Platinum Casting Investments
Robert Grierson Ph.D.
Mikhail Spasskiy Ph.D.
Kerr Corp.


Investment Types
When considering the type of mold material that can be used to cast a specific metal, it is necessary to first consider the properties of the metal being cast.  With platinum and its alloys the most important of these are high melting point and high specific gravity. Due to their high melting points, platinum alloys have to be transferred rapidly into the mold, and this is generally achieved with high speed centrifugal casting.  This, coupled with the high specific gravity, places a considerable force on the investment.  As there is a need to get the metal into the cavity rapidly, there is also a need to get the air out rapidly.  The permeability of the investment is therefore of interest.

Not all about casting platinum alloys is negative, however.  Platinum and its alloys, in particular those with other precious metals, are relatively chemically inert, and are thus less likely to attack the investment. Standard gypsum investments contain eight chemically different components. They set by a chemical reaction between the binder and water. This gives a distribution of fine silica particles held together by the gypsum.  When you mix a gypsum investment, the amount of water that is used is much greater than that needed for the chemical bonding reaction, and its function is to give a workable mixture. The excess water departs in the early stages of burnout.

There is another somewhat similar system which is used to a limited extent in jewelry manufacturing. This is the phosphate system.  It is used to cast non precious metal school rings and has much higher temperature stability than gypsum.  It is also used in the dental lab to work with higher melting point metals.

The investments used with platinum are chemically much simpler than those that I have just described.  They are basically about 99% fine silica with up to 1% additives. The liquid itself may be water or a dilute acid, generally phosphoric.  The mold that results from using platinum investments depends, for its structure, on both what happens when it is mixed and allowed to set, and also on changes as it is burnt out. The additives and the dilute acid can have an effect in both stages.  Initially they allow the particles of sand to be uniformly dispersed in the liquid at a low liquid/powder ratio. They also participate in the formation of a gel structure which eventually sets to give the structure, which provides green strength. During burnout they react with the silica to form mixed oxides which bond the particles to each other.

The most commonly used sintering agent was sodium silicate.  This itself can be considered as a mixed oxide of sodium and silicon.  Phosphoric acid is essentially a solution of phosphorous oxide in water and therefore also has the capability to form mixed oxides.

In the simplest of these materials we do not use deliberate acid additions, although the sintering agent which we do use performs some of the same functions that a deliberately added acid would in terms of making the mixed investment more fluid. This material I will denote as A.  It partially sets by settling, which means that as it sets, the sand particles can move around and take up their final position. This material needs up to 16 hours to set and at the end of setting there is a layer of water on the surface which is decanted off. It can be used under conditions where some of the water is wicked off during settling or it can be used with a rubber base like gypsum.

At the other end of the range are materials which use diluted phosphoric acid as the liquid and which use a sintering agent which gives a more refractory mixed oxide than does sodium silicate. I will describe this material as C.  It does not show the settling behavior of the A type and its structure is basically established during the mixing process. It is always used with a system which allows the removal of water from the investment and requires a shorter setting time than A type. This material uses a finer sand than does the A material.

Intermediate between the A and C type is one which I will call B. It is similar to A except that it contains a small amount of a powdered acid.  This makes its incorporation somewhat easier. There is some settling during its setting process but not to the extent to which the A material settles. It should also be used in such a way that water can be wicked out of it.

There is no well defined chemical reaction occurring in these materials as there is in gypsum and phosphate bonded materials. There are some weaker electro—chemical interactions occurring between the silica particles and between these particles and their liquid environment.  These lead to gelling and this gives the material sufficient green strength to allow it to be handled.

Test Procedures
The tests which we carry out are: 
(A) Fluidity (Zahn Cup)
(B) Permeability
(C) Fired Strength
(D) Thermal Expansion
(E) Casting
(F) Particle Size

The full length version of this article goes through these test procedures in detail.

