Casting procedures and defects/prosthodontic courses

CASTING PROCEDURES
AND DEFECTS
INDIAN DENTAL ACADEMY
Leader in continuing Dental Education
www.indiandentalacademy.com

CONTENTS
 Introduction
 Definition
 Wax pattern & master die
 Spruing principles & technique
 Casting ring
 Casting ring liner
 Investing procedure
 Burnout procedure
 Casting of different alloys
 Ringless casting systems
 Cleaning of casting
 Review of literature
 Summary & conclusion
 References www.indiandentalacademy.com

INTRODUCTION
 Reproducing the wax up in metal with
predictable results has always been a
problem.
 The progress that has been achieved from
time has been a search for a standardized
system capable of consistently providing
precision at highest level.

DEFINITIONS
 CASTING:- Is defined as something that has
been cast in a mold, an object formed by the
solidification of a fluid that has been poured or
injected into a mold.(GPT)
 WAX ELIMINATION:- The removal of wax from
a mold, usually by heat.(GPT)
 INVESTING:- 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.(GPT)

WAX PATTERN
 The fabrication of a restoration in wax is
convenient because it involves few material &
is inexpensive, fast, reversible & customizable.
 There are 2 fundamental ways to prepare a
wax pattern for dental restorations:-
1) Direct method- Pattern is prepared on the
tooth in the mouth( used for small inlays).
2) Indirect method- Model of tooth is 1st
made on
the die.

PREPARATION OF MASTER DIE
 Most commonly used Die materials are:-
- Type IV – Dental stone, high strength
- Type V – Dental stone, high strength, high
expansion
 To produce relief space for cement, it is
common to use a die spacer with a stone die.
 Most common die spacers are resins.
 Epoxy die materials & divestments are also
used as die materials.

Spruing Principles & Techniques
 Accurate dental castings are produced by the
lost wax process whereby the wax pattern is
used to produce a precise refractory mold into
which molten alloy is cast.
 1st
step in the production of refractory mold is
termed “spruing” the pattern.
The wax pattern is attached to a conical base
by a short extention of wax, plastic or metal.
 Connector is termed as “SPRUE FORMER.”
 Proper selection of the size & configuration of
sprue former is critical to the production of a
dense, complete & accurate casting.

Sprue Former
 Purpose:- To provide a channel through which
molten alloy can reach the mold in an invested
ring after the wax has been eliminated.
 3 Basic requirements of sprue:-
1) Must allow the molten wax to escape from the
mold.
2) Sprue must enable the molten metal to flow
into the mold with as little turbulence as
possible.
3) Metal must remain molten slightly longer than
the alloy that has filled the mold.

 Sprue former gauge selection is often
empirical, yet it is based on the following 5
principles:-
1) Select the gauge sprue former with a
sufficient diameter.
- If large sprue former attached to a thin
delicate pattern cause DISTORTION.
- If small sprue former, then SUCK BACK
POROSITY may develop.
- Size – should be atleast 1.7mm in diameter
unless the pattern is extremely small.
- 2.5mm diameter(10 gauge) may be used for
very large patterns such as“bulky full crowns.”www.indiandentalacademy.com

SPRUE FORMER

2) Sprue former be attached to the portion of
pattern with largest cross-sectional area.
- Flow of alloy from thick to surrounding thin
areas e.g. margins, minimizes the risk for
turbulence.
- Orientation should minimize the risk of metal
flow onto flat areas of the investment or small
areas such as line angles.

3) Length of sprue former should be long enough to
properly position the pattern in the casting ring & yet
short enough so that the molten alloy does not solidify
before it fills the mold.
- Length of sprue former should be adjusted so that the
pattern is approx.3-6mm from the open end of the ring.

4) The type of sprue former selected influences
the burnout technique used.
- Either wax, plastic or metal sprue formers
may be used.
- 2 stage burnout technique should be used
whenever plastic sprue formers are involved
to ensure complete carbon elimination,
because plastic sprues soften at room temp.
above the melting point of inlay waxes. Metal
sprue cannot be burned out, but must be
carefully removed after investing of the
pattern.

5) Patterns may be sprued either:-
- Directly
- Indirectly
- Direct Spruing- The sprue former provides a
direct connection between the pattern area &
sprue base or crucible former area.
- Indirect Spruing- A connector or reservoir bar
is positioned between the pattern & crucible
former.
It is common to use indirect spruing for multiple
single units& FPD’s.

