Gypsum – Investment Material

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The investment material forms the mould into which an alloy will be cast.

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Requirements of Investment Materials

  • The investment should be capable of reproducing the shape, size and detail recorded in the wax pattern. The accuracy of the casting can be no better than the accuracy of the mould.
  • Thermal stability: the investment mould should be capable of maintaining its shape, integrity and have a sufficiently high value of compressive strength at the casting temperatures.
  • Compensating expansion: the investment mould should compensate for the casting shrinkage, achieved by a combination of setting, hygroscopic and thermal expansion.

Selection of Investment Material

The main factors involved in the selection of investment material are:

  • The casting temperature to be used.
  • The type of alloy to be cast.

The investment which is best able to retain its integrity at the casting temperature and able to provide the necessary compensation for casting shrinkage is chosen.


Composition of Investment Materials

Basic Components

Investment materials consist of a mixture of:

1. Refractory material: Silica is the refractory material of choice, it is available in three crystalline forms quartz, cristobalite and tridymite.

  • It adequately withstands the temperatures used during casting.
  • It is responsible for producing much of the expansion which is necessary to compensate for the casting shrinkage of the alloy.

2. Binder material: which binds the refractory particles, and may provide additional expansion to compensate for the casting shrinkage of the alloy.

The nature of the binder characterizes the material:

  • Gypsum-bonded Investment material
  • Silica-bonded Investment material
  • Phosphate-bonded Investment material

Gypsum-bonded Investment Material Composition

These materials are supplied as powders which are mixed with water and are composed of a mixture of silica (SiO2) and calcium sulphate hemihydrate (gypsum product) with minor components including powdered graphite or powdered copper and various modifiers to control setting time.

The calcium sulphate hemihydrate reacts with water to form calcium sulphate dihydrate (gypsum) which effectively binds together the refractory silica.

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 320°C, is significantly reduced in investment materials by the incorporation of sodium chloride and boric acid.

The setting expansion of the calcium sulphate dihydrate, when mixed with water partially compensate for the shrinkage of the alloy which occurs on casting.

Further compensation can be achieved by employing the hygroscopic setting expansion.

Silica-bonded Investment Material Composition

These materials consist of powdered quartz or cristobalite which is bonded together with silica gel.

On heating, the silica gel turns into silica so that the completed mould is a tightly packed mass of silica particles.

The binder solution is generally prepared by mixing ethyl silicate or its oligomers with a mixture of dilute hydrochloric acid and industrial spirit (improves the mixing of ethyl silicate and water). A slow hydrolysis of ethyl silicate occurs producing a sol of silicic acid with the liberation of ethyl alcohol as a byproduct.

Stock solutions of the silicic acid binder are normally made and stored in dark bottles. The solution gels slowly on standing and its viscosity may increase noticeably after three or four weeks, when this happens it is necessary to make up a fresh solution.

(C2H5O)4Si + 4H2O → Si(OH)4+ 4COH2H5

The silicic acid sol forms silica gel on mixing with quartz or cristobalite powder under alkaline conditions achieved by the presence of magnesium oxide in the powder.

It is necessary to incorporate as much powder as possible into the binder solution to have sufficient strength at the casting temperature. This process is aided by a gradation of particle sizes such that small grains fill in the spaces between the larger grains. 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.

Phosphate-bonded Investment Material Composition

These materials consist of a 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. This binds the silica together to form the set investment mould.

The formation of the magnesium ammonium phosphate involves a hydration reaction followed by crystallization. A small setting 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.

On heating the investment prior to casting, mould enlargement occurs by both thermal expansion and inversion of the silica.

At a higher temperature some of the remaining phosphate reacts with silica forming complex silicophosphates.


Thermal Stability

One of the primary requirements of an investment is that it should retain its integrity at the casting temperature and have sufficient strength to withstand the stresses set up when the molten alloy enters the investment mould.

Gold alloys are cast at relatively low casting temperatures of around 900°C whilst some chromium alloys require casting temperatures of around 1450°C.

Phosphate and silica-bonded materials have sufficient strength at the high temperatures used for casting higher melting base metal alloys.

Gypsum-bonded Investment Material Thermal Stability

Gypsum-bonded investments decompose above 1200°C by interaction of silica with calcium sulphate to liberate sulphur trioxide gas. This not only causes severe weakening of the investment but would lead to the incorporation of porosity into the castings.

CaSO4 + SiO2 → CaSiO3 + SO3

Thus, gypsum-bonded materials are generally restricted to use with those alloys which are cast well below 1200°C. This includes the majority of the gold alloys and some of the lower melting base metal alloys.

