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Dr Mumtaz ul Islam
M Phill Scholar 1 st year

12/16/2013

1
 Investment of the pattern to form a mould or die
 When investment becomes hard and wax pattern is

removed by burning an empty mould is left which will be
filled with any suitable material
 In acrylic resin denture wax pattern or base plate wax is
invested in a two part split mould by using hard or soft
plaster as an investment

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2
 The investment mould for casting alloys and some

castable ceramics are from a material which retains its
integrity at very high casting temperatures
 Unmodified dental plasters or stones are not suitable for
this purpose

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3
 Closely related to the investment materials are the

refractory die materials
 Used for making dies on which ceramic restorations (e.g.
porcelain crowns) are constructed
 Refractory die materials should retain their structural
integrity at the temperatures used to fire ceramics
 Phosphate-bonded die materials are most commonly used
for the construction of refractory dies

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5
 Accuracy
 Reproduction of the shape size and details of wax pattern
 Mould should not distort or fracture at very high casting

temperatures
 Should withstand high compressive strength
 Compensation for casting shrinkage of metals
A. Setting expansion of the investment mould
B. Thermal expansion during the heating of the mould to
the casting temperature
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 The casting temperature
 Type of alloy to be cast
 Some gold alloys are cast at relatively low casting

temperatures of around 900ºC
 Chromium alloys require casting temperatures of around
1450ºC

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7
 A mixture of a refractory material
 Silica is capable of withstanding very high temperatures

without degradation
 A binder which binds the refractory particles together
 The nature of the binder characterises the material

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 Gypsum-bonded
 Silica-bonded
 Phosphate bonded

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 Presentation is in the form of powder
 Liquid used is water
 A mixture of silica (SiO2) and calcium sulphate

hemihydrate (gypsum product) with graphite or powdered
copper and various modifiers to control setting time

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 A refractory material
 Adequately withstands the temperatures used during

casting
 Three allotropic forms
1. Quartz
2. Cristobalite
3. Tridymite
 Quartz and cristobalite are used extensively in
investments
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11
 Accomplished by a combination of simple thermal

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

expansion coupled with a crystalline inversion which
results in a significant expansion
Quartz undergoes inversion at a temperature of 575ºC
Low form or α-quartz to high form or β-quartz
For cristobalite conversion from the low to the high form
occurs at a lower temperature of around 210ºC
The expansion is probably due to a straightening of
chemical bonds to form a less dense crystal structure

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 Quartz and Cristobalite reverts back to the low form on

cooling
 The overall thermal expansion and inversion expansion of
materials containing cristobalite is greater than those
containing quartz

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 The calcium sulphate hemihydrate is an essential

component since it reacts with water to form calcium
sulphate dihydrate (gypsum) which effectively binds
together the refractory silica

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 The setting expansion of the calcium sulphate dihydrate is

used to partially compensate for the shrinkage of the alloy
which occurs on casting
 Further compensation can be achieved by employing the
hygroscopic setting expansion which occurs if the
investment mould is placed into water at the initial set
called as water immersion hygroscopic expansion
technique
 Results in an expansion five times the normal setting
expansion
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 Water added technique in which a measured volume of

water is placed on the upper surface of the investment
material within the casting ring
 This produces a more readily controlled expansion
 Hygroscopic expansion can be further encouraged by
lining the casting ring with a layer of damp asbestos
which is able to feed water to a large surface area of the
investment mould
 This is routinely employed technique

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 The mechanism of hygroscopic expansion is not fully

understood
 Water is attracted between crystals by capillary action and
that the extra separation of particles causes an expansion
 The magnitude of the hygroscopic setting expansion
which occurs with gypsum bonded investments is greater
than that which occurs with gypsum model and die
materials

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 Gypsum alone is not satisfactory as an investment for

alloy casting since it contracts on heating as water is lost
and fractures before reaching the casting temperature
 The magnitude of the contraction which occurs rapidly
above 320ºC is significantly reduced in investment
materials by the incorporation of sodium chloride and
boric acid

