Metal Detection vs. X-Ray

The choice of whether to deploy conventional metal detection or X-Ray inspection as your primary method of detection of foreign material is sometimes a simple one, sometimes not. This page discusses the various considerations.

First off, the choice may be out of your hands -- if your customer dictates one or the other, then absent your ability to convince them otherwise (and if there's a logical argument to be made, Pack & Inspect Group can help with that discussion), the choice is made for you. Otherwise, consider these factors:

  1. Primary Contaminant Risk Profile

  2. Your Packaging Type

  3. Your Product Type

  4. Other Value

  5. Cost and Regulatory Burden

1. Primary Contaminant Risk Profile

This comes down to the obvious -- why are you buying detection equipment in the first place? This whole exercise is about preventing harmful foreign material contaminants getting to the consumer so an obvious first consideration is "what types of contaminant are at most risk of ending up in your product?". If the answer to this is a broad spectrum of metals and that there's little to no risk of any other contaminant types, then a metal detector may suffice (subject to other considerations below). If, however, you are putting product in glass jars for example, then glass is most certainly a likely contaminant and X-Ray should be used.

Generally, for metal contaminants, X-Ray and metal detection will delivery broadly similar detection capabilities for most metals, but even here there are exceptions -- specifically, for aluminum contaminants, metal detection technology may be better while for lead (buckshot) and non-magnetic stainless steel, X-Ray technology may be better; see here for a comparison of various non-magnetic metal types and their relative detection capability with both technologies.

Outside the world of metal contaminants, there are many other contaminant types (but not all!) that can be detected with X-Ray technology, while metal detection technology is limited to metals:

 

2. Your Packaging Type

Here's the bottom line here - metal detectors do a great job, but they don't discriminate; any metal, including in the packaging, will be considered a defect so packaging of metal, or that includes metal, prohibits use of metal detection, but X-Ray can ignore those metal package contents easily. So, for foil pouches, glass jars with steel lids, foil seals, pies in foil trays etc, X-Ray inspection is really the only option, regardless of the contaminant risk profile.

 

One note of clarification here, pouches made of metalized film (evaporated aluminum) or microwave susceptors, as opposed to true aluminum laminate film, can, in theory, be scanned by a metal detector but with compromised results; while this approach was common in the 90's before X-Ray was widely available, the prevailing opinion today is that these packages should be limited to scanning with X-Ray for best results.

 

3. Your Product Type

For metal detectors, the Achilles heel is 'product effect' -- this is signal the metal detector picks up from moist/wet and especially salty or warm products that 'looks like' metal to the metal detector; typically it is 'learned' or 'phased out' as part of metal detector setup but the challenge here is that 'learning' is usually specific to a particular product effect characteristic -- as the size or the product, or it's moisture/salt content or temperature changes, so too can the product effect, causing false positives (false rejects). While this doesn't prevent use of metal detection technology on these more difficult product types (dough, fresh bread, meats, cheeses etc.), it does increase the likelihood of false rejects, complicate the set-up and, as a result, make optimum detection capability difficult to achieve. These composition and temperature changes that make product effect challenging in the metal detection world have no impact on X-Ray operation.

Conversely, for X-Ray systems, the more difficult products are those where the density can vary -- this may be due to the dimension through which X-Rays are travelling varying, or maybe due to inconsistent presentation of product contents (non homogeneous); while homogeneous products will present to the X-ray as a fairly consistent shade of gray, those non-homogeneous products will show variation. Again, this does not prevent use of X-Ray (indeed, for the other reasons discussed here, X-Ray may still be the best technology), but can make the set-up a little more challenging:

 

4. Other Value

There's little doubt that both metal detectors and X-Ray systems are usually justified and purchased for the primary task of inspecting for foreign material contaminants. However, with both technologies, X-Ray in particular, there's other capabilities that ca sometime be deployed to add additional value.

For metal detectors, being the simpler device, this is mainly limited to 'reverse metal detection' where the metal detector is configured to expect some 'metal signal' (either from something metallic like an added premium or metallic pouch, or from something non-metallic but detectable like an oxygen scavenger) as confirmation of presence of that component, rejecting any package that's missing that component that provides the 'metal' signal.

For X-Ray, there are, depending on application, a number of other capabilities that can be deployed concurrently with inspecting for foreign material. Examples include:

  • Fill level inspection for simple contents verification

  • Check-weighing, where the X-Ray estimates weight from density and performs functionality similar to that of a dedicated check-weigher

  • More complex contents verification, looking for specific components (e.g. a chocolate in a box of chocolates) even when the missing components doesn't create enough weight change to be detected by a check-weigher

  • Counting of discrete components to verify a count in a package

  • Verification of presence of a cap/lid on a container

  • Checking for dents in cans

  • Checking of the presence of an easy-open ring-pull on a can or composite container

  • Checking of product/package dimensions

 

5. Cost and Regulatory Burden

For the most part, the various considerations above tend to indicate superior capability and greater flexibility for X-Ray inspection technology. So why then is there still a huge and continuing use of metal detection technology? All this extra capability with X-Ray comes at a cost:

  • Capital cost: While X-Ray technology has come done in cost substantially over the last fifteen years as the technology has matured and volumes have increased, you should still expect to pay more for X-Ray inspection technology in an application that you would for a corresponding metal detector.

  • Continuing cost: Metal detectors have been typically reliable, trouble free and relatively lost cost to 'fix' when something does go wrong. X-Ray, ten years or so ago, had something of a reputation for unreliability though that is largely a thing of the past. However, when a part in an X-Ray system does fail (this shouldn't happen early in the life, but, unlike metal detectors, X-Ray systems do have some parts that have a finite life) this can be costly -- the real world of the technology and physics employed requires X-Ray tubes and detectors specifically that will one day need replacing, and will be more costly than most metal detector parts -- accruing maintenance cost provisions to anticipate those needs, or subscribing to an appropriate service plan can soften the blow of these nasty surprises!

  • Regulatory burden: From a safety perspective, there is no regulatory oversight associated with metal detection technology. For X-Ray technology, while largely benign, there are individual state requirements for registration, training, labeling and radiation monitoring. Most X-Ray providers, and some third party providers will provide support as required, but the obligation for compliance is on the user company. While compliance is generally not difficult, and much less burden than the regulatory oversight of radioactive isotopes some will be familiar with from other inspection devices, there is some burden here.

 

For more discussion on the pros and cons of metal detection or X-Ray inspection, or to schedule education programs on the principles of operation of these technologies, please contact us.

© Pack & Inspect Group, 2020

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