Update on Platinum Casting

June 1999

Precious Metals & Bench:Metalsmithing

Update on Platinum Casting

It's not as difficult as it used to be thanks to new equipment and technology

The perception: It's more difficult to cast platinum than other metals. The reality: The exploding popularity of platinum has led to new products and technologies that make the process more efficient, consistently better and more profitable.

This is the consensus of many of the experts who spoke at the recent Platinum Day VI Symposium in New York City, sponsored by the Platinum Guild International. The symposium was timed to coincide with the Manufacturing Jewelers and Suppliers Association's Expo New York. To help you cast platinum with confidence and consistency, here's a look at the latest technologies and some tips gleaned from the symposium and from conversations with exhibitors at Expo New York.

Your Goal
The goal of casting platinum, or any other metal, is to get consistently good quality. A cast that isn't of adequate quality has to be fixed, recast or discarded then refined. These operations waste time and money. Therefore, says Roger Green of Eisinger Enterprises, Newark, NJ, the goal of good casting should be a piece that has:

  • No defects.
  • Sufficient mechanical strength.
  • Good surface.
  • No porosity.
  • A homogenous mix of platinum and alloy.

Preparing To Cast: The Wax Model
The first step in the casting process is the wax model. It can be carved, created by injecting wax into a rubber mold or a combination of the two steps. The quality of the model is crucial – the finished cast won't be any better than the wax model from which it's made.

New waxes developed specifically for platinum casting result in a model that is smooth and shiny, says Peter Romanofsky of I. Shor, Newark, NJ. Given the very high temperature of the platinum at the time it enters the mold, this smoothness is critical to a good cast finish. Imperfections on the inner surface of the mold will lead to cracking, finning or inclusions in the finished cast piece.

The wax model then is placed on a wax sprue tree (a wax tube to which the wax models are attached). The tree is placed in a flask (a metal can that slips over the sprue tree and onto a base) for investing (a process in which a plaster-like material is poured over the sprue tree and allowed to harden). Proper preparation of the sprue tree, sprue button (a small pyramid-shaped piece of wax that holds the sprue tree to the base) and flask are all critical, says Robert Romanoff of Romanoff International, Amityville, NY. All three components have been improved. Romanoff says the 3-by-3-in. flask is the most popular, but you can choose from various sizes based on what or how many pieces you want to cast at one time.

He suggests you use a paper rather than a rubber base for the flask because it absorbs water (powder and water are mixed to make the investment). While rubber works fine for gold and silver casting, you need paper for platinum because of differences in platinum investment. New papers designed specifically for use with platinum investment are available. When you are preparing the paper base, cut a half-inch hole for the wax to drain out. Romanoff advises that when preparing the sprue tree and button, use a wax that melts at a lower temperature than the model wax. The sprue wax should melt at 125°F while the model wax should melt at 165°F. This way, the sprue tree and button will melt out first and the model wax will follow smoothly.

Investing in Investment
Probably the most important developments have been in the investment itself. One of the biggest causes of defects in casting is improper preparation of the investment. The three basic types of investments for platinum casting have drying times ranging from 15 minutes to 18 hours. I. Shor sells a quick-setting investment called RPT Rapid. It has a phosphate binder and sets in an hour. Another product uses a phosphoric acid binder that sets in two hours and hardens fully during the burnout(when the sprue tree and wax models melt out the bottom of the flask and the investment becomes completely hardened).

So what's the difference and which one is right for your casting needs? The quick-setting investments are more expensive, but they offer convenience. The slower-setting investments can be mixed for longer periods, which results in a more homogenous mix and, therefore, a smoother mold. The downside, in addition to the waiting time, is that slow-set investment has to be kept in a vibration-free environment with consistent temperature and humidity during the setting period. Many bench jewelers find it difficult to work this into their casting schedule and opt instead for fast-setting investment.

The powder/water ratio of the investment is another issue with platinum casting. If there is too much water, it may turn to steam in the investment and cause cracks during the burnout. If there is too little water, the investment will dry and crack.

Follow Instructions
Whatever investment you choose, says Romanofsky, you must follow the directions that come with it. There is very little margin for error, so minute changes in the powder/water ratio will affect the investment and ultimately the cast quality. He recommends the use of ionized water or at very least distilled water; never use tap water, which lacks the correct ph balance and has minerals that can contaminate the platinum.

Robert Romanoff adds that exact measurements are crucial. Measuring by eye may work with gypsum-based investments used for gold, but not for the new phosphate-based investments for platinum. After you've worked with a particular investment and kept good records of results, he says, you will be able to vary the liquid amount slightly (1%-2%) to accommodate for variations in humidity.

