casting procedure, casting defects, casting machines, finishing & polishing of casting

1. CASTING
PROCEDURE &
DEFECTS

2.  HISTORY
 BASIC STEPS OF CASTING
PROCEDURE
 SPRUE FORMER AND ITS
ATTACHMENT
 CRUCIBLE FORMERS
 CASTING RINGS AND LINERS
 INVESTING PROCEDURE

3.  WAX BURNOUT
 CASTING OF ALLOY INTO MOLD
 HEAT SOURCE
 MACHINES TO INDUCE CASTING
FORCE
 RECOVERY AND CLEANING OF
CASTING

4.  11th Century Theophilus Described
lost wax technique, which was a common
practice for making jewellery
 1558 – B. Cellini - claimed to have
attempted use of wax and clay for
preparation of castings
 1884 – A. de saran used 24K gold to form
Inlay

5.  1897 - Phillibrook described a method of
casting metal filling
 1907 -Taggart -devised a practically useful
casting machine.
 1959 - Strickland et al stated the
importance of the type, shape location &
direction other than the size of the sprue
 Apart from this various studies conducted on
the properties of investment materials and
casting alloys have lead a path for a better,
practical and useful processing methods.

6.  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-8)

8. 1) To heat the alloy as quickly as possible
to a completely molten condition.
2) To prevent oxidation by heating the
metal with a well adjusted torch.
3) To produce a casting with sharp details
by having adequate pressure to the well
melted metal to force into the mold

9.  The process of attaching a sprue
former/sprue pin to the wax pattern is
called as spruing
Purpose:-
 To provide a channel through which
molten alloy can reach the mold in an
invested ring after the wax has been
eliminated..

10.  1) Must allow the molten wax to escape
from the mould.
 2) Sprue must enable the molten metal
to flow into the mould with as little
turbulence as possible.
 3) Metal must remain molten slightly
longer than the alloy that has filled the
mould

11. Principles of selecting an
appropriate sprue

13.  uses single stage burnout.
 More preferred because it melts at the
same rate as the pattern, & allow for
easy excape for molten wax.

14.  used for castings of alloys which use 2
stage burn out with Phosphate bonded
investment.
 Their main disadvantage is its softening
temperature, which is higher than wax
pattern. And may block escape of wax.
 They may be used for casting FPD’s
because of their high rigidity, which
minimizes distortion.
 Plastic sprues may be completely solid (or)
hollow plastic.

15.  should be a non-rust metal to avoid
contamination of wax.
Hollow metallic sprue increase contact
surface area and strengthen the
attachment between the sprue and pattern.
They are removed from the investment at
the same time as the crucible former.

16. A d v a n t a g e s o f h o l l o w
s p r u e f o r m e r :
 I t i n c r e a s e s t h e
c o n t a c t a r e a .
 I t h o l d s l e s s h e a t
t h a n t h e s o l i d s p r u e
f o r m e r .
“C a r e m u s t b e t a k e n t o
e x a m i n e t h e o r i f i c e
f o r s m a l l p a r t i c l e s

17.  The diameter and length of the sprue former depends
on:
- The type and size of the pattern.
- The type of casting machine to be used.
- The dimensions of the casting rings in which casting
is made.
 Pre fabricated sprue former are available in a wide
variety of gauge from 6 to 18.
 The diameter of sprue should be equal to the
thickest portion the wax pattern.

18.  Usually for molar and metal ceramic restoration
- 10-gauge (2.5mm)
Premolars and partial coverage
restoration - 12-gauge (2.0mm)
 A narrow sprue may be useful in air pressure casting
procedure where the metal is melted in conical
depression formed by crucible former. so narrow
sprue prevents premature metal flow into mold.

