space usually favours metal detection, as
do applications where the speed of the
product passing through the machine is
very fast or very slow.
Maintaining a reliable performance
over time demands periodic calibration
and validation checks on both types of
technology. In some cases, the process
can halt production several times each
shift. Users may therefore want to reduce
the frequency, but they need to bear in
mind that the longer the interval, the
more product may have to be discarded or
recalled if a problem with the inspection
system is discovered.
While a well-maintained x-ray system
Metal detectors can identify all types of Unlike ferrous metals, stainless steel is
metal based on magnetic and conductive usually non-magnetic and a poor electrical
properties, while an x-ray system conductor.
depends on density differentials. This
means that an x-ray system may struggle Consequently, a stainless steel swarf,
to detect aluminium, including foil or metal shard or narrow wire hidden in a dry
metallised film. Equally, foil packaging can product typically needs to be 50% larger
present a challenge for metal detectors,
which can only detect magnetic or ferrous
metal contamination inside the package,
while to some extent an x-ray system can
than a ferrous sphere to generate a similar
signal size. This disparity can rise to 300%
in wet products, such as ready meals,
meat, fish, sauces, preserves and bread,
detect all metals. because moisture acts as a conductive
Orientation effect occurs if a contaminant swamped by product effect. Solutions,
is non-spherical, such as a piece of wire,
in which case the ability to spot it may
depend on the orientation it presents
to the detector. In the case of x-rays,
the presenting face must be equal to or
greater in size than the base resolution of
the detector diodes, which is analogous
to the pixel resolution in a camera. Metal
and the metal detection signal can be
such as simultaneous multi-frequency, are
now available on the market to address
this longstanding issue.
Both product density and how uniform it is
can affect the sensitivity of an x-ray.
THINK ABOUT PRACTICALITIES
detectors are certainly not immune to the The physical size of machinery is a major
orientation effect, but they do not have consideration, driven in part by rising
this absolute cut-off.
Product effect may be produced by the
product itself or by its packaging. Until
recently, anything wet or conductive
impacted the performance of metal
detectors, especially in the case of
stainless steel contaminants. This
is because metal detectors work by
spotting materials that create a magnetic
or electrical disturbance as they pass
through an electromagnetic field.
16 FDPP - www.fdpp.co.uk
prices of industrial real estate. Limited
presents a minimal hazard, some systems
require protective lead curtains to contain
the x-rays. These in turn need condition
monitoring and must not come into
contact with unpackaged product. Care
must also be taken that they don’t impede
the passage of lightweight products
through the system.
Summing up, Phil comments: “It can be
quite easy to overthink the options. In
some applications, it is obvious which
type of inspection technology to go for,
since only one of them can reliably detect
the contaminants that pose the risk you
are trying to mitigate. As food factories
strive to curtail rising food prices through
widespread adoption of automation and
sourcing ingredients through different
suppliers, the risks increase incrementally.
Installing a metal detection system is
the first line of defence, with today’s
technology on a par with, if not better
than, most x-ray features.”
www.fortresstechnology.com