 SPRUE FORMER ATTACHMENT:-
- Sprue former is generally attached to the most bulky
part of the pattern.
- The sprue former connection to the wax pattern is
generally flared for higher density gold alloys but is
often restricted for lower density alloys.
- Flaring of the sprue former may act in same way as
reservoir, facilitating the entry of the fluid alloy into
the pattern area.www.indiandentalacademy.com

- Overheating of wax could distort the pattern. For this
reason, hollow metal or hollow plastic sprue formers are
preffered, since they hold less heat than a solid sprue
former.
- Sprue former should be smooth & produce no sharp
angles at the juncture of the wax pattern or the crucible
former.

 Sprue Position:-
- Some clinicians prefer placement of the sprue
former at the occlusal surface, whereas others
choose sites such as a proximal wall or just
below a non functional cusp to minimize
subsequent grinding of occlusal anatomy &
contact areas.
- Ideal area for the sprue former is the point of
greatest bulk in the pattern to avoid distorting
thin areas of wax during attachment to the wax
pattern & to permit smooth flow of the alloy.

 Sprue Direction:-
- The sprue former may be directed away from any thin
or delicate parts of the pattern, because the molten
metal may abrade or fracture investment in this area.
- It should not be attached at a right angle to a broad flat
surface.
- Such an orientation of the sprue former leads to
turbulence within the mold cavity & severe porosity in
this region.
- When pattern is sprued at a 45 degree angle to the
proximal area, a satisfactory casting is obtained.

 Sprue Length:-
- If too short – Wax pattern may be so far
removed from the end of the casting ring that
gases cannot be adequately vented to permit
the molten alloy to fill the ring completely.
resulting in
porosity
-Long & thin sprue formers:-
Disadvantage:- causes Internal Porosity
called as Shrinkage Porosity.
- When alloy fills casting ring, the pattern area
should solidify 1st
& then the reservoir.
- The metal will freeze in the sprue before it
freezes in the inlay & crown, thus preventing
more metal from entering the mold.www.indiandentalacademy.com

- Overcome by using a Reservoir or larger sprue.
- Reservoir – must have greater bulk than the
adjacent portion of the pattern into which sprue
former is inserted.
 Wax Pattern Removal:-
- The sprue former should be attached to the
wax pattern with the pattern on the master die,
provided the pattern can be removed directly in
line with its path of withdrawl from the die.

CRUCIBLE FORMER
 The sprue is attached to a crucible former
usually made of rubber, which constitutes the
base of the casting ring during investing.
 The exact shape of crucible former depends
on the type of casting machine used.

CASTING RING CASTING RING:- The inferior portion of a refractory
flask that provides a “negative likeness or dimple” into
which a metal is cast in the refractory investment.
 Identification of preferential sectors of casting:-
- The position of the pattern placed in casting ring is of
vital importance.
- Pattern must not be placed in the thermal center of the
ring, it should be situated 3-5mm from the top of the ring
- Distance from walls of the ring is about 5mm.

 Reason for such positioning are:-
1) Placing pattern in an area that is relatively cooler
than the thermal center, gives us the possibility of
having a reservoir of molten metal available.
2) Ideal position of pattern in ring is being able to
position the pattern in an area where the force of
dimensional expansion of investment can be easily
controlled.
3) Necessity of controlling the cooling. Cooling should
be from periphery towards the center.
 To build proper supply system:-
1) Casting ring dimension
2) Position of central thermal zone- is traditionally,
position in the geometric center of casting ring.
3) This zone where the temp. is higher & remains so
even after heat source is removed.www.indiandentalacademy.com

This central thermal zone moves towards the
sprueing system as soon as fluid metal has
entered the mold as a consequence of temp.
of alloy.
4) Identification of direction of movement- imp.
which represents the movement of the
casting ring undergoing during casting
process.

CASTING RING LINER
 With the use of solid metal rings or casting
flasks, provision must be made to permit
investment expansion. The mold may actually
become smaller because of the reverse
pressure resulting from the confinement of
the setting expansion.
 The effect can be overcome by using a split
ring or flexible rubber ring that permits the
setting expansion of the investment.
 Most commonly used technique to provide
investment expansion is to line the walls of
the ring with a ring liner. Earlier, asbestos
was the material of choice, but it can no
longer be used because its carcinogenic
potential makes it a biohazard.www.indiandentalacademy.com

 2 types of non-asbestos ring liner materials are:-
- An aluminium silicate ceramic liner
- A cellulose paper liner.
 To ensure uniform expansion, a liner is cut to fit the
inside diameter of the casting ring with no overlap.
 The dry liner is tacked in position with sticky wax, &
then is used either dry or wet.
 Wet liner technique- The lined ring is immersed in
water for a time & excess water is shaken away.
 Sqeezing the liner should be avoided, because this
leads to variable amounts of water removal &
nonuniform expansion.
 Although a ceramic liner may not absorb water like a
cellulose liner, its network of fibers can retain water on
the surface. www.indiandentalacademy.com

 Not only does the liner afford greater normal setting
expansion in the investment, but also the absorbed
water causes a semi- hygroscopic expansion as it is
drawn into the investment during the setting. Use of
one liner increases the normal setting expansion
compared with no liner.
 The thicker liner material or 2 layers of liner provide
greater semi-hygroscopic expansion & also afford a
more unrestricted normal setting expansion of the
investment.
 In any case, the thickness of the liner should not be
less than approx. 1mm.
 Cellulose liners, being paper products, are burned
away during the burnout procedure, so a technique
must be found to secure the investment in the ring.