Product Development
The Zahn Cup test is a very simple and useful test to allow us to both understand what is happening in the investment and also to be used as a quality control tool. When we mix these investments, we are setting up a distribution of the particles in the liquid. This process can be followed by mixing the material for a given period of time and then measuring the Zahn Cup flow time.

The type A and type B materials can be made using the same type of sand in theory, the only difference being the use of a small amount of acid. One way to use the Zahn Cup test as a Q.A. tool of the quality of sand being used is to carry out the test with two different mixing times, the shorter one being less than the recommended and the longer one being the recommended mixing time.

When we first started to work on a type C material we looked at a large range of sands.  We wanted to use a finer sand than in the A or B type material because we thought it was necessary to give a smoother surface.  Going back to the gypsum bonded materials; the binder fills in the gaps between the sand particles and thus can give a smooth surface; while in the platinum investment material we have to rely on the sand particles themselves with the smaller ones filling in the gaps between the larger ones.  We collected samples of all the readily available sands which were as fine as or finer than the ones used in A and B types and tried to use them with a phosphoric acid based liquid.

Properties of Investments
The materials were mixed according to the manufactures instructions. A and B use water while the others use phosphoric acid—based liquids. From our field testing the permeability is sufficient and the finer particle size gives smoother surfaces. We do think that there is a relationship between particle size and these properties.

The strength numbers need more work before we can claim to understand them.  The fact that some materials were stronger when burnt out at 1400ºF than at 1600ºF surprised us, but we know that some people use 1400ºF in preference to 1600ºF while others use even higher temperatures.

Fluidity is an important property and we believe that it should not be too long but we can't say what is acceptable and what is not. It depends on the material and how it is being used. Finally, thermal expansion.  All the listed materials are fairly similar.  Even though they do not contain cristobalite, they have higher expansions than the gypsum investments.

Use of the Investment
The first important step in using these investments is to use them at the correct liquid/powder ratio and to mix them long enough. This is necessary to achieve the required structure in the slurry. If this is not achieved, then it will have a negative effect on both the strength of the investment and on the surface quality of the casting.  We know that larger mixes take a longer mixing time than smaller ones from our measurements of fluidity and also from problems encountered by people who use this type of investment to cast medical and dental devices out of superalloys and who have scaled up the size of there mix.

The slurry should be well vacuumed.  In general, there is not a problem of air bubbles being trapped on the surface of the patterns as these investments wet the patterns well, but if air is trapped in the investment, it can weaken it.

The bench setting of these investments is very important.  The material forms a structure as it sets.  I don't claim to fully understand what is happening but it is likely that a silica gel is being formed. They form over a period of time and how they form will be dependent on the chemistry within the slurry.  As water is removed from the setting investment, the formation of the gel is sped up. The structure of the gel will be a function of how rapidly it is formed, and therefore the behavior of the investment will be a function of the drying rate.  Faster is not necessarily better. Different users have different methods to remove water from the setting investment and this is one area where variations in technique can give variable results. To a greater or smaller extent, some settling of the investment occurs during setting.

During burnout, care has to be taken not to heat the investment too rapidly at lower temperatures when the wax is being removed and the excess water driven off. This is no different than with a gypsum investment.  From the thermal expansion curve we know that the expansion, as a function of temperature, is relatively smooth compared with gypsum investments.  This means that the expansion behavior of the material should not limit the rate at which it can be heated. One topic that we have not yet evaluated is how the strength of the investment varies with temperature as it is burnt out.  This will be interesting to know, as it could influence the way in which the investment should be burnt out.

There is a lot that we don't know or fully understand about these materials and we continue to work on them. Hopefully we will be able to continue to improve them. A very important part of the effort is the feedback that we get from field testers and other users.
 

V4N4

Platinum Casting Investments
Robert Grierson Ph.D.
Mikhail Spasskiy Ph.D.
Kerr Corp

This is an abbreviated version of the original work. For full technical details, please consult the original paper.