With metal detectors, metals with one or two conductive and magnetic properties (such as brass, bronze, ferrous metals, etc.) will generate detectable signals. Non-magnetic stainless steel is more difficult to pick up-it is a poor conductor-certain foods with added iron, moisture, salt and acid tend to obscure metal detection. This is called "product effect", which affects the detection performance.

However, technology has moved forward, and the detector is now suitable for wet and dry applications. Even conductive products that have always been presented in small packages will be selected.

With an X-ray machine, light waves pass through the product to be inspected. Anything very dense will block the waves. However, low-density materials, such as wood, feathers, bugs, some stones/glass, magnesium and aluminum, allow light waves to pass through directly.

When working together, the X-ray machine will help to detect the position online, while the metal detector is more suitable at the end of the production line, usually where the product is packaged. Gravity systems are an exception to online processing, because the product passes through the machine at such a fast speed, this type of application is not suitable for X-ray inspection.

X-ray machines are getting better, but they are usually limited to 250 feet, which is about 75 meters per minute. In gravity applications, the product passes at a speed of 800-1500 feet per minute and over 450 m, so at this point, the metal detector is the best performance.

The product and its packaging obviously affect the detectability. Until recently, the performance of metal detectors in detecting products with iron and any salt/moisture added was still limited. Manufacturers of "wet" products often have no choice but to calibrate their metal detection in a way that reduces the margin of error. The result of this is that it often triggers too many false positive rejections. Simultaneous multi-frequency scanning now makes it easier for food manufacturers to identify metals that are difficult to find in high-conductivity foods because it can "tune" the product effect.

Products with different densities will affect the performance of X-ray machines. Although it can be calibrated for the total density of the product, grain ingredients, raisins, and bran have variable densities. The raisin will have high spots, low bran and the same density as the box it is in. Since X-rays are measuring total density, if there are contaminants in the box and the machine falls on low-density bran flakes, it may not be able to detect the sample as it does when it falls on higher-density raisins.

In terms of packaging, recycled cardboard may cause problems for both machines. This kind of cardboard often contains ferrous and non-ferrous metal contaminants, especially aluminum foil (a large contaminant that X-rays will ignore) and staples.

Both machines have potential rewards for specific products and packaging. Metal detectors provide "reverse detection". For example, a cereal box may have a toy, usually identified by metalized film. The detector will ensure that the toy is there, confirming the strap while checking other metals. Similarly, it can detect metal contaminants while ensuring that oxygen absorbers are packaged into meat products.

X-rays have greater potential: it can look for missing ingredients, such as missing ingredients in ready-to-eat foods or identify damaged or incorrectly packaged products. The two machines can work well together, but the metal detector should be the key control point on the production line, and the X-ray machine is a bonus.

For more details, listen to the latest Fortress Technology Food Safety Podcast hosted by Steve Gidman.

You must enable JavaScript to view a limited number of articles for the next 30 days.

For 25 years, the Global Bag Forum has been an event to obtain the latest information from experts and establish contacts with leaders and innovators who develop, supply, purchase or recommend flexible materials (including bags, films, and machinery).

Copyright ©2021. All rights reserved by BNP Media.

Design, CMS, hosting and web development :: electronic publishing