 If the length of the liner is somewhat shorter than the
ring itself, the investment is confined at one or both
ends of the ring.
 The longitudinal setting & hygroscopic expansion are
thereby restricted, as compared with an end where
the liner is flush with the end of the ring.
 The expansion of the investment is always greater in
the unrestricted longitudinal direction than in the
radial direction, i.e. towards the ring.
 Therefore, it is desirable to reduce the expansion in
the longitudinal direction.
 Placing the liner somewhat short(3.25mm) of the
ends of the ring tends to produce a more uniform
expansion, thus there is less chance for distortion of
the wax pattern & the mold.

INVESTING PROCEDURE
 The wax pattern should be cleaned of any debris,
grease or oils.
 For this we can use either:-
- A commercial wax pattern cleaner, or,
- A diluted synthetic detergent.
 The pattern is left to air dry while the investment is
being prepared.
 The thin film of cleanser left on the pattern reduces the
surface tension of the wax & permits better ‘wetting’ of
the investment to ensure complete coverage.
 Wetting agent reduces the contact angle of a liquid with
wax surface.
 Lower contact angle indicates that treated wax surface
has an affinity for water, which results in investment
being able to spread more easily over wax.www.indiandentalacademy.com

MIXING
 Mechanical mixing under vacuum removes air
bubbles created during mixing & evacuates any
potentially harmful gases produced by the
chemical reaction of the high heat investments.
 For investing by hand- entire pattern is
painted(inside & out) with a thin layer of
investment with the help of brush which should
not touch the pattern.
- Casting ring is positioned on the crucible
former,& the remainder of the investment is
vibrated slowly into the ring.
 With vacuum investing- the same equipment
used to mix the investment is employed to
invest the pattern under vacuum.www.indiandentalacademy.com

 The amount of porosity in the investment is
reduced by vacuum investing.
 Adv. - The tensile strength of vacuum mixed
investment is also increased.
 Freedom of any surface imperfections is
highly important, because even a small
nodule on a casting may damage a fragile
enamel margin when casting is evaluated for
fit in the prepared cavity.
 Air bubbles that remain in the mix, even with
vacuum mixing, can be entrapped on flat or
concave surfaces that are not oriented
suitably for air evacuation.
 Tilting the ring slightly aids in releasing these
bubbles so that they can rise to the surface.www.indiandentalacademy.com

 Excessive vibration should be avoided,
because it can cause solids in the investment
to settle & may lead to free water accumulation
adjacent to the wax pattern, resulting in surface
smoothness.
 Excessive vibration may also dislodge small
patterns from the sprue former, resulting in a
miscast.
 If hygroscopic technique is employed, the filled
casting ring is immediately placed in a 37ºC
water bath with the crucible former side- down.
 For thermal expansion or high-heat technique,
the invested ring is allowed to bench set
undisturbed for the time recommended by the
manufacturer. www.indiandentalacademy.com

INVESTING OF GYPSUM
BONDED INVESTMENTS
 Require very specific W:P ratio’s .
 A variation of only 1ml of H2O can significantly alter the
setting expansion & the character of the casting surface.
 ing W:P ratio makes investing process easier but
investment will lose strength,
cause cracks to occur during heating
surface of casting inferiors.
 After the casting ring has been filled with investment
material, any excess should be removed before the
material sets. www.indiandentalacademy.com

 The filled ring is now set aside to allow the
investment material to complete its setting
reaction & the accompanying setting
expansion.
 Setting is complete in 30-40min.
 Hygroscopic technique is used.
- Freshly filled investment ring is immediately
placed into water bath for 30min. & kept at
100ºF(38ºC).