Romanoff says casting materials account for only 5% of jewelry production costs, so it's good to buy the best-quality investment, wax, paper and flasks you can find. The small amount you would save on lower-quality products would be offset by the cost of repairing defects or having to recast. Even if a poorly cast piece is usable, it may require far more polishing time.

Once you have the flask filled with investment, the next step is the burnout, the removal of wax from the investment and the hardening of the investment so it will take the molten platinum. This is done in a furnace. One important recent change in platinum furnaces (gas and electric) is the addition of oxygen flow-through. According to Christopher Cart, former technical adviser to PGI and now working for Weston Beamor Ltd., a platinum casting company in Birmingham, England, this is one of the most important requirements for a platinum furnace to prevent ash in the mold that can ruin the cast.

Another important recent development is the addition of more computerized temperature and timing controls. These advancements make it easier to attain the sequence of temperature changes critical in the platinum furnace for a good mold. Linus Drogus of A.U. Enterprises, Southfield, MI, stresses the importance of following the instructions of the investment supplier and the furnace manufacturer. In choosing a furnace, Romanofsky says it should be able to go from minimum to maximum temperature in under an hour.

Casting Methods and Equipment
Once the burnout is complete, the mold is ready for casting. Casting machines have become increasingly diverse in terms of options, capacity and consistency of the quality of casts you can achieve. Choosing one for your bench will require careful consultation with your supplier. He or she can help you decide which method is best for the amount of casting you expect to do and the quality you want. Here are the three major types of casting machines you're likely to use:

  • Spin or centrifugal casting machine. This machine comes in horizontal-rotation, vertical-rotation and 45°- slanted-rotation versions that are spring-driven or motor-driven. They force platinum into the mold by centrifugal force. The speed at which the platinum enters the mold is controlled by the temperature of the mold, the temperature of the metal and the speed of the centrifuge at the time the metal is released. Flaws will show up if the spin caster hasn't reached 850-1,000rpms at the time of release. If you use a spring-driven caster and have non-fill problems, you probably need to change your spring. Simple versions of these machines start at $1,000 to $1,200 and rise in price depending on whether you use a torch or want a casting machine that melts the platinum for you.
  • Vacuum-assisted caster. These machines pull liquid platinum into the mold. They are more expensive but give better flow of the liquid metal and result in more consistent casting with fewer porosity and surface problems.
  • Pressure caster. The liquid metal is forced into the mold under pressure. According to Roger Green of Eisinger Enterprises, this type of casting gives the most complete control and helps to reduce the chance of defects that occur with the other methods. These machines are considerably more expensive, many costing $60,000 to $70,000. But they enable the bench jeweler to produce a high volume of quality casting that requires a minimum of surface polishing or repair. Romanoff says many of his company's customers lease this type of machinery and find it affordable and profitable despite the high price tag. He also points out that when shopping for casting equipment, you should ask your supplier for customer references. Ask what they like and don't like about a particular machine and whether they would buy another one.

Whichever method you decide is right for your bench, the good news is the price has come down across the board and the machines are available with increasingly sophisticated controls for temperature and metal flow, all designed to maximize your results.

After the casting is finished, even the problem of devesting has been greatly improved. The old method was to use hydrofluoric acid, which was dangerous to handle and created noxious fumes. Newly developed solutions don't contain hydrofluoric acid, but are effective at removing investment from the cast.

  • Eisinger Enterprises, Newark, NJ; (800) 282-1980.
  • Peter Romanofsky/I. Shor, Newark, NJ; (973) 648-6779.
  • A.U. Enterprises Inc., Southfield, MI; (800) 637-CAST.
  • Romanoff Int'l. Supply Corp., Amityville, NY; (800) 221-7448.
  • Platinum Guild International, Newport Beach, CA; (949) 760-8882, jmaerz@earthlink.net.

Platinum Bench Help

The Platinum Guild International USA has published the third edition of its popular Platinum Source Book. Copies are available by calling PGI at (949) 760-8279.

This comprehensive directory is divided into easy-to-use sections that cover such topics as chains and components, services, metal suppliers, equipment, schools and associations. It includes a reference guide.

The book is available also on CD-ROM. The CD version includes the published papers from previous Platinum Day Symposiums.

Jurgen J. Maerz, PGI's manager of technical education, is available to answer questions on the Platinum Hotline at 949-760-8882 or by e-mail to jmaerz@earthlink.net. PGI's Web site [www.pgi-platinum-tech.com] also has a range of technical information on platinum use.

Copyright © 1999 by Bond Communications.


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