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 Type 1 thermal expansion type for casting inlays and

crowns
 Type 2 hygroscopic expansion type for casting inlays and
crowns
 Type 3 for casting complete and partial dentures

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 The ISO Standard describes all of these materials as

Dental gypsumbonded casting investments for gold alloys
 They are suitable only for a limited range of alloys

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 Powdered quartz or cristobalite
 Bonded together with silica gel
 On heating silica gel turns into silica and mould is a

tightly packed mass of silica particles

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 Ethyl silicate or one of its oligomers with a mixture of

dilute hydrochloric acid and industrial spirit
 The industrial spirit improves the mixing of ethyl silicate
and water which are otherwise immiscible
 A slow hydrolysis of ethyl silicate occurs producing a sol
of silicic acid with the liberation of ethyl alcohol as a
byproduct
 (C2H5O)4Si + 4H2O → Si(OH)4 + 4C2H5OH

12/16/2013

24
 The silicic acid sol forms silica gel on mixing with quartz
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or cristobalite powder in alkaline conditions
For pH regulation magnesium oxide is added
Stock solutions of the hydrolysed ethyl silicate binder are
normally made and stored in dark bottles
The solution gels slowly on standing after three or four
weeks
When this happens it is necessary to make fresh solution

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25
 Hydrolysis and gelation can be promoted by amines such

as piperidine
 Shrinkage occur as a result of the hydrolysis reaction
 To achieve sufficient strength at the casting temperature it
is necessary to incorporate as much powder as possible
into the binder solution
 This process is aided by a gradation of particle sizes such
that small grains fill in the spaces between the larger
grains

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 A very thick almost dry mix of investment is used and it is

vibrated in order to encourage close packing and produce
as strong an investment as possible

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27
 A small shrinkage occurs during the early stages of the

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





heating of the investment prior to casting
This is due to loss of water and alcohol from the gel
The contraction is followed by a more substantial thermal
expansion and inversion expansion of the silica similar to
that for gypsum-bonded investments
Ethyl-silicate bonded investments do not expand on
setting in the same way that gypsum-bonded and
phosphate-bonded materials do
Total linear expansion is identical to the linear thermal
expansion
12/16/2013

28
 Powder containing silica magnesium oxide and

ammonium phosphate
 On mixing with water or a colloidal silica solution a
reaction between the phosphate and oxide occurs to form
magnesium ammonium phosphate
 NH4·H2PO4 + MgO + 5H2O →+ Mg·NH4·PO4·6H2O
 This binds the silica together to form the set investment
mould

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 The formation of the magnesium ammonium phosphate

involves a hydration reaction followed by crystallization
similar to that for the formation of gypsum

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30
 As in the case of gypsum a small expansion results from

the outward thrust of growing crystals
 The material is also able to undergo hygroscopic
expansion if placed in contact with moisture during setting
 Moisture adversely affects the unmixed material and the
container should always be kept closed when not in use

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31
 The use of colloidal solution of silica instead of water for

mixing with the powder has the dual effect of increasing
the setting expansion and strengthening the set material

12/16/2013

32
 On heating the investment prior to casting mould

enlargement occurs by both thermal expansion and
inversion of the silica
 Thermal expansion is greater for the colloidal silica-mixed
materials than for the water-mixed materials
 At a temperature of about 300ºC ammonia and water are
liberated by the reaction
 2(Mg·NH4·PO4·6H2O) → Mg2·P2O7 + 2NH3+ 13H2O

12/16/2013

33
 At a higher temperature some of the remaining
 phosphate reacts with silica forming complex

silicophosphates
 These cause a significant increase in the strength of the
material at the casting temperature

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 Type 1 for inlays crowns and other fixed restorations
 Type 2 for partial dentures and other cast removable

restorations

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35
 Thermal stability
 It should retain its integrity at the casting temperature
 Gypsum-bonded investments decompose above1200ºC by

interaction of silica with calcium sulphate to liberate
sulphur trioxide gas
 CaSO4 + SiO2 → CaSiO3 + SO3
 On heating gypsum-bonded investments calcium sulphate
and carbon reacts to produce carbon mono oxide
 CaSO4 + 4C → CaS + 4CO