INVESTING OF PHOSPHATE
BONDED INVESTMENTS
 Expansion of the mold cavity can be ed by:-
1) ing the no. of layers of asbestos or fibrous ceramic
lining the casting ring.
2) ing the special liquid : water ratio.
3) ing the total L:P ratio.
4) Placing the investment in contact with water during
setting.
5) Burning out the mold at a higher temp.
 The mold cavity may accordingly be ed in size by the
reversal of any or all of these methods.
 3mm on each end is left as it serves to lock the
investment within the ring & equalize radial & axial
expansion. www.indiandentalacademy.com

 Residual, hardened investment in an unclean
mixing bowl will greatly accelerate the set of
newly mixed investment.
 Phosphate investment should not be mixed in
an apparatus that has been used for gypsum
investment. Residual gypsum will also
accelerate the set & will break down at temp.
above 2400ºF(1300ºC) liberating sulfurous
gases that can be detrimental to the casting.
 Ammonia gas is given off during mixing, & it is
important to hold the mixed investment under
the vacuum after mixing ceases to dissipate
some of this gas & thereby reduce the
incidence of bubbles adhering to the wax
pattern ( this additional holding time will vary
from 15-45sec).www.indiandentalacademy.com

 Initial set of the phosphate bonded investment
is generally rapid with the liberation of heat.
 If burnout is not carried within 1-2hrs, the ring
should be stored in a humidor at 100%
humidity, not soaked in water since excessive
hygroscopic expansion may result.
 Rapid steam release from a water saturated
ring can fracture the investment.
 Carefully grinding or scraping the shiny “skin”
off the end of investment just prior to burnout is
advisable. This removes a relatively impervious
layer, opening the pores of the investment &
facilitating gas release as the alloy is cast into
the mold. www.indiandentalacademy.com

BURNOUT PROCEDURE:-
 Once the investment has set for an appropriate period
45min. it is ready for burnout.
 A crucible former or any metal sprue former are
carefully removed.
 It is advisable to begin the burnout procedure while the
mold is still wet, because water trapped in the pores of
investment reduces the absorption of wax & as water
vaporizes, it flushes wax from mold.
 This burnout after 45min. determines with a gradual
increase in temp. with wax elimination & phenomena of
crystalline inversion that accounts for volume increase
on thermal expansion.
 Temp. of investment must be increased in successive
stages & be well defined in terms of time.
 These time intervals bet. various successive burnout
temp. levels must be followed.www.indiandentalacademy.com

 For expansion phenomena to take place in the best
possible conditions, it is necessary that internal temp.
of casting ring gradually reach prescribed level.
 The interval between successive temp. level is
indispersible to permit the external heat to reach the
internal areas of casting ring.
 Final burnout temp. of casting ring must satisfy
fundamental principles:-
1) Give a degree of expansion that is in harmony with
the shrinkage of alloy.
2) Maintain the viscosity of alloy at a level necessary
for complete filling of thinnest area in mold.
3) Permit controlled cooling.
1st
thinnest parts with cooling towards thermal
center. www.indiandentalacademy.com

Burnout Procedure For Gypsum Investments:-
 These investments are relatively fragile & require the
use of metal ring for protection during heating.
 So, the molds are usually placed in a furnace at room
temp. & slowly heated to 650ºC-700ºC in 60min. & held
for 15-30min. at the upper temp.
 At 468ºC for hygroscopic technique the investment
obtains its compensation expansion from 3 sources:-
1) 37ºC water bath expands the wax pattern
2) Warm water entering the investment mold from top
adds some hygroscopic expansion.
3) Thermal expansion at this temp. provides the needed
expansion.
 Advantage:- 1) Less mold degradation.
2) Cooler surface for smoother castings.
3) Convenience of placing molds directly at 468ºCwww.indiandentalacademy.com

 Rate of heating has some influence on the smoothness
& in some instances on overall dimensions.
 Rapid heating can generate steam that can cause
flaking of the mold walls.
 Too many patterns in the same plane within the
investment often cause separation of a whole section of
investment because, expanding wax creates excessive
pressure over a large area.
 Too rapid heating may also cause cracking of the
investment. In such case, outside layer of the
investment becomes heated before the center sections.
 Outside layer starts to expand thermally, resulting in
compressive stress in the outside layer that counteracts
tensile stresses in the middle regions of the mold.
 This stress distribution causes brittle investment to
crack from the interior outwardly in the form of radial
cracks. www.indiandentalacademy.com

 Cristobalite- low inversion temp. & rapid rate
of expansion during inversion, makes it
especially imp. to heat the investment slowly.
 The mechanism of this investment
decomposition & alloy contamination is
related to a chemical reaction between
residual carbon & CaSO4 binder.
 CaSO4 does not decompose unless it is
heated above 1000ºC.
 However, reduction of CaSO4 by carbon takes
place rapidly above 700ºC.
CaSO4 + 4C CaS + 4CO
3 CaSO4 + CaS 4CaO + 4SO2www.indiandentalacademy.com

 This reaction takes place whenever gypsum
investments are heated above 700ºC in the presence
of carbon.
 Sulfur dioxide as a product of this reaction
contaminates gold castings & makes them extremely
brittle.
 After casting temp. has reached, the casting should be
made immediately.
 Maintaining a high temp. for a considerable length of
time may result in sulfur contamination, rough surface.
 Methods for rapid burnout procedure are:-
- Placing the mold in a furnace at 315ºC for 30min. &
then rapid heating. Or
- Directly place into a furnace at the final burnout temp.
held for 30min. & cast.

