Foreign body detection

There’s little doubt that the task of choosing an inspection system for your production line can, at best, seem confusing and, at worst, appear daunting. There will be many factors to take into consideration. One may need to assess the feasibility of different approaches based on detailed knowledge of the physical space available at the various locations in the process. Or it may be necessary to prepare a careful economic justification, based on up-front lifetime costs.

There may well be corporate guidelines regarding the contaminant size that must be detected – or pressures from customers who demand increasing quality control standards. A review the various types of contaminant along with the defects that occur in each production process and how best to combat them will be necessary. Any contaminant or defect detection and rejection solution should only be part of an overall plan to prevent the contamination and the cause of defects.

Non-metallic contamination

Metal detectors are well established across the food industry and installed in food production facilities of all kinds around the world. Their reliability can be depended upon and the cost of installation, set-up and running is well understood. But what do you do if you need to find non-metallic contamination?

It is well known that X-ray systems are capable of much more than just detecting metal, they can detect non-metallic contamination, for example stones, glass and PVC. However, if these types of contaminants don't figure in your contamination log, what else can X-ray offer in terms of a viable quality control tool?

X-ray is capable of defect detection, finding missing or misshapen parts, and checking the mass of food in individual compartments of a multi-compartment package. So contamination may not be the only consideration in choosing X-ray equipment.

A full appreciation of the significant differences between metal detection and X-ray solutions is a key step in making the right choice. For example, if wanting to detect metal (included in this “catch all” statement is the detection of small pieces of aluminium), then metal detection is probably the only option, as it is if the food product is gravity fed, as in the case of a grated cheese packing line.

However, if it is vital that non-metallic dense contaminants are detected – or that part counts must be checked, misshapen products detected or mass of product estimated- then X-ray is the most likely choice.

In other instances, there may be a case for metal detection at one stage of the process and X-ray at other locations. Another factor to consider will be the desire to equal or preferably exceed the quality requirements of the customer (the retailer) while minimising the total cost of ownership of the equipment.

If the requirement is for the detection of metallic-only contamination in product packaged in non-metallic materials, then in most cases, metal detection technology will offer the best alternative. However, if a product is highly conductive due to high moisture levels - and certain cheese and yoghurt type products can fall into this category - then X-ray inspection should be investigated as it may well out-perform metal detection on a basic contaminant sensitivity specification, but the cost will be higher.

Aluminium contaminants

Aluminium is a good electrical conductor, but has lower X-ray density than other commonly occurring metal contaminants. This means that if aluminium is a contaminant threat, then metal detection will be best suited to identifying and rejecting aluminium contaminant.

An example of where the use of X-ray in the food industry would be challenged is the detection of the ubiquitous 'blue plasters'. Specifically designed to be metal detectable, it is the use of aluminium that makes these plasters detectable and, therefore, undetectable by X-ray.

However, where aluminium is included in the packaging, either in the form of metallised film or aluminium foil (for example, where foil membrane lids are used on some yoghurt and milk-based drinks), then its low X-ray density can be exploited. This is because the X-ray system will largely ignore it, and will do a superior job of inspecting the contained product therein for the metal contaminants.

Product effects are normally associated with metal detectors. However, in the same way certain product characteristics limit metal detection performance, X-ray systems can equally suffer from “challenging product characteristics”. An example is those products that contain a reasonably high level of salt, and especially those with free salt crystals. The salt crystals can appear as dense particles potentially limiting the X-ray’s performance capabilities.

The above are just some of the issues to consider, before making the decision as to which technology best suits any particular application. One thing to remember, however, is that "ignorance isn't bliss", it can be extremely costly and if something sounds too good to be true, it probably is. My advice to anyone considering either technology is to seek the help of a reputable supplier, and preferably one without a vested interest.