12/16/2013

36
 The carbon may be derived from the residue left after







burning out of the wax pattern or may be present as
graphite in the investment
Another reaction can occur liberating sulphur dioxide
3CaSO4 + CaS → 4CaO + 4SO2
These reactions occur above 700ºC
Their effects can be minimized by ‘heat soaking’ the
investment mould at the casting temperature to allow the
reaction to be completed before casting

12/16/2013

37
 The presence of an oxalate in some investments reduces

the effects of gypsum decomposition products by
liberating carbon dioxide at elevated temperatures
 Phosphate- and silica-bonded materials have sufficient
strength at the high temperatures used for casting base
metal alloys
 The strength of the phosphate-bonded materials is aided
by the formation of silicophosphates on heating

12/16/2013

38
 The wax burn-out temperature is varied to suit the type of
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






alloy being cast
Typical wax burn-out temperatures
Gold alloys 700–750ºC
Palladium-silver alloys 730–815ºC
Base-metal alloys 815–900ºC
The temperature is held for 30 minutes for small moulds
and 1 hour for larger moulds before the metal is cast
Burn-out times need to be extended when resin-based
pattern materials are used
12/16/2013

39
 Mechanical
 Strength to withstand the stresses set up when the molten

alloy enters the investment mould
 Heat causes weakening of the investment and leads to the
incorporation of porosity into the castings
 Gypsum-bonded materials are generally restricted to use
with those alloys which are cast well below 1200ºC

12/16/2013

40
 The cohesive strength of the phosphate investments is

such that they do not have to be contained in a metal
casting ring
 The material is generally allowed to set inside a plastic
ring which is removed before heating

12/16/2013

41
 The higher strengths of the phosphate-bonded materials

mean that these products are becoming widely used for
casting all types of alloys (precious, semi-precious and
base-metal)

12/16/2013

42
 The gypsum-bonded and phosphatebonded materials are

sufficiently porous to allow escape of air and other gases
from the mould during casting
 The silica-bonded materials are so closely packed that
they are virtually porosity-free and there is a danger of
‘back pressure’ building up which will cause the mould to
be incompletely filled or the castings to be porous
 These problems can be overcome by making vents in the
investment which prevent the pressure fromation

12/16/2013

43
 The accuracy of fit of a casting depends primarily on the

ability of the investment material to compensate for the
shrinkage of the alloy which occurs on casting
 The magnitude of the shrinkage varies widely but is of the
order of 1.4% for most gold alloys 2.0% for Ni/Cr alloys
and 2.3% for Co/Cr alloys
 Achieved by a combination of setting expansion thermal
expansion and the expansion which occurs when silica
undergoes inversion at elevated temperatures

12/16/2013

44
 Used to supplement the setting expansion of gypsum-

bonded materials
 This is also possible for phosphatebonded materials but is
rarely used in practice for these products
 The setting expansion of a typical gypsumbonded material
is of 0.3% which may be increased to around 1.3% by
hygroscopic expansion

12/16/2013

45
 Depends on the nature of the silica refractory used and the







temperature to which the mould is heated
Investments containing cristobalite undergo greater
thermal expansion than those containing quartz
If hygroscopic expansion has been used to achieve
expansion it is likely that the magnitude of the thermal
expansion required will be relatively small
If thermal expansion is primary means for achieving
compensation of a cristobalite-containing investment
Mould should be heated to around 700ºC
12/16/2013

46
 Undergo a slight contraction during setting and at early

stages of heating
 This is due to the nature of the setting reaction and the
subsequent loss of water and alcohol from the material
 Continued heating causes considerable expansion due to
the closepacked nature of the silica particles
 A maximum linear expansion of approximately 1.6% is
reached at a temperature of about 600ºC