Phosphate Investments:-
 Total expansion of 2% or more is required for
porcelain bonding alloys, since gold & base
metal alloys require higher melting &
solidification temp.
 These investments- harder & stronger than GBI.
 Disadvantage:-
- Quite brittle & are subject to the same unequal
expansion of adjacent sections as phase
changes occur during heating.

 PBI require:-
1) Higher burnout temp. for total elimination of
wax patterns.
2) Completion of chemical & physical changes.
3) Prevention of premature solidification of
higher melting alloys. Usual burnout temp.
range from 750ºC-900ºC.
 Heating rate is usually slow to 315ºC & is quite
rapid thereafter, reaching completion after a
hold at upper temp. for 30min.

 Some investments that can be subjected to 2
stage heating more rapidly, are placed directly
in the furnace at the top temp. held for
20-30min., & then cast.
TIME ALLOWABLE FOR CASTING:-
 The investment contracts thermally as it cools.
 When high heat technique is used, the
investment loses heat after the heated ring is
removed from the furnace & the mold contracts.
 Because of the liner & low thermal conductivity
of the investment, a short period can elapse
before the temp. of the mold is appreciably
affected. Under average condition approx.1min.
can pass without a noticeable loss in
dimensions. www.indiandentalacademy.com

 In low- heat casting technique, temp. gradient
between the investment mold & the room is
not as great as that employed with high- heat
technique.
 Thermal expansion of investment is not as
imp. to the shrinkage compensation.

CASTING CRUCIBLES
 3 types of casting crucibles are available:-
1) Clay(high noble & noble types)
2) Carbon(high noble alloys& high fusing,gold
based metal ceramic alloys)
3) Quartz(high fusing alloys of any type&suited
for alloys that have high melting range
& sensitive to carbon contamination).

CASTING OF LOW FUSING
GOLD ALLOYS
 3 methods are available:-
- Compressed air & gas
- Oxygen & gas
- Electric casting machine
 When torch is used with compressed air & gas, the
valves should be adjusted until the reducing cone is
approx. 40mm. The tip of the reducing cone is used to
make a rapid & clean melt.
 When Oxygen & gas combination is used, a hotter flame
is produced. A small multiorifice O2-gas tip is ideal for
rapid melting of the gold ingots. Single orifice tips may
be used if they are large enough. Small soldering tips
should not be used to melt the casting alloy.www.indiandentalacademy.com

 When reducing zone is in contact, the surface of the
gold alloy – bright & mirrorlike.
 When oxidizing portion of the flame is in contact with
the alloy, there is a dull film of “dross” developed over
the surface.
 The alloy 1st
appears to be spongy & then small
globule of fused alloy appears. Molten alloy soon
assumes a “spheroidal shape”.
 At proper casting temp., the molten alloy is light
orange & tends to spin & follow the flame when latter
is moved slightly.
 The use of an electric furnace casting machine
eliminates the need for a torch & the judgment
required in heating gold alloy.
 When torch is used, alloy should be preheated before
the mold is removed from the burnout oven.www.indiandentalacademy.com

 This will reduce the time, the investment has to cool
before the molten alloy is forced into the mold.
 If the investment cools prior to the casting, the mold
will become smaller through cooling contraction of
investment. True for quartz investment.
 Casting produced with large amounts of alloy in the
remaining sprue & button will usually produce better
detail & definition than will castings made with mini.
amount of metal.
 The larger amount of molten alloy, times the force
generated by casting machine, create more pressure
on the metal in the mold.
 Reducing flux should be used in melting the alloy.
The flux(50% boric acid powder & 50% fused borax)
increases the fluidity & reduces the potential of
oxidation. www.indiandentalacademy.com

 Centrifugal machine – mini. of 4 turns is
necessary.
 There is no evidence that too much casting
pressure, within reasonable limits, can be
used, but insufficient pressure is a common
cause of casting failure.
 When alloy has solidified & cooled for approx.
1min., the investment may be placed into room
temp. water & the alloy quenched. This will aid
in removing the adhering investment which can
be cleaned from casting with a lab. brush.
 The casting should be its characteristic gold
color & is now ready to be used with copious
amounts of water prior to further finishing.www.indiandentalacademy.com

CASTING HIGH FUSING METAL-
CERAMIC ALLOYS:-
 It requires that the metal be heated approx.
200ºF(100ºC) higher than its upper melting
point.
 This is generally in range of 1200ºC-1370ºC.
 These temp. can be achieved with either:
- A gas oxygen torch, or
- Induction heating.

