Eyes to See

How is Foreign Material (FM) detected?

The question perhaps begs another question: where in the line do you want to detect (and remove) FM? Particularly, what can best be done post-packaging (where no further FM can be entrained) and what needs to be placed in other line locations?

Most folks are very aware of metal detectors- those little boxes that check packages for ferrous metals. They work fairly well, but of course have limitations: They don't work on non-metallic FM, nor on most stainless steel. And of course their resolution requires the FM to be large enough to be detected, making some wire pieces, etc. to be undetectable to these systems. So metal detectors have their purpose and role, but are not a panacea.

A few frozen potato processors have tried x-ray on packaged goods. X-ray can detect a wide range of materials, not metal only, a valuable improvement over metal detection only. The challenge for frozen product is that x-ray works on density, using through-beam transmission, looking at how much energy gets through the object. More dense objects block more of the x-ray, providing a differential signal vs. less dense objects. Problem is, most frozen product is bagged, with the individual pieces randomly oriented. So the density through the bag at any one point is highly variable- it depends on exactly how many pieces the beam must penetrate going through the bag at that point. This density variation effectively acts as "noise" to the x-ray detector: not a problem for large, very dense objects such as sizable stones and large pieces of glass. But this "noise" makes small pieces and less dense objects undetectable.

There have been some attempts to scan pre-packaged, bulk flow frozen potato products with x-ray technology. In this case, the ability to detect many foreign materials is high. The drawbacks include high cost relative to throughput, and x-ray is unable to detect foreign materials of similar density to good product, such as some plastics, rubber, wood and extraneous vegetation such as weeds.

So, like metal detectors, x-ray perhaps has its place, but does not thoroughly address the need for zero tolerance of FM. To do so, it seems we must scan the product prior to packaging, using technologies that look at reflected energy, primarily lasers and cameras. Next time, we will dig into the details of those technologies, and discuss where in the line they belong.

Tim

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