Smart systems for error-free metal detection

Prepared Foods, March, 1992 by Jack Mans

Metal detectors are not new to food processors. Metal contamination has always been a major problem. Sources include mechanic's tools and pieces broken off equipment, tags and staples carried into the plant with ingredients, and personal effects like jewelry and pens. This has been exacerbated by the fact that contamination often is non-magnetic stainless steel, which has been difficult for metal detectors to pick up.

Companies have installed systems in their plants for years to protect expensive production equipment from damage and to produce metal-free finished products for their customers. I was introduced to metal detectors in the '60s when I installed units at a citrus plant to remove metal from orange peel handling systems.

The applications are even greater today. Besides those earlier requirements, more food processors are demanding that the ingredients they buy from other processors are certified free of metal. This has opened a new area of applications on bulk food products.

To meet this broadened demand, equipment manufacturers have developed a wide variety of metal detectors. In addition to the long-used package inspection systems, unique metal detectors today inspect free-falling powders and liquid flowing through a pipe. Very thin units are available for installation in restricted spaces such as above the forming tube on vertical form-fill-seal machines. Some units can even detect ferrous contamination (but not stainless steel) of products packed in aluminum foil containers.

For harsh environments, metal detectors are available in NEMA 4 or 12 configurations that will withstand direct washdown. Another development is systems that combine a metal detector and a checkweigher on a common frame. These minimize space requirements and problems with transferring product from one machine to another. In addition, you might be able to use one reject mechanism for products that either are underweight or contain metal, rather than requiring separate reject systems.

The latest developments in this never-ending effort is the new smart metal detector. These units, with their sophisticated microprocessor-based digital electronics, are enabling food processors to achieve levels of metal-free products never before possible.

SENSITIVITY:

WHERE THE METAL MEETS THE ROAD

The first requirement of a metal detection system is obvious. It must detect product containing metal contamination and reject it. However, there is a second requirement that is sometimes overlooked. The system must not reject product that isn't contaminated.

Achieving the first without overdoing the second comes down to the sensitivity of the metal detector. Sensitivity usually refers to the smallest sphere of a particular metal that the unit can detect as the sphere passes through the center of the metal detector aperture.

The sensitivity of a metal detector is adjustable, and the user generally sets it to the largest setting that will detect the expected contaminants. This sounds simpler than it is. Set it too high and metal-contaminated product goes undetected; too low, and you have continual false rejections.

Many factors affect sensitivity:

TYPE OF PRODUCT. Dry products, such as sugar, salt and cereals, are relatively easy to inspect and generally do not pose any unusual problems. Wet products, on the other hand, create an interference signal in the metal detector that will show up even when no metal is present. The signal is caused by salt or acid that makes the product electrically conductive, and the detector's electronics must be able to eliminate or at least reduce it for the detector to perform satisfactorily.

TYPE OF METAL CONTAMINANT. Ferrous metal ("black iron"), which is both magnetic and a good electrical conductor, is easy to detect. Stainless steel, particularly the grades used in the food industry, is non-magnetic and a poor electrical conductor; thus, it is much more difficult to detect. Non-ferrous metals, such as aluminum and copper, are non-magnetic but are good conductors; they are generally easy to detect.

SHAPE OF METAL CONTAMINANT. As long as the smallest dimension of the metal contaminant is larger than the spherical sensitivity of the detector, shape is not a concern. However, for thin pieces of metal, such as a piece of wire where the diameter is smaller than the spherical sensitivity of the detector, the orientation of the piece in the product will affect its likelihood of detection. For example, ferrous wire is easiest to detect when it is in line with the direction of movement and most difficult to detect when it is across the direction of movement. Stainless steel and non-ferrous wire are just the opposite.

APERTURE DIMENSIONS. A small aperture is more sensitive than a large aperture, and the sensitivity increases from the center to the corners. Therefore, for best performance, the aperture should be sized as close as possible to the dimensions of the largest product that will pass through it. Futhermore, extra care should be given if you need a very wide aperture, because you could get false reject signals from the higher sensitivities at the edges of the opening.