19.  Large diameter sprue:
this improves the flow of molten metal
into the mould.
 less diameter sprue:
causes localized shrinkage porosity

20.  The length of the sprue former - it keeps the
wax pattern 6mm from the end of the ring

21. Very short sprue : porosity in casting
at the junction of sprue and pattern.
Very long sprue : sprue solidifies first
leading to casting shrinkage and
incomplete casting

22.  The ideal area- point of greatest bulk in the
pattern.
 The point of attachment should permit stream of
metal to be directed to all parts of the mold
without having to flow opposite the direction of
casting force

23.  Full veneer crown - sprue is attached to
Maxillary buccal and mandibular lingual
cusp.
 Partial veneer crown - sprue is attached
to cusp that encompasses the preparation.
 If attached to cusp tips near margins of
wax pattern, distortion and restriction of
flow of molten metal into mold occurs.

24. Attached 45 degrees to the walls
of mold, which decreases the
turbulence of molten alloy.

25.  The attachment of sprue former to the wax
pattern should be smooth and do not posses
pits or irregularities.
 Irregularities produces tags of investment
which is prone for fracture by molten alloy
leading to casting failure.

26. TYPES OF ATTACHMENT
D i r
e c t
I n d i r e c t

27.  In indirect spruing -
a connector or reservoir bar is
positioned between the pattern and
crucible former.
 It is common to use indirect
spruing for multiple single units and
fixed partial dentures.

28.  The sprue former should be straight to
reduce chances of creating turbulence
in molten metal entering the mold.
 High turbulence of alloy cause porosity

29. Usually a single sprue is used for small
castings.
 When two thick sections of a pattern
are connected by thin part of wax, 2
separate sprues should be attached to
each thick portion.

30.  The double sprue design is more
effective than the single sprue design
in decreasing the internal porosity
(jpd vol 78 no 4 oct 1997)

31.  Reservoir is a small amount of additional
wax which is added to the sprue former
near the junction of wax pattern
 It prevents localized shrinkage porosity
as the alloy in this part solidifies last
after the solidification of metal in mold

32.  It is used in direct spruing.
 The horizontal running bar of indirect
spacing provides the same function;
they are used when the distance
between the crucible and pattern is
high.
 The reservoir is present in
prefabricated plastic sprues also.

34.  Small auxiliary sprue/vents are applied
to thin wax pattern to improve the
quality of casting. Usually 18- gauges
sprues are used. It is indicated with
extremely thin/thick casting to produce
nonporous castings.

35. They help in escape of gases during casting
and ensure beginning of solidification in
critical areas by acting as a heat sink.
It is attached to the wax pattern directly
opposite to larger sprue former.

36.  The sprue is attached to crucible
former which constitutes the base of
casting relation with casting ring during
investing.
 It also helps by holding sprue in
desired ring.

37.  Crucible formers are basically of 2
types---
a)Steep-sided cone: used with metal
when casted using centrifugal casting
force.
b)Shallow cone: used to cast metal using
stream/air pressure

38.  They are available as----
 Rubber crucible former
 Metallic Crucible former
 Plastic crucible former

39.  They form a conical depression in
investment, which guides flow of molten
metal.
 It should be clean and petroleum is
applied to prevent formation of rough
investment tag.
 Then the end of sprue former is passed
into the hole and held in position till the
molten wax sets.

40.  Casting rings are used to confine the
fluid investment around the wax
pattern while the investment sets.
 It also allow the hardened investment
to be safely handled during burnout and
casting

41.  They are available as---
1) Shapes - Round
- Oval
2) Complete rings
I) - Rigid
- Metal (stainless steel)
- Plastic
II) Flexible - Rubber
3) Split rings
I) metal
II) plastic

42.  Considerations in selection of castings
rings
1) The internal diameter of casting ring
should be 5-10mm greater than the widest
measurement of the pattern and about 6 mm
higher.
2) For single crown/inlay - small rings as
used. Diameter - 32 mm
3) For large fixed partial denture – 63mm
round/oval shaped casting ring are used

43. Plastic ring with rubber crucible formers
are used. The ring is conical in shape with
tapering walls. As the investment sets the
investment is tapped out of ring. Then
burnout is done with out casting ring, this
causes greater expansion

44.  They are commonly used to produce
expansion of mold. Various materials
used as ring liners ----
1. Asbestos liner
2.Cellulose (blotting paper) liner
3.Ceramic ring liner
4.Combination of ceramic and cellulose
ring liner

45. 1. Allow uniform setting expansion of
investment by decreasing the confinement
of rigid casting ring.
2. In case of wet liner technique ---The
absorbed water help in hygroscopic
expansion.

