A vertical form, fill, and seal machine It fills bags at a steady pace, but it relies on its own dosage: the machine itself does not check what the screw or checkweigher dispenses. The checkweigher does that—every bag, on the conveyor belt, immediately after sealing. Combined with a metal detector, this ensures the entire quality chain from dosage to approved bag.
What the checkweigher measures
Each bag passes over the weighing belt at a rate of 30 to 60 bags per minute; the load cell reads the weight in milligrams within a window of approximately 200 ms. From the series of readings, the control system calculates the average, the spread, and the percentage outside tolerance. You set this tolerance around the target net weight: for example, 500 g with a tolerance range of minus 2 to plus 4 g, depending on what is permitted by law and your customer.
Feedback to the dosing system
The strength lies not in measuring but in making adjustments: the checkweigher reports the running average to the dosing control system, and the screw adjusts the number of revolutions per cycle—the hybrid approach takes the equation out of the picture screw feeder or multihead weigher. This ensures that the average remains at the target weight even if the product's density varies due to humidity or batch differences. Without this link, your operator would have to calibrate manually, and that would happen only as often as work permits.
Metal Detection and X-ray Screening
Weighing is followed by detection: a metal detector scans the bag for ferrous and non-ferrous particles starting at approximately 1.5 mm for steel and 2.5 mm for non-ferrous metals in wet products; an X-ray scanner also detects stone, glass, and dense bone. Which of the two you choose depends on your product and target market—retail chains often require detection. Both devices belong in the category auxiliary equipment around the core machine.
Discharge and Logging
A bag that’s out of tolerance isn’t allowed to pass: a pusher or flip-over device removes it from the line into a sealed bin. The logging system does the rest—every weighing is time-stamped, and every rejection is logged with a reason. After an hour of production, you’ll see whether the rejections represent normal variation or a trend indicating wear and tear—a sign that you’ll also notice in the maintenance schedule finds.
| Station | Measure or do | Response in Case of Deviation |
|---|---|---|
| VFFS with dosing unit | prepare a dose | — |
| Checkweigher | weigh each bag | adjust, drain |
| Metal Detector | metal scanning | Remove, Block batch |
| Coder | date and time | traceability |
Frequently Asked Questions
Q: Does every bag have to be weighed?
No, random sampling is also acceptable—but 100% weighing is the standard as soon as the line allows for it. Random sampling only reveals a trend after dozens of bags; inline weighing detects the deviation in the bag that’s still there.
Q: How much does a checkweigher cost per Tempo?
Practically nothing: the weighing conveyor runs at the same speed as the production line and adds milliseconds per bag. The real limitation is synchronization—at 120 bags per minute, the control system requires tight synchronization with the VFFS, which you specify when placing the order, just as with instant coffee packaging machine-projects are common.
Q: Can the checkweigher be integrated into an existing production line?
Yes, provided there is sufficient cable length and an available control connection. Have the supplier specify the communication between the checkweigher and the dosing control system before placing the order—the integration is the project; the devices are the components.
Related pages
vertical form, fill, and seal machine | auxiliary equipment | screw feeder or multihead weigher | maintenance of a packaging machine | instant coffee packaging machine
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