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1. Definition
An investment can be described as a ceramic material which is suitable
for forming a mold into which a metal or alloy is appropriately cast. The
procedure for forming the mold is described as “investing” (wax pattern).
Depending on the melting range of the alloy and the preference of the
clinician, generally two types of investment:
1. Gypsum – bonded and
2. Phosphate – bonded investment are employed.
- The gypsum based materials represent the type traditionally used for
conventional gold alloys.
- The phosphate based invest are designed purely for alloys used in the metal
ceramic restoration.
A 3rd
type is the ethyl silicate bonded invest used periapically in the
casting of R.P.D. with base metal alloys.
According to ADA specification No.2 for casting investments for dental
gold alloys are of three types of determined by whether the appliance is to be
fabricated is fixed or removable and the method of obtaining the expansion
required to compensate for the contraction of the molten gold alloy during
solidification.
1

2. Type I – are used for the casting of inlays or crowns, and the
compensation of casting shrinkage is principally by thermal expansion of the
investment.
Type II – are also used for the casting of inlays or crowns but major
mode of compensation is by the hygroscopic expansion of the investment.
Type III – are used in the construction of partial dentures with gold-
alloys.
Gypsum-Bonded Investment
The essential ingredients are :
1. α-hemihydrate of gypsum and
2. A form of silica.
α-hemihydrate gives greater strength to the material and acts as a binder
to hold the other ingredients together and provide rigidity. Although depends
on amount of binder – may contain 25% - 45% and is used for alloy with
melting ranges below 1000°C (i.e., gold-containing).
When heated to the required temperatures it shrinks considerably and
frequently fractures all form shrink considerably after dehydration between
200°C and 400°C. A slight expansion then occur between 400°C and
approximately 700°C, and then a large contraction occur. This is most likely
carried by decomposition and sulfur gases such as sulfur dioxide are emitted
2

3. which contaminates the castings (with the sulfides of non habit allogest
elements such as silver and copper). Thus not to be heated above 700°C.
α-hemihydrate requires less mixing water and shrinks less.
Silica
Added to provide a refractory during the heating of the investment and
to regulate the thermal expansion.
It exists in an allotrophic form.
1. Quartz
2. Tridymite
3. Cristobalite and
4. Fused quartz.
When heated a change in crystalline form occurs at a transition
temperatures, characteristics of the particular form of silica.
- When heated quartz invasion from a ‘low’ form α-quit to high form to
quartz at 570°C.
- Cristobalite undergoes – between 200°C-270°C from α –β cristobalite.
- Tridymite – 117°C to 163°C.
- α-allotropic form are stable only above the transition temperature, inversion
to the lower form occur on cooking in each case. The density decrease as
the α -changes to β -form resulting increase in volume.
3

4. Fused quartz is amorphous and glucobites in character exhibits inversion
at any temperature below its fusion points has an extremely low coefficient of
thermal expansion and is of little use in dental expansion.
Quartz, cristobalite, or a combination of the two forms may be used in a
dental investment.
Modifiers
Such as coloring matter reducing agents such as carbon powdered
copper to provide a non-oxidizing atmosphere in the mold when the gold alloy
is cast.
Some of the added modifiers such as toxic acid and sodium chloride not
only regulate setting expansion and the setting time, but also prevent most of
the shrinkage of gypsum when it is heated above 300°C.
Setting Time
According ADA specifications No.2 for dental alloy casting, S.T. should
not be shorter than 5 minutes nor longer than 25 minutes the modern inlay
investments set initially in 9-18 minutes.
Normal Setting Expansion
A mixture of silica and gypsum hemihydrate results in setting expansion
greater than that of the gypsum product when it is used alone. The silica
4