12/16/2013

47
 A combined setting expansion and thermal expansion of

around 2.0% is normal provided the special silica liquid is
used with the investment
 Many manufacturers of phosphate-bonded investments
supply instructions which enable the expansion to be
varied so that the casting shrinkage of the alloy can be
compensated more precisely
 This variation is achieved by diluting the special liquid
with water

12/16/2013

48
12/16/2013

49
 The expansion reaches a maximum at 700ºC and remains

the same to 1000ºC
 The lowest permissible burn-out temperature for any
particular alloy normally gives the best results so it is
essential to follow the directions given for any particular
alloy

12/16/2013

50
 Consideration of the relatively large casting shrinkages

which can occur with some base-metalalloys in
comparison with the compensating expansions possible
with the investments may suggest that ideal compensation
is not always possible

12/16/2013

51
 Compensation may take place during other stages in the

production of the casting
 A small contraction of the impression, for example, may
give the required compensation
 Because of greater expansion of the base metal alloys

12/16/2013

52
12/16/2013

53
 Phosphate-bonded products are most widely used
 Silica-bonded materials are rarely used nowadays because

of its manipulation than the other products
 Ethanol produced in the liquid can spontaneously ignite or
explode at elevated temperatures
 phosphate-bonded materials are used even for gold
castings
 A single material is sufficient for all purposes

12/16/2013

54
 The phosphate-bonded refractory die materials are used in a

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





different way to the investments
A duplicating impression of the working die is made usually
using a proprietary material
The mixed refractory die material is poured into this
impression and allowed to set
It is then removed from the impression and consolidated by
firing at about 1000ºC
The surface is then coated with a thin layer of glaze and the die
refired at a slightly lower temperature (e.g. 970ºC)
The surface glaze helps to prevent moisture from the porcelain
from being soaked up into the porous die material
12/16/2013

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12/16/2013

56

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Investments and refrectory dies in dentistry

