Every bag starts with a dose, and every dose follows a specific method: volumetric measurement or static weighing. The screw feeder measures volume, the multi-head weigher weighs a certain amount. Both hang above the same vertical form, fill, and seal machine, but they solve different problems. This guide will help you choose the route that's right for your product.
The screw feeder: measuring volume
A screw rotates a fixed number of revolutions per cycle, dispensing a fixed volume per revolution. This works for products with a constant density—powder, instant coffee, sugar. Accuracy is around ±1%, and the speed is 30 to 50 bags per minute. If the density varies with humidity, the weight will vary as well—calibration per batch is the solution. The full operating procedure is described in the powder packaging machine-guide.
The multihead weigher: determining weight
Ten to fourteen weighing heads weigh in parallel: each head fills toward the target weight, and the control system selects the combination that is closest to the dose. This results in an accuracy of ±0.5 g, regardless of density fluctuations. The weigher handles nearly any product that falls—granules, beans, snacks, frozen pieces—and processes 60 to 120 bags per minute. The trade-off: more moving parts and a higher initial investment than a screw feeder.
The Comparison at a Glance
| Aspect | Screw Feeder | Multihead Weigher |
|---|---|---|
| Principle | measure volume | combine weight |
| Product | powder, constant density | pellets, beans, snacks |
| Accuracy | ±1% | ±0.5 g |
| Speed | 30 to 50 bags per minute | 60 to 120 bags per minute |
| Density-sensitive | Yes, calibration is needed | no |
| Route is suitable for | powder packaging solution | reliable packaging solution |
The hybrid approach: screw conveyor with checkweigher
Many powder production lines combine a screw feeder with a checkweigher downstream of the machine: each bag is weighed, and any deviation causes the screw to adjust. This brings accuracy to ±0.5% without the investment in a separate weigher. For products weighing less than approximately 200 g, this often offers the best value for money; heavier doses cause the adjustment to be too slow.
When neither option works
Liquids and pastes should be used with a piston-type dispenser — see Liquid and Paste Packaging Machine. And products containing large pieces—half nuts, dried fruit—can clog a screw conveyor and bounce out of weighing hoppers; these require a specially designed weigher with deep hoppers and a low speed, as discussed in VFFS for snacks.
Frequently Asked Questions
Q: Can a single line have both types of dosing devices?
Yes, with a modular frame design, you can swap out the screw and weigher as modules. Expect a changeover time of half an hour to an hour and a higher machine price. Only do this for products that truly justify both options.
Q: Which is more accurate for duration powder?
The multihead weigher, because it measures weight rather than volume. For high-value products—such as supplements and spices—that level of accuracy pays for itself in reduced overfilling: ±0.5 g instead of ±1% results in a difference of several metric tons of product per million bags.
Q: How many load cells do I need?
More heads mean faster combination selection and a higher output rate: 10 heads handle approximately 60 bags per minute, while 14 heads can handle up to 120. Choose based on your target output, not the maximum—the maintenance schedule increases with each additional pair of heads.
Related pages
powder packaging solution | reliable packaging solution | vertical form, fill, and seal machine | multi-head weigher | powder packaging machine
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