Metal Detection and X-Ray in Pressed Candy OEM: Sensitivity, Reject Checks and CCP Control
Foreign body complaints are not the most frequent issue in tablet contract manufacturing, but they are the most damaging: a consumer who finds metal fragments in a bottle does not ask for a replacement, the retailer pulls the batch and the claim lands on the brand. Nearly every tablet plant owns a metal detector, and owning one is not the same as controlling the hazard. During audits we routinely find an expired calibration sticker, test pieces buried in a drawer, and rejected tablets going straight back into the hopper. Physical hazard control works only when source analysis, equipment capability, verification frequency and exception handling form one closed loop. On pressed candy programmes we run metal detection as a critical control point, not as a formality before despatch.
1. Where the metal actually comes from
Start from sources, because they determine where monitoring effort belongs. Raw materials are one route: botanical extracts and mineral ingredients can carry screen wire, bolt fragments or washers from milling and packing, which is controlled upstream through supplier qualification and sieving before charging. Equipment wear is another, and it is the most common source in practice: punches, dies, scraper blades and feeders generate fine metal over time, while a broken sieve mesh in the granulator is the classic culprit, which is why mesh integrity is checked and recorded every batch. Packaging contributes foil slivers and cap burrs, handled through incoming inspection. Rank these by likelihood at your own plant and the monitoring plan designs itself.
2. What a metal detector can and cannot do
A metal detector works on balanced coils: metal passing the head disturbs the field and triggers a signal. Understanding the limits matters more than memorising specifications. Sensitivity falls as aperture grows, so a machine with a large tunnel claiming sub-millimetre iron detection is usually quoting an empty-aperture figure that will not survive contact with product. Metal type changes everything: ferrous is easiest, non-ferrous such as copper and aluminium comes next, and non-magnetic stainless, particularly the 300 series, is hardest, sometimes producing a signal a third the size of iron for the same particle. Product effect is the big one for tablets: moisture, salt and mineral content make the product itself conductive, and common tablet ingredients such as calcium carbonate, dicalcium phosphate, iron salts and electrolytes raise the background noise so much that sensitivity has to be backed off to stop false rejects. Orientation matters too: a wire travelling lengthwise is far harder to see than the same wire side-on. These four factors mean selection and sensitivity must be proven on the real product rather than copied from a brochure.
3. Setting sensitivity and running test pieces
The correct method uses finished tablets. Place standard test pieces at several positions, front, middle and back of the product stream, and at the centre of the aperture as well as near the edges, then pass them through and confirm the machine alarms and rejects every time. Three materials are standard: ferrous, non-ferrous and non-magnetic stainless, sized to the risk and the machine, typically around 0.8 to 1.5 mm ferrous, 1.2 to 2.0 mm non-ferrous and 1.5 to 2.5 mm stainless, with mineral-heavy tablets needing the larger end of each range. A workable verification rhythm is all three pieces before start-up, again at batch change or at a defined interval during the run, and once more at the end. Every check is recorded with result, time and operator, and a failed check triggers containment of everything produced since the last passing check, back to a known good point. Reject confirmation must be part of the same test: the alarm firing is meaningless if the flap or air blast does not fire, and we have seen installations where the lamp lit and the reject valve was dead, so alarm and reject are always verified together.
4. When X-ray is the only sensible answer
Four situations push you to X-ray. Product effect that cannot be overcome, where mineral-loaded tablets force metal detector sensitivity down to a meaningless level. Shielding packaging, because foil blisters and metallised laminates cannot be inspected by coil-based detection at all while X-rays pass straight through. Non-metallic foreign bodies such as glass, stone, dense plastic, rubber and bone fragments, which a metal detector simply cannot see and X-ray identifies through density difference. And added inspection value, since X-ray can simultaneously check for missing tablets, broken tablets, fill anomalies and pack integrity, which is particularly useful on blister lines. A common and sensible configuration is to run metal detection before packing and X-ray after packing on foil formats, so the two technologies complement rather than replace each other.
5. Handling rejects without creating a loophole
The reject stage is where controls quietly fail. The reject bin must be lockable and opened only by authorised staff, which removes the temptation to tip material back into the line. A workable flow is to record time and quantity, open the bin under quality oversight, examine the contents to confirm whether metal is actually present, and if it is, identify the source: screen wire or a machine part means stopping the line and tracing back to the last verified check with nothing found. If no metal is found, record it as a suspected false reject and track the rate, because a rising false-reject rate usually means sensitivity settings or product effect have drifted and need re-evaluation. Everything goes into a record: photographs, particle size, probable source, affected batch range and disposition. An auditor who finds any of these will treat the whole system as uncontrolled.
6. Running it as a real CCP
Once hazard analysis places metal detection at a critical control point, it has to be managed like one. Critical limits must be stated: every test piece, at every defined position, is detected and rejected. Monitoring is the recorded result of each check. Corrective action is written in advance, covering segregation, recalibration and the conditions for release. Verification covers periodic sensitivity review, reject mechanism confirmation and preventive maintenance. Records cover monitoring, corrective action and verification. Two points are regularly missed: fail-safe behaviour so that a fault stops the line rather than passing product, and belt speed matching the speed at which the machine was qualified. The technology here is not difficult. Doing it identically on every shift, every day, is the hard part, and it is precisely what a brand should be assessing when it chooses a manufacturing partner.
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About Guangzhou Mesen Health Biotechnology Co., Ltd
Guangzhou Mesen Health Biotechnology Co., Ltd is located in the Huashan Science and Technology Park, Huadu District, Guangzhou. The company has focused on OEM and ODM services for the big health industry for nearly 20 years, operating dedicated research, production, sales and management centres, and maintaining long-term strategic partnerships with well-known Chinese medicine colleges and research institutions. Capabilities span pressed candy, gummies, capsules, solid beverages, herbal teas, plant beverages, protein powders, dietary supplements and enzyme jelly - from formulation development through contract manufacturing. For details on minimum order quantities, sampling lead times and quotations, see our manufacturing advantages and product range, or contact us for a tailored proposal.
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