5. particles probably interfere with the inert washing and interlocking of the
crystals as they form. Thus the thrust of the crystals is outward during growth
and they increase expansion.
ADA specification No.2 for type I invest permits maximum setting
expansion in air of only 0.6%, that of modern invest is approximately 0.4%.
The purpose of setting expansion is to aid in enlarging the mold to
compensate partially for the casting shrinkage of the gold.
The effectiveness of the setting expansion in enlarging the mold
containing the wax pattern may be related to the thermal expansion of the
pattern caused by the heat of reaction that occurs coincidentally with the setting
of the investment. It follows from such a theory that the setting expansion and
effective only to the extent that the exothermic heat is transmitted to the
pattern.
The amount of heat present depends on the gypsum content of the
investment ; therefore the setting expansion of the invest with comparably high
content of gypsum more effective in enlarging the mold than is a product with a
lower gypsum content. Likewise manipulative conditions that increase the
exothermic heat increase the effective setting expansion, (eg, the lower the
water powder ratio for the investment, the greater is the effective setting
expansion).
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6. Other variables are:
As the investment sets, it eventually gains sufficient strength to produce
a dimensional change in the wax pattern as setting expansion occurs.
The inner wall of the investment within a MOD wax pattern can actually
force the proximal walls outward to a certain extent. If the pattern has a thin
wall then the effective setting expansion, is somewhat greater than for a pattern
with thicker walls because the investment can move the thinner wall more
readily. Also the softer wax, the greater the effective setting expansion because
the softer wax is more readily moved by the expanding investment.
Hygroscopic Setting Expansion
The hygroscopic setting expansion differs from the normal setting
expansion in that it occurs when the gypsum product is allowed to set under or
in contact with water and that it is greater in magnitude than the normal setting
expansion.
This is related to the additional crystal growth permitted and not to any
differences in chemical reaction.
In normal setting condition, the water around the particle is reduced by
the hydration and the particle are brought more closely together by the surface
tension action of the water.
6

7. In hygroscopic reaction the setting is taking place under water, the water
of hydration is replaced and the distance between the particles remain same.
As the crystals of dehydrate grow they contact each other and the setting
expansion begins in normal setting reaction the crystals being inhibited become
intermershed and entangled much sooner than those on hygroscopic reaction
which grow much more freely during the early stage before the intermeshing
finally prevents with further expansion, the hygroscopic setting expansion is
one of the methods for expanding the casting mold to compensate for the
casting shrinkage of the gold alloys.
Commercial investments exhibit different amounts of wax expansion.
ADA specification No.2 for such type II investments requires a minimum
setting expansion in water of 1.2%, the wax expansion permitted is 2.2%. the
factors controlling hygroscopic expansion.
Effect of Composition
Proportional to the silica content of the investment the fine the particle
size of silica the greater hygroscopic expansion α-hemihydrate produce more
with silica than pH. Should have enough binder with silica, at least 15% of
binder is necessary to prevent and drying shrinkage.
Effect of the Water / Powder Ratio (W:P)
Higher the W:P ratio less the hygroscopic expansion.
7

8. Effect of Spatulation
- Mixing time is reduced hygroscopic expansion decreased.
- Older the inert lower is the setting expansion.
Effect of Time of Investment
The greater amount of hygroscopic expansion is observed if the
immersion take place before the initial set, the longer the immersion of the
investment in the water both is delayed beyond the time of the initial set of the
invest. The lower is the hygroscopic expansion.
The effect of confinement
Both the normal and hygroscopic setting expansion are confined by
exposing forces such as walls of the container in which the investment is
placed on the walls of a wax pattern, the confining effect on the hygroscopic
expansion is much more pronounced than the normal setting expansion.
The increase in the effective setting expansion when the investment is
immersed in a 38°C water bath is caused mainly by the softening of the wax
pattern at the water bath temperature permitting an increase in effective setting
expansion, softened conditions of wax reduces its confining effect on the
expansion of the setting expansion.
8

9. Effect of the Amount of Added Water
The magnitude of the hygroscopic setting expansion can be controlled
by the amount of water than is added to the investment.
Magnitude is in direct proportion to the amount of water added during
the period until a maximum expansion occurs also further expansions to
evident regardless of any amount of water added, the hygroscopic setting
expansion is a continuation of the ordinary setting expansion because the
immersion water replaces the water of hydration and thus prevents the
confinement of the growing crystals by the surface tension of the excess water.
Because of the diluent effect of the quartz particles the hygroscopic expansion
in these invest is greater than that of the gypsum binder when used alone.
The phenomenon is purely physical, the hemihydrate binder is not
necessary for the hygroscopic expansion. Investment with other binder exhibit
similar expansion when allowed to set under water. Expansion can be detected
when water is poured into a vessel containing only small smooth quartz
particles, the water is drawn between the particles by capillary action and thus
causes the particles to separate, creating an expansion. Any water insoluble
powder that is wettable can be mixed with hemihydrate and hygroscopic
expansion results. The greater the amount of silica or inert filler the more easily
the added water can diffuse thus the setting material and the greater is the
expansion.
9

