A metal detector is a mandatory tool for foreign body inspection equipment in food factories. In Japan, HACCP guidelines will be fully institutionalised from June 2021, so the metal detector—which is a critical control point (CCP) for foreign body inspection process—is attracting attention again. By Hidehiro Ueyama, Marketing Department, Anritsu Corporation.

Figure 1: Tablet Metal Detector

While a belt conveyor is generally used as a product transport method, in the manufacturing process of products that cannot be conveyed using conveyor belts, a free-fall method within a covered chute is used to transport dry and loose products including tablets. 

For pumped products, a pipeline method is generally used. In this article, we’ll explore the tablet metal detector which adopts a free-fall chute for product transport method. The typical applications for the tablet metal detector are healthcare products, functional foods, as well as tablets and capsules in the pharmaceutical industry. 

Product Transport

As you can see in Fig. 1, the Metal Detector is equipped with a free-fall chute instead of a belt conveyor. A tablet is about a few mm up to 10 mm in diameter and several mm in thickness, which is very small compared to general food products. It is placed in a mold and has a dry characteristic (Fig. 2). For this type of application, in order to avoid moisture, it is common to pack dozens to hundreds of pieces together into an aluminium foil package or a bottle-shaped container.
For the above mentioned products, there are many packages using metal sheets or caps, and for the final product form in the hands of consumers, a metal detector with a belt conveyor is not suitable or the detection performance is very limited due to the extremely strong product effect from the packaging materials. That is why the tablet metal detector performs inspection only on the contents itself before packaging.

Please refer to Fig. 1. The inspected product is put directly into this inlet and reaches the sorting part through a tunnel-like detection head while sliding on the inner wall of the chute. What is remarkable here is the angle of the chute. The inspected product glides through the inner wall of the chute in the direction of gravity, but the element that controls the speed of the slide is the angle of the chute. 

Figure 3: Adjusting the chute angle (tool-free)

The speed of gliding is directly related to the processing amount, and the processing amount increases as the angle approaches vertically. However, in the case of a metal detector, since the detection sensitivity of the metal decreases when the inspected product passes the detection head extremely fast or extremely slowly, it is necessary to determine the angle of the pipe by looking at the balance between the detection sensitivity and the processing amount. 

The other aspect is the relationship between the properties of the product and the friction with the chute. When product has a large frictional resistance and the

angle of the chute is shallow, the flow speed slows down and finally stops, causing clogging. In our latest model, the angle of the chute can be adjusted from 20 ° to 45° (Fig. 3) without using tools, but a product test is recommended at the stage of considering equipment introduction.

2. Environmental Resistance

Compared to X-ray inspection machines used in the same foreign body inspection process, metal detectors have features that are prone to performance differences due to good or bad installation environments. Vibration is the most important installation environment factor related to the performance of metal detectors. In the case of weighing scales, floor vibration can make weighing accuracy worse, as the zero point is not determined, and the load is not physically stable. The measurement principle is different in the metal detector, but since the reference value equivalent to the zero point of the weighing scale rises, it becomes difficult to detect small metals.

Anritsu has developed an advanced technology that analyses the characteristics of vibration and separates and measures the reaction of metal-contaminated products from there, so that false detection does not occur after setting an appropriate detection limit, even if the metal detector is hit or shaken. This feature can be effective in countermeasures against vibration caused by tablet presses used in the manufacturing tablets.

The next environmental resistance performance is antistatic measures. Static electricity is not recommended for metal detectors, and even belt conveyor type metal detectors are sometimes equipped with antistatic brushes on conveyors, sprayed with ion mist, and devise ways to prevent static electricity from being charged. However, with the tablet metal detector, taking such countermeasures is usually difficult as product (tablets) glide on the chute. In addition, the product is almost completely dry and prone to get electrified. In response to this problem, Anritsu can supply antistatic chutes and ionisers that neutralise statics by mixing conductive substances in the chute itself and suppressing the static electricity. Since the generation of static electricity is a cause of false detection and causes clogging in the chute, it is recommended to install an antistatic chute or an ioniser according to the characteristics of the product.

3. Simplified Initial Setup & Detection Performance

The metal detector needs to measure the reference value at the initial setting stage because the product itself has a certain amount of product effect and the detection limit setting is to be done based on the reference value. However, Anritsu Tablet Metal Detector basically does not require the initial setting. This means that regardless of the product composition, the detection performance remains unchanged. Our latest model, the KDS1004PSW, shows detection performance of feφ0.25mm, NonFeφ0.30mm, SUS316 φ 0.40mm for common tablets. Only if the product includes any substances that increase the variation of the magnetic field line, it is necessary to implement the initial setting.

4. Magnetism Analysis Function

Figure 4: Magnetism Analysis Function

When a foreign body is detected, knowing its characteristics helps to identify where it originated. It is common to use destructive testing with a dedicated analysis device, but in this case, the results cannot be obtained in a short time.

Our latest model has an in-built magnetism analysis function (Fig.4). After production, manufacturers can activate this function and feed in the contaminated product. The system will then analyse the degree of magnetic and non-magnetic contaminants. Although it is a rather auxiliary function, it would be convenient because it can determine whether the metal is magnetic or non-magnetic just by feeding the contaminated product.

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