1) INDUCTION CASTING:-
 Can be used for alloys of any composition & provides
for excellent control of the casting temp.
 The alloy is rapidly melted in a carbon crucible, heated
by electrical induction
2) TORCH CASTING:-
 All alloys can be cast with a gas-oxygen or
oxyacetylene torch.
 Best melted by placing it on inner side wall of crucible
& there is greater opportunity for any gases in the
flame to be reflected from the surface of the metal
rather than to be absorbed.
 The temp. of gas air flame is greatly influenced by the
nature of gas & the proportion of gas & air in the
mixture. www.indiandentalacademy.com

 Parts of flame can be identified by conical areas:-
1) Long cone- emanating directly from the nozzle is the
zone in which the air & gas are mixed before
combustion. No heat is present.
2)Combustion Zone- Which is green & immediately
surrounding the inner cone.
- Here, gas & air are partially burned.
- Zone is oxidizing & should always be kept away from
molten alloy during fusion.
3)Reducing Zone- Dimly blue & is located just beyond
the tip of the green combustion zone.
- It is the hottest part of the flame.
- This area should be kept constantly on the metal
during melting.
4) Oxidizing Zone- Outer cone is the area in which
combustion occurs with oxygen the air.
- Not employed to melt the alloy.
- Its temp. is lower than the reducing zone & also
oxidizes the alloy.

 The casting of porcelain-bonding alloys differs
from casting low-fusing gold alloys in following
respect:-
1) No asbestos is used in the high heat
crucibles, since it may react with the alloy
constituents at high temp.
2) There should be one high heat crucible for
each brand of alloy being cast in order to
avoid cross contamination.
3) No casting flux is used on porcelain bonding
alloys, since flux may alter its composition &
diminish the bond strength.True for gold alloys
4) The visual uses for assessing casting temp.
are different for low-fusing gold alloys since
high fusing alloys are heated white-hot before
they are released into the mold.www.indiandentalacademy.com

- Base metal alloys for instance should not
“pool” like gold alloys.
5) Greater casting pressures are generally
required to force porcelain-bonding alloy into
the mold.
This is dictated by:-
- The greater degree of chilling found with high
fusing alloys.
- A very thin section often have to be cast in
copings.
- Lower density of the semiprecious &
particularly, the base metal alloys.

 Gas oxygen flame, generated with a single
orifice tip, should form an inner reducing cone
about ¾ -1 inch (2-2.5cm) long.
 The multiorifice tip will have inner cones about
1.3cm long.
 For Large masses of alloys the casting force
may be reduced by 1or2, since the force
pushing the metal into the mold
is α to both the mass of the alloy
no. of turns of machine

 Excessive casting pressure can blow the alloy through
the distal end of the ring, thereby ruining the casting.
 High fusing gold alloys(both precious & semi
precious):- are generally supplied as flat tabs & will
melt into a pool that is cast when it has reached a
“white hot”, almost vaporous state.
 Non precious Ni-Cr alloys:- are generally supplied as
sharp cornered ingots of various shapes & heated until
corners are round over. They are cast at this point &
should not be allowed to “pool” which would overheat
this alloy which becomes highly oxidizable & form an
oxide skin which tends to encapsulate the molten
metal.

 There is little change in external shape.
 When cast, molten metal breaks through the
skin & quickly fills the mold with oxide trailing it.
 All alloys should be melted as quickly as
possible to avoid excessive oxidation, which
interfere with - Bonding of porcelain
- Color
 For semiprecious alloys, overheating the alloy
often leads to peripheral yellowing of the
porcelain-metal junction.
 High fusing gold alloys are least sensitive to
overheating or prolonged heating.

TITANIUM CASTING
 Titanium casting is highly technique sensitive.
 The problem associated with casting Ti is due
to its extreme tendency to interact with
atmospheric components & with the majority
of elements at melting temp.
 Factors that affect casting of Ti are:-
- High melting point (1670ºC), its strong
chemical affinity with O2, N & H.
- High reduction capacity.
- Low density.

Ti CASTING:-
 In all these machines main characteristic is
that the melting occurs inside an inert gas
atmosphere such as Argon gas, because of
the high reactivity of molten Ti, which would
absorb O2 & nitrogen from the air.
 The electric arc or high frequency induction is
used to reach the high melting point of Ti.
 To pour the molten metal into the mold
cylinder, either pressure from different gases
or centrifugal method is used.
 Copper ceramic or carbon based crucibles are
used to contain molten metal.www.indiandentalacademy.com

 Casting system by gas pressure, consists of 2
chambers:-
1) Upper melting chamber- housing a copper
crucible & a tungsten electrode.
2) Lower casting chamber- connected to the 1st
one by placing the mold cylinder in a way that
the open side meets exactly the center of the
crucible.
 The necessary power is generated by an
electric arc.
 After placing the Ti load to be melted over the
crucible, the air in both the chambers is
drained off by means of hard vacuum & Argon
gas is introduced in the melting chamber.

