Customize Your Idea
ISO Process for Custom Test Pieces
June 1, 2022
Bone Simulate
The Problem with Bone Detection in X-ray Inspection
July 21, 2022
Customize Your Idea
ISO Process for Custom Test Pieces
June 1, 2022
Bone Simulate
The Problem with Bone Detection in X-ray Inspection
July 21, 2022

Metal Detector Guide Notes

In providing you with a set of metal detector guide notes, we start with usage, which can be broken into three main categories.

  • Packaging – where higher sensitivity (the smallest piece of metal) is desired,
  • Bulk Processing – where gravity drop or pipeline metal detectors are used to protect equipment (e.g. blenders, mixers, cutters, choppers, etc.),
  • Industrial – where industries have lower sensitivity requirements (mining, aggregate, gravel, plastic, lumber, etc.).

All metals have either magnetically conductive or electrically conductive properties, or both. When they enter an electro-magnetic field, they create a detectable disturbance (or “signal”). Three coils in a static-shielded head loop around an aperture through which product passes. The head houses a transmitter coil that broadcasts a radio frequency signal and generates an electro-magnetic field. On either side of that transmitter coil, two equally spaced receiver coils detect whatever might be passing through. The signals from the receiving coils connect in opposition to each other.  Without any disturbance, the net signal across the coils amounts to zero – they are balanced.

Balance stability

“False triggering” can be caused by:3 Coil Design

  • Very small movements
  • changes in temperature
  • vibration from nearby equipment
  • instability in the construction

Foil lids and trays require an alternative method: “Ferrous in Foil” detector. Non-ferrous and stainless steel contaminants cannot be detected with this type of detector.  Metal detectors measure electrical conductivity and magnetic permeability. Thus, iron-enriched products (e.g. cereals) create a large magnetic signal.  Products w/high moisture & salt content (e.g. bread, meat, cheese) are electrically conductive.  Signals from ferrous metal are larger than signals from the same size piece of nonferrous or stainless metal. Signals caused by vibration are always along the same lines as non-ferrous metals.  The detector can “rotate” the signals and map out or create a “detection envelope” for the product so that it can be ignored.  This is called “phasing out.”

Metal Free Zone

Generally, the practical size of the leakage is 1½ times the (smaller) than the aperture dimension.  No metal should be allowed in this area. Large moving metal should be kept 2 times away.

Sensitivity

In theory, aperture size determines the sensitivity of a given metal detector.  The center of the aperture is the least sensitive point of the metal detector.

Ferrous:

Ferrous materials are any metal easily attracted to a magnet and usually the most common contaminant outside of food processing plants.

Non-ferrous:

Non-ferrous materials are highly conductive non-magnetic metals (e.g. copper, aluminum, brass, etc.). When inspecting conductive products, increasing the test sphere size by at least 50% is a good practice.

Stainless Steel 316 is the most difficult metals to detect and the industry standard.  This is commonly used metals in the food processing and pharmaceutical industries. When inspecting non-conductive products, a stainless steel test sphere typically needs to be 50% larger than a ferrous sphere.  When inspecting conductive products, a stainless steel test sphere needs to be 200% – 300% larger than a ferrous sphere to produce the same size signal.

The Four “Ps” of Metal Detection:
  • Preventing Contamination
  • Profitable Production
  • Protecting Customers
  • Promoting Brand Confidence

Metal Detection Terms

In the absence of any industry standards for detector testing, each company must establish their own test criteria within this broad range of Metal Detector Guide Notes.

Typical guidelines for sensitivity:

Environmental conditions may affect the performance of the detector:

Airborne electrical interference – static, radio, earth loops, etc.,

Vibration – moving metal,

Temperature fluctuation – ovens, freezing tunnels, etc.

 

 

Detector Performance

Manufacturers will generally supply recommendations for achieving the best performance from the metal detector. Always apply these recommendations;

  • Isolated rollers – to prevent ground loops,
  • High quality belt – metal free, interlocked finger joints, plastic modular belt,
  • Low vibration and static-free area,
  • Adequate metal free zone.
Reliable Reject Device
Type Suited for Max Wt Notes
Air blast Light consumer packs (i.e. biscuits, chocolate bars, etc.) High throughput 2 lbs. (1 kg.) Unsuitable for loose product, boxes, curved surfaces, and some bagged product
Divert arm Medium to light packs.  Medium throughput 10 lbs. (5 kg.) Product generally enters bin diagonally — must ensure it will fit.
Push arm Medium packs. High throughput 15 lbs. (7 kg.) Unsuitable for loose or fragile product.
Stop on detect Large bags or boxes, hand fed or bulk material.  Slow throughput. 50 lbs. (25 kg.) Requires an operator to remove contaminated product.
Retracting pulley or flop gate. Small product in lines or irregular shape.  Medium throughput. 5 lbs. (2 kg.) Dimensions are for whole line or batch of products.
Gravity Drop / Drop-Through Metal Detectors

Existing piping often determine the pipe and detector size. This formula is useful if you know the peak expected flow rate of the product and its density:

Area of aperture required (in2)    =   0.024 X Flow rate (#/hr) / (divide by) Bulk density (#/ft3)

Example: A product with a flow rate of 20,000 #/hr and a bulk density of 18 # /ft3 will require a calculated pipe area of .0.024 X 20,000 / 18 = 26.7 in2. (or a pipe with a diameter of about 5.8”)

Metal detection can be tricky.  We hope this set of Metal Detector Guide Notes will assist you in determining how  best to protect your customers and your business against metal contaminant.   At Testrods.com, we can provide you with all the necessary test pieces to keep your systems fully tested and safe.

4 Comments

  1. Chad says:

    “Stainless Steel 316 is the most difficult metals to detect and the industry standard.”

    Do you have a scholarly article or data to support this statement?

    • brian says:

      Chad,
      Good question.
      I would refer you to our article at https://www.testrods.com/stainless-steel-316-vs-stainless-steel-304/, but I will also send you a couple of articles from other sources. It’s generally understood that the 300 series Stainless Steels are the least conductive and least magnetic, therefore producing the least signal in a metal detector, so the discussion usually focuses on the difference between 304 and 316. Because 316 is less detectable, it is the food industry standard, and why we only offer it. Look for the articles in your inbox soon and let me know if you still have questions.

  2. Steve Smith says:

    It’s interesting when you said that understanding the proper usage of metal-detectable equipment could help implement the safety process. My friend mentioned a few days ago that he was looking for a metal-detectable food hygiene sleeve that could eliminate allergen transfer. He asked if I had thoughts on the best detectable product-finding planning. I appreciate this enlightening detectable products article, I’ll tell him that consulting a trusted metal-detectable products supplier company can provide details about the products they offer.

    • brian says:

      Steve, thanks for your question and the kind words. Unfortunately, at Testrods.com, we don’t sell anything but metal detection and x-ray inspection test pieces and I’m not familiar with how to handle allergens. I don’t believe you would want a metal detectable sleeve. The only thing you want being detected in a metal detector is contaminant in the product. As for the “best detectable product-finding planning,” I’m not sure I have a clear idea about what you mean. Each company can develop its own HACCP plan and food safety programs based on various aspects of production, including, of course, the product itself. Sorry I can’t provide more information. Feel free to contact us directly at testrods@testrods.com.

Leave a Reply

Your email address will not be published. Required fields are marked *


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.