  • 1. Dr Mumtaz ul Islam M Phill Scholar 1 st year 12/16/2013 1
  • 2.  Investment of the pattern to form a mould or die  When investment becomes hard and wax pattern is removed by burning an empty mould is left which will be filled with any suitable material  In acrylic resin denture wax pattern or base plate wax is invested in a two part split mould by using hard or soft plaster as an investment 12/16/2013 2
  • 3.  The investment mould for casting alloys and some castable ceramics are from a material which retains its integrity at very high casting temperatures  Unmodified dental plasters or stones are not suitable for this purpose 12/16/2013 3
  • 4.  Closely related to the investment materials are the refractory die materials  Used for making dies on which ceramic restorations (e.g. porcelain crowns) are constructed  Refractory die materials should retain their structural integrity at the temperatures used to fire ceramics  Phosphate-bonded die materials are most commonly used for the construction of refractory dies 12/16/2013 4
  • 6.  Accuracy  Reproduction of the shape size and details of wax pattern  Mould should not distort or fracture at very high casting temperatures  Should withstand high compressive strength  Compensation for casting shrinkage of metals A. Setting expansion of the investment mould B. Thermal expansion during the heating of the mould to the casting temperature 12/16/2013 6
  • 7.  The casting temperature  Type of alloy to be cast  Some gold alloys are cast at relatively low casting temperatures of around 900ºC  Chromium alloys require casting temperatures of around 1450ºC 12/16/2013 7
  • 8.  A mixture of a refractory material  Silica is capable of withstanding very high temperatures without degradation  A binder which binds the refractory particles together  The nature of the binder characterises the material 12/16/2013 8
  • 9.  Gypsum-bonded  Silica-bonded  Phosphate bonded 12/16/2013 9
  • 10.  Presentation is in the form of powder  Liquid used is water  A mixture of silica (SiO2) and calcium sulphate hemihydrate (gypsum product) with graphite or powdered copper and various modifiers to control setting time 12/16/2013 10
  • 11.  A refractory material  Adequately withstands the temperatures used during casting  Three allotropic forms 1. Quartz 2. Cristobalite 3. Tridymite  Quartz and cristobalite are used extensively in investments 12/16/2013 11
  • 12.  Accomplished by a combination of simple thermal      expansion coupled with a crystalline inversion which results in a significant expansion Quartz undergoes inversion at a temperature of 575ºC Low form or α-quartz to high form or β-quartz For cristobalite conversion from the low to the high form occurs at a lower temperature of around 210ºC The expansion is probably due to a straightening of chemical bonds to form a less dense crystal structure 12/16/2013 12
  • 13.  Quartz and Cristobalite reverts back to the low form on cooling  The overall thermal expansion and inversion expansion of materials containing cristobalite is greater than those containing quartz 12/16/2013 13
  • 14.  The calcium sulphate hemihydrate is an essential component since it reacts with water to form calcium sulphate dihydrate (gypsum) which effectively binds together the refractory silica 12/16/2013 14
  • 15.  The setting expansion of the calcium sulphate dihydrate is used to partially compensate for the shrinkage of the alloy which occurs on casting  Further compensation can be achieved by employing the hygroscopic setting expansion which occurs if the investment mould is placed into water at the initial set called as water immersion hygroscopic expansion technique  Results in an expansion five times the normal setting expansion 12/16/2013 15
  • 16.  Water added technique in which a measured volume of water is placed on the upper surface of the investment material within the casting ring  This produces a more readily controlled expansion  Hygroscopic expansion can be further encouraged by lining the casting ring with a layer of damp asbestos which is able to feed water to a large surface area of the investment mould  This is routinely employed technique 12/16/2013 16
  • 17.  The mechanism of hygroscopic expansion is not fully understood  Water is attracted between crystals by capillary action and that the extra separation of particles causes an expansion  The magnitude of the hygroscopic setting expansion which occurs with gypsum bonded investments is greater than that which occurs with gypsum model and die materials 12/16/2013 17
  • 20.  Gypsum alone is not satisfactory as an investment for alloy casting since it contracts on heating as water is lost and fractures before reaching the casting temperature  The magnitude of the contraction which occurs rapidly above 320ºC is significantly reduced in investment materials by the incorporation of sodium chloride and boric acid 12/16/2013 20
  • 21.  Type 1 thermal expansion type for casting inlays and crowns  Type 2 hygroscopic expansion type for casting inlays and crowns  Type 3 for casting complete and partial dentures 12/16/2013 21
  • 22.  The ISO Standard describes all of these materials as Dental gypsumbonded casting investments for gold alloys  They are suitable only for a limited range of alloys 12/16/2013 22
