SAFELINEMetal Detection & X-ray Inspection

Contaminant Detection:Making the right choice

Choosing between Metal Detection and X-ray Inspection

1

Met

al D

etec

tor o

r X-r

ay In

spec

tion

Choosing Between MetalDetection and X-ray Inspection: making an informed choice

The choice of an inspection system for yourproduction line may seem confusing, perhapsdaunting. Often there are many factors to takeinto consideration. On the one hand, you mayneed to assess the feasibility of differentapproaches based on detailed knowledge of thephysical space available at different locations inthe process. Alternatively, you may need toprepare a careful economic justification basedon up-front and lifetime costs.

There may be corporate guidelines on thecontaminant size that must be detected - canyou meet them? You may need to review thetypes of contaminant and defects that occur inyour production process and assess how bestto combat them. To be sure, any contaminantor defect detection and rejection machineshould only be part of an overall plan toprevent the contamination and the cause ofdefects.

Metal detectors are well established in the foodindustry. There are several hundreds ofthousands installed throughout the world. Theirreliability can be depended on and the cost ofinstallation, set-up and running is wellunderstood. But what do you do if you need tofind non-metallic contamination?

You know that x-ray systems are capable ofmuch more than just detecting metal, but howdo you assess the risks in applying newmethods for your application? X-ray can detect

non-metallic contamination, for example stones,glass and PVC.

X-ray is capable of defect detection, findingmissing or misshapen parts, and checking theweight of food in individual compartments of amulti-compartment package. Is the increase incost of x-ray justified by these capabilities?

Understanding the Technologies: the core principlesUnderstanding the important differences betweenmetal detector and x-ray technology is a keystep to making the right choice. In the nextsection you will find a description of each of thetechnologies. This is followed by a comparisonof their relative merits.

X-ray

In addition to purchase costs, understanding lifetime costs are often an important factor in purchase decisions:

Tota

l Cos

t of O

wne

rshi

p

Direct Costs Installation Commissioning Operator training Performance verification Product changeover Sanitation Downtime Annual maintenance

Indirect Costs Brand Image Supply Chain Confidence Product Recall Costs Scrapped Product

The Conventional Metal Detector

In a conventional metal detector, the productpasses through a tunnel, or aperture, in theunit. It might be taken through on a conveyorbelt, or it may be dropped through. Inside themetal detector, surrounding the aperture arethree encircling coils, usually made of copper.

One coil, the transmitter, has an alternatingelectric current applied, which produces analternating magnetic field. The other two coils,which form the receiver, are spaced equally oneither side of the transmitter. A voltage isinduced in each of these coils, and, ifsymmetrical to each other, the voltages in bothreceiver coils will be equal.

By connecting the coils back to back, thevoltage will cancel. However, when anythingdisturbs the magnetic field, the voltages nolonger cancel. If amplified, this differentialvoltage can be used to detect whatever disturbs

the magnetic field. This could be magneticmaterial, but can also be conductive material,since, in an alternating field, electricalconductors will have an eddy current induced inthem, making them look like small alternatingmagnets.

The electrical conductivity of your product mayproduce a disturbance of the magnetic field,and this effect - the 'product effect' - must besuppressed before any metallic contaminationcan be reliably found. Similarly, if the food is tobe inspected within its packaging, and if thatpackaging contains metal (for example,aluminum foil trays, metalized film wrap), theeffect of this must also be avoided.

Robust Design: essential for reliable operationA metal detector designed for the food industryis capable of finding a pinhead in a loaf ofbread. To be this sensitive, and to operatereliably in a production environment withoutfalse rejects, the detector must be verymechanically stable.

If the casework of the metal detector moves onlya few microns relative to the coil system, themagnetic field will be disturbed causing a falserejection of product. Metal detectormanufacturers use extreme measures to achieve

Output

Receiver Receiver

Transmitter

ProductFlow

Balanced Three Coil System

2

As the pack size increases, the aperture and coil inside must also increase. At larger coil sizes, metal detector sensitivity reduces.

Aperture size

76x22

100 circular

350x175

1000x400

1000x1000

3 x 7/8

4 circular

14 x 7

40x16

40x40

0.15

0.4

0.8

2.0

5.0

(mm) (inches)

Ferrous ball size that can be detected (diameter mm)

Rectangular Aperture Metal Detector

Section through metal detector, showing coils

3

Met

al D

etec

tion

the necessary mechanical stability.

