There is one key ingredient to casting platinum that is the root source of most problems and dictates just about everything else that we must do. That key is heat.

The melting temperature of platinum is quite high, particularly when compared to other precious metals that we are more used to handling, such as gold and silver. Not only does it take extremely high temperatures to melt platinum, but once the source of heat is removed, there is a very small window open during which the platinum stays fluid enough to cast.

Let's take a moment to examine the ways in which we melt platinum. The first method is the rosebud torch. There are some major drawbacks to using a torch to melt platinum. It is nearly impossible to approximate the actual temperature of the melt to within 100 degrees. Some of the platinum alloys have a maddening ability to attract gases and contaminants.  Another source of contamination in torch melting are the crucibles themselves.  Unfortunately, the melting temperature of the fused silica is less than the melting temperature of the platinum. There is no way to prevent some of the melted crucible from getting into the flowstream of the metal  when it is cast.

Luckily there is another method to melt platinum that gives us much more precise controls over many variables. That method is induction melting. An induction melting unit, when used with an accurate temperature measuring device, can take much of the guesswork out of casting platinum and brings us closer to the goal of having statistically reliable castings through the use of repeatable processes. However, induction melting does not resolve the issue of crucible contamination. Most of the crucibles used in induction casting machines for platinum also use a fused silica material. You must still be careful to keep the crucible material from getting into your metal when casting. There is only one significant drawback to an induction casting unit suitable for platinum: the price! They range in price from around $20,000 all the way up to $100,000 and beyond.  Because of the new interest that has been created for platinum, newer products are starting to be introduced. These products range from better casting machines, to new and improved investments and crucibles. In addition, an assortment of new and improved polishing compounds have been introduced. All of these products have made it easier to manufacture platinum and bring the final product to market.

Design
As with any original concept, careful consideration should be given to the fact that a particular design is going to be manufactured in platinum. In particular, when assembly is involved, thought should go into the design process to account for nuances of cleaning the castings and being able to assemble the piece to come out looking like the design.

Model Making
The exact concepts and theories that have become quite popular in dealing with gold casting run contrary to what can be done with platinum.  That is to say that the concept of filling parts through thinned or reduced areas does not allow enough metal to flow into thicker areas of a piece cast in platinum. How do we address this all too common problem? In my opinion, the best way to deal with transition areas of thick to thin to thick again is to cast the piece in smaller parts and assemble it afterwards. This allows each part to be properly designed, sprued and cast to achieve optimum quality. 

The second-best solution is to leave the design as a one-piece cast, but reinforce the thin areas to allow for better metal flow through those areas. A third way in which we try to address this problem is with creative sprueing techniques. I have found this to be a hit or miss solution, however.

To sum up the model discussion, it is an important part of the learning curve to understand how your piece will be worked on and changed throughout the manufacturing process and properly address those issues in the model construction. If your models are made for repeated production purposes, the job of making molds for these pieces is quite routine. Regular rubber production molds are still the best and most economic way to go.

Waxes
The next step of casting is making the waxes. There are injection waxes and handmade or hand-carved waxes. For injection waxes for the application of platinum casting, I believe that the natural, vegetable-based waxes work better than petroleum-based wax. The problem with hand-carved wax is that unlike an injection wax which we use everyday and can experiment with and learn to deal with, the various carving waxes have many different melting points and different properties of expansion when they are heated in the investment mold before they melt and are burnt out.

Sprueing
Sprueing is the next step, and is perhaps the single most important factor in determining whether or not your casting will result in a successful piece. By correctly sprueing your waxes with large enough diameter sprues to the heaviest sections of your pieces, you may be able to avoid the problem of porosity. The goal should be to keep metal flowing freely into the mold cavity and eventually choke off the sprue.  Remember, our casting window is very narrow, and the process happens very quickly. Solidification occurs very soon after the heat source is removed from the crucible. Keep your sprues thick, short, and well aimed.  The next step, as in any lost wax casting process, is setting up the waxes to be cast on either a tree or a button. The general method when torch melting, is to use a button.  In other words, set the waxes directly into the button. This will achieve the shortest distance from the button to the cavity to be filled.  If you are lucky enough to own one of the newer induction melting centrifugal casting units, you will find that setting up on trees will enable you to have more production per flask, thus giving you a better product-to-scrap ratio.

