Casting procedures

1. Casting procedures
& defects
Aisha Habeeb
SVS Institute of Dental Sciences

2. Contents
Introduction
History
Steps in casting procedure
Sprue formers
Crucible formers
Casting rings and ring liners
Investing procedure

3. Wax burnout
Casting of alloys into mold
Casting of titanium alloys
Cleaning of casting
Casting defects
Conclusion

4. Introduction

5. ACCORDING TO GPT (7th edition): 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
ACCORDING TO CRAIG (13th Edition): Casting is the process
by which a wax pattern of a restoration is converted to a
replicate in a dental alloy
ACCORDING TO WILLIAM J. OBRIEN (3rd edition): Casting is a
process of forming objects by pouring molten metals in molds that
are cooled to cause solidification.

8. Copper was cast in
Mesopotamia in about
3000 B.C

9. lost wax moulding process
was first developed.

10. Benevento Cellini (1500-1571) in his autobiography described his method
of casting in both gold and bronze by coating his finely detailed wax model
with a reinforced refractory shell

11. The oldest dental castings were gold inlays

12. W.H. Taggart
1907
described a casting process which launched the cast
restoration as it is known today.

13. direct wax pattern of the
lost tooth structure
His process was
sprue
Plaster of Paris 37.5%
silica 57.5%
graphite 5%.
Invest
cast gold into space formed using air pressure

14. setting expansion - 0.45%
hygroscopic expansion -
1%
thermal expansion- 0.77%
Investment
Taggart was unable to overcome the problem of casting shrinkage
which resulted in restorations which were under size.
Lane believed that all castings made by Taggart method were undersize and attributed this to
the shrinkage of gold.

15. Van Horn introduced a different method of compensation recommending
that the wax pattern be invested at a temperature equal to mouth
temperature
the idea of casting into an enlarged mould which he achieved by using an
investment containing high percentage of silica (75% approx) plus plaster of
Paris, heated to about 650ºC.
Lane
1910
High silica
content
investment
warm
water
(46ºC)
sealed inlay ring in a water bath held at 43ºC until the investment had set

16. The low heat casting technique was finally abandoned in 1928 when
Coleman published his research paper No.32 for the United States National
Bureau of Standards in which he demonstrated the great shrinkage which
occurred when investment is cooled after heating.
Weinstein in 1929 found that adding boric acid to customary investment
mixtures prevented the shrinkage which occurred when the investments
were heated to about 370ºC and with the addition of this chemical quartz
based investments expanded approximately 0.9% when heated to 700ºC.
Moore(1993) discovered by the addition of chlorides to investments he
could obtain a thermal expansion of as much as 1.1% from quartz based
investment.

17. Two important developments occurred in the next few years were
use of Cristobalite in investment
Hygroscopic setting
expansion technique
Sweeney, Paftenbarger (1930-33)
use of a “Control powder” mixed with a cristobalite investment in varying
proportions designed to give different thermal expansion to the investment.
Phillips
(1935)

18. In 1932 Scheu presented a end technique which used the setting expansion of
investment to compensate totally for the gold alloy shrinkage.
Dr. Wilmer souder recognized that the thermal expansion of
the investment was greatly inhibited by the rigid metal casting
ring and advocated lining the ring with soft asbestos to act as a
heat resisting cushion which would permit the investment to
undergo its full thermal expansion.

19. Lost wax technique

20. Dimensional changes in the Lost wax Technique
Can be summarized by the equation
Wax shrinkage + metal shrinkage = wax expansion + setting expansion
+ Hygroscopic expansion + Thermal expansion
Goldberg (1937) recommended the use of the wax with 0.1% Contraction for
the direct technique and the wax with 0.38% contraction for the a patterns
made on a die at room temperature in the indirect technique

21. Casting shrinkage

22. The shrinkage occurs in 3 stages:
• The thermal contraction of the liquid metal between the temperature to which it is
heated and the liquidus temperature.
• The contraction of the metal inherent in its change from the liquid to the solid state.
• The thermal contraction of the solid metal that occurs on further cooling to room
temperature.