In addition, since the metal detectormust operate at radio frequencies, it isalso a very sensitive radio receiver,and a false rejection can be causedby electrical or radio interference.Again, great care is taken bymanufacturers to minimize thisproblem by designing electroniccircuits that are not disturbed byexternal fields.

Product Effect: handling conductive productsThe signal from a conventional metaldetector is a continuous voltagecorresponding to disruption of thealternating magnetic field in the metaldetector's coil system. Two signalscan be detected.

One originates from reactive effectsand is shown in red in the chartbelow: this can be caused by casemovement, large pieces of metalmoving in the vicinity of the metaldetector and by magnetic materialpassing though the aperture.

The second signal occurs as a resultof resistive effects and is shown inblack in the chart below: this iscaused by absorption of energy whenan object with electrical conductivitypasses through the aperture.

Some food products have electricalconductivity as a result of their salt oracid content. Small metalcontaminants produce both reactiveand conductive effects in varyingproportions that depend on the type ofmetal and on the size and shape ofthe particle.

There are two important potentialcauses of false triggering in a metaldetector: (a) vibration causing verysmall movements of the metaldetector case with respect to the coilsystem and appearing predominantlyon the reactive signal; and (b)electrical conductivity of food productswhich appears mostly on the resistivesignal, particularly at highfrequencies.

The signal processing in the metaldetector must be arranged so as toreject these two effects, but maintainthe best possible sensitivity to a realmetal contaminant. When there is noproduct signal, the resistive signalcan be used at very high sensitivity todetect the very smallest metalparticles and the reactive signal islargely ignored, since it contains onlyvibration signals. When the producteffect is large, the reactive signal mustbe used, but at reduced sensitivity, inorder to avoid the effects of vibration.

Case Study: Snack FoodA major snack food producer wasbuilding new plant as part of anexpansion program to meetincreased demand. They had foryears used ‘Throat’ metaldetectors, which require as littleas 200mm of vertical installationspace.

These are inserted between themulti-head weigher and theforming tube or ‘throat’ of thevertical form, fill and seal (VFFS)machine. The performance deficitusually associated with metalizedfilm packaging is avoided byperforming the metal detectionjust before the product enters thepackaging.

The new development consistedof eight multi-head weighers eachfeeding a VFFS machine. Thesefeed into a single case packingline and the feasibility of utilizinga single x-ray machine at thispoint was investigated. One x-raymachine could potentially replaceeight metal detectors.

Choice of technology came downto contaminant detectionperformance. The metal detectorswill detect down to 1.0mm Feand 1.5mm SS test ball. Inpractice, the x-ray system couldnot match the 1.0mm ferrousmetal ball test and the decisionwas made to stay with the metaldetection technology.

Metal detector signals from three ready meal products passing through the aperture

Volta

ge

Time

READY MEAL #1 READY MEAL #2 READY MEAL #3

Red signal from reactive effects - vibration, good conductors and magnetic contamints

There is metal contamination in the third pack - detected by the red signalBlack signal

from conduction - primarily product effect but also from metal contaminants

Metal Detectors

4

Packaging Effect: managing conductive packagingWhile a metal detector is extremely sensitive todetecting metal, its ability to ignore metal orconductive packaging, or parts that are meantto be there, is limited.

There is an increasing use of metalized film inpackaging which acts as a good gas barrierand extends product shelf life. Metalized filmemploys a thin layer of aluminum which canhave a considerable effect on metal detectorperformance. In order to combat the effect, themetal detector may have to operate at a lowfrequency where it will be less sensitive tostainless steel.

The toughest challenge for metal detectorperformance comes from aluminum foil traysfor goods that are to be baked at home. In thiscase the volume of metal passing through themetal detector with each product isconsiderable.

Over the years, metal detector manufacturershave developed various alternatives to thethree-coil radio frequency technology to helpdetect metal contamination in foil trays. So-called ferrous-in-foil metal detectors useconstant magnetic fields so that the eddycurrent effect is minimized. As a result, thesesystems can only really detect magneticmaterials. Austenitic stainless steel grades suchas 304 or 316 cannot be found very efficientlybecause they are mostly non-magnetic.However, these materials are used extensivelyin the food industry and they frequently turn upas contaminants.

Performance Verification: maintaining detection standardsMetal detectors are traditionally tested usingball bearings, usually ferrous, non-ferrous andstainless steel. In practice these are made from,respectively, a low alloy chromium steel (case

hardened for bearing applications), brass, andnon-magnetic stainless steel; either grade 304or 316. These are readily available in a varietyof sizes. Using a sphere means that it does notmatter what the orientation of the test piece is, itwill always perform equally well. It is thenpossible to check the performance of a metaldetector in a test that takes just a few minutes.