46. 3.Thickness of the liner should be < 1mm
4.The amount of expansion depends on
the number of liners used. The expansion
seen with 2 liners is greater than one liner

47. 1. Asbestos liner: Asbestos is refractory
to high temperature, they show a
sufficient amount of water absorption.
There are 3 types of asbestos-
 White asbestos (least toxic) – this type
is used in dentistry
 Blue asbestos (most toxic)
 Brown asbestos (Intermediately toxic)

48.  Asbestos is no longer used in dentistry.
As produces 3 types of diseases
1) Asbestosis
2) Bronchogenic lung cancer
3) Mesothelioma – fatal tumour

49.  This material shows adequate water
absorption.
 It is burnt during burnout procedure.
So to keep the investment in contact
with ring after burnout,the liner is kept
3mm short of ring ends.
 This also restricts the longitudinal
setting and hygroscopic expansion.

50.  A long cellulose liner is carefully adapted on
the walls of casting ring and is tucked in
position with sticky wax.
 If wet liner technique is used, the lined ring
is immersed in water for some time. Then
excess water is shaken away.
Squeezing of liner should be avoided.
The liner should end 3mm short of the
casting ring end.

51. They are basically alumino-silicate
fibrous material.
 They do not absorb water to large
extent, but its network of fibres can
retain small amount of water on its
surface.

52.  They are refractory to high temperature.
 The binders used in ceramic liner (Ex –
neoprene-latex) can contribute to toxicity
(stimulate fibrosis/ act as adsorbent
surface for carcinogenesis).
 They show potential for development of
Mesothelioma .They posses fibers of ---
Length 5.3-17.8 mm.
Diameter 0.2- 0.97 mm

55.  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.

56.  T h i n f i l m o f c l e a n e r
o n p a t t e r n
a )r e d u c e s
s u r f a c e t e n s i o n o f
w a x
b )b e t t e r
“w e t t i n g ” o f w a x
p a t t e r n b y t h e
i n v e s t m e n t
 The wax pattern should not stand for
more than 20-30 min before being

57. Mixing of investment may be done
either by ----
i) Vacuum mixing
ii) Hand mixing
The incidence of bubble free casting
with different investing technique
– Open investing - 17%
– Vacuum investing - 95%

58.  The incidence of nodules on casting is
more in hand mixing then vacuum
mixing. Application of surface tension
reducing agent decreased the nodules
(Johnston, IJP, 1992, 5; 424-433).
 The best method is vacuum mix and
vacuum pour technique. But most
popular method vacuum mix and open
pour.

59. Advantages of vacuum mixing-----
1) Remove air bubbles
2) Produce smooth castings
3) Increase tensile strength of
investment
4) 95% of castings free of nodules.
5) Removes all the gaseous by products
of chemical reaction of investment
material

60.  Hand mix for 15 seconds
 Vacuum mix for 60 seconds
 Working time: 2-3 minutes

61. Mixing Ratios General ---
•
•More investment liquid, less water =more
expansion
• Less investment liquid, more water =less
expansion
• Begin with a dry bowl Use a maximum of
27ml of liquid
Using more liquid results in a
weak mold
• For 100gms of investment:-
Crowns/veneers: 22ml liquid,
5ml distilled water

62.  • Inlays/Onlays: 16ml liquid, 11ml
distilled water
 • Follow instructions on investment
packet

63. Second part

64.  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.
 Increasing 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

65.  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).