 The Argon gas pressure is adjusted & then, the arc
generated between Ti load & the tungsten electrode
heats up causing the Ti to melt.
 A considerable amount of thermal energy is kept at the
center of Ti load for 60sec. to compensate for the low
thermal conductivity of Ti.
 During melting & casting, the molten metal contacts the
surface of the crucible & solidifies immediately as the
crucible behaves as a heat dessipator.
 As a result, the molten Ti which is cast soon after,
slides over a layer of already solidified Ti &
contaminations are thus avoided.
 Due to reaction between molten Ti & the investment
components, a hard & visible layer of α case forms.
This layer must be removed as it complicates the
subsequent working steps of Ti & also prevents micro
cracks. www.indiandentalacademy.com

RINGLESS CASTING SYSTEMS
 To provide maxi. expansion of investment, a
ringless system is available commercially.
 The system called “powder cast ringless
system,” consists of 3 sizes of rings &
formers, preformed wax sprues & shapes,
investment powder, & a special investment
liquid.
 The tapered plastic rings allows for the
removal of the investment mold after the
material has set.
 This system is suited for the castings of alloys
that require greater mold expansion than
traditional gold based alloys.

CLEANING THE CASTING
 After casting has been completed, the ring is
removed & quenched in water as soon as the
button exhibits a dull red glow.
 Advantages of quenching:-
1) The noble metal alloy is left in an annealed
condition for burnishing, polishing & similar
procedures.
2) When water contact with hot investment, a
violent reaction ensues. The investment
becomes soft & granular & the casting is more
easily cleaned.

 Surface of casting appears dark with oxides &
tarnish. Such surface film can be removed by
a process known as “pickling,” which consists
of heating the discoloured casting in an acid.
 For gypsum – 50% HCl solution
- It aids in removal of any residual investment
as well as of the oxide coating.
 The disadvantage of use of HCl is:-
1) The fumes from the acid are likely to corrode
lab. metal furnishings.
2) Health hazard & should be vented via a
fume hood.
 Sulfuric acid is more advantageous.

 Ultrasonic devices available for cleaning &
casting,
 Best method for Pickling is to place the casting
in a test tube or dish & pour the acid over it.
 May be necessary to heat the acid, but boiling
is avoided, because of considerable amount of
acid fumes involved.
 Pickling solution should be renewed frequently,
since it is likely to become contaminated.
 In no case the casting be held with steel tongs
because, when both the casting & tongs come
into contact with pickling solution, it may
contaminate the casting.www.indiandentalacademy.com

 The pickling solution usually contains small
amounts of copper dissolved from previous
castings. When steel tongs contact with this
electrolyte, a small galvanic cell is created &
copper is deposited on the casting at the point
where tongs grip it.
 Copper deposition extending into the metal is
a future source for discoloration.
 Delicate margins may be melted or casting
may be distorted by sudden thermal shock,
when heated casting is plunged into acid.
 Gold based & palladium based metal ceramic
alloys & base metal alloys are bench cooled to
room temp. before casting is removed from
investment.

 All the investment should be removed before
further surface refinement.
 Precious alloys(Gold-Platinum-Palladium) can
be soaked with hydroflouric acid.
 Semiprecious alloys(Gold-Silver-Palladium or
Silver-Palladium) are only acid treated.
 Nickel Chromium should never be placed in
acid because of high reactivity. Part of the
casting is not to be veneered with porcelain,
finished 1st
& can be polished to smooth finish
by conventional techniques, but grease based
agents must not contact the surface to which
porcelain is added because, the organic
residue could easily lead to gas bubbles,
porcelain detachment & discoloration.www.indiandentalacademy.com

 Greasy films are removed by cleaning the
casting with household ammonia followed by,
rinsing with distilled water.
 Small nodules & residual oxidation on the
porcelain bearing surface can be removed by
rotary carbide burs & ceramic bound stones.
 Once porcelain bearing surface is clean,
sandblast it with a clean 25-50µm aluminium
oxide abrasive to texturize it.
 Casting can now be cleaned in distilled water
in an ultrasonic cleaner.

