at 1-2 mm from point of attachment.
❖ A reservoir bar is used when multiple sprues are used. It acts similar to a
reservoir.
❖ The sprue axis should not be directed towards the thin or extreme
sections of the wax pattern.
❖ A plastic or ceramic crucible former is attached to the other end of the
sprue former.
Use of reservoir bar for
multiple casting

19. Single vs Multiple sprue
❖ Usually single sprues are used for small
castings.
❖ But when either the pattern is too large or
multiple sections of the pattern are connected by
thin wax, multiple sprues must be used.
❖ Use of multiple sprues ensures adequate flow of
molten metal to all areas of the mold during
casting and prevents internal porosities.

20. Venting
❖ Small auxiliary sprues or vents may be added
in order to improve casting in the thin sections
of the mold. They help in:
❖ Escape of gasses during casting.
❖ Compensate for shrinkage during
solidification
❖ Induce solidification in critical areas by
acting as heat sink.

21. Crucible former
❖ According to GPT-9 Crucible formers is defined as “the
base to which a sprue former is attached while the wax
pattern is being invested in refractory investment.”
❖ It is convex in shape and made of rubber, plastic or
metal.
❖ Shape of the base is round or oval depending on the
casting machine used.
❖ It is removed after investment and creates a concave
depression or crucible in the refractory investment.

22. 3. Casting ring
❖ According to GPT-9 a casting rig is “a metal or silicone tube in which a
refractory mold is made for casting dental restorations.
Classification
According to shape:
❖ Round
❖ Oval
Complete ring:
❖ Rigid: Metal, Plastic
❖ Flexible: Rubber
Split Ring:
❖ Rigid: Metal, Plastic
❖ Flexible: Rubber

23. Casting ring
❖ They confine the investment material and allow it to set.
❖ The internal diameter of the ring should be at least 5 - 10mm greater than the
widest measurement of the pattern and about 6mm higher.
❖ For single crowns and inlays, generally small castings rings are used
(diameter 32mm).
❖ For large FPD patterns, large (diameter 63mm) casting rings or round or oval
shape are used.

24. Casting ring liner
❖ Previously asbestos sheets were used as casting ring
liners.
❖ They were laid inside the ring leaving around 3mm at the
ends.
❖ But due to potential health hazards due to inhalation of
asbestos dust or vapours (carcinogenic), the use of
asbestos was discontinued.
❖ Now cellulose or alumina silicate ceramic ring liners are
used.
Ceramic Ring Liner

25. Casting ring liner
❖ Functions:
❖ Allow lateral expansion of the mold (investment) during setting
and heating.
❖ Minimises distortion by reducing the amount of longitudinal
expansion of investment.
❖ When wetted, provide water for hygroscopic expansion of
investment.
❖ For high fusing metals, split casting rings or plastic casting rings
are used to allow sufficient lateral expansion of investment. For
base metal alloys, ringless casting is used.
Removal of casting ring and
crucible former after
investing, for ringless
casting

26. Ring-less Casting system
❖ This has been designed to allow unrestricted
expansion of the mould during burnout.
❖ Tapered casting rings made of plastic or paper are
used.
❖ Once the investment sets, the casting ring is removed
before burnout.
❖ This is most commonly used with phosphate bonded
investments and with alloys that require greater mold
expansion than traditional gold alloys.

27. Ring-less Casting system
❖ The PowerCast Ringless casting system by Whipmix Corporation is
a very well known ringless casting system.
❖ It consists of three sizes of rings and formers, preformed wax
sprues and shapes, investment powder and special investment
liquid.
❖ The PowerCast investment material is a carbon free phosphate
bonded investment material.
❖ The w/p ratio is 23ml/100g and working time is 8-9min.
❖ Setting expansion 0.8% and thermal expansion 1.0%.
❖ Resistant to rapid increase in temperature and easy to breakout for
faster cleanup.