66. Expansion of the mold cavity can be
increased by--
1) increasing the no. of layers of asbestos
or fibrous ceramic lining the casting ring.
2) increasing the special liquid : water
ratio.
3) increasing the total L:P ratio.

67. 4) Placing the investment in contact with
water during setting.
5) Burning out the mold at a higher temp.
3mm on each end is left as it serves to
lock the investment within the ring &
equalize radial & axial expansion.

68.  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

69.  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).

70.  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

71.  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.

72.  Once the investment has set for an
appropriate period 45min. it is ready for
burnout.
 A crucible former is then 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.

73.  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.
 when the alpha form is converted into
beta form at inversion temp.
density decreases
volume increases, k/a
volumetric expansion

74.  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 such that it permits the
external heat to reach the internal
areas of casting ring

75.  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.

76.  These investments are relatively fragile
& require the use of metal ring for
protection during heating.
 So, the mould are usually placed in a
furnace at room temp. & slowly heated
to 650ºC-700ºC for 60min. & held for
15-30min. at the upper temp.

77.  At 468⁰C 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
mould from top adds some hygroscopic
expansion.
3) Thermal expansion at this temp. provides
the needed expansion.

78. Advantages -----
1) Less mold degradation.
2) Cooler surface for smoother castings
3) Convenience of placing molds directly
at 468ºC
 Rapid heating can generate steam that
can cause flaking of the mould walls.

79.  Too rapid heating may also cause cracking
of the investment. In such case, outside
layer of the investment becomes heated
before the centre sections.
 Outside layer starts to expand thermally,
resulting in compressive stress in the
outside layer that counteracts the tensile
stresses in the middle regions of the mold.

80. Decomposition & alloy contamination is related to a
chemical reaction between residual carbon &
CaSO4 binder.
 CaSO4 + 4C CaS + 4CO
3 CaSO4 + CaS 4CaO + 4SO2
 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.

81.  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.

82. 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 750⁰C-1030⁰C.
.

83.  PBI obtain their EXPANSION by
1)EXPANSION OF THE WAX PATTERN-
this is considerable because the setting
reaction raises the mold temp substantially.
2)SETTING EXPANSION-
this is usually greater than gypsum,
especially because special liquids are used to
enhance such expansion.
3)THERMAL EXPANSION-
this is greater when taken to temp higher
than those used for gypsum investment

84.  Heating rate is usually slow to 315ºC &
is quite rapid thereafter, reaching
completion after a hold at upper temp.
for 30min.

85.  Casting of an alloy into the mold space
uses 2 basic requirements.
A) Heat source – to melt the alloy
B) Casting force – to force molten alloy
into mold

86. casting force > surface tension of alloy
+ resistance offered by gas in the
mold
This can be done by use of following
different type of force-
 Vacuum force
 Air or Gas Pressure
 Centrifugal force

87.  Melting temp of pure gold –1063⁰c
 Melting temp of gold alloy-924-960⁰c
 Melting temp of base metal alloy-1155-
1304⁰c

88.  A) Heat Source: Different types of
materials and method are used as heat
source to melt alloy. Two basic modes are
by using
1) Torch flame--
Gas air
Gas oxygen
Air acetylene
Oxygen acetylene.
hydrogen oxygen generator
2) Electricity --

89. Two type of torch tips:
1. Multi-orifice
2. Single-orifice
Zones of the blow torch
flame:
 Zone 1 - colorless zone
 Zone 2 – Combustion
zone
 Zone 3 - Reducing
zone
 Zone 4 - oxidizing zone

90.  Gas air torch: -Gas-air torch is used
to melt conventional noble metal alloys
(used for inlays, crown and bridge)
whose melting points less than 1000⁰c

91.  Used to melt metal ceramic alloys of
higher temperature up to 1200⁰c
 The tip of torch is available as single
orifice/multiorifice.
the oxygen pressure is adjusted to
10-15 psi

92.  The flame is directed onto metal
with the nozzle of the torch about
1.5 cm away from the metal.
 Complete fluid should be obtained
within 30 second at which point the
metal is poured into the
mould.

