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Metal Detection Basics

We'll show you the Metal Detection Basics

GETTING DOWN TO THE BASICS – An overview of metal detection basics

The Coils

Metal detectors used in the food industry operate on the concept of a balanced coil, full loop system.  So let’s review some metal detection basics.
For the detector to function, three coils are wound around the aperture (the opening) which the product passes through.  It should be noted that the transmitting coil,  in the center, broadcasts a radio frequency signal, generating an electromagnetic field.

Then, equally spaced on either side of the transmitter coil (inside the metal detector head) are two receiver coils.

The Field

The field is generally trapped inside the shielded enclosure of the detector.  But some field escapes from the aperture on both sides of the detector. Anything Magnetic or Electrically Conductive that enters the field will cause a disturbance in the field strength around it.  Of course, all metals have either one or both of these characteristics. And this will be detectable if the size of the signal is large enough.
The signals from the receiving coils are connected in opposition to each other.  Therefore, when no disturbance is occurring there will be a net signal of zero across the coils – they are balanced. This, then, forms the electrical equivalent of a balanced weigh scale.

As metal contaminant passes through the aperture, the balance will be offset. This disturbance is amplified and analyzed by the control electronics and detection occurs if the sensitivity threshold has been exceeded.

Best Practices

But what is the recommended usage? What are the best practices for installation and the latest features? Which type of metal detector is right for your application?
These can be complex questions.  Regal Packaging Services has the answer to these and many more in depth questions regarding metal detection technology.  Call us at (630) 942-8461.  With decades of experience, we can help you understand Metal Detection Basics.

Testing Metal Detectors

For any quality control/HACCP (Hazard Analysis & Critical Control Points) planto be successful, regular testing must be done.   And this is true no matter how complex or reliable a metal detection system may claim to be. Alongside this, It is crucial that a systematic recording procedure be established. This is especially true since there are no industry standards for detector testing and validation.  Each company must determine its own assessment criteria.

Consider the following when writing or altering your HACCP plan:

1) Test Standard

Traditionally, metal detectors have been checked with ferrous, non-ferrous, and stainless steel test samples. The type of test piece used will vary depending upon the product and application.  In addition to a variety of card types, a ¾”x¼” puck to a 20 “whip” may be used as well. These are just a sample of the many options.  See a full listing here. Some applications will require a custom test piece. Regal Packaging Services has custom designed and manufactured a broad array of test pieces, including a 24 ft “rope,” 6 ft testrods, ½” “top hats” and 1”x5” cylinders.

Also, when it comes to metal detection basics, the size of the metal (seed/standard) sample must be clearly established.  This is true so it can consistently be detected inside the product passing through the centerline of the detector. This is the least sensitive and ideal point.  Placing a test piece on top of or underneath the product moves it closer to the aperture, making it easier to detect. Testing should always be done as close to the center of the aperture as possible. If validation is proven here, it can be safely defined as happening in all other parts of the aperture.

2) The Application

The application for each company and product will be different, so the samples should be adapted to each detector. If the seed (metal) is too small, it will produce unnecessary failures and an unwanted level of frustration level for the operators on the line.  When the seed is too large, it will not accurately test the operation of the detector. Using a range of metal sizes, confirm a repeatable performance level.

Typical guidelines for sensitivity:

Aperture Height               Dry Product                                       Wet Product

                                    Ferrous & Non Ferrous                Ferrous          Non Ferrous

50 – 124mm                            1 mm                                    1.5mm             2.mm

125- 199mm                          1.5mm                                   2 mm               2.5mm

200 mm and up                      2 mm                                    2.5mm             3.mm

3) Test Frequency
The quality control tame needs to determine the frequency of testing the metal detector. Ordinarily, detectors are checked at:

    • Shift Change
    • Product Change, and
    • Hourly or other interval

There is a clear tradeoff exists between the cost of testing versus the risk of a detector failure. A company opens itself to litigation, ROI and sales loss, not to mention the danger to the end-customer. An automatic system can boost the regularity of detector performance testing without additional cost.

