In September 2025, New Zealand Food Safety and Tasti Products Ltd recalled more than 20 snack bars, nut and snack-ball products over possible small, wire-like metal pieces. The company said its monitoring had picked up exceptionally thin metal wire in some imported raw materials (Waatea News; recall later extended to more batches, per 1News).
Snack bars like these are often wrapped in metallised film. That creates a real problem for inspection, because the wrapper itself looks like metal to an ordinary detector. If you need a metal detector for aluminium foil packaging, this guide explains why standard detectors struggle, how foil-pack detectors work, and when X-ray is the smarter choice.
Why foil interferes with standard detectors
A standard balanced-coil metal detector looks for anything conductive that disturbs its field. Aluminium is an excellent conductor.A foil pouch, blister lid or metallised wrapper therefore produces a strong signal of its own, often far larger than the contaminant you want to find.
The result is a bad choice between two failures:
Snack bars like these are often wrapped in metallised film. That creates a real problem for inspection, because the wrapper itself looks like metal to an ordinary detector. If you need a metal detector for aluminium foil packaging, this guide explains why standard detectors struggle, how foil-pack detectors work, and when X-ray is the smarter choice.
Why foil interferes with standard detectors
A standard balanced-coil metal detector looks for anything conductive that disturbs its field. Aluminium is an excellent conductor.A foil pouch, blister lid or metallised wrapper therefore produces a strong signal of its own, often far larger than the contaminant you want to find.
The result is a bad choice between two failures:
- Set sensitivity high and every good pack is rejected.
- Turn sensitivity down until good packs pass, and the detector is too blind to find real contaminants.
Neither is acceptable on a production line.
How foil-pack metal detectors work
Foil-pack detectors avoid the problem by working on a different principle. Instead of a high-frequency balanced coil that responds to all conductors, they use a magnetic-field approach that responds mainly to ferromagnetic material.
Aluminium is not magnetic, so the foil is effectively invisible. Iron and steel fragments, which are magnetic, still produce a clear signal.
TCT's Aluminium Foil Metal Detector is built for exactly these packs: aluminium sealing bags, foil bags, foil-wrapped products and aluminium-tinned sausage. Its publishedsensitivity is 1.0 mm ferrous and 2.0 mm stainless steel. It is also designed for hot, salty, sugary and water-containing products, which often cause false signals on ordinary
detectors.
Limitations
Foil-pack detectors solve one problem well, but they are not universal. Be clear about what they may miss:
How foil-pack metal detectors work
Foil-pack detectors avoid the problem by working on a different principle. Instead of a high-frequency balanced coil that responds to all conductors, they use a magnetic-field approach that responds mainly to ferromagnetic material.
Aluminium is not magnetic, so the foil is effectively invisible. Iron and steel fragments, which are magnetic, still produce a clear signal.
TCT's Aluminium Foil Metal Detector is built for exactly these packs: aluminium sealing bags, foil bags, foil-wrapped products and aluminium-tinned sausage. Its publishedsensitivity is 1.0 mm ferrous and 2.0 mm stainless steel. It is also designed for hot, salty, sugary and water-containing products, which often cause false signals on ordinary
detectors.
Limitations
Foil-pack detectors solve one problem well, but they are not universal. Be clear about what they may miss:
- Non-ferrous metals such as aluminium, copper and brass are not targeted, because the detector is designed to ignore aluminium.
- Non-magnetic stainless steel (such as some 316 grades) gives a much weaker signal, so detectable sizes are larger.
- Non-metal contaminants such as glass, stone, bone and hard plastic are invisible to any metal detector.
- Thin wire can be hard to detect depending on its orientation as it passes through the field.
That last point is relevant to the Tasti case. Fine wire is a notoriously difficult contaminant for any metal detector, which is why raw material screening matters as much as end-of-line checks.
When X-ray is the better choice
X-ray inspection does not care whether the pack is foil, film or carton. It looks at density differences in the image. Aluminium foil is thin and light, so it adds very little to the image, while dense contaminants stand out.
Choose X-ray when:
When X-ray is the better choice
X-ray inspection does not care whether the pack is foil, film or carton. It looks at density differences in the image. Aluminium foil is thin and light, so it adds very little to the image, while dense contaminants stand out.
Choose X-ray when:
- You need to find non-ferrous metal in foil packs.
- You need to find glass, stone, bone or dense plastic.
- You also want product checks such as missing items or fill level
TCT's Standard X-Ray Inspection System detects stainless steel balls down to 0.3 mm, stainless wire down to 0.2 x 2 mm and glass or ceramic down to 0.8 mm under best conditions.
Inspecting before wrapping
There is a third option that is often overlooked: inspect the product before it goes into the foil.
If the product is unwrapped, an ordinary high-sensitivity detector can find ferrous, non-ferrous and stainless steel. Typical inspection points include:
Inspecting before wrapping
There is a third option that is often overlooked: inspect the product before it goes into the foil.
If the product is unwrapped, an ordinary high-sensitivity detector can find ferrous, non-ferrous and stainless steel. Typical inspection points include:
- Raw materials: a Gravity Feed Metal Detector on nuts, seeds or powders before mixing.
- Before the wrapper: a Conveyor Metal Detector on bars or tablets before the flow- wrapper or blister machine.
- After tablet compression: a Pharma Metal Detector directly after the tablet press, before blistering.
The limitation is that contamination introduced during wrapping, or by the packaging line itself, will not be caught. Many plants therefore combine an early unwrapped check with a final foil-pack or X-ray check.
[Insert diagram comparing the two detection approaches: balanced coil vs foil-pack (ferrous-only) detector, with the foil signal shown on each.]
Frequently asked questions
[Insert diagram comparing the two detection approaches: balanced coil vs foil-pack (ferrous-only) detector, with the foil signal shown on each.]
Frequently asked questions
- Can a normal metal detector inspect aluminium foil packs? Usually not well. The foil produces a strong signal that forces sensitivity down too far.
- What does a foil-pack metal detector detect? Mainly ferrous metal. TCT's model is specified at 1.0 mm ferrous and 2.0 mm stainless steel.
- Can a foil-pack detector find aluminium or copper? No. It is designed to ignore aluminium, so non-ferrous metals are not its target.
- Is X-ray better than a foil-pack detector? X-ray finds more contaminant types, including non-ferrous metal and glass, but costs more. The right choice depends on your risk assessment.
- Where else can I inspect foil-packed products? Before wrapping, on raw materials or after the tablet press, using a standard metal detector.
Find the right inspection point for your foil packs
Talk to a TCT application engineer about which technology fits your product.