93. The actual production of flame can be
done by adjusting the pressure and flow
of individual gases .
commonly advised pressure for acetylene
nozzle is 3.5 N/cm2 and oxygen nozzle 7-
10 N/cm2
 one part of acetylene + 2 and half part
of oxygen

94.  The best results are obtained when
flame is used with a distance of 10cm
between the face of blow torch nozzle
and the base of crucible.
 If distance is reduced to –
- 7.5 mm -slight porosity
- 5 mm -increased porosity due to
occluded H2 gas

95.  When the reducing zone is in contact,
the surface of the gold alloy is bright
and mirror like.
 When the oxidizing portion of the
flame is in contact with alloy there is a
dull film developed over the surface

96.  The Melting of alloy requires a crucible
to act as a platform on which the heat
can be applied to the metal. There are
three types of casting crucibles
available---
Clay
Carbon
Quartz
Zirconia-alumina

97.  Clay crucibles are used with high noble
and noble metal alloys
 used for crown and bridges.

98.  Quartz crucibles are recommended for
high-fusing alloys of any type of base
metal alloys and palladium alloys

99.  Carbon crucibles –
for high noble crown and bridge and also
for higher fusing gold-based metal
ceramic alloys.

100.  Carbon crucibles should not be used in
melting of high palladium, palladium
silver alloys (to be melted above
1504⁰c) and also with nickel-
chromium/cobalt chromium base metal
alloys
 The crucibles used with noble metal
alloys should not be used for melting
base metal alloy

101.  Copper –containing gold alloys and non-
copper gold alloys for use with porcelain
should not be melted in the same
crucible
 Crucible should be discarded if it
contains large amount of oxides and
contaminants from previous metals

102.  Traditionally a wet lining of asbestos sheet
was used on casting crucible. The moistened
asbestos sheet provides a clean and good
surface on which the alloy could be melted.
 Advantages is, prevent alloy contamination
with oxides and residuals that may be present
in the crucible

103. Sufficient mass of alloy must be present
to sustain adequate casting pressure---
 6g m i s t y p i c a l l y
a d e q u a t e f o r
p r e m o l a r a n d
a n t e r i o r c a s t i n g
 10g m i s a d e q u a t e f o r
m o l a r c a s t i n g
 12 g m i s a d e q u a t e f o r

104. A) Electrical resistance-
 It is used to melt ceramic alloys. Here
the alloy is automatically melted in
graphite crucible.
 Provides best means of temperature
control. It is quite convenient as
compared to blow torch.

105.  B) Electrical arc melting:
 is used to melt higher fusing alloys.
 It used to create a electrical arc at the
end of two electrodes
 The apparatus requires a
high electrical input (30A)

106.  Device for forcing the molten alloy into
the mould under pressure after wax has
been eliminated

107.  Alloy is melted in situ in crucible, followed by
applied air pressure.
 Pressure of 10-15 psi

108.  Alloy is melted in a crucible, and forced in to
mold by centrifugal force.

109.  It is used to melt ceramic alloys. Here the alloy
is automatically melted in graphite crucible.
 The crucible in the furnace is always against the
casting ring. So the metal button remain molten
slightly longer and ensures complete
solidification.
3)Electrical resistance - heated
casting machine

110.  Direct current is produce between two electrodes:
the alloys and the water cooled tungsten electrode.
 Temp between the arc rapidly increases to 4000°C –
alloy melts very quickly.
 High risk of over heating of the alloy.
 Damage may occur even after few seconds of over
heating.
Direct-current arc melting
machine:-

111.  Metal is melted by an induction field that developed
with in the crucible surrounded by water- cooled
metal tubing.
4. Induction melting machine:

112.  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 as much as
other machines

113.  Consider the gold crown & bridge alloys.
 After casting has been completed, ring is
removed & quenched in water.
Advantages:
1. Noble metal is left in an annealed condition
for burnishing & Polishing.
2. When water contacts hot investment, violent
reaction ensues. Investment becomes soft,
granular & casting is more easily cleaned.