4) Test Procedure

The method of testing should be as simple as possible, however it must take into consideration that test pieces and procedures:
a) Should move through as close to the centerline of the aperture as possible. This is the least sensitive point. If the test is successful at this point, it should be safe at any other point in the aperture.
b) Need to be positioned within the product when possible.
c) Ought to include the action of the reject mechanism so that the entire system is checked. This can include testing (documented):

    • With the test piece at the leading, middle and trailing edges
    • Sequential packages or product
    • Alternate packages

5) Test Records

The format for documentation (.docx, .xlsx, .pdf or even handwritten) is not critical, however it should be easily accessible to an auditor and include:
a) Production line or detector identification
b) Date and time
c) Sample used , including metal size, type and certificate number.
d) Identification of the operator
e) Pass or fail result
f) Remedial action taken if result was a fail

6) Automatic Test Systems

Automatic Testing systems are intended to complement, and in some applications replace, manual testing procedures. The effectiveness of these systems should be examined carefully to ensure the testing is relevant and feasible for the application. When installed effectively, they can offer significant savings through reduced labor and product waste.

Sources of Interference

The location, including all environmental conditions, may influence the performance of the Metal Detector.  There can be a significant challenge where high levels of sensitivity are required. Wherever possible, the detector should be situated to avoid or minimize the effect from such considerations.

Sensitivity and performance can be altered by a number of sources:

  • Electrical interference – static, radio, ground loops
  • Vibration – moving metal
  • Temperature variation – ovens, freezing tunnels

While the detector may be capable of filtering some of this interference through features such as “Automatic Balance.”  Still, in some instances, the only option is to decrease the sensitivity level. This is a vital consideration when comparing the capabilities of metal detection systems.

See also:

TestingMetalDetectors.pdf

More info at Regalpack

IFSQN

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Some applications involve large tubs, vats, or some other type of product travel where it would be nice to have a test piece that floats. Once again, our thermoform cards provide the simplest solution. We simply insert some type of structure that forces the thermoform card to retain air, which, of course, is what makes it float. We have also manufactured floating rods, which are more difficult because there is less space to close in enough air to make the rod float but is doable.

We took an ordinary plastic “chip clip” as defined by the customer and embedded the contaminant into the clip. The clip can be attached to the product on the line without damaging or opening the product for testing purposes.

Bone is a difficult contaminant to find because it can vary widely based on the size, age, and calcium content (as well as feed used) of the animal. After working with several customers, Regal Packaging Services offers Pork, Beef and Chicken Bone Simulate. We have a small range of sizes at a variety of depths to simulate whatever you might expect to find in the product. Our recommendation (assuming your x-ray has visual display) is always to start with a multi-card. Using a multi-card gives you an easy way to test several sizes and depths to determine what matches the bone you’re using. Once you’ve made a comparison, testing the card against the bone you’re looking for, you can purchase a card with a single contaminant for regular testing.

This small pill shaped test piece is used in a system with a vacuum tube that shoots the test piece through the aperture and returns it to the “home” base. Your system may never be like it, but the point is that we can make a customized solution no matter how unusual or impossible it might seem.

As you may have seen in some of the other thermoform card descriptions, we can put nearly as many seeds as you can imagine on a single card. Pictured here you will find a wide variety of configurations and contaminants. Just let us know your specs: size of the card (up to 8” x 8”) along with size and type contaminant(s) and we’ll get you a quote as quickly as possible. Working with a multi-card is especially helpful when you are looking for multiple types of contaminants or when you want to determine what size of the same contaminant you are able to detect. In that event, once the size is chosen, you can order individual card(s) for regular, standard inspection and detection.

There are a wide range of applications that require a test piece much longer than our standard 5” or 6” Testrods. We can make up to 3/8” x 34” or 1/2” x 34” rigid test pieces which can then be extended well beyond that length with a “handle” made of acetal cylinder larger than the 3/8” or 1/2” extension. In the photos, we used a 1” x 24” cylinder handle with a 3/8” x 24” inch extension.

