Metallic inclusions are the No. 1 contaminant in food products, causing product quality and consumer safety issues; however, the orientation of a metal contaminant can affect a metal detector, as the size, shape, and symmetry of metal contaminants cannot be controlled.
As an odd-shaped piece of metal passes through a machine in different orientations, the response to each one will be different. For this reason, we use spheres to test a metal detector: A sphere does not exhibit orientation effect and will always produce the same signal when passed through the same position within a metal detectors aperture. But if you flatten out the metal or roll it into a needle or wire shape, there will be a significant difference in signal, depending on how it passes through, due to the physics of disturbing the electromagnetic field.
The general rule for like metals is that if any of the dimensions are less than the detectable metal’s sphere size, the machine may have trouble detecting it in the hardest-to-detect orientation. Depending on the orientation in which it passes through, the signal will likely be much larger than that of the sphere.
These spherical test samples showcase advances in sensitivity and provide machine suppliers and buyers with a comparative benchmarking tool. They provide a solid and reliable gauge to measure machine sensitivity against. So, when a supplier reports a sensitivity improvement of 0.5 mm, this is a major concern.
Overcoming Orientation Effect
Orientation effect is a result of asymmetrical metal contaminant shards being more easily detected if they pass through the metal inspection system in one direction rather than another. Often, it’s easier to detect stainless steel and nonferrous wires when they pass through the aperture space sideways or upright, rather than in alignment with the conveyor. The reason for this is related to the magnetic permeability of the metal, which for stainless steel is much lower than for other metals.
One solution could be to position several metal detectors at various angles along the conveyor; however, this often results in a significant increase in aperture size, which diminishes the performance and sensitivity of the metal detector. Placing systems upstream throughout the process is usually more advisable.
Reducing the aperture size is another simple and effective way to increase metal detector sensitivity. Because sensitivity is measured at the geometric center of the aperture, the ratio of the aperture to the size of the product should be considered. Maximum sensitivity occurs when the contaminate is closest to the aperture walls where the electromagnetic field is strongest. It therefore makes sense that as the size of the aperture decreases, the performance of the metal detector improves.
During regular testing of food metal detectors, manufacturers should insert FDA-approved test pieces in various locations along the product—for example in the front, center, and back—and then run consecutive tests in which the metal sphere is travelling as close to the geometric center of the aperture as possible. These tests should be performed for all package sizes and configurations. This provides extra assurance that metal detectors are performing as they should, picking up the test contaminants, regardless of metal type, size, or product masking.
Know Your Metals
The type of metal contaminant also needs to be factored into the equation. All industrial metal detectors will exhibit a different level of sensitivity for the three main groups: ferrous (such as iron or steel), nonferrous (including aluminum foil), and stainless steel. Because metal detectors work by spotting materials that create a magnetic or conductive disturbance as they pass through an electro-magnetic field, stainless steel (300 series) is typically the most difficult to detect.