114. A) Trimming is done from the button
end of the ring.
B) Investment is being pushed out of
the casting ring

115. C) The mold is broken open.
D) Investment is removed from the
casting. Care must be taken to avoid
damaging the margin

116.  The casting is held in a sandblasting
machine to clean the remaining
investment from its surface.

117.  Surface of the casting appears dark with oxides
and tarnish. Such a surface film can be removed
by a process called Pickling.
 Best method for pickling is to place a casting in a
dish & pour acid over it.
 Heat the acid but don't boil it.
 50% Hydrochloric acid
 Sulfuric acid
 Ultrasonic devices
 Gold and palladium based metal ceramic alloys and
base metals, these alloys are not generally
pickled.

118.  Pickling solution should be renewed
frequently, since it is likely to become
contaminated
 Precious alloys(Gold-Platinum-Palladium)
can be soaked with hydroflouric acid
 Nickel Chromium should never be placed
in acid because of high reactivity

119.  The casting is trimmed , shaped and
smoothen with suitable burs or stones.
 The sprue is sectioned off with a
cutting disc.

120.  Minimum polishing is required if all the
procedures from the wax pattern to
casting are followed meticulously.
 White stone ,rubber wheels, rubber
disks, and fine grit are included in the
finishing and polishing agents

121. Third part

122. Error in the procedure often results in
defective casting, these defects are
known as casting defects.

123. According to philips
 Distortion
 Surface roughness and irregularities
 Porosity
 Incomplete or missing details
Based on location
 Internal
 external

124. According to Rosensteil
 Roughness
 Nodules
 Fins
 Incompleteness
 Voids or porosity
 Marginal discrepancy
 Dimensional inaccuracies

125.  Distortion of the casting is probably related to
distortion of the wax pattern.
Causes:
 Can occur from the time of wax pattern preparation to
the time of investing due to stress relaxation.
 Distortion of the wax pattern occurs during the
investment procedure.
Minimized by:
 Application of minimum pressure
 Manipulation of wax at high temperature
 Investing pattern immediately
 If storage is necessary, store in refrigerator

126. Surface roughness
Defined as relatively finely spaced surface
imperfections whose height, width and direction
establish the predominant surface pattern.
Surface irregularities
Isolated imperfections such as nodules that are not
characteristic of the entire surface area
 The surface roughness of the casting is greater than
the wax pattern from which it is made, because
- the particle size of the investment and
-its ability to reproduce the pattern in microscopic
detail

127.  Small nodules on the casting are caused by air bubbles,
that become attached to the surface during or
subsequent to the investing procedure.
Prevented By:
 Proper investment technique
 Vibration of mix or by vacuum mixing
 Application of wetting agent properly
and correctly – important that it be
applied in a thin layer.
Air bubbles:

128. Water films:
 Wax is repellent to water, & If the Investment becomes
separated from the wax pattern, a water film may form
irregularly over the surface.
 Appears as minute ridges or veins on the surface.
Prevented By:
1.Use of wetting agent
2.Correct L/P ratio (Too high L/P ratio may produce
these irregularities)

129. Rapid Heating Rates
It produces
 Fins or spines on the casting
Cause-
because of Flaking of the investment
Prevented by:
 Heat gradually at least 60min from room
temperature to 700 c.
 Greater the bulk – more slowly heated.

130. Under heating
 Incomplete elimination of wax residues may occur, if
the heating time is too short.
 This factor is mainly important for low heat
technique.
Prolonged heating
 During high heat technique, decomposition or
disintegration of the investment occurs & the walls
of the mold are roughened.
 Product of decomposition are sulphorous compounds,
which contaminates the casting, this is the reason
why the surface of the casting does not respond to
pickling sometimes.