Test pieces can be made in a variety of sizes and given a pointed end in applications that require the contaminant (metal) to be inserted into a food package such as sugar or some other soft packaged food product. This allows the metal to reach the center of the aperture.

Double-End Test Whips are also available, with metal in both ends. One consideration is to make certain that the whip is long enough to keep the metal on the non-testing side of the whip outside the metal free zone of your detector. If you insert, for instance one end of a whip with 2.0mm and 3.0mm Ferrous, you need to make certain that the 2.0mm metal doesn’t affect the detection of the 3.0mm metal (and vice versa). If the 3.0mm metal is inside the metal free zone, then the detector will be reacting to the 2.0 as well as the 3.0 as if they were combined.

Multi-seed laminate test cards are designed for seeds 4.0mm or less. Multi-cards are useful in x-ray inspection to determine what sizes and types of contaminants your system can achieve. They can be especially useful in temperate environments and applications. If your application is rugged or wet, or your standards are larger, we recommend our thermoform multi-cards.

We can simulate a variety of packaging in an application where you need the test piece to mimic what is traveling down the production line. In this case, we manufactured a clear “package” with the seed inside. With this method, the customer is able to reject the container appropriately, and, in the event the package is not rejected, it is easily seen by the line worker for manual removal.

There are two types of “candy bars” pictured here – one similar to a flat candy bar and the other in a custom, near perfect copy of a candy bar. Since we’re now able to produce many variations using colored material, the copy was made in red (Fe), green (NFe) and blue (SS 316). We have the equipment to accomplish nearly anything you can imagine, and these kinds of designs are proof positive.

Need a test piece that looks and handles like chicken nugget? We can do it. Interestingly enough, in this situation, our first version for the customer left too much “void” in the hole where the metal was placed. Given the sensitivity the customer system was using, the void had the effect of causing false positives. We were able to minimize that hole and supply the test piece with almost no void. It’s an example of how we can work with you before, during and after manufacturing to ensure all the specifications were met.

Our standard acetal card is 2 1/4"x3 3/8,” but if needed, we can cut that size down to as small as 1 1/4” x 1 1/4” (with limited engraving). If a larger card or a card with multiple seeds is needed, see our Custom Shape-Size Blocks and Tablets.

This distinctive test piece was created to mimic an actual hamburger patty. The customer supplied photos of the raw product, which we were able to reproduce and then we placed blue glove contaminant. Using this configuration, the customer was able to determine what size piece the vision scanning system could detect with the raw burger as the background. Had they simply used a blank card with the glove, the system could easily have achieved contaminant rejection, but they needed to be sure the system could “see” the blue against the product itself.

A card with 100 seeds in a 10x10 configuration is used to place beneath whole chickens on a production line. This gives the x-ray system operator the ability not just to see IF the equipment can detect the metal in the card, but also WHERE the equipment can detect it. If there is a problem, the operator (or company) can make any changes necessary to achieve the end goal – safe chicken in this case. This card really highlights another valuable facet of our thermoform cards.

In this scenario, a company processing sugar needed to have a “bag” simulated with the same weight in order to accurately fall through the reject mechanism. A lighter test piece had the risk of passing over the reject mechanism.

Our standard cubes are 1” and 1.5,” but we can also manufacture other sizes if needed. The cubes shown here are 1/2" in dimensions. Cubes can be helpful in applications where a ball or cylinder may bounce or roll away during testing, making their retrieval difficult, or, in some cases, present a hazard to the product or production equipment.

This is a test piece that is dropped into a bottle. It’s designed so that the metal appears in the center of the aperture when testing the metal detector. In addition, the round stop makes the test piece easily removable after testing.

Hexagonal Shaped test pieces can be very useful in places where a cylinder or ball might roll or bounce around a production floor whether dropped by a line worker or ejection from a reject mechanism. Hexagons are similar in weight to the 1inch cylinders without having the smooth, rounded cylinder edge.

Our customer in this plant needed a test piece with a thickness less than 5.0mm. Because we have the capacity to customize sizes and shapes, we were able to mill down a product that met their specifications. That’s always our goal, meeting the specs you need for your product, testing environment and contaminants.