28. 4. Investing Procedure
❖ According to GPT-9, investing is “the process of covering or enveloping,
wholly or in part, an object such as a denture, tooth, wax form, crown,
etc., with a suitable investment material before processing, soldering,
or casting.”
❖ Before investing is done, a surfactant is applied on the surface of the
hydrophobic wax pattern to aid in better wetting of the surface and avoid
formation of air bubbles which cause nodules of casting.

29. Investing Procedure
❖ The investment material is either hand mixed or mixed with the help
of a vacuum mixing machine.
❖ First the mixed investment material is painted on the surface of the
wax pattern for better wetting and then the remaining investment
material is vibrated carefully into the casting ring to fill completely.
❖ After allowing the investment material to set for 1 hr, the crucible
former is carefully removed.
❖ If a metal sprue former is used, it is also removed. The exothermic
setting reaction softens the layered wax and makes it easy to
remove.
Investment vacuum
mixing machine

30. Investment materials
❖ They are of mainly three types: Gypsum bonded, phosphate bonded and
ethyl-silicate bonded.
❖ Gypsum Bonded: Decomposes at 700℃. Thus used for casting gold alloys
that have low melting range.
❖ Phosphate bonded: used for casting base metal and gold alloys for copings
and frameworks for metal-ceramic prostheses. Also used for cast ceramics.
❖ Ethyl-silicate bonded: used for casting high fusing metal partial denture
alloys

31. 5. Wax burn-out 🔥
❖ The invested rings are placed in a room temperature furnace and then heated to
a maximum prescribed temperature.
❖ Gypsum bonded investment: upto 500℃ for Hygroscopic Expansion technique
and 700℃ for Thermal expansion technique.
❖ Phosphate bonded investment: Maximum temperature may range from 700℃ to
1030℃ according to the type of alloy used.
❖ Temperature of Gypsum Bonded investment should be carefully controlled as
gypsum bonded investment is more prone to decomposition under high
temperatures.

32. ❖ When the temperature reaches 300℃, the
casting ring is inverted and placed facing
upwards for circulation of oxygen. This prevents
carbon formation.
❖ During burnout, wax has a tendency to get
absorbed by the porous investment and some
residual carbon gets trapped. So it is advised to
carry out burnout procedure, when the
investment is still wet.
❖ When the water evaporates, it flushes out the
absorbed wax.
❖ Also the heat converts the residual carbon into
CO or CO2.

33. Two-stage Burnout
❖ This process is used when plastic is used as sprue former.
❖ Since plastic takes longer to melt as compared to wax, the first stage involves
prolonged heating to melt the plastic without putting pressure on the mold.
❖ Then the ring is inverted with sprue-face facing downwards and second stage
is carried out.
❖ Second stage involves regular wax-burnout.

34. Temperatures for wax burnout for various
investments
Taken from S. Mahalaxmi

35. Precautions
❖ The casting ring should be placed in a furnace at room temperature and then
heated.
❖ Rapid heating should be avoided.
❖ Over heating and prolonged heating, both should be avoided.
❖ Cooling and reheating of the investment should be avoided.

36. Thermal expansion of mold
❖ This can be achieved by two ways:
❖ Low heat Hygroscopic expansion
❖ High heat thermal expansion

37. Hygroscopic Low heat technique
❖ Hygroscopic expansion of Gypsum bonded investment takes place from
three sources:
❖ 37℃ water bath expands the wax pattern.
❖ Warm water entering mold from the top adds some hygroscopic expansion
❖ The thermal expansion at 500℃ provides the remaining required
expansion.

38. Advantages
❖ Less chances of investment degradation
❖ Cooler surface provides smoother castings
❖ The mold can be directly placed in a furnace heated at 500℃
❖ The mold may be held in the furnace even upto 5 hours with negligible
damage.
Disadvantages
❖ Low heat leads to greater retention of carbon, which may lead to reduced
venting and cause back pressure porosity.

