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From: Food Quality & Safety magazine, October/November 2008

Protect Your Product From Contaminants

Metal detection and X-ray inspection can help cut product loss

by Sarrina Crowley

Today’s processing lines are complex and, increasingly, automated, making the detection of foreign contaminants in both raw materials and finished products more challenging than ever. Companies need comprehensive programs to ensure that contaminants never reach the consumer. Good manufacturing processes are an essential component of any overall program.

While no single technology can guarantee that contaminants will not reach consumers, utilizing a variety of technologies can reduce the likelihood dramatically. Two types of technology that can effectively contribute to an overall contamination control program are metal detection and X-ray inspection. While neither of these technologies is new, both offer a high degree of reliability and have undergone dramatic improvements in terms of software, automation, and ease of use.

In the past, a human operator watched the monitor, flagging errors in products when the machine detected them. Today, no operator is required; instead, the system automatically rejects any predetermined setting or contaminated package. Today’s systems are also extremely user friendly. An operator with very little training can easily perform the initial product setup using a touch screen color monitor. With recent software advances and these ease-of-use improvements, plants are able to operate more efficiently while improving product safety.

Of course, all line operators must be trained in proper equipment use. Imagine if this key process was not in place. A device might be in place without a proper reject bin, for example, so that contaminated product simply dropped onto the floor. An untrained operator entering the scene might simply pick the product up and put it back on the line, not realizing it had been rejected due to contamination.

It is important to inspect the product at several stages of the production line. Even when a contaminant is found and rejected at the beginning of the line, it is still possible for a different contaminant to be introduced at a later point, resulting in a contaminated end product. Inspecting only at the end of the line does not make sense, either; this strategy results in much greater product waste. In addition to these procedures, each company must make certain that all ingredients are purchased from a supplier with equally strict inspection policies.

Metal Detection

One of the most common product contaminants is metal. Metal contaminants can be introduced to product as shards that break off of machinery in the production line. If left in the product, such metal pieces can destroy equipment farther down the line, creating a much larger problem, with multiple fragments in more product. This kind of problem can grow exponentially if it is not caught early.

The potential for metal contamination makes a metal detector essential to any foreign object control program. With many installed throughout the world, metal detectors are well established in the food industry. Vast improvements in software have made them highly reliable and capable of detecting a variety of metal types, including the non-magnetic stainless steel that other forms of contaminant detectors like magnets cannot find.

The most common types of metal contamination found in a broad range of industries include ferrous (iron), copper, aluminum, lead, and various types of stainless steel. Of these, ferrous metal is the easiest to detect; relatively simple metal detectors, or even magnetic separators, can perform this task well. Stainless steel alloys, which are extensively used in food processing equipment, are the most difficult to detect, especially the common non-magnetic grades like 316 (SN58J) and 304L (EN58E). Non-ferrous metals such as copper and lead fall between these two. Only metal detectors that use a balanced three-coil system have the capability to detect small non-ferrous and stainless steel particles. A variety of factors influence a detector’s sensitivity and stability. The metal case itself has an effect, while minute movements of the coils in relation to each other can cause an out-of-balance voltage and a false detect signal. One of the major design issues to be addressed is the need for a totally rigid and stable system, one unaffected by vibration from motors, pulleys, auto-reject devices, temperature changes, transportation, and machinery nearby. Potting material, coils specification, and case design are crucial.

Automatic balance control and quartz crystal control are some of the software features that contribute to unit stability. Automatic balance control continuously monitors the out-of-balance voltage and automatically corrects it to zero. This feature eliminates the need for periodic fine-tuning by an operator and ensures that the detector is permanently set to its optimum operating condition. Similarly, quartz control allows for control of the frequency of the oscillator with great accuracy, eliminating frequency drift.

Other factors that influence sensitivity include type and shape of metal, orientation, operating frequency, and throughput speed. A good indication of a detector’s all-around capability is the sensitivity ratio between ferrous and the most difficult-to-detect grade of stainless steel 304L (EN58E). This ratio, which can be as good as 1:1? and as poor as 1:2?, has a major affect on the detector’s ability to detect forms of contamination, like slivers and screen wire, that exhibit an “orientation effect.” Non-spherical metals such as wire are easier to detect when they pass in one orientation than they are in another.