10. The term hygroscopic is a misnomer, although the added water may be
drawn into the setting material by capillary action, the effect is not related to
hygroscopy.
On the basis of theory the hygroscopic expansion is a normal
phenomenon as that which occurs during normal set expansions the terms have
gained general acceptance by usage.
Thermal Expansion
The thermal expansions of a gypsum bonded investment is directly
related to the amount of silica present and to the type of silica employed, the
contraction of the gypsum is entirely balanced when the quartz content is
immersed to 75%. The thermal expansion curves of the quartz is influenced by
particle size of the quartz, the type of the gypsum binder and the resultant water
powder ratio necessary to provide a workable mix.
Much greater expansion occurs during the inversion of cristobalite, the
normal contraction of the gypsum during heating is easily eliminated. The
expansions occurs at a lower temperature because of the lower inversion
temperature.
Investments containing cristobalite expand earlier and to a greater extent
than those containing quartz.
10

11. ADA specifications no.2 requires that the thermal expansion must be not
(0-66%) less than 1% nor greater than 1.6%.
Maximum thermal expansion is obtained at a temperature not higher
than 700°C.
W:P Ratio
More water that is used in mixing the investment the less is the thermal
expansion that is achieved during subsequent heating.
Effect of Chemical Modifiers
The addition of small amounts of sodium, potassium or libuim chlorides
to the investments eliminates the contraction caused by the gypsum and
increase the expansion without the presence of excessive silica.
Strength
The strength of the investment must be adequate to prevent fracture or
chipping of the mold during heating and casting gold alloy. When the alloy is
still quite hot and weak the investment and resist alloy shrinkage by strong and
constant dimension. After burnout of the pattern (mold), the strength need be
no greater than that required to resist the impact of the metals containing the
mold.
11

12. ADA specifications no.2 the compressive strength for the inlay
investments should not be less than 2.4 Mpa for gypsum.
Other Gypsum Considerations
Investments fineness affect the setting time, the surface roughness of the
casting, a fine silica results in higher hygroscopic expansion.
Porosity
As the molten metal enters the mold, the air must be forced out ahead of
it. If not a back pressure builds up to prevent the gold alloy from completely
filling the mold, the common method for venting the mold is though pores of
investment, the more gypsum crystals, the less is its porosity lower the
hemihydrates content and the greater the amount of gauging water used to mix,
the more porous it becomes.
More uniform the particles size, the greater the porosity.
Storage
Phosphate Bonded Investment
The rapid growth of use of metal ceramic restorations and the increased
use of higher melting alloys have resulted in an increased use of phosphate or
silica bonded investment.
12

13. Composition
Consists of refractory fillers and binder, the filler is silica, in the form of
cristobalite, quartz or a mixture of two – 80% concentration approximately.
The purpose of silica is to provide high temperature thermal shock resistance
and a high thermal expansion.
The binder consists of magnesium oxide (basic) and a phosphate that is
acid in nature.
Originally phosphoric acid was used, but mono ammonium phosphate
has replaced it, because it can be incorporated into the powdered investment.
Newer gold-containing alloys and other alloys used for metal ceramic
have higher melting temperature ranges and then contraction during
solidification is also greater. This necessitate greater expansion, can be
achieved by using colloidal silica suspensions with the phosphate investments,
in place of water colloidal silica liquid suspension freeze, should be assessed
before winter, freeze solid act low temperature.
Some are made to be mixed with water, for predominantly base metal
alloys, a 23% dilution of the colloidal silica is required.
Carbon is often added to the powder to produce clear castings and
facilitates the divesting of the casting from the mold, appropriate when the
13

14. casting alloys is gold not with silver containing and base metal alloys. It is
believed carbon embrittles the alloys.
Latest evidence palladium reacts with carbon if heated above 1504°C in
this case investment without carbon should be used.
Setting and Thermal Expansion
There is a slight expansion during the reaction compared to gypsum
products, and this can be increased considerably by using a colloidal silica
solution instead of water.
When phosphate investments are mixed with water this exhibit a
shrinkage within essentially the same temperature range as gypsum inert
(200°C-400°C). this contraction is practically eliminated when a colloidal silica
solution replaces.
(Some users of phosphate bonded) expansion can be decreased by the
increasingly the liquid : powder reaction ratio also by decreasing the
concentration of the special liquid or by they may use a combination of these
methods.
Working and Setting Time
Phosphate investments are markedly affected by temperature. The
normal the mix, the faster it sets the setting reaction itself gives off heat (this
itself gives heat) and this further accelerates the rate of setting.
14