39. High Heat Thermal expansion technique
❖ It relies completely on a high heat burnout process to obtain the required
expansion, while at the same time eliminating the wax.
❖ In this process small amount of hygroscopic expansion may be achieved by
using wet-liners.

41. Time lapse between burnout and
casting
❖ After burnout casting should be carried out as soon as possible.
❖ Thus before casting, the casting ring is maintained at the maximum
temperature in the furnace. This is known as “heat soak”.
❖ Between removal of the mould from furnace and casting, a maximum of 1
min is allowed. This prevents dimensional change of the mold due to change
in temperature for High heat technique.
❖ Even in Low Heat technique, casting should be done ASAP to prevent any
chance of dimensional change.

42. Casting Crucible
❖ According to GPT-9, the crucible is “a vessel or
container made of any refractory material
(frequently ceramics) used for melting or
calcining any substance that requires high
degree of heat.
❖ Important features:
❖ It must not react with the alloy during casting
❖ It must be able to withstand the temperature
of the melting tech and molten alloy.
Crucible types suitable for different types of alloy.

43. 6. Melting of the alloy
❖ The alloy should be heated as quickly as possible to a completely molten
state above its liquidus temperature.
❖ But it should not be so hot that it begins to oxidise or crystallisation is delayed
when it reaches the extremities of the mold, damaging the mold wall.
❖ Gold alloy: when the gold alloy is ready for casting, it becomes spherical with
a shiny surface, orange in colour and appears to spin following the flame.
❖ Base metal alloys: these are ready for casting, when the sharp edges of the
ingot becomes rounded.

44. Flame
❖ Two types of torches are available: single orifice tip and
multiple orifice tip.
❖ Fuel sources: acetylene, natural gas and propane.
❖ Electric heating prevents undesirable changes in the alloy
components which may be caused by volatilisation of low
melting component elements.
❖ A properly adjusted torch develops adequate temperatures
from 870℃ - 1000℃ for melting dental alloys whose
melting ranges are within these temperatures.

45. Zones of the flame
❖ Zone 1: Air and fuel mix in this zone. This zone has no
heat.
❖ Zone 2: This zone is green in colour. It is known as
combustion/oxidising zone. Here gas and air are partially
burned.
❖ Zone 3: This zone is suitable for the melting of alloy. This is
dimly blue in colour and known as reducing zone. It is the
hottest part of the flame.
❖ Zone 4: This zone is also an oxidising zone where
combustion takes place with oxygen and air.

46. 7. Types of casting
❖ Centrifugal casting
❖ Electrical resistance heated centrifugal casting
❖ Induction casting
❖ Direct current arc melting casting
❖ Vacuum or pressure assisted casting

47. Centrifugal casting
❖ Alloy is melted in a separate crucible and molten metal is
forced into the mold using centrifugal force.
❖ The crucible is attached to the free arm (also called broken
arm) of the casting machine and the casting machine is
spring wound and locked.
❖ Once the heated casting ring is placed and the alloy is
melted to casting temperature, the spring is released which
initiates rotational motion.
❖ The free arm helps to accelerate the initial rotation and
helps to increase the speed of molten metal entering the
mold.

48. Electrical resistance heated centrifugal
casting
❖ Uses same principal, for the movement of molten metal alloy from crucible to
mold, as that of centrifugal casting machine.
❖ Here a graphite or ceramic crucible is used which is located flush with the
casting ring.
❖ The alloy is melted with the help of electrical resistance based heating unit.
❖ It allows the alloy button to stay molten for a slightly longer time and ensures
continuous flow of molten metal and also proper solidification from the tip of
the casting to the button surface.

49. Induction casting
❖ The alloy is melted in an electric induction
furnace, within the crucible surrounded by
water cooled metal tubings.
❖ As soon as the metal reaches casting
temperature in air or vacuum, it is cast into
the mold using centrifugal force.