Another reaction may occur on heating gypsum-bonded investments above 700°C is that between calcium sulphate and carbon (from the residue left after burning out of the wax pattern or graphite present in the investment). Further reaction can occur liberating sulphur dioxide.

The effects of these reactions and can be minimized by:

  • ‘Heat soaking’ the mould at casting temperature to allow the reaction to be completed before casting commences.
  • The presence of an oxalate in some investments reduces the effects by liberating carbon dioxide at elevated temperatures.

Phosphate-bonded Investment Material Thermal Stability

The use of colloidal solution of silica instead of water for mixing with the powder increase the strength of set material.

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.

The formation of silicophosphates on heating cause a significant increase in the strength of the material at the casting temperature.

The higher strengths of the phosphate-bonded materials promote them becoming widely used for casting all types of alloys (precious, semi-precious and base-metal).

The wax burn-out temperature is varied to suit the type of alloy being cast. This temperature is normally 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.


Porosity

The gypsum-bonded and phosphate-bonded 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 from increasing.


Compensating Expansion

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.

The compensating expansion is achieved by a combination of:

  • Simple thermal expansion.
  • Expansion caused by silica crystal inversion at elevated temperatures.
  • Setting expansion.
  • Hygroscopic expansion.

Thermal Expansion

The expansion is 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 575°C from the so-called ‘low’ form or α-quartz to the so-called ‘high’ form or β-quartz.

For cristobalite, conversion from the low to the high form occurs at a lower temperature of around 210°C.

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The expansion is due to a straightening of chemical bonds to form a less dense crystal structure. The change is reversible and both quartz and cristobalite revert 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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Hygroscopic Expansion

Hygroscopic expansion can be used to supplement the setting expansion of gypsum-bonded materials. This is also possible for phosphate-bonded materials but is rarely used in practice.

The mechanism of hygroscopic expansion may be envisaged that water is attracted between crystals by capillary action and that the extra separation of particles causes an expansion.

Hygroscopic Expansion Techniques:

A) Water immersion technique:

The investment mould is placed into water at the initial set stage, this can result in an expansion of five times the normal setting expansion.

B) Water added technique:

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 is 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 technique is routinely employed even when no attempt is made to maximize hygroscopic expansion by immersing in water or adding water.

Gypsum-bonded Investment Material Compensating Expansion

The setting expansion of a typical gypsum-bonded material is of the order of 0.3% which may be increased to around 1.3% by hygroscopic 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.

If hygroscopic expansion has been used to achieve expansion it is likely that the magnitude of the thermal expansion required will be relatively small.

When thermal expansion is used as the primary means of achieving compensation a cristobalite-containing investment mould heated to around 700°C is required.

Three types of gypsum bonded investments can be identified as follows:

  • 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.

Silica-bonded Investment Material Compensating Expansion

Silica-bonded investments undergo a slight contraction during setting and the early stages of heating due to loss of water and alcohol from the gel material.

Continued heating causes considerable expansion due to the close packed nature of the silica particles. A maximum linear expansion of approximately 1.6% is reached at a temperature of about 600°C.

The total linear expansion is therefore identical with the linear thermal expansion.

Phosphate-bonded Investment Material Compensating Expansion

For phosphate-bonded materials, the use of colloidal silica solution instead of water for mixing with the powder has the dual effect of increasing the setting expansion and thermal expansion of the material.

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,

by selecting the most appropriate liquid dilution the investment can be made to compensate for casting shrinkages of both base-metal alloys and gold alloys.

Special liquid : water

Expansion (%)

Neat liquid

1.9-2.1

3 : 1

1.7-1.9

1 : 1

1.5-1.7

1 : 3

1.3-1.5

The expansion reaches a maximum at 700°C and remains the same to 1000°C. 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.

Two types of phosphate-bonded investment can be identified as follows:

  • Type 1 for inlays, crowns and other fixed restorations.
  • Type 2 for partial dentures and other cast, removable restorations.

Consideration of the relatively large casting shrinkages which can occur with some base-metal alloys in comparison with the compensating expansions possible with the investments may suggest that ideal compensation is not always possible.

It should be remembered, however, that further 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.


Applications

Investment Primary use
Dental plaster or stone Mould for acrylic dentures
Gypsum-bonded materials Mould for gold casting alloys
Silica-bonded materials Mould for base metal casting alloys (rarely used)
Phosphate-bonded materials Mould for base metal and gold casting alloys;

mould for cast ceramics and glasses

Refractory die for ceramic build-up

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