Casting Platinum
On the market today there are essentially two different types of platinum investments available. The first of these is a dry, phosphate-bonded material to which only water is added to activate it. This type of investment is quick drying (about the same as gold investment), and can be put in the burnout oven within an hour or two of when it has been invested. The other type of investment that is used has a phosphoric acid binder system. Even though this material can give you a smooth, strong mold, it is very unforgiving.

A few tips that I can pass along when working with phosphoric acid systems would be:
1. Always carefully measure your ingredients. When making binder solution from
concentrate, when measuring your binder solution for the mix, and when measuring the
amount of dry powder you wish to add.

2. There are differing opinions about whether to use a flask liner, and if so, what the best material is to use. This is an area that requires careful study and analysis on the part of the platinum caster.

3. Work clean. Don't let material build up on your mixing utensils that can fall off into a mix at a later time.

4. Try de-ionized water. It has helped me.

5. Once a flask has been invested, put it on a solid, vibration-free work area until you are ready to move it. Prematurely moving the flask can lead to finning and cracking or poor surface quality on the parts.

6. When you have found a system that works well for you in your shop, commit the procedures to writing and don't allow variation to creep in. To properly cast platinum, discipline is required.

Burnout
Regarding the burnout procedure, I have found that long and slow is the way to go. The same would apply to the burnout cycle. The initial de-waxing is the first critical phase. Because of the different types of waxes that are used on the market today and the variables in melt point as well as thermal expansion, burnout would bring you to a long, gradual rise of temperature over a period of 10 to 12 hours. Silica goes through a phase change at about 1062o F. This then becomes one of the most critical points of the burnout cycle and you should slowly nurse the molds through this phase. After that point, a somewhat more aggressive rise in oven temperature should do no harm.

Variables
Regardless of the type of casting machine that you use, there will be variables. And certain variables you can control. The first is flask or mold temperature. We can easily control the mold temperature by controlling our oven temperature. What you should keep in mind is that the difference between the temperature of the metal and that of the mold is very wide apart, unlike gold, where the metal temperature is reasonably close to the flask temperature. When you combine that fact with the extremely narrow liquidus to solidus range of platinum, we have a situation where essentially we are putting a quick-chilling metal into a cold mold. Obviously this will continue to enhance the quick chilling properties of platinum. What this all means to the caster is that he must manipulate other variables in the equation to compensate for this quick-chill aspect. The most convenient approach would be to vary metal temperatures and also to vary the speed of the centrifugal rotation as well as the ultimate rpm. If you can combine these variables with appropriate mold temperature and adequate sprueing, you will greatly enhance your acceptable percentage of parts cast.

The following are some of the universal guidelines that you may draw from when casting platinum:

1.The heavier and thicker a piece, the lower your mold temperature should be.  Conversely, the lighter and thinner pieces will require a higher mold temperature.

2.A second universal guideline that I would like to share with you is that most crucibles have a melting temperature lower than the platinum itself. So whether you are melting with a torch or with an induction melter, the better job you do of segregating your molten charge from the crucible itself, the less chance you will have of crucible material being caught in the flowstream of metal on its way into the flask, thus leaving inclusions in casting surfaces.

3.Another truth that I live by is: you must keep your metal clean. When platinum is either molten or very hot it will have a strong propensity to combine with certain other materials with which you are working, resulting in casting defects. Work clean, work neat, and be careful about adding anything to your melt charge that could contaminate it. These types of things would include but not be limited to: iron or steel from flasks, files from saw blades and tweezers, or every other tool, for that matter.

Finally, although we know that because platinum tends to not oxidize we can continually reuse our scrap, it is still important to add a percentage of fresh metal to each melt. Another point that is more critical with platinum than with any of the other precious metals is strict adherence to procedures.  Once the procedures are established, the process of casting platinum is a very delicate balance and can be extremely unforgiving.  Once you have established the specific parameters that work in your shop, you must be dogmatic about strictly sticking to the rules that you have established.

Removing the Metal from the Mold
The final phase of the casting process involves removing the metal from the mold and cleaning it so that it can go on to the finishing process. Unfortunately, because of the durable ceramic natures of platinum investment, it just doesn't fall away in water like we are used to seeing with gold or silver investments.

Ways to remove the tree from the flask are as follows:

Tap the sides of the steel flask firmly with a hammer. This will begin to dislodge the investment and the tree will slip out of the cylinder with a lot of investment still stuck to it. By gently tapping on the bottom of the button you should be able to dislodge a considerable additional amount of investment.
Another way to dislodge the platinum from the flask is to drop the whole thing into a very large container of cold water. This will result in a small explosion which will leave the tree quite free from investment.