Test pieces are inserted into samples of theproduct, usually in three positions: front. middleand back. The test ball is always positioned atthe least sensitive part of the aperture, which isin the middle. A routine test can be performedon a regular basis, e.g. every 4 hours. Thespiked product must go into the reject bin everytime in order to pass the test.

These tests have become standard practice inthe food industry because they are relativelyeasy to perform and produce very consistentresults.

The Metal Free Zone: achieving and maintainingperformance in the plant environmentWith a metal detector, because it is so sensitiveto metal, a metal free zone must be createdaround the detector. Conveyor components,including belt materials, must be metal free andreject mechanisms must be positioned at asufficient distance to prevent their movementfalsely re-triggering the metal detector.

Where space on the production floor isparticularly limited, metal detectors with reducedmetal free zones are available, but cancompromise the sensitivity of the detector.

Metal Free Zone: Plan view of detector

Metal Free ZoneDetectorHead

Conveyor Belt Product

5

X-ra

y In

spec

tion

X-ray Inspection

X-ray inspection machines operate usingtechnology essentially the same as that usedfor medical x-ray and baggage security x-rayimaging, but with one significant difference: Forhigh-speed production lines in the food andpharmaceutical processing industries, werequire these systems to operate fullyautomatically. This necessitates a computercontrolled system to apply the pass or rejectcriteria.

The x-ray image is formed by scanning theproduct in two directions: one direction is themovement of the pack on a conveyor belt orpumped through a pipe. The second direction isperpendicular to the motion of the product,generated by the x-ray tube on one side of theproduct, and detected by a linear detector arrayon the other side. Data from the detector ispassed to a computer where it is assembledinto a complete image of the product. Imageprocessing algorithms are then used todetermine if contaminant or defects are present.The images are complex, and require wellproven algorithms to achieve sensitive andreliable results.

Data Collection:A wealth of informationCompared with metal detection, the datacollected by x-ray inspection on a product ismuch more diverse, and potentially far more

useful to the line management team. A 2-dimensional image - essentially a crosssection of the product - is captured. Softwarealgorithms can then exclude packaging andknown features, isolate individual componentswithin the pack, check the number, shape andsize of these individual components and evencalculate the mass of material in individualcompartments. In some cases, x-ray offers theonly method of check-weighing certain types ofproduct, i.e. individual compartments in a webof connected products such as milk powderpouches.

Larger Pack Sizes:maintaining sensitivityWith large product sizes, an x-ray system will

End View of X-ray Inspection Machine

X-ray tubein tank

X-ray collimator

X-ray fanshaped beam

Product in aperture

Com

pute

r

X-ray linear array detector

Increasing Pack Size

The small increase in pack size from A to B has resulted in twice the operating distance for the x-ray tube requiring four times the power. The length of the detector has also increased.

Detector

X-ray Tube

B

Product

A

Product

Front View of X-ray Inspection Machine

Food packs

X-ray tubein tank

X-ray collimator

X-ray beam

X-ray cabinet

X-raydetector

6

often out-perform metal detection. The signalfrom a given piece of metal contaminant getssmaller when its distance from the coil of ametal detector increases. So for larger size coilsneeded for larger apertures, the sensitivity at thecentre of the aperture degrades, lowering theoverall metal detector performance.

As pack size increases on the x-ray system, sothe voltage of the tube is increased to 'see'through the increasing thickness of the product,and sensitivity is maintained. The power of thex-ray tube must increase as the square of thedistance from tube to detector. The higher powerx-ray system does increase the cost of the x-raycomponents.

X-ray's sensitivity to detect small contaminantdepends on the resolution, or pixel size, of theproduct image generated. This can be improvedby decreasing the size of diodes used in thedetector, giving an increased number of diodesin the detector array, and an increased imageresolution. Again, this can increase thecomponent cost of the system.

However, with these improvements, thesensitivity does not degrade with size increaseas it does with metal detection.

Higher Line Speeds:maintaining sensitivityMetal detectors can operate at up to 5000ft/min (1500 m/min) for blown pipelineapplications and can handle high speed flow

wrap lines, running at up 2000 packs perminute or more. Modern X-ray systems operateat up to 600 ft/min (180 m/min)and forconveyorized applications, they can tackle allbut the fastest flow wrap lines. At the highestspeeds, they may require increased power tomaintain good image quality.