23. Coleman used the expression ‘casting shrinkage’ to include the contraction which
occurred during all the three stages already described and employed the term ‘net
casting shrinkage’ to describe the casting contraction measured in his experiments.
Earnshaw (1957) suggested the following nomenclature to give a more precise
meaning for the term “casting shrinkage”.
Inherent casting
shrinkage
Actual casting
shrinkage
Net casting
shrinkage

24. Effect of alloy composition on casting shrinkage: varies with alloy composition, the
variation amongst gold alloys normally used for ordinary cast inlays, crowns and bridges is not
large enough to warrant changes in investment expansion (Morey 1991).

25. Effect of shape of the casting
on casting shrinkage

26. Interferences could have a greater effect in castings of more complex shapes,
especially if those shapes allow interlocking of the solidified alloy and the mould.
Size and shape of the casting effect the linear casting shrinkage (Paffenbarger).
An investment with high compressive strength at the
casting temperature will be more effective than a weaker
one in restricting casting shrinkage of a complex casting
and the effect will be greatest when interlocking of casting
and mould in maximal (Earnshaw, R) 1969.

27. least in buccolingual and occluso-
gingival directions
MOD inlay casting
greatest in the mesiodistal direction
Restraint imposed by the walls of the mould

28. Effect of alloy shrinkage on the fit of MOD
inlay casting
masked by the marginal bevel

29. For a given alloy the inherent casting shrinkage is determined by its coefficient of
thermal expansion and its solidus temperature, the actual or observed casting
shrinkage will be lower by a varying amount depending on the size and shape of the
casting and the compressive strengths of the investment mould at its burnout
temperature (Moorey 1991)
Linear solidification shrinkage of casting alloys
Type I (Au-based) 1.56%
Type II (Au-Based) 1.37%
Type III (Au-based 1.42%
Type IV (Ni-Cr based) 2.30%
Type IV (Co-Cr) 2.30%

30. Theory of compensation
Compensation for the shrinkage of wax and gold by
investment expansion was studied scientifically in the early
1920s when Weinstein and Coleman at the United States
National Bureau of Standards.
Contraction occurs both during cooling of the liquid alloy
and during solidification.
Price, Coleman, Sonder

31. Setting expansion
 occurs as a result of normal crystal growth

35. The following factors tend to increase hygroscopic
expansion:
 Increased silica content of the investment & hemihydrate as the binder.
 Increased setting expansion of the binder
 Thicker mix of investment
 Increasing mixing time (spatulation)
 Immersion of the investment at or before its initial set length of time it is
immersed.
 Temperature of water bath
 Lining the ring with asbestos liner
 Using a split ring or one made of flexible rubber
 Storage at 100% humidity
 Hygroscopic technique with the pattern in the upper apart of the ring.

36. Wax pattern expansion
while the investment is still fluid occurs when the wax is
warmed above the temperature at which it was formed

37. Heat may come from
chemical reaction of the investment warm water bath

38. Sprue Direction

39. Should be directed away from thin or delicate
parts of the pattern, because the molten
metal may abrade or fracture investment in
this area and resulting in casting failure.
It should be directed away from or at 45º to
these details.

40. Length of the sprue

41. (1/4”) 6 mm
3.25 mm (1/8”)
Major factor governing the sprue length
is the length of the ring

43. Patterns may be sprued
directly or indirectly

44. Reservoir

45. Wax is added around the sprue former 1-2 mm from the
pattern in order to create an area in the mold (a reservoir)
with dimensions far exceeding that of the thickest portion
of the pattern.