You may also set up a press to actually press the ceramic cylinder from out of the steel casing. Once it is out you can either tap with a hammer or use an air hammer on the button to shake loose larger chunks of investment.

No matter what you do, there will still be investment stuck to the castings that needs to be removed. You can try using different types of blasters such as a sand blaster or a bead blaster or a wet/dry sand blaster.  The way to get the cleanest castings is to use hydrofluoric acid. Please be aware that this is a very dangerous and powerful chemical. You should thoroughly familiarize yourself with all safety and handling procedures before you use it.

Materials
Ceramics play a key role in the casting of platinum. The two primary areas would be in your investment and in your crucibles. There are some newer zirconia products that are either calcium- or yttria-stabilized, allowing them to be somewhat less sensitive to thermal shock than traditional zirconia products. Continued research into this area should eventually yield positive results for the platinum caster. A big part of today's reality still revolves around silica- or silica-related products. In the area of thermal expansion, silica products do extremely well. Just be sure to be aware of the limitation of silica—its relatively low melting point. Don't let molten platinum sit in a silica crucible for too long.
I've already discussed investments and how ceramics are used in that application.    Part of your routine maintenance should be to periodically check the accuracy of all of your temperature measuring devices. In particular, be aware that thermal couplers do wear out and must periodically be replaced. One final area of material selection to be aware of is the type of wax that you are using. Learn what is out there and how it will affect the outcome of your process.

Choosing an Alloy
There are many alloy combinations that are used around the world.  Each one has been designed and developed by metallurgists because it has properties that are particularly advantageous when being used to perform specific tasks.  For many years my company and probably many others cast in one alloy only: 90% platinum/10% iridium. Yet there has come along a need for something new. There are many companies who want to export their products beyond the United States, and different countries have
different requirements for trade marking and purity standards. There arose a need for a
95% platinum alloy. The one that gained popularity in the United States was 95%
platinum/5% ruthenium. The primary drawback of 95% platinum/5% ruthenium was its intense desire to absorb gases, oxygen in particular. In my research I learned that an alloy consisting of 95% platinum/5% cobalt would fill a number of the criteria I had been searching for in an appropriate alloy to be used for casting. For starters, platinum/cobalt has a better fluidity than the other platinum alloys. I was able to fill even the finest filigree. In the area of color, platinum/cobalt was unique. When final rouging is completed, the metal has a faint bluish hue to it. Finally, the melting temperature of platinum/cobalt is lower than platinum/iridium or platinum/ruthenium. The significance of this was that we could cast platinum/cobalt at lower metal temperatures than the other alloys and in turn we seemed to get less contraction and shrinkage than had been the norm. Which alloys you decide to cast is a matter between you and your customers.
However, it pays to learn the nuances of each and handle them accordingly.

Porosity
There are many comparative types of porosity. The first is shrinkage porosity, caused by inadequate flow of metal to a given section of a design. The best ways to deal with this is the use of good models, adequate sprueing, and correct coordination between the mold and the temperature of the metal. The other type of porosity we encounter is gas porosity. Gas porosity is most easily recognized by inclusions in the casting that are usually round or spherical in shape. The reasons and the cures for gas porosity are much more varied and complex than shrinkage porosity. Suffice it to say that there is some type of reaction which is taking place. This reaction is possibly between the metal and the crucible, between the metal and outside gases, between the metal and the mold, or some other impurity which has worked its way into your system due to poor procedure or metal contamination. If you feel you are having a problem with gas porosity, you should closely examine what you are doing and how you are doing it.
Cracking and surface texture is another problem. Poor surface texture can be a result of a bad model, a bad mold, a bad wax, an improper investing technique, or using the wrong combination of metal and mold temperatures. Some things that can be a cause of cracking might be improper sprueing, or casting with temperatures that are too cold, causing improper joining of metal. Another cause is contamination. A sure way to get your platinum to crack is to contaminate it with foreign materials such as braze alloyed gold.  Even a poor gas and oxygen mixture in your torch can cause problems.

Further Recommendations
Research, experiment, and do testing on your own. Try to be as scientific about your approach as possible. If you add too many variables to an experiment even if you get a positive result, you won't know why. Be patient. There is a progressive learning curve in platinum casting. It's hard to rush.
 

V1N5

Platinum Casting Techniques
Richard Atkin
Lou Atkin Castings

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