Water Depth and TextureX-ray imaging performance is limited by twomain factors: the x-ray density of the materialthrough which the x-rays must pass, and thetexture of the object.

X-ray density is often referred to as ‘waterdepth’, that is, the equivalent depth of water toattenuate, or absorb, x-rays to the same degree.

Contaminantdetection in glass enclosure using dual beam system

X-ray Image Examples

High-resolution imageof nutraceutical product

Missing part detection in chocolate assortment

The texture of the product is the ultimate limit to contaminant detection - if the contaminant is

not distinguishable from the natural variation in the product, then it cannot be detected

i.e. ready meals chicken and rice

7

X-ra

y In

spec

tion

Since water is a major constituent ofdenser foods, then the water depth isoften similar to the physical depth of theproduct. If a very high proportion of thex-rays are being absorbed by a givenwater depth, then the x-ray energy levelmust be increased to improvepenetration. If the energy levels have tobe increased, the detection of fine detailin the image reduces and lower densitycontaminants are more difficult to find.

Product texture, or variations in the x-ray density, caused by the granularityof a non-homogeneous product, canlimit performance. The power setting ofthe x-ray tube may be correct for theaverage density of the product, but willbe too low for more dense elements,but also too high for less denseelements, reducing the sensitivity todetect contaminants and productfeatures. Modern x-ray systems haveelectronic controls for the voltage andpower of the x-ray tube, so that thesecan be optimized for each product typeto be inspected.

Mass estimationX-ray density is not the same as atomicmass, so estimating mass from x-rayabsorption is not exact. For applicationswhere the proportion of differentelements remains reasonably constant,

then veryaccurate massestimation ispossible.Somecompensationfor ingredientchanges canbe achievedusing dualenergytechnology,but this doesnot ensuretotal accuracy.

Performance TestingThe test procedure using metalspheres employed routinely withmetal detectors will not yieldconsistent results with x-ray. This isbecause there is considerably morevariation in signal from small testspheres in x-ray than there is in metaldetection. in the graph below, theprobability of detection of metalspheres is plotted against their size. Itis easy to specify a ball size that willalways be detected in a metaldetector. For x-ray, even though theaverage size of test sphere that canbe detected is smaller, the test spheresize that will always be detected ismuch larger.

If an x-ray machine is testing formissing or deformed components,glass and plastic contamination, (inaddition to metal), then each of thesecapabilities must also be tested withappropriate test pieces andquantified. Validating the performanceof an x-ray system on a regularbasis, and to the same standards asis routinely done in metal detection,requires significant resource.However, performance testing of theinspection equipment is a vital part ofachieving product integrity. Yourequipment vendor will be able toadvise you on how this can beachieved at minimal cost for yourapplication.

Probability of Detection for Metallic SpheresComparing Metal Detector and X-ray Technology

Prob

abili

ty o

f Det

ectio

n (%

)

Metal Sphere Size (dia. mm)

0

10

20

30

40

50

60

70

80

90

100

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Metal DetectorX-ray

Mass attenuation coefficients for common elements at 20 keV and 50 keV compared with the atomic mass

H

C

N

O

Na

Al

Cl

K

Ca

Fe

Cu

Zn

20 keV

0.369

0.442

0.618

0.865

2.06

3.44

7.74

10.9

13.1

25.7

33.8

37.2

50 keV

0.336

0.187

0.198

0.213

0.280

0.368

0.648

0.868

1.02

1.96

2.61

2.89

Atomic

Mass

1

12

14

16

23

27

35

39

40

56

64

65

Case Study: FlowWrapping inmetalized FilmA baked goodsmanufacturer supplyingflow wrapped product fora premium brand neededto implementcontamination detection.

Two main alternativesbecame apparent: a metaldetector offering down to6mm stainless steeldetection capabilitybecause of the metalizedfilm wrap or an x-raysystem that could achieve1.5mm stainless steel.

Although the x-ray systemwas significantly moreexpensive, the companychose x-ray. “We will beable to exploit the x-raymachine's capability tocheck correct placement ofchocolate toppings to thecakes; this, and the vastlyimproved detection ofstainless steelcontaminant meant thatinvestment in x-ray wasthe correct decision forus."

8

Comparison of the Technologies

Choosing between x-ray and metal detectiontechnologies may require a careful analysis ofall the factors although in some cases thechoice is simple (see flow chart on next page).If you need to detect shredded aluminum drinkscans, then metal detection is probably your onlyalternative, as it is if your product is gravity fed.If it is vital that you detect non-metalliccontaminants or that you check part counts,detect misshapen product or estimate weight ofproduct, then x-ray is your choice. In othercases, however, the whole production operationneeds to be considered; there may be a case formetal detection at one stage of the process andx-ray at other locations.