The higher the detector’s sensitivity, the more capable it is of handling orientation effect. With software features that include automatic balance control and quartz crystal control, newer systems are extremely reliable.

Today’s systems are also quite user friendly, some with full color touch screen monitors that are very easy to operate. All systems also have security safeguards: Users have passwords for management that determine what settings each operator has permission to alter. A wide variety of metal detector styles are designed for specific products, including rectangular aperture for packaged and conveyed products, gravity flow detectors for powdered or bulk products, and pipeline detectors for pumped liquids, semi-solids, or slurries. All types of automatic reject systems for any type of detector can be used to fit any line configuration.

X-Ray Detection

Although metal detectors are extremely reliable, they can only detect metals and, as mentioned earlier, the size of metal contaminant detected varies with the type of metal. X-ray inspection detects all types of metal, as well as calcified bone fragments from chicken and other meats and glass particles from packaging. Also, with larger product sizes, an X-ray system will often outperform metal detection. The signal from a given piece of metal contaminant gets smaller when its distance from the coil of a metal detector increases. For the larger size coils needed for larger apertures, sensitivity at the center of the aperture degrades, lowering the overall metal detector performance. Because of these factors, most plants need to supplement their metal detection equipment.

A product’s thickness, homogeneity, density, and size determine the accuracy of detection of a contaminant; with most products, the accuracy rate of contaminant is +/- 1% at 2 Sigma. As with metal detectors, there are many types of X-ray systems available to suit any product, including designs for various sizes of packaged, bulk, and conveyorized products; pumped liquids and slurries; and systems for inspecting plastic composite containers, glass jars, and cans.

Today’s X-ray technology is not affected by vibration, wind, or electrical noise and can detect many contaminants other than metal, including glass, stone, bone, and dense plastics. Also, new modular designs enable easy, quick repair.

Because X-ray systems operate by measuring density, they treat all metallic contaminants in the same manner, whether they are ferrous, non-ferrous, or stainless steel. X-ray inspection can offer additional benefits over contaminant detection. It is a very useful tool for quality assurance, for example, because it can verify product count, dimension, malformed products, product voids, fill level, and package seal—even for a product packaged in metal foil. X-ray systems measure mass or density and can therefore determine if a package is under- or overfilled, errors that can result in loss of profits. In addition, an X-ray machine can determine if the seal of a package is damaged—even one in metal foil—and can also identify a missing component intended for a particular area of a package.

While X-ray technology has been around for many years, it has improved radically. “Today’s X-ray machines are extremely user friendly,” says Wyeth Thomas of Mettler-Toledo Safeline (Columbus, Ohio). “In the past, a lot of training was required to set up and operate X-ray units, but now just about anyone can do it.” And despite popular perception, X-ray machines are very safe to use. “X-ray machines use an electrically generated X-ray source—not radiation,” says Oscar Jeter, national sales manager for Mettler-Toledo Safeline.

Return on Investment

Regardless of the technology chosen for a contaminant control program, a primary consideration is determining return on investment. Both tangible and intangible factors should be considered. Tangible costs include acquisition, operation, and maintenance, while a less tangible but very important aspect is brand protection.

Clearly, protecting consumers from harm is of utmost importance. Consumers expect their products to be safe and will accept nothing less. A consumer who finds a contaminant is likely to simply stop using the product, knowing that competitive products are available. In fact, consumer complaints received most likely represent only 5 to 10% of actual consumer issues.

A foreign object detection program is necessary in today’s market. The detection of foreign objects requires more than one detection device and occurs, ideally, at multiple locations along the processing line. A program that includes various technologies, including metal detection and X-ray inspection for detecting contaminants at each stage, can cut costs by reducing unnecessary product loss, thus providing a clear return on investment.

Crowley is a market communications specialist at Mettler-Toledo Safeline, Inc. Reach her at sarrina.crowley@mt.com or (813) 342-9166.

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