  • 23.  Powdered quartz or cristobalite  Bonded together with silica gel  On heating silica gel turns into silica and mould is a tightly packed mass of silica particles 12/16/2013 23
  • 24.  Ethyl silicate or one of its oligomers with a mixture of dilute hydrochloric acid and industrial spirit  The industrial spirit improves the mixing of ethyl silicate and water which are otherwise immiscible  A slow hydrolysis of ethyl silicate occurs producing a sol of silicic acid with the liberation of ethyl alcohol as a byproduct  (C2H5O)4Si + 4H2O → Si(OH)4 + 4C2H5OH 12/16/2013 24
  • 25.  The silicic acid sol forms silica gel on mixing with quartz     or cristobalite powder in alkaline conditions For pH regulation magnesium oxide is added Stock solutions of the hydrolysed ethyl silicate binder are normally made and stored in dark bottles The solution gels slowly on standing after three or four weeks When this happens it is necessary to make fresh solution 12/16/2013 25
  • 26.  Hydrolysis and gelation can be promoted by amines such as piperidine  Shrinkage occur as a result of the hydrolysis reaction  To achieve sufficient strength at the casting temperature it is necessary to incorporate as much powder as possible into the binder solution  This process is aided by a gradation of particle sizes such that small grains fill in the spaces between the larger grains 12/16/2013 26
  • 27.  A very thick almost dry mix of investment is used and it is vibrated in order to encourage close packing and produce as strong an investment as possible 12/16/2013 27
  • 28.  A small shrinkage occurs during the early stages of the     heating of the investment prior to casting This is due to loss of water and alcohol from the gel The contraction is followed by a more substantial thermal expansion and inversion expansion of the silica similar to that for gypsum-bonded investments Ethyl-silicate bonded investments do not expand on setting in the same way that gypsum-bonded and phosphate-bonded materials do Total linear expansion is identical to the linear thermal expansion 12/16/2013 28
  • 29.  Powder containing silica magnesium oxide and ammonium phosphate  On mixing with water or a colloidal silica solution a reaction between the phosphate and oxide occurs to form magnesium ammonium phosphate  NH4·H2PO4 + MgO + 5H2O →+ Mg·NH4·PO4·6H2O  This binds the silica together to form the set investment mould 12/16/2013 29
  • 30.  The formation of the magnesium ammonium phosphate involves a hydration reaction followed by crystallization similar to that for the formation of gypsum 12/16/2013 30
  • 31.  As in the case of gypsum a small expansion results from the outward thrust of growing crystals  The material is also able to undergo hygroscopic expansion if placed in contact with moisture during setting  Moisture adversely affects the unmixed material and the container should always be kept closed when not in use 12/16/2013 31
  • 32.  The use of colloidal solution of silica instead of water for mixing with the powder has the dual effect of increasing the setting expansion and strengthening the set material 12/16/2013 32
  • 33.  On heating the investment prior to casting mould enlargement occurs by both thermal expansion and inversion of the silica  Thermal expansion is greater for the colloidal silica-mixed materials than for the water-mixed materials  At a temperature of about 300ºC ammonia and water are liberated by the reaction  2(Mg·NH4·PO4·6H2O) → Mg2·P2O7 + 2NH3+ 13H2O 12/16/2013 33
  • 34.  At a higher temperature some of the remaining  phosphate reacts with silica forming complex silicophosphates  These cause a significant increase in the strength of the material at the casting temperature 12/16/2013 34
  • 35.  Type 1 for inlays crowns and other fixed restorations  Type 2 for partial dentures and other cast removable restorations 12/16/2013 35
  • 36.  Thermal stability  It should retain its integrity at the casting temperature  Gypsum-bonded investments decompose above1200ºC by interaction of silica with calcium sulphate to liberate sulphur trioxide gas  CaSO4 + SiO2 → CaSiO3 + SO3  On heating gypsum-bonded investments calcium sulphate and carbon reacts to produce carbon mono oxide  CaSO4 + 4C → CaS + 4CO 12/16/2013 36
  • 37.  The carbon may be derived from the residue left after     burning out of the wax pattern or may be present as graphite in the investment Another reaction can occur liberating sulphur dioxide 3CaSO4 + CaS → 4CaO + 4SO2 These reactions occur above 700ºC Their effects can be minimized by ‘heat soaking’ the investment mould at the casting temperature to allow the reaction to be completed before casting 12/16/2013 37
  • 38.  The presence of an oxalate in some investments reduces the effects of gypsum decomposition products by liberating carbon dioxide at elevated temperatures  Phosphate- and silica-bonded materials have sufficient strength at the high temperatures used for casting base metal alloys  The strength of the phosphate-bonded materials is aided by the formation of silicophosphates on heating 12/16/2013 38
  • 39.  The wax burn-out temperature is varied to suit the type of       alloy being cast Typical wax burn-out temperatures Gold alloys 700–750ºC Palladium-silver alloys 730–815ºC Base-metal alloys 815–900ºC The temperature is held for 30 minutes for small moulds and 1 hour for larger moulds before the metal is cast Burn-out times need to be extended when resin-based pattern materials are used 12/16/2013 39