CASTING DEFECTS
DISTORTION
 Due to distortion of wax pattern
 Casting fits best when pattern is
invested immediately
 Distortion results due to thermal
changes and relaxation of stresses in
wax

SURFACE IRREGULARITIES
 Isolated imperfections, such as nodules, that
are not characteristic of the entire surface
 Surface roughness are finely spaced surface
imperfections
 Generalized casting roughness may indicate
a breakdown of the investment from
excessive burnout temperature

 Small nodules on casting are caused by air
bubbles that become attached to the pattern
during or subsequent to the investing
procedures
 Water film may form over the surface and
cause appearance of ridges or veins on the
surface. Wetting agent prevents such
irregularities

 Incomplete elimination of wax residues occur
if heating time is too short or due to
insufficient air in the furnace. Makes gold
castings brittle or the casting may be covered
with carbon which is impossible to remove by
pickling.

 Micro porosity/pinhole porosity/gas inclusions

Suck-back porosity
 “suck-back” porosity may be due to
improper pattern position or narrow long
sprue

FINS
 Too rapid a heating rate may cause cracking
of the investment. It results in fins or spines
on the casting Increased water powder ratio
 Pattern too near the investment
 Premature heating
 Too rapid heating
 Dropped mold

INCOMPLETE CASTING
 Wax pattern too thin
 Cool mold or melt
 Inadequate melt

 Incomplete castings with shiny, rounded
defects result from incomplete wax
elimination
 Inclusion porosity due to particle of
investment dislodged during casting

 Marginal discrepancy
 Wax pattern distortion
 Uneven expansion

 Inadequate/excessive expansion
 Improper water powder ratio
 Improper mixing time
 Improper burnout temperature

 Multiple nodules
 Inadequate vacuum during investment
 Improper brush technique
 Lack of surfactant

REVIEW OF LITERATURE
 J.David Duncan,1982- conducted a study to
compare the casting accuracy of 4 different
commercially available Ni-Cr alloys with that of
a precious alloy. It was concluded that:-
1) The Ni-Cr alloys tested, did not cast as
consistently or as accurately as precious alloys
2) Alloy composition & technique parameters
may also influence the accuracy of the casting
but further study was needed to determine their
singular effects.

 K. Asgar & A.H. Arfaei,1985 –
evaluated & compared the castability of
base metal alloys, high fusing noble
metal alloys & conventional type III gold
alloys with 5 different casting machines.
Results showed that there was a
significant difference in the castability
among different alloys & that the
casting machines had a stronger effect
on castability.

 A study conducted in our department under the able
guidance of DR.N.P.PATIL Sir, in 2004 was to
observe directly the actual dimensional changes
taking place inside the mold space due to variations in
temp. & its position in casting ring. A 2-D finite
element model of an invested wax pattern was
constructed using a computer software. 2 additional
finite element models were constructed by altering the
position of the wax pattern in relation to central axis of
ring.
It was concluded that the wax patterns should be
located towards the periphery rather than in the center
to achieve maxi. expansion of the mold space so that
it may compensate adequately for the casting
shrinkage. It was also advocated that the temp. of the
invested casting ring must be mini. or above 800ºC in
order to have the benefit of maxi. expansion.

 A study conducted in our department under the able
guidance of DR.N.P.PATIL Sir, in 2004 to observe
the effect of casting ring temp. on accuracy of fit of
cast Ti implant suprastructure which was based on
the assumption of the hypothesis that different
temperatures of the invested casting ring at the time
of casting would result in different magnitude &
direction of inaccuracy of the fit horizontally as well
as vertically. The patterns for the framework having
copings & connecting bar constructed & cast at 3
different temp. of the casting ring. It was concluded
that significant difference between the mean values
of linear & marginal discrepancy between 3 groups
were least at 700ºC.

SUMMARY & CONCLUSION

REFERENCES
1) Anusavice K.J. –“Phillips’ science of dental materials”
11th
edition, 2003
2) Asgar K., Arfaei A.H.- “Castability of crowns & bridge
alloys” JPD 54(1985); 60-63.
3) Craig R.G. & Powers J.M.- “Restorative dental
materials” 11th
edition 2001
4) “Dimensional changes in the mold space due to
variations in temperature & its position in the casting
ring- FEM Study” Sept. 2004
5) Duncan J.D.- “The casting accuracy of nickel-
chromium alloys for fixed prostheses”
JPD 47(1982) ; 63-67
6) “Effect of casting ring temperature on the accuracy of
fit of cast titanium implant suprastructure” Sept 2004www.indiandentalacademy.com

7) Martignoni M. – “Precision fixed prosthodontics”
clinical & laboratory aspects 1990
8) Phillips R.W. – “Skinners science of dental materials”
9th
edition 1992
9) Rosenstiel – “Contemporary fixed prosthodontics”
2002
10) Rudd K.D., Morrow R.M. – “Dental laboratory
procedures” FPD 2nd
edi, 1986
11) Stananought D.- “Laboratory procedures for inlays,
crowns & bridges”, 1975

Casting procedures and defects/prosthodontic courses

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