15. Increased mixing time and mixing efficiency results in a faster set and a
greater rise in temperature. The ideal technique is to mix as long as possible yet
have just enough time for investing. Mechanical mixing under vacuum is
preferred.
Ethyl-Silicate – Bonded Investments
Involves more complicated and time consuming procedures involved.
Used in the construction of the high fusing base metal palladium alloys.
The binder is a silica gel, that reverts to silica cristobalite on heating.
Several method may be used to produce the silica or silicic acid gel
binder. When the pH of sodium silicate is lowered by the addition of an acid
salt, a bonding silicic acid gel forms. The condition of magnesium oxide
strengthen the gel. An aqueous suspension of colloidal silica can be converted
to a gel by the addition of an accelerator, such as ammonium chloride.
Another system for binder formation is based on ethyl silicate. A
colloidal silicic acid is first formed by hydrolyzing ethyl silicate in the presence
of hydrochloric acid, ethyl alcohol and water. The solution is then mixed with
the quartz or cristobalite to which is added a small amount of finely powdered
magnesium oxide to render the mixture alkaline. A coherent gel of polysilicic
acid then forms accompanied by a shrinkage. The soft gel is dried at a
temperature below 168°C. During the drying process, the gel losses alcohol and
water to form a concentrated hard gel, a volumetric contraction accompanies
15

16. the drying which reduces the size of the mold. This contraction is known as
“green shrinkage”, and it occurs in addition to the setting shrinkage.
The gelation process is slow and time consuming certain types of
amines can be added to the solution of ethyl silicate so that hydrolysis and
gelation occurs simultaneously.
The Sprue Former
The purpose of a sprue former or sprue pin is to provide a channel
through which molten alloy can reach the mold in an invested ring after the
wax has been eliminated.
With large restorations or prosthesis, such as removable partial denture
frame works and fixed partial dentures, the sprue former are made of wax. For
smaller casting metal pins can be used, plastic sprue forms are also available.
The diameter and length of the sprue former depends to a larger extent
on the 1. Type and size of the pattern, 2. The type of casting machine to be
used, 3. And the dimensions of the flask ring in which the casting is to be
made.
Prefabricated sprue formers are available in a wide range of gauges or
diameters sprue former gauge selection is often empirical, yet it is based on the
following five general principles (Skinner’s).
16

17. • Select the gauge sprue former with a diameter that is approximately the
same size as the thickest area of the wax pattern.
If the pattern is small the sprue former must also be small because a
large S.F. attached to a thin, delicate pattern could cause distortion.
However, if the sprue former diameter is too small, this area will
solidify before the casting itself and localized shrinkage porosity (“suck back”
porosity) may develop. Reservoir sprues are used to help overcome this
problem.
• If possible, the sprue former should be attached to the portion of the pattern
with the largest cross-sectional areas, it is best for the molten alloys to flow
from a thick section to surrounding thin areas not the reverse. This
minimizes the risk for turbulence.
Porosity :
Also, the sprue former orientation should minimize the risk of metal
flow on to flat areas of the investment or small areas such as line angles.
• The length of the sprue former should be long enough to be within 6 mm of
the trailing end and yet short enough so the molten alloy doesn’t solidify
before it fills the mold.
• The type of sprue former selected influences the burnout technique used it
is advisable to use a two-stage burnout technique, whenever plastic sprue
17

18. former or pattern are involved, to ensure complete carbon elimination
because plastic sprues soften at temperature above the mounting point of
inlay wax.
• Patterns may be sprued either directly or indirectly.
For direct spruing the sprue former provides a direct connection between
the pattern area and the sprue base or crucible former area.
In indirect spruing, a connector or reservoir bar is positioned between
the pattern and the crucible former – commonly used for multiple single units
and fixed partial dentures.
Also several single limits can be sprued with multiple direct sprue.
Reservoir should be added to a spruing network to prevent localized shrinkage
porosity.
When the molten alloy fills the heated casting ring, the pattern area
should solidify first and reservoir lost. Because of its large mass of alloy and
position in the heat centre of the ring, the reservoir remains molten to furnish
liquid alloy into the mold as it solidifies. Resulting solidification shrinkage
occurs in the reservoir bar and not in the restorations.
Sprue Former Attachment
18