  • 40.  Mechanical  Strength to withstand the stresses set up when the molten alloy enters the investment mould  Heat causes weakening of the investment and leads to the incorporation of porosity into the castings  Gypsum-bonded materials are generally restricted to use with those alloys which are cast well below 1200ºC 12/16/2013 40
  • 41.  The cohesive strength of the phosphate investments is such that they do not have to be contained in a metal casting ring  The material is generally allowed to set inside a plastic ring which is removed before heating 12/16/2013 41
  • 42.  The higher strengths of the phosphate-bonded materials mean that these products are becoming widely used for casting all types of alloys (precious, semi-precious and base-metal) 12/16/2013 42
  • 43.  The gypsum-bonded and phosphatebonded materials are sufficiently porous to allow escape of air and other gases from the mould during casting  The silica-bonded materials are so closely packed that they are virtually porosity-free and there is a danger of ‘back pressure’ building up which will cause the mould to be incompletely filled or the castings to be porous  These problems can be overcome by making vents in the investment which prevent the pressure fromation 12/16/2013 43
  • 44.  The accuracy of fit of a casting depends primarily on the ability of the investment material to compensate for the shrinkage of the alloy which occurs on casting  The magnitude of the shrinkage varies widely but is of the order of 1.4% for most gold alloys 2.0% for Ni/Cr alloys and 2.3% for Co/Cr alloys  Achieved by a combination of setting expansion thermal expansion and the expansion which occurs when silica undergoes inversion at elevated temperatures 12/16/2013 44
  • 45.  Used to supplement the setting expansion of gypsum- bonded materials  This is also possible for phosphatebonded materials but is rarely used in practice for these products  The setting expansion of a typical gypsumbonded material is of 0.3% which may be increased to around 1.3% by hygroscopic expansion 12/16/2013 45
  • 46.  Depends on the nature of the silica refractory used and the     temperature to which the mould is heated Investments containing cristobalite undergo greater thermal expansion than those containing quartz If hygroscopic expansion has been used to achieve expansion it is likely that the magnitude of the thermal expansion required will be relatively small If thermal expansion is primary means for achieving compensation of a cristobalite-containing investment Mould should be heated to around 700ºC 12/16/2013 46
  • 47.  Undergo a slight contraction during setting and at early stages of heating  This is due to the nature of the setting reaction and the subsequent loss of water and alcohol from the material  Continued heating causes considerable expansion due to the closepacked nature of the silica particles  A maximum linear expansion of approximately 1.6% is reached at a temperature of about 600ºC 12/16/2013 47
  • 48.  A combined setting expansion and thermal expansion of around 2.0% is normal provided the special silica liquid is used with the investment  Many manufacturers of phosphate-bonded investments supply instructions which enable the expansion to be varied so that the casting shrinkage of the alloy can be compensated more precisely  This variation is achieved by diluting the special liquid with water 12/16/2013 48
  • 50.  The expansion reaches a maximum at 700ºC and remains the same to 1000ºC  The lowest permissible burn-out temperature for any particular alloy normally gives the best results so it is essential to follow the directions given for any particular alloy 12/16/2013 50
  • 51.  Consideration of the relatively large casting shrinkages which can occur with some base-metalalloys in comparison with the compensating expansions possible with the investments may suggest that ideal compensation is not always possible 12/16/2013 51
  • 52.  Compensation may take place during other stages in the production of the casting  A small contraction of the impression, for example, may give the required compensation  Because of greater expansion of the base metal alloys 12/16/2013 52
  • 54.  Phosphate-bonded products are most widely used  Silica-bonded materials are rarely used nowadays because of its manipulation than the other products  Ethanol produced in the liquid can spontaneously ignite or explode at elevated temperatures  phosphate-bonded materials are used even for gold castings  A single material is sufficient for all purposes 12/16/2013 54
  • 55.  The phosphate-bonded refractory die materials are used in a      different way to the investments A duplicating impression of the working die is made usually using a proprietary material The mixed refractory die material is poured into this impression and allowed to set It is then removed from the impression and consolidated by firing at about 1000ºC The surface is then coated with a thin layer of glaze and the die refired at a slightly lower temperature (e.g. 970ºC) The surface glaze helps to prevent moisture from the porcelain from being soaked up into the porous die material 12/16/2013 55

Hinweis der Redaktion

  1. Production of ethyle alcohol