Targeting Defects – Technical Articles

Using analysis tools to get to the root of manufacturing defects can save time and money.

Metals prices are higher than they have been for years, raising expenses and reducing margins throughout all manufacturing industries. In the jewelry industry, this problem is compounded when failures occur in the production process, costing manufacturers even more time and money.

Most attempts to solve such problems are educated guesswork. In many cases, manufacturers use a trial-and-error technique until the problem goes away. But there is another way: Instead of spending countless hours (and dollars) trying and trying again to fix the problem, manufacturers can get to the root of it right away by taking a more scientific approach. With the help of analysis tools, they can troubleshoot failures, understand why problems occur, and prevent costly recurrence.

WHY ANALYZE?

Should jewelers and manufacturers be artists, scientists, or a mixture of both? There is no doubt that artistic design is a major success factor in the jewelry industry: No matter how well-made a piece is or how ingenious the assembly method, no one will buy it if it doesn’t look good. But in addition to a good design, jewelers and manufacturers need some scientific knowledge (or access to those who have it) to avoid expensive mistakes and even more expensive repetitions of those mistakes. Troubleshooting and understanding problems is a necessary part of jewelry manufacturing for both bench jewelers and volume manufacturers.

For example, when a casting fails, wouldn’t it be better to understand what is happening so that you can avoid the problem in the future? Or, if it does occur again, wouldn’t you like to be able to recognize it and know how to avoid it? Re-processing in the hope that a problem will go away is sheer madness.

The following mini case studies illustrate how some of the analytical tools available to jewelers and manufacturers can help you get to the root of a defect.

WATCH YOUR TEMPER

The Defect: Jewelers typically ask for sheet stock to be annealed so they can work with it right away. This is ideal for most fabrication applications, but it’s not always advantageous to have sheet stock in the annealed condition for die striking, particularly when a blanking operation is involved. To achieve a crisp, burr-free edge, you should instead opt for sheet stock that has been cold worked to a certain degree. If the sheet is too soft, excess metal moves into the clearance gap between the punch and the die, creating unwanted burrs on the piece. Poor edge retention is also a common problem, resulting in beveled edges.

To avoid such problems, take sheet stock samples of various tempers, stamp out pieces, and decide which sheet works best. You can then measure the hardness of the chosen sheet and use that data as a material specification for quality control purposes. If future problems occur, a simple hardness test will determine if the temper of the sheet isn’t suitable for producing acceptable stampings.

The Tool: Several types of hardness tests, called “scales,” are available. The most common hardness scale used in the jewelry industry is the Vickers Hardness Scale, which is arguably the most versatile and user-friendly.

All hardness-testing machines are relatively simple to use, but they can be expensive, ranging from a few thousand dollars to tens of thousands. Most are cost-prohibitive to jewelers and small manufacturers. If you wish to have your metal tested, check with your metal supplier, as most provide this service, or do a search online for a lab near you.

TOO HOT TO HANDLE

The Defect: When problem solving, the obvious answers might be the toughest to find. A 14k nickel white gold ring blank that has been annealed in a belt furnace as part of a batch process. The blanks were originally stamped from plate and were being annealed prior to lathe turning and sizing. When the batch exited the furnace, the blanks had an unusual look to them. Closer examination revealed that the undersides of the blanks had indentations from the mesh furnace belt. It appeared as if the blanks had been overheated during the annealing cycle, but this theory required confirmation.

Enter the stereo microscope. Examination under this tool revealed partial liquation of the surface, confirming the theory that the temperature inside the furnace was higher than the solidus for the alloy. The furnace thermocouple was not functioning properly and needed to be replaced. The new thermocouple produced perfect annealing results.

The Tool: The stereo microscope is a tool that can be used by both bench jewelers and manufacturers to examine a metal specimen in 3-D. Typically low-resolution, low-powered instruments, stereo microscopes seldom have magnifications above 100x. The advantage is in the “depth of field” or “depth perception.”

While a highly specified stereo microscope can cost around $10,000, an entry- level model that will magnify up to 50x can be purchased for about $500.

CUT ‘EM SOME SLAG

The Defect: The manufacturer was getting surface pores on its 14k palladium white gold investment-cast rings. These defects were not occurring in the same place on each casting, and the manufacturer was concerned that its casting process was incorrect for the alloy.

They took a micro-section of the cast ring circumferentially, so that they could examine the complete cross-section, and sent it to a lab for analysis. Using a metallurgical microscope, the analyst could see that the majority of the cast ring section was free from defects such as shrinkage porosity, micro-porosity, and any gas/mold reaction. The castings were actually perfect throughout the majority of the section, suggesting that the sprue/gate design, the metal temperature, and the flask temperature were all correct.

The surface pores were identified and examined under higher magnification. They had round shapes and smooth surfaces, but some contained translucent glassy inclusions. The analyst recognized the porosity and inclusions to be casting slag.

When trees and defective castings are recycled into a melt, any residual investment must be completely removed. If any remains, it may become included in the new castings, or react at high temperatures with any oxides present to form crucible slag, as was the case here.

After receiving the diagnosis, the manufacturer said that because 14k palladium white gold was not quantity business for him, he had a tendency to over-recycle scrap and swap out crucibles less often. When he cast new rings using a casting charge of new and cleaned metal, as well as a new crucible, he no longer had the problem of surface pores.

The Tool: What is commonly referred to as a metallurgical microscope is a typical compound microscope. Compared to a stereo microscope, it provides a relatively high image resolution and magnification, but in 2-D rather than 3-D. Depending on the sophistication of the tool and the number and magnification of the lenses, metallurgical microscopes cost between about $10,000 and $30,000.

To have the complete image in focus in this 2-D microscope, the specimen must be planar. Specimens are often mounted in a resin, ground down, and polished flat. This resulting specimen is referred to as a micro-section. The micro-section can be placed on the movable stage on the microscope table and moved from left to right and backward and forward without actually touching it.

The key to using a metallurgical microscope is in interpretation and experience. It is relatively easy to learn how to operate the microscope and look at samples and details surrounding a defect or phenomenon, but the analyst must be able to interpret the results, recognize what he is seeing, and draw sensible conclusions based on this evidence that will help solve the problem. These skills usually (but not exclusively) come from education, training, and experience.