Powder is not granules
Flour, spices, milk powder, cocoa, or protein powder do not flow freely—they clump, create dust, and trap air. Volumetric dosing via free fall fails in this case; the standard dosing system for powders is the screw feeder. It feeds the product into the bag in measured portions using a screw conveyor and achieves accuracies of ±0.5 %.
Which powder requires which dosage?
| Powder Behavior | Dosing Solution | Typical Accuracy |
|---|---|---|
| free-flowing (e.g., sugar-powder mixture) | Volumetric screw | ±0.5 to ±1 % |
| poorly flowing (e.g., flour, cocoa) | Screw Conveyor with Forced Feed | ±1 % |
| Prone to separation (e.g., spice blends) | gravimetric net dosing | ±0.3 to ±0.5 % |
| dusty and fine (less than 50 µm) | Screw Conveyor with Suction System | ±1 % |
How the screw feeder works
The screw rotates at a defined number of revolutions per bag—the number of revolutions multiplied by the conveying volume equals the fill quantity. Volumetric systems dose based on this volume; gravimetric systems weigh each batch and adjust the number of revolutions accordingly. For powders with fluctuating bulk density, the gravimetric method is the safer choice, because otherwise fluctuations in bulk density would directly affect the fill weight.
Integration into the line: The screw feeder is mounted above the filling chute of a vertical filling and capping machine. He feeds the bag in two stages—a rough feed for the bulk, a fine feed for the target weight—and then steps back so the sealing jaws can close.
Ventilation: The Underestimated Issue in Powder Handling
Fine powder brings up to 50 % of air volume into the bag. Without venting, three problems arise: The bag looks fuller than it actually is, particles get into the seal, and during transport, the powder settles—the bag appears empty, and the customer files a complaint. Two solutions are commonly used:
- Mechanical venting: A dip tube or brush valve compacts the powder as it is filled into the bag.
- Vacuum Bleeding: The machine lightly evacuates the air from the bag before sealing it—a standard procedure for milk powder and instant products, which are also flushed with nitrogen.
Hygiene and Dust Control: What Sets the Powder Line Apart
Powdered products are primarily foodstuffs—the choice of production line depends on hygiene details: SS304 stainless steel for product-contact surfaces (SS316 for products containing salt), quick disassembly of the product path without tools, and smooth weld seams without dead spaces. Spices and cocoa require CIP-compatible designs that can be cleaned without disassembly. And: Flour dust is the critical issue—encapsulated junctions and extraction ports on the filling chute are mandatory, and explosion protection requirements are clarified with the operator on a product-by-product basis.
Selection in Five Steps
- Classifying powders: Particle size, bulk density, flow behavior, dustability—the dosing principle is derived from these four values.
- Specify accuracy requirements: ±1 % is sufficient for 500 g of all-purpose flour; for 50 g of protein powder, ±0.3 % determines the yield.
- Set the fill weight range: The screw must be able to handle both the lightest and heaviest loads—very wide ranges require two screws.
- Determine the bag size and film type: Powder requires films that are free of fine pores; the inner layer must be resistant to the grease from the spices.
- View the cleaning plan: How long does it take to switch from cocoa to vanilla? Dismantlable product lines can do it in minutes; bolted-together lines take hours.
When the screw feeder Reaches Its Limits
For very large fill weights exceeding about 5 kg, the auger slows down—in this case, a net weighing station with a hopper is faster. With highly fibrous products (shredded coconut), the fibers wrap around the screw. And for liquid pastes (e.g., tomato paste), a screw feeder is not suitable; instead, a piston or tubular conveyor system is required—which is a different machine configuration.
Frequently Asked Questions
Just how accurate is a screw feeder, really?
Volumetric: ±0.5 to ±1 % at a uniform bulk density; gravimetric: ±0.3 to ±0.5 %. These values apply under stable product conditions; freshly mixed or moist powder reduces accuracy.
Why does my fill weight fluctuate when using powder?
In most cases, the bulk density fluctuates: Volumetric dosing measures volume, not weight. Solution: gravimetric readjustment or tighter control of the product's bulk density.
How many cycles does the venting process take?
Mechanical venting takes up hardly any cycles; vacuum venting reduces output by about 10 to 20 % because the vacuum step takes time. At a base rate of 40 bags per minute, 32 to 36 bags per minute is a realistic figure with vacuum venting.
Can a powder filling line also fill granules?
Yes—the screw feeder meters free-flowing granules just as effectively, only more slowly than a multihead weigher. Conversely, handling powder on a granule production line requires a retrofitted screw feeder; the issue of retrofitability should be clarified before purchase.
Related pages: What is a vertical form-fill-seal (VFFS) machine? | Powder Packaging Solution | Small Powder Filling Machine | Format Change on Packaging Machines
Request technical specifications and a quote for your production line. Please specify the product, bag size, and target output (bags/min.) – Request Technical Specifications and a Quote.
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