You will always seek to match or excel thequality requirements of your customer whileminimizing the total cost of ownership of theequipment. This section highlights some of theapplication issues that may influence yourdecision.

Metallic Contamination Detection in non-metallic packagingIf your requirement is the detection of metalliconly contamination in product packaged in non-metallic materials, and if your budget is limited,then in many cases metal detection technologywill offer the best alternative.

However, if your product is large, then x-rayinspection will out perform metal detection on abasic contaminant sensitivity specification in themajority of cases, but the costs will be higher.

The Effects of Aluminum: as a contaminant and as a packagingmaterialAluminum is a good electrical conductor buthas lower x-ray density than other commonlyoccurring metal contaminants. Therefore, ifaluminum is a contaminant threat, than metaldetection will do the best job of identifying andrejecting aluminum contaminant. However, ifaluminum is included in the packaging, eitherin the form of metalized film or as a foil tray,then its low x-ray density can be exploited - thex-ray system will largely ignore it and will do asuperior job of inspecting the contained productfor the metal contaminants.

Available SpaceThe space available for installation of yourinspection machine may be an importantconsideration. A significant number ofmetal detectors are used whereperformance has to be balanced againstthe space available.

Although very small in footprint, rememberthat with a metal detector you must alsotake account of the metal free zone, soeven though the detection unit is small, itmay not be possible to position it as closeto other machinery as hoped.

Where x-ray machines must be fitted intosmall spaces, the focus is often onreducing the in feed and out feed tunnel.However, under the regulations in force inmany countries, the tunnel must besufficiently long to prevent a personreaching into the unit. This problem can beovercome in some cases by incorporatingadjacent machinery into the guardingprovision. However, the operators safetymust always take precedent.

Metallic Contamination DetectionWithout Metallic Packaging

For metallic contaminant detection, metal detectors offer greater performance at small apertures than x-ray systems, and the cost is lower. However x-ray may be the correct choice if

other inspection capabilities are needed. X-ray outperforms metal detection at large apertures.

X-ray

MetalDetection

MetalDetection

X-rayPerfo

rman

ce

Aperture Size

Higher Cost Lower Cost

9

Met

al D

etec

tor o

r X-r

ay In

spec

tion

Some Initial Questions on Choice of X-ray or Metal detection

Start Here

No

No

No

No

YesIs your product gravity fed?

YesIs your product in metal or metalized film packaging?

YesDo you need to detect

aluminium contamination?

YesDo you need to detect for glass,stone, bone? Or measure mass

or identify shape defects?

A more complexevaluation of

options is needed.

X-ray Inspection could be

the right choice.

Metal detection could be

the right choice.

Key Differences

Metal DetectionProduct can be on conveyor, in pipe or drop fed

Costs increase only slightly with increased aperture size

Significant sensitivity reduction with increase in aperture size

Reduced performance with a short conveyor length or insertion distance with incorporation of metal free zone (MFZ)

Detectable contaminants must be magnetic or electrically conductive

Product texture not important

Product electrical conductivity may limit performance

Aluminium packaging limits performance - you may only be able to detect magnetic material

Can detect aluminium contaminant

Can operate at very fast speeds

X-ray InspectionProduct must be at constant speed - cannot be drop fed

Costs increase with increased aperture size

Only slight reduction in sensitivity with increase in aperture size

May need special guarding with short conveyor length

Detectable contaminants must have a different X-ray densityfrom product

Product texture may limit performance

Product electrical conductivity not important

Aluminium packaging generally does not reduce performance

Aluminium can't easily be detected

May be limited in speed

10

Many factors go into the melting pot before achieving the solution that exceeds your customer's expectations

and minimizes your total lifetime costs.

1. Exceed customer’s requirements2. Minimize costs

Contaminant size and line speed?

Inspection of shape, weight or

integrity needed?

Space available at different locations

through production?

Contaminant type: Metal, glass,

plastic, aluminium?

Lifetime cost of ownership analysis

Mettler-Toledo Safeline, Inc.Metal Detection and X-ray Inspection6005 Benjamin Road, Tampa, FL 33634Tel. 1-800-447-4439 • 1-813-889-9500Fax 1-813-881-0840

Subject to technical changes©12/2005 Mettler-Toledo Safeline, Inc.Printed in United States

Visit for more information

Safeline - Making your products safer.

www.mt.com/safelineus