131.  Prevented by- when thermal expansion technique is used,
the mold should be heated to the casting temperature &
NEVER HIGHER.
Liquid/Powder Ratio
 The amount of water and powder measure should be accurate.
 Too little water- investment too thick & cannot be applied to
the wax pattern
 Too much water- making investment easier but reproduces
poor casting.
Casting pressure
 To high pressure – rough surface of the casting
 To low pressure – incomplete casting
 Average – 0.10 to 0.14 Mpa in an air pressure machine and
- 3 to 4 turns of the spring in centrifugal casting
machine.

132. Foreign bodies
 Any casting that shows sharp, well- defined deficiencies
indicates the presence of some foreign particles in the
mold. They may be:
- Pieces of the investment
- Bits of the carbon from the flux
- Sulfur components from – decomposition of the
gypsum investment and high sulfur content torch flame.
Pattern position
 Should not place too close together
 Should not place many patterns in same plane
 Space between the pattern is atleast 3mm

133. Impact of metal alloy
Cause:
 The direct impact of molten alloy on the weak portion of
the mold surface, may fracture or abrade the mold
surface regardless of its bulk.
Prevented by:
 This type of surface roughness or irregularities can be
avoided by proper spruing.
 Placement of sprue at 45 degree
Carbon inclusions
 Carbon from- carbon crucible,
- carbon containing investment,
- improperly adjusted torch– can be
absorbed by the alloys during casting results in
formation of carbides or visible carbon inclusion.

134. Classified as follows:
I. Solidification defects
A. localized shrinkage porosity
B. Micro porosity
II. Trapped gases
A. pin hole porosity
B. gas inclusion porosity
C. sub surface porosity
III. Residual air

135. Localized shrinkage porosity
It is caused by premature termination of the molten
metal during solidification.
It mainly occurs at sprue-casting
junction.
Cause:
 Diameter is too narrow
 Length of the sprue is too long
 Absence of reservoir
 Direction of sprue at 90 degree

136. Prevented by-
 Using sprue of correct thickness
 Attach sprue to the thickest portion of the
wax pattern
 Flaring the sprue at the point of attachment
 Placing reservoir close to the attachment

137. A hot spot is created by the hot metal impinging on the
mold wall near the sprue.
This hot spot causes this region to FREEZE LAST
Since the sprue is already solidified, NO MORE MOLTEN
MATERIAL IS AVAILABLE, resulting in
shrinkage k/a SUCK BACK POROSITY
Suck back porosity

138. It often occurs at OCCLUSOAXIAL OR
INCISOAXIAL LINE ANGLE
PREVENTED BY-
 Flaring the point of sprue attachment
 Reducing the temperature between the mold
& molten alloy

139. Pin hole and Gas inclusion
porosity
 Characterized by spherical contour, but gas inclusion
porosities are much larger than pin hole porosity.
 Occur primarily because most metals dissolve gases
when molten, these gases expelled during
solidification..
 Eg- copper & silver dissolves oxygen
platinum & palladium dissolves hydrogen
 Also be caused by gas occluded from a poorly
adjusted torch flame or use of oxidizing zone
rather than reducing zone.
 Casting is usually black, do not clean easily on pickling

140. Sub surface porosity
 Caused by simultaneous nucleation of solid grains and
gas bubbles at the first moment that the alloy freezes
at mold walls
 Prevented by controlling the rate at which the molten
metal enters the mold.

141. Back pressure porosity
Some times referred to as entrapped-air
porosity.
 found on the outer surface of the casting when
the casting or mold temperature is low, that
solidification occurs before the trapped air can
escape.

142. Causes
 Inability of the air in the mold to escape through the
pores in the investment
Prevented by:
 Proper burnout
 Sufficiently high casting pressure
 Adequate L/P ratio
 Thickness of investment between tip of pattern and end of ring
is not greater than 6mm.

143.  Factors that inhibit the mold filling is:
1. In sufficient venting
2. In sufficient casting pressure, pressure should be applied
atleast for 4 sec
3. Incomplete elimination of wax
4. Lower L/p ratio
5. Viscosity of the fused metal

144. THANK YOU