Investment materials

Dr. Waseem Bahjat Mushtaha

Specialized in prosthodontics

Introduction

When a restoration or appliance is being made by a (loss wax) process, the wax pattern is embedded in an investment material. The wax is then removed from this mould, and the space which it occupied is filled by the material of which the restoration or appliance is to be made

Requirements for investment materials

All investment materials contain:

1 )A refractory substance: a material that will not decompose or disintegrate on heating.

2 )A binder: a material which will set and bind together the particles the refractory substance.

In addition the following properties are desirable:a) The mould must expand to compensate for the shrinkage on cooling of the alloy.

b) The powder should be of a fine particle size to ensure a smooth surface on the casting.

c) The mixed unset material should have a smooth consistency.d) The material should have a suitable setting time.

e) The set material should be permeable to allow air to escape as the molten alloy enters the mould.

f) The strength of the material should be sufficient to withstand the force of the molten alloy entering

the mould .

Types of investment material

Three types are available. They all contain silica (SiO2) as the refractory constituent. The chief difference between them is the type of binder used, as follows:

a) gypsum-bonded investments are widely used for gold alloys, but are unsuitable for alloys which melt at temperatures approaching 1200C

b) Phosphate- bonded materials are used for casting cobalt-chromium alloys, since they can withstand higher temperatures

c) Silica-bonded investments are an alternative to the phosphate-bonded materials for high temperature casting

Gypsum-bonded investments

Gypsum-bonded investments constituents

a. Silica is present in one of its allotropic forms (cristobalite or quartz) to:

1 )Act as refractory.

2 )Provide mould expansion by thermal expansion and inversion

B- Autoclaved calcium sulphate hemihydrate, for the following purposes:

1 )To react with water and on hydration to bind the silica together

2 )To impart sufficient strength to the mould.

3 )To contribute to the mould expansion by the setting expansion which occurs.

C- A reducing agent such as powdered charcoal, to reduce any oxide formed in metal.

D- Modifying chemicals such as boric acid or sodium chloride to inhibit shrinkage on heating .

Manipulation

a) The mixing of an investment material is similar to that of dental stone. Use of the correct water/powder ratio is important to ensure that the correct strength, setting time and expansion are obtained.

b) Before investing the wax pattern , it is washed with a non-foam detergent to remove any oil or grease, and to facilitate the wetting of the pattern

by the investment mix .

C) The casting ring is usually lines with a wet asbestos strip. This does two things:

1 -It facilitates mould expansion, since it can be compressed as expansion occurs, whereas a rigid ring on its own cannot do this.

2 -It contributes the pattern may be carried out

a) Under vacuum to prevent trapping air on the surface of the pattern

b) painting investment material on to the pattern with a brush before carefully inserting it into the filled

casting ring

E- The mould is heated through 150-200C; this dries out the excess water and burns off the wax. The mould is then slowly heated to above the temperature of inversion- usually to 700C it is held at this temperature for 30 minutes to ensure that the wax is

completely burn out .

Dimensional changes of the mould

A) Setting expansion is caused by the crystal growth of gypsum.

b) Hygroscopic expansion. In one technique the investment is immersed in water after setting has begun. A greatly increased setting expansion occurs. So less thermal expansion is required. Increased hygroscopic expansion is obtained in the following cases:

1 )When a lower water/powder ratio is used2 )For an investment material of greater silica content.

3 )If water of higher temperature is used.4 )For longer immersion in water.

c) Thermal expansion. Investment materials containing cristobalite and quartz show rapid expansions between 200-300C and 500-600C respectively, due to displacive transformation of the silica. The amount of thermal expansion therefore depend on:

1 -Temperature.2 -Quantity of silica in the material

3 -Allotropic form of silica used, for example the thermal expansion of cristobalite is greater than that of quartz at most temperatures.

4 -water/powder ratio- thicker mixes have greater thermal expansion.d) Shrinkage on heating. This occurs due to the dehydration of the set gypsum in two stages:

2CaSO4,2H2O (CaSO4)2,H2O + 3H2O(CaSO4)2,H2O 2CaSO4 + H2O

The investment shrinkage is eliminated or reduced by the presence of small quantities of additives such as sodium chloride or boric acid.Other properties:

a) The total expansion of the mould is generally sufficient to compensate for the shrinkag on cooling of cold alloys ( about 1.5% by volume).

b) The investments containing finer particles of silica and calcium sulphate hemihydrate give smoother surfaces on the finished casting.

c) gypsum-bonded investments are easy to manipulate giving a smooth consistency mix.

d) The setting time of theses materials can be easily controlled, as for stone and dental plaster

e) The set investment is porous, as is set gypsum. This helps to prevent back-pressure porosity in castings.

f) The strength of these materials, when set, if mixed at the correct water/powder ratio, is sufficient to withstand the forces of the molten alloy as it enters the mould. The autoclaved hemihydrate is used in preference to the calcined material for this reason

Limitations

Above around 1200C, a reaction can occur between calcium sulphate and silica:

CaSO4 + SiO2 CaSiO3 + SO4The sulphur trioxide gas that is evolved:

1 )Causes porosity in the casting.2 )Contributes to the corrosion of the casting.

For this reason gypsum-bonded investments are not used for the higher fusing dental alloys, such as cobalt-chromium. In this case phosphate or silica

bonded materials are chosen .

Phosphate-bonded investments

Phosphate-bonded investments composition and setting

Magnesium oxide can react with a phospharic such as an ammonium phosphate in an aqueous system as follows:

MgO + NH4H2PO4 MgNH4PO4 +H2O

The crystals of the magnesium ammomium phosphate bind together the particles of the

silica refractory .

Manipulation

These materials are mixed with water, similar to gypsum-bonded investments. However, the following difference in manipulation should be noted:

a) Because of the strength of the set material, metal casting rings are not needed. In their place, plastic rings can be used, they are removed after the material has set, but before the investment is heated.

b) The investment is heated to 1000-1100C

Properties

a) Expansion. The setting reaction is accompanied by an expansion, analogous to the crystal growth of gypsum. Also, thermal expansion occurs in heating.

b) Porosity : a set phosphate-bonded material shows a certain degree of porosity, again similar to the gypsum containing investments.

c) strength: the set materials increases in the strength during heating, possibly due to chemical interaction between silica and the binder, giving complex silicophoshates.

Silica-bonded investments

Setting reaction:a) Stage 1 : hydrolysis. Ethyl silicate can be hydrolysed to silica acid, with liberation of ethyl alcohol:

Si(OC2)4 + 4H2O Si(OH)4 + 4C2H5OHIn practical, a polymerised form of ethyl silicate is used, yielding a sol of polysilicate

acid .

b) Stage 2: gelation. The sol is mixes with cristobalite or quartz, then gel formation is made to occur under alkaline conditions by adding magnesium oxide. There is a slight shrinkage at this stage.

c) Stage 3: drying. on heating, considerable shrinkage occurs and there is a loss of alcohol and water, leaving a mould made of silica particles tightly packed together.

As alternative of the above, simultaneous hydrolysis and gel formation can occur, when an amine such

as piperidine is incorporated .

Properties

a) Dimensional changes. There is a large amount of the thermal expansion, due to the very large percentage of silica in the final material. This

expansion is usually sufficient to compensate for :1 -The setting shrinkage of the investment material .

2 -The casting shrinkage of the alloy.b) porosity. The particles of the set material are packed so closely together that the porosity is negligible. Air spaces or vents must be left in the investment to permit escape of the air from the mould.

Venting