50. Direct current arc melting casting
❖ A direct current arc is produced between two
electrodes, the alloy and the water cooled
tungsten electrode.
❖ The alloy is vacuum melted and cast by
pressure in an argon atmosphere.
❖ Since the temperature in the arc exceeds
4000℃, it is a major disadvantage as the alloy
can quickly become over heated.

51. Vacuum or pressure assisted casting
❖ This is required mainly for the casing of Titanium alloys.
❖ Melting point of CP Titanium is 1671℃ while the liquidus
temperature of other constituents is less than 1500℃.
❖ Thus a protective environment of argon or vacuum is
required to prevent absorption of gasses in molten state.
❖ In order to be able to withstand such high temperatures,
either a graphite or a water cooled copper crucible is
used.

52. 8. Divestment
❖ This process refers to the removal of the casting from the investment mold.
❖ The process differs according to the type of alloy used for casting.

53. Noble alloys
❖ Once casting is done, the ring is immediately removed and quenched in water
as soon as the button exhibits a dull red glow.
❖ As soon as the hot investment comes in contact with water it reacts violently and
crumbles. This makes it easy to remove the casting from the investment.
❖ Plus after quenching, the noble metal is in an annealed condition which enables
it to be burnished, polished etc.
❖ After removing from the investment the dark oxide layer from the surface of the
casting is removed by the process of pickling which involves heating the casting
in 50% HCl.

54. Palladium based alloys and base metal
alloys
❖ Castings using phosphate bonded
investment are allowed to bench cool
before removal from investment.
❖ The residual investment is removed
with the help of sandblasting using
fine alumina and using an ultrasonic
cleaner.

55. Cleaning the casting
❖ Cleaning the casting is an important part of divestment.
❖ Residual investment is removed using abrasive blasting
using Al2O3 or glass bead powders with the help of a
pencil-tip air abrasive unit.
❖ Precious (gold-platinum-palladium) gold alloys are
soaked in hydrofluoric acid to remove investment
materials. Semi precious alloys (gold-silver-palladium,
silver-palladium) may be acid treated according to
manufacturers instructions.
❖ Non-precious alloys must never be acid treated due to
their high reactive nature with acid.

56. Xcavavator by WhipMix
❖ Automatic divestment machine developed
by WhipMix Corp.
❖ It automatically removes 95% investment
material and alerts the operator when the
process is over.
❖ This is accomplished by high velocity air
abrasion using glass beads and dry air.

57. 9. Inspection and finishing of casting
❖ Once the casting is cleaned of all debris, the accuracy inner surface is
carefully examined under magnification.
❖ It is then tried on the die to check for the accuracy of fit and margins.
❖ After this the casting is finished and polished.

58. Finishing and polishing of cast metal
❖ Initial contouring is done carbide burs, silicon
carbide green stones, or heatless polymer stones.
❖ Initial finishing is done with aluminum oxide (pink)
stone or medium grade impregnated rubber wheels
and points (brown and green)
❖ Final finish is achieved with finer abrasive
impregnated rubber cups, wheels and points.
❖ Polishing is done with Tripoli or rouge with leather
wheels.

59. Degassing
❖ For ceramo-metal castings, after the coping or frameworks is cleaned it is
subjected to degassing or surface treatment procedures.
❖ It removes embedded contaminants on the surface and entrapped gas. In
addition it creates a oxide layer on the surface for bonding of ceramic.
❖ Precious gold alloys: rapidly heated in air to 1950 ℉ (1065℃), held there for 5-
10min and rapidly cooled to room temperature.
❖ Semi-precious and base metal alloys: rapidly heated to 1950℉ and held in this
temperature for 5-10min in mercury vacuum. The vacuum is then broken and
rapidly cooled to room temperature.

61. Casting Defects
❖ Distortion
❖ Surface roughness
❖ Surface Nodules
❖ Fins
❖ Porosity
❖ Incomplete casting
❖ Contaminated casting