46. Crucible former
It is cone shaped

47. Casting ring
Holds the investment in place during setting
and restricts the expansion of mold with the
use of solid metal rings

48. To overcome this
Split ring flexible rubber ring ceramic paper liner

49. • Stainless steel has been found to produce the most
acceptable rings.
• The thermal expansion of SS is 1.20% at 700ºC which
is compatible with the expansion of investments
provided a liner is used (Ray 1933).
29mm
38mm

50. Ringless casting system
• This method uses paper or plastic casting ring
and is designed to allow unrestricted expansion
(Engelman 1989)
• Also called as power cast ringless system
consisting of three sizes of rings and formers,
preformed wax sprues and shapes, investment
powder, and a special investment liquid.
• The crucible former and plastic ring are removed
before wax elimination, leaving the invested wax
pattern.

51. to provide a buffer of pliable material against which the
investment can expand to enlarge the mould
Ring liners
Need
Liner
semi hygroscopic expansion of the
investment

52. Asbestos liner
• no longer be used because its carcinogenic potential
can cause
• asbestosis
• bronchogenic lung cancer
• mesothelioma
The currently accepted threshold limit value for asbestos fibers range from 2 x 105 – 20×105
fibres/m3
Material of liner

53. manufactured by standard
paper making technique from
fibers of alumino silicate glass
derived from Kaolin
Cellulose fibers readily absorb
water when immersed and
therefore like asbestos wetted
before use
absorbent
cellulose
non absorbent
ceramic
Alternatives to asbestos liners have been introduced

54. How to use liner
can be used dry or wet
The maximum thickness of liner is 1 mm

55. Preparing the wax pattern for investment
wax pattern should be cleaned of any debris, grease or oils
thin films of cleanser left on the pattern reduces the
surface tension of the wax

56. The distortion of the wax pattern after its removal from the die is
a function of the temperature and time interval before investing.
The nearer the room temperature
approaches the softening point of the
wax the more readily internal stresses are
released
a pattern should not stand for more
than 20 to 30 minutes before being
invested

57. Investment Techniques

58. Hand investing
Double investing
Vacuum investing

60. INVESTMENT MATERIALS
A heat resistant or a refractory material used to form a mold into which a metal or
alloy is cast .
(OR)
A molding material that surrounds the pattern & subsequently hardens & forms the
mold after the wax pattern is eliminated.
The operation of forming the mold is called Investing .
Types – Gypsum bonded investments
Phosphate bonded investments
Ethyl silicate bonded investments

61. Casting procedure

62. Once the investment has set for approximately 1 hour
for most gypsum and phosphate bonded investments it
is ready for burnout
The crucible former and any metal sprue
former are carefully removed

63. If the burnout procedure does not immediately follow the
investing procedure, the invested ring is placed in a humidor
at 100% humidity
Dehydration of set investment that has been stored for
an extended period in 100% humidity may not replenish
all of the lost water

64. Casting Machines

65. The alloys are melted
separate crucible by a torch flame and cast into mold by centrifugal force
electrically by resistance heating or induction furnace then cast into the
mold by motor or spring action
vaccum arc melted and cast by pressure in an argon atmosphere
induction heating then cast into the mold centrifugally by motor or
spring action

66. All casting machines accelerate the molten metal into the
mold either by centrifugal force or air pressure

67. Centrifugal machines
Each of these machines depends on the centrifugal force applied to the molten
metal to cause it to completely fill the mold with properly melted metal
Advantages
• Simplicity of design and operation
• the opportunity to cast both large and small castings on the same machine

68. Operative Dentistry Modern theory and practice-Marzouk
Dental materials – Philiphs – Anusavice
Restorative Dental materials – Craig
Fixed prosthodontics – Rosenstiel
Fundamentals of fixed prosthodontics – Schillinburg.
REFERENCES

69. Dental Materials – Ferra cane
Dental Materials –Anderson
Australian Dental Journal 1991 36 (5): 391-6
1992 37(1) 93-54
1991 36(4): 302-9
1992 37 (2): 91-7
Notes on Dental Materials – E.C. Coombe
British Dental Journal 1972: 428-435.

70. CASTING DEFECTS
To be continued in PART II