19. The sprue former connection in the wax pattern is generally flared
(telescopic) for higher density gold alloy, but is often restricted for lower
density alloys.
Flaring act much in the same way as a reservoir, facilitating the entry of
the fluid alloy into the pattern area.
Sprue Former Position
sprue former attachment is often a matter of individual judgement, based
on the shape and form of the wax pattern.
Some prefer at the occlusal surface, others choose sites such as a
proximal wall or just below non functional cusp to minimize subsequent
grinding of occlusal anatomy and contact areas, as indicated earlier the ideal
area for the sprue former is the point of greatest bulk in the pattern to avoid
distorting this areas of wax during attachment, and to permit a smooth flow of
the alloy.
Sprue Former Direction
The sprue former 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 result in a casting failure.
It should not be attached at a right angle to a broad flat surface, this will
lead to turbulence within the mold cavity and serve porosity in this region, if it
19

20. is sprued at a 45° angle to the proximal area a satisfactory casting can be
obtained.
Sprue Former Length
Length depends on the length of the casting ring, if sprue is short, the
resulting mould space may be far from the end of the casting ring that gases
can not be adequately vented to permit the molten alloy to fill the ring
completely (mould space), thus may result in the porosity.
For gypsum bonded should be within 6mm of the open end of the ring,
with the higher strong phosphate bonded investments position may be within 3-
4 mm of the top of the investment.
For reproducibility of casting accuracy, the pattern should be placed as
close to the center of the ring as possible.
Wax Pattern Removal
Sprue former should be attached with the pattern on the master die,
provided the pattern can be removed directly in line with its path of withdrawal
from the die.
Preparation of the Master Die
The most commonly used die materials are type-IV (dental stone, high
strength) and type-V (dental stone, high strength, high expansion).
20

21. Relatively, inexpensive, easy to use and generally compatible with all
impression materials. Type-IV stones have a setting expansion of 0.1% or less
whereas the harder type-V stones expand as 0.3% this greater expansion is
useful for compensation of the relatively large solidification shrinkage of base
metal alloys.
To increase the abrasion resistance several means including silver
plating, coating the surface with cyanoacrylate and adding a die hardner to the
gypsum. However each may also increase the die dimensions, thus reducing
accuracy.
Methods of Altering Die Dimensions
To reduce the setting expansion of the type-IV die stone to less than
0.1% there by reducing diameter additional accelerator (potassium sulfate) and
retarder (borax) can be added to the gauging water.
To produce relief space for cement, die spacer can be used with a stone
die, the most common die spacers are resins. Although proprietary point on
liquids are sold for this purpose, model paint, colored nail polish or
thermoplastic polymers dissolved in volatile solvents enjoy wide spread
popularity.
These spacers are applied in several coats to within 0.5mm of the
preparation finish line to provide relief for the cement luting agent and to
ensure complete seating of an otherwise precisely fitting casting.
21

22. Die Stone – Investment Combination
In this technique (that has been developed) the die material and the
investing medium have a comparable composition. A commercial gypsum
bonded material, called divestments (whip mix corporation, Louisville KY) is
mixed with a colloidal silica liquid. The die is made from this mix and the wax
pattern constructed on it. Then the entire assembly (die and pattern) is inserted
in a mixture of divestment and water thereby eliminating the possibility of
distortion.
Casting Ring Liners
With the use of solid metal rings or casting flasks, the mold may
actually become smaller rather than larger because of the reverse pressure
resulting from the confinement of the setting expansion.
This effect can be overcome by using a split ring on flexible rubber ring
that permits the setting expansion of the investment.
The most commonly used technique to provide investment expansion is
to line the walls of the ring with ring liner.
Traditionally, (earlier) asbestos was the material of choice, no longer be
used because of its carcinogenic potential.
Two types of non-asbestos ring liner used are aluminium silicate
ceramic liner and a cellulose (paper) liner.
22

23. To ensure uniform expansion, the liner is cut to fit the inside diameter of
the casting ring with no overlap. The cut liner is added in position with stick
wax and then is used with a dry or wet, with a wet liner technique the liner ring
is immersed in water for a time and the excess water is shaken away.
Squeezing the liner should be avoided because this leads to variable
amounts of water removal and uneven expansion.
Ceramic liner doesn’t absorb water like a cellulose liner, its network of
fibres can retain water on the surface.
In the liner the absorbed water causes a semihygroscopic expansion as it
is drawn into the investment during setting. A thicker liner material or two
layers of liner provide even greater semihygroscopic expansion and also affect
a more unrestricted normal setting expansion of the investment in any case, the
thickness of the liner should not be less than approximately 1mm.
The length of the liner remains a matter of controversy. If the liner is
shorter than ring, the investment is confined at one or both end of the ring, the
longitudinal setting and hygroscopic expansion are thereby restircted as
(hygroscopic expansion are thereby restricted as) compared with the end where
the liner is flush with the ends of the ring.
The expansion of the investment is always greater in the unrestricted
longitudinal direction than in the lateral direction that is toward the ring itself.
Therefore it is desirable to reduce the expansion in the longitudinal direction.
23

