Two testers, one task: safety after the machine
The packaging line fills and seals—but the responsibility doesn’t end at the end of the line. Two types of inspection equipment are standard in food and pharmaceutical packaging: the metal detector to detect foreign metal in the product and the checkweigher to ensure fill volume compliance. Both are part of the Optional Equipment modern production lines and are firmly integrated into IFS and HACCP systems.
The Metal Detector: Three Types of Metal, One Principle
Metal detectors for packaging lines operate on the principle of electromagnetic induction: The bag passes through a ring of coils; metallic contaminants alter the magnetic field, and the control electronics trigger the bag to be ejected. Detection performance is categorized into three types of metal:
| Type of metal | Typical detection limit | Difficulty |
|---|---|---|
| Iron (Fe) | starting at about 0.8 mm ball | The simplest—ferromagnetic |
| Non-ferrous (NFe: brass, copper, stainless steel) | starting at about 1.2 mm | moderate effort |
| 316 Stainless Steel (ST) | from about 1.5 to 2.0 mm | the heaviest—conductive but not magnetic |
The actual detection limit depends on the product: Moist, salty, and metal-containing products (e.g., mineral salt mixtures) generate a signal of their own (product effect) that limits sensitivity. The detector is therefore calibrated for the specific product—acceptance testing is performed using sample bags and test balls.
Designs in the Line
- Tunnel detector on the conveyor belt: Standard configuration following the packaging machine; the bags pass through the spool ring one at a time and are discharged via a pusher or diverter.
- Downspout Detector: In the product feed line before dosing—checks the free-flowing product before it enters the bag; common for powders and granules.
- Combination units with a load cell: Detector and reweighing in a single conveyor section—saves space when the line is short.
The Reweighing Check: Fill Level in Accordance with Regulations
The reweighing check weighs each bag after sealing and compares it to the target weight. Two approaches:
- Dynamic checkweigher: Each bag passes over a belt scale; accuracy of ±0.1 to ±0.5 g for package sizes up to 1 kg, with cycle rates of up to 150 bags per minute. Light and heavy bags are diverted, and trends are reported back to the dosing system.
- Random Sample Reweighing: End-of-line scale, manual operation—sufficient for slow lines; only a continuous-feed scale provides seamless weighing.
The regulatory framework is the Prepackaged Goods Ordinance: fill quantities must correspond to the nominal fill quantity, the average value must not be undershot, and variation is limited. The reweighing check automatically generates proof of compliance—a report for each batch—and triggers an alarm in the event of drift.
Feedback: Testing Technology Improves Dosing
The real benefit lies in the closed-loop control: The reweighing check reports any drift in the average value to the Multi-head scale, with the dosage adjusted accordingly (fine-tuning of the target weight). This keeps the fill level just above the minimum—the most effective way to prevent product loss: Anyone who consistently overfills by 1 % wastes 5 g per bag for a 500-g product; without regulation, overfilling by 1 to 3 % is standard in the industry. The inspection technology pays for itself through the overfilling it prevents, not just through compliance.
Position in the line
- Metal detector after the packaging machine: Each bag is inspected, and any defective bags are removed before the cartoner. This is the most common setup.
- Additional downspout detector before dosing If there is a high risk of metal contamination in the raw material—better safe than sorry, but the second investment comes at a cost.
- Re-weighing check after the detector: Separate devices are easier to maintain than combination units; the sequence “detector → scale” prevents weight inaccuracies caused by bags being ejected from the system.
- Case packer and palletizer: Only after passing the inspection does the package go into the carton—a rejected package is cheaper than a recalled carton.
Validation and Operation
For audit purposes, the inspection system includes: validation during commissioning (detection limits logged using test balls), regular testing with test interference per shift (documentation of triggered rejections), and annual calibration of the load cells. It’s worth analyzing the data on rejections: An increase in metal detections indicates wear upstream—it’s cheaper to find a loose screw in the conveyor line than to deal with a customer damage claim.
Frequently Asked Questions
Does every packaging line need a metal detector?
In the food and pharmaceutical packaging industries, it is standard practice under HACCP and during audits, and is specified as a control point in most procedures. For technical products (screws, plastic parts), it is optional on a risk-based basis.
What can't a metal detector find?
Non-metallic foreign objects such as glass, stone, plastic, and bone—this is where X-ray technology comes in as a complementary method, which also inspects metals in film laminates with an aluminum top layer, where the induction detector reaches its limits.
How accurate is a reweighing check?
±0.1 to ±0.5 g for bags weighing up to 1 kg, depending on conveyor speed and product density. Accurate enough to comply with the Prepackaged Goods Regulation for all standard portion sizes.
Can the reweighing check adjust the bags?
It weighs and sorts the product—but it cannot readjust the sealed bag. Corrections are made based on feedback to the dosing system: average value drift leads to fine-tuning of the target weight on the scale or the screw feeder.
Related pages: Multi-Head Scale Guide | Optional Equipment | OEE of Packaging Lines | VFFS Guide
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