Metal Detectors: Pre-Packaging Inspection Of Raw Material

Tuesday, July 19th, 2022

There is an increasing demand to inspect raw material for foreign body contamination. APFI Magazine speaks to Hidehiro Ueyama, Global Business Promotion Division of Anritsu Corporation, to learn more about the free-fall metal detector.


METAL detectors have been used for 40 years in food factories. Generally, they are installed and used to inspect finished products as a final check point before the products are sent to consumers. Recently in Japan, the number of suppliers has increased due to a surge in raw material prices, and it is becoming more difficult to maintain quality. This increases the risk of contamination. If a raw material becomes contaminated, there is a chance that it is due to the contaminants being crushed during the downstream process, making it harder to detect the pulverised and scattered bits. Even if foreign bodies are detected after packaging, it could lead to an enormous amount of damage as a result of re-inspecting or discarding the entire production lot. Production equipment could also become damaged during crushing. Thus, it is important to find foreign bodies in the upstream processes. A free-fall type metal detector (more simply “Free-Fall Metal Detector”) detects metals in raw material in the form of powder or granule that falls through a pathway such as a pipe. This type of detector offers great flexibility in installation, as well as superior detection sensitivity. In addition, since a free-fall metal detector is used before the packaging process, it is especially effective for vertical form fill seal packaging lines where metallised film packaging materials are used.


Features Of Free-Fall Metal Detectors

Free-fall metal detectors are used to inspect powders such as wheat and seasonings, and granules such as grain and rice. These materials are not suitable for transportation on a conveyor belt because they will spill off from the sides. Therefore, a free-fall detector is better able to inspect these types of free-falling products that go through a pipe.

parts of a metal detector External appearance of a free-fall metal detector


Let’s look at the features of the exterior. Fig. 1 shows the basic configuration of a free-fall metal detector. The material from the upstream equipment falls through the pipe and passes the detection head, and then goes through a pipe called the “entry chute”, and finally reaches the rejecter. A typical metal detector often has an indicator that shows the settings and results of detection on its detection head. However, the detection head of a free-fall metal detector is separate from the detection head and is connected by a single cable, so it is possible to install the indicator at a convenient location. The detection head, indicator, and rejecter are supplied separately as shown in Fig. 2, which allows for flexible integration into your existing production lines.


Next, let’s discuss performance. To increase the efficacy of a metal detector, the effects of the target material and the environment in which the detector is installed should be minimised to simplify the detection of contaminants. Specifically, the vibration of the detection head is the most significant factor, as it causes magnetic fluctuation, which adds to the effects from possible contaminants, resulting in
lower sensitivity and misdetection. As shown in Fig. 3, Free-Fall Metal Detector is often installed around production equipment such as a multi-head weigher, VFFS packaging machine, and filler. Because of this, it is often exposed to vibrations from the pipe and the platform, which are usually larger than that of a normal metal detector. To cope with this, a free-fall metal detector contains new signal processing technology to reduce vibration noise and maintain stable sensitivity. Signal processing is optimised against physical vibration and electromagnetic noise from the aforementioned equipment, which is also effective to ensure stable detection performance.

Powders and granules tend to generate electrostatic effects due to abrasion against the inner walls of the pipe and between the materials, and a discharge in or near the head can cause misdetection. Antistatic measures are thus important when developing a free-fall metal detector. Usually, these measures are taken inside the transportation path to prevent the target from being discharged. However, Anritsu resolved this problem by wrapping an antistatic sheet around the pipe. We have been experiencing successful installations by this countermeasure.


When it comes to functions, a normal metal detector requires product setup process (auto-setting) to set optimal magnetic-field frequencies and other parameters by feeding good products multiple times. In the case of a free-fall metal detector, most targets are powders and granules, which makes performing auto-setting unnecessary. Contaminant detection is ready to go simply by turning on the equipment and hitting the start button. In rare cases when powders and particles have significant effects on the product, the auto-setting should be performed as with a general metal detector.


A Fail-Safe Design

The most important function of a free-fall metal detector is to reliably reject metal contaminants. It is highly recommended that a rejector for a free-fall metal detector has the following fail-safe mechanisms and functions:
1. The rejection gate stands by in the reject direction even with no power and air supplied.
2. The gate operation is monitored by sensors.
3. An air pressure failure switch is mounted to constantly monitor the air supply to ensure the function of the rejecter.


When the metal detector is switched on, the system automatically implements a test with opening and closing the rejection gate several times. This will be done while checking the positions via integrated sensors, which also checks that the rejector is functional.


It will become increasingly common for inspection of contaminants to be performed in two stages: First on raw materials, and then on finished products after packaging. This advanced method can contribute to improving both product quality and production efficiency, reducing food waste at the same time.


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