24. Placing the liner somewhat shorter of the end of the ring tends to
provide a more uniform expansion; thus there is less chance for distortion of
the wax pattern and the mold.
Investing Procedure
The wax pattern should be cleaned of any debris, grease or oils. A
commercial wax pattern cleaner or a diluted synthetic detergent is used. Any
excess liquid is shaken off and the pattern is left to air dry while the investment
is being prepared. The thin film of cleaner left on the pattern reduces the
surface tension of the wax and permits better “wetting” of the investment to
ensure complete coverage of the intricate portions of the pattern.
While the wax pattern cleaner is air drying, the approximate amount of
distilled water (gypsum investment) or colloidal silica special liquid (phosphate
investment) is measured. The liquid is added to a clean dry mixing bowl, and
the powder is gradually added to the liquid care should be taken to minimize air
entrapment, mixing be started gently until all the powder has been wet, or the
unmixed powder may inadvertantly be ejected from the bowl. Hand mixing is
an option.
It is far more common place to mechanically mix all casting investments
under vacuum.
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25. Vacuum Mixing
Mechanical mixing under vacuum removes air bubbles created during
mixing and excavators alloy potentially harmful gases produced during
chemical reaction of the high heat investment.
Once the mixing is completed, the pattern may be hand invested or
vacuum invested. For investing by hand, the entire pattern is painted (inside
and out) with a thin layer of investment. The casting ring is positioned on the
crucible former, and 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.
Amount of porosity in vacuum investment is reduced the texture of the
cast surface is smoother with better detail reproduction and tensile strength also
increases.
In one study it has found 95% of vacuum invested castings were free of
nodules where as 17% castings made in hand investment molds were entirely
free of defects.
Air bubbles that are remain in the mix, can be entraped on flat or
concave surfaces that are not orientated suitably for air evacuation tilting the
ring slightly aids in releasing these bubbles so they can rise to the surface.
25

26. Excessive vibration is to be avoided it can cause solids in investments to
settle and may lead to free water illumination adjacent the wax pattern.
Resulting surface roughness. Excessive vibration may also dislodge small
pattern from the sprue former with miscast.
If the hygroscopic technique is employed, the filled casting ring is
immediately placed as 37°C water bath with crucible former side down. For
high heat expansion, the invested ring is allowed to bench cool undisturbed for
the time recommended by the manufacturers.
Compensation for Shrinkage
A number of factors influence the mold size:
1. two liners allows a greater setting and thermal expansion than does a single
liner.
2. Setting thermal and hygroscopic expansion can be controlled to a certain
extent by varying the liquid : powder ratio of the investment.
3. Lower the L:P ratio greater the potential for expansion, thinner mixes
reduces the expansion.
With some investment minor adjustments with L:P ratio is insignificant.
There is a limit to which L:P can be altered if it is too thick, it can’t be
applied to the pattern without distorting the pattern and producing air voids. If
the mixture is too thin, a rough surface on the casting may result.
26

27. In controlling hygroscopic expansion along with L:P ratio can also be
regulated either by reducing the time of immersion of the setting ivnestment or
by controlling the amount of water to be added during the setting process.
The longer the delay before immersion in the water bath, the less the
hygroscopic expansion that occurs.
Increasing the burnout temperature and the water bath temperature
increases the expansion and vice versa.
Controlled Water – Added Technique
Another technique, in which the shrinkage compensation is controlled
by the addition of water during the setting of the investment.
Here the linear hygroscopic expansion increases directly with the
amount of water added until a maximal expansion is attained.
The compositions of investments in this technique ensure maximal
expansion during immersion in water. The amount of hygroscopic expansion
needed is then obtained by adding may enough water to provide the desired
expansion.
A soft, flexible rubber ring is employed instead of the usual asbestos
lined metal ring. The pattern is invested as used, a specified.
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