There are a number of schools of thought on mass-finishing plati¬num. Over the past decade, platinum as a commodity has re-emerged in popularity making the mass-finishing of it an important issue among jewel¬ers and their suppliers.

WHY IS PLATINUM SO DIFFICULT TO MASS FINISH?
Platinum is a heavy metal and can be difficult for many manual as well as mechanical polishing operations to process because few understand the characteristics or properties of the metal.

Many try to use the same proc¬ess with platinum as they would with gold or silver and then wonder why the results were not as impressive. You need to actually have a different mind-set when running platinum parts compared with other metals, and un¬derstand what is transpiring on the surface of your parts in order to have any success.

In short, platinum is a rather soft metal in its pure form and needs to be alloyed with another metal. Even al¬loyed, the platinum smears and pushes when being worked with. Most of the time it work-hardens and can be less forgiving than other precious metals.

Certain influences can affect the results of the smoothing and polishing of platinum. Some of these would in¬clude the alloy used in the metal, the purity of the platinum, the process be¬ing used to smooth and polish it, as well as if the proper equipment and media are being used for a specific application. It is also extremely im¬portant to not let the platinum surface get contaminated. We recommend that you keep a clean work area and always run your platinum parts in its own separate media to avoid this problem. If contaminated, it would need to be reclaimed through refining, which is costly.

WHAT ARE SOME OF THE ALLOYS USED WITH PLATINUM AND WHICH ONES WORK BEST?

From a mass-finishing point of view, we have found the following platinum alloys work best in me¬chanical mass-finishing equipment:
• Tungsten
• Cobalt
• Copper

Other types of platinum alloys studied with less favorable results:
• Iridium
• Ruthenium

Results with the above alloys proved to be less positive after the fi¬nal polish. 
 


IT'S IN THE PREPARATION!
The quality of the mass-finish de¬pends greatly upon the steps preced¬ing it. Mass-finishing is, or is close to, the final-step in many cases and is not a cure-for-all process. If the alloy, the quality of metal, the casting and preparation (removing the sprue and a light sanding if need be) are not prop¬erly performed, chances are that a high luster or quality finish will also not be achieved.

MACHINES, MEDIA AND COMPOUNDS RECOMMENDED FOR MASS- FINISHING PLATINUM:
New processes, machinery, media and compounds are evolving faster now than ever for the mass finishing of platinum. Depending on the parts to be processed, there are a couple of machines, media and compounds that we have found to work best. For cast platinum parts such as rings we rec¬ommend a high-energy machine in order to do the first cut down step and remove the outer skin and as much porosity (if present) as possible.  

MACHINES:
The two most popular for this would be the high energy Centrifugal Barrel Finisher, or CBF, and the disk finishing machine. The CBF Machine can produce highest speeds and energy requisite for the removal of surface metal, burrs and parting lines.

The machines can be run for both wet as well as dry processing to in¬clude wet cutting, dry smoothing and dry polishing. Disc-finishing machines, (also regarded as high-energy machines) are now becoming another popular means of mass-finishing platinum. The action of the disc-finisher is not as rigorous as with the CBF machine, but it does a good job on platinum and has the advantage of having an open bowl to view parts. Results from polish¬ing platinum in these machines have not been as favorable as with the CBF or the drag-finishing machines.

The drag-finishing machine is ideal for parts needing fixturing to avoid collision and scratching. Its' principle is simple. The fixtured parts lower into a processing bowl with dry (smoothing or polishing) media and then rotate for a desired time. Once the process is complete the fixtures rise out of the media and can be removed.

Vibratory bowls can also be used with good success for burnishing platinum parts. The size and shape of the steel media should be considered when using this method depending on the shape, size, and how intricate the design of the parts is. Depending on the parts, I have found vibratory bowls a very good secondary process (tighten¬ing and giving shine to the surface) before putting the parts into a drag-finisher to give the part a high luster. In some cases, the process has even given parts a final polish.

MEDIA AND COMPOUNDS:
In the cut down process, or first step, it is important to find a media which is aggressive enough to remove the surface layer in a relatively timely fashion. Always make sure the media size does not present any lodging problems and can reach all of the necessary areas of the part. The com¬pound used with this media should assist in cutting and/or cleaning the parts.

For dry media, it is also important to find a size (i.e. of walnut shell) which will be suitable to perform its primary function but not lodge in the parts. A good rule to remember is that the larger the media the more aggres¬sive it is. There are a variety of cut¬ting or polishing pastes, powders and liquids available for dry media, which are used to enhance the media per¬formance and to keep the life span of the media longer. Combinations or media sizes, when processing either wet or dry, is also a consideration.

PROCESSING (CAST VS. TURNED):
The process used for cast parts compared to turned parts is relatively the same except for the time needed to process cast parts is longer. In some cases, turned parts were processed using strictly a dry cut and dry polish in the CBF and drag-finishing machines. This was accomplished where the milling machine was able to cut the platinum so fine it was dif¬ficult to detect the turn lines.

MAGNETIC FINISHING:
There is some misunderstanding in the industry as to what magnetic finishing can accomplish. This equipment is not capable of produc¬ing a high-end polish. Its purpose is to have the fine pins reach areas where standard media cannot reach such as the inside of ring prongs. It is also ideal for removing casting ves¬tige found inside of concave or pronged areas. 

IN CONCLUSION:
With the growth of platinum in recent years along with rising costs of manually manufacturing quality products in a timely fashion, we are at an important juncture where the me¬chanical means of finishing products should be considered. Machinery can assist or, in some applications, com¬pletely finish platinum jewelry saving time and money.

The processes for finishing plati¬num are now available, and it is im¬portant to ponder if your product can benefit by one of these mechanical means of mass finishing in order for you to be more competitive and prof¬itable.

V9N3

Platinum Mass-Finishing
Jim Fowler • Dreher Corporation

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