Why VFFS Lines Need Metal Detection Before Bag Forming

Why Metal Detection Should Happen Before Final Packaging

In most packaging lines, metal detection is positioned after the bag is sealed. This is common practice, but it creates an inefficiency that adds up quickly: every contaminated pack that reaches the end of the line as a finished product must be rejected and destroyed entirely — including the packaging material, the product inside, and the labor that went into forming and sealing the bag.

Detecting metal contamination before the bag is formed eliminates this waste. When a contamination event is identified in the falling product stream before it enters the film tube, only the affected portion of product is diverted — not a sealed, labeled, or counted finished unit. This reduces reject waste, lowers the cost per contamination event, and avoids unnecessary downstream product loss.

Metal contamination in packaging lines originates from multiple upstream sources: processing equipment such as mixers, grinders, and cutters; conveyor components including belts, scrapers, and fasteners; worn machine parts that shed metal fragments over time; and raw material contamination from suppliers. Food, chemical, pharmaceutical, and dry ingredient packaging lines all face these risks continuously. Detecting metal before bag forming addresses the risk at the most cost-effective point in the process.

What a Throat-Type Metal Detector Does

A throat-type metal detector is installed around the product falling path — the vertical channel through which product drops from the weighing or feeding system into the packaging machine below. Unlike conveyor-based metal detectors that inspect product moving horizontally on a belt, a throat detector works with free-falling product in a vertical stream, making it directly compatible with the operating principle of vertical form-fill-seal (VFFS) machines.

The detector's aperture surrounds the falling product channel. As product passes through, the detector continuously scans for ferrous, non-ferrous, and stainless steel fragments. You can explore the full range of checkweigher and metal detector combinations to see how inspection systems integrate into complete packaging lines.

When contamination is detected, the system triggers an alarm and activates a rejection mechanism appropriate to the line configuration. Common rejection methods include diverter gates that redirect contaminated product away from the film tube, or line stop signals that halt production until the contamination is cleared. The specific rejection logic depends on product type, line speed, and hygiene requirements.

Best Product Types for Throat Metal Detection

Throat-type metal detectors perform best with free-flowing materials that travel consistently through the vertical channel without bridging, clumping, or irregular flow. Suitable product types include:

• Granules and small pellets
• Powders and fine materials
• Seeds and grains
• Snack pieces and extruded products
• Seasoning and spice blends
• Pet food kibble and small treats
• Frozen small pieces and IQF products
• Dry ingredients and composite mixes

Free-flowing materials are easier to inspect in the falling channel because they pass through the aperture in a consistent, predictable stream. Products with irregular shapes, high moisture content, or strong clumping tendency may require additional consideration for throat detection to be effective, as inconsistent flow can create signal variation that affects sensitivity.

Where to Install It in a Packaging Line

The throat-type metal detector is installed between the weighing or feeding system and the vertical form-fill-seal machine below it — specifically in the transition zone where product leaves the weigher discharge chute and enters the film forming tube of the VFFS machine.

This positioning places detection before forming, filling, and sealing. Metal fragments identified at this point have not yet been enclosed in packaging, making rejection straightforward and waste-free compared to post-seal detection.

Correct installation requires adequate vertical space for the detector aperture, the rejection mechanism, and maintenance access. Detection sensitivity is affected by the aperture size relative to the product stream diameter — a tighter aperture improves sensitivity but must match the actual product flow width. Installation height must also allow for proper product velocity through the detection zone; product falling too fast or too slowly relative to the detector's scan frequency may reduce detection reliability.

Buyer Checklist

Use the following parameters to specify a throat metal detector for your VFFS line:

• Product type: Free-flowing granule, powder, flake, small piece — affects aperture design and sensitivity calibration.
• Package weight: Determines product stream volume and flow rate through the aperture.
• Falling speed: Product velocity through the aperture affects detection window and scan frequency requirements.
• Aperture size: Must match actual product stream diameter with appropriate margin.
• Required sensitivity: Minimum detectable fragment size for ferrous, non-ferrous, and stainless steel contaminants.
• Installation height: Available vertical space between the weigher discharge and the VFFS forming tube inlet.
• Rejection method: Diverter gate, line stop, or other rejection logic depending on production layout and hygiene requirements.
• Packaging format: VFFS, premade pouch, or other filling format — affects integration requirements and timing synchronization.

Contact Keypack Intelligent

Want to add metal detection before bag forming in your vertical packaging line? Contact Keypack Intelligent to discuss your product type, line speed, and installation requirements for a throat-type metal detection solution.