Duraflex Loss-in-Weight & Volumetric Screw Feeders

What loss-in-weight feeding actually measures

Loss-in-weight feeding is a gravimetric dosing method: instead of assuming that each turn of a screw delivers the same volume of powder, the feeder continuously weighs itself and measures how quickly its weight is falling. The controller compares that real discharge rate with the set point and adjusts the screw speed to close the gap. The result is a feed rate expressed in kilograms per hour that stays on target even when the bulk density of the powder changes.

This matters because powders rarely behave consistently. Density shifts as a hopper empties, as material aerates after conveying, or as a new batch of raw material arrives. A volumetric feeder cannot see those changes, while a loss-in-weight feeding system corrects for them automatically.

How a loss-in-weight feeding cycle works

Every gravimetric feeder runs through the same basic cycle, whether it doses a minor additive into an extruder or a major ingredient into a continuous mixer.

1. Gravimetric phase. The hopper, screw and drive sit on load cells. As the screw discharges powder, the controller tracks the weight loss over time and trims the motor speed to hold the required rate.
2. Refill trigger. When the hopper reaches a low-level weight, the controller prepares to refill it.
3. Volumetric hold. During refill the incoming material disturbs the weight signal, so the feeder briefly holds the last good screw speed rather than reacting to false readings.
4. Rapid refill. A valve, screw feeder, vacuum receiver or bag station tops up the hopper quickly to a high-level weight.
5. Return to gravimetric control. Once the refill settles, the feeder resumes weight-based control and the cycle repeats.

Keeping the refill short and predictable is one of the most important parts of the design. A slow or inconsistent refill means the feeder spends longer in volumetric mode, which reduces the overall accuracy of the process.

Where loss-in-weight feeding pays off

Gravimetric control costs more than a simple volumetric screw, so loss-in-weight feeding is worth using where accuracy has a clear value. Typical examples include:

• Dosing colour masterbatch, additives and fillers into plastics extruders and compounding lines.
• Adding vitamins, minerals, flavours or functional ingredients into continuous food and dairy blending.
• Metering active ingredients and excipients in pharmaceutical and nutraceutical processes.
• Feeding several ingredients in ratio into a continuous mixer, where each feeder follows a master rate.
• Controlling reagents and chemicals where overdosing wastes money and underdosing affects quality.
• Recording actual quantities delivered for traceability and batch reporting.

For major ingredients with steady flow properties, a volumetric feeder may still be the most practical choice. Many lines combine both, using loss-in-weight feeding only on the ingredients where precision drives product quality or cost.

5 proven tips for accurate loss-in-weight feeding

1. Isolate the weighing system. Use flexible connections at the inlet and outlet so pipework, chutes and refill equipment do not carry weight onto the load cells.
2. Control vibration and air movement. Mount feeders on a rigid, stable structure away from heavy vibration, and shield them from draughts or pressure changes in connected equipment.
3. Match the screw to the powder and rate. An oversized screw running very slowly, or an undersized screw running flat out, both reduce stability. Interchangeable screws let you tune the feeder to the job.
4. Keep the powder moving in the hopper. Cohesive materials that bridge or rat-hole cause erratic discharge. The Duraflex flexible hopper is massaged to keep material flowing to the screw.
5. Plan the refill properly. Size the refill device so the hopper tops up quickly, and set sensible low and high levels so the feeder spends most of its time under gravimetric control.

Choosing a loss-in-weight feeding system

Selecting a gravimetric feeder starts with the material and the rate. We look at the bulk density, particle size, flow behaviour, temperature and moisture of each ingredient, together with the minimum, normal and maximum feed rate the process requires. The ratio between the smallest and largest rate guides the screw size and drive selection.

Information to have ready

• Product name and safety data sheet, plus a description of how the powder behaves in storage.
• Required feed rate range in kg/h and the accuracy the process needs.
• What the feeder discharges into: an extruder, mixer, conveyor or another vessel, and whether it is under pressure or vacuum.
• How the feeder will be refilled: bags, bulk bags, mini bulk containers, a silo or a pneumatic conveying line.
• Cleaning requirements and how often the product changes.
• How the feeder will integrate with existing PLC, SCADA or batching controls.

If the powder is unusual or difficult, a trial is the best way to remove guesswork from loss-in-weight feeding. KPM can run a sample through a feeder at our Kings Park workshop so you can see how the material behaves before committing to a design.

Integrating gravimetric feeders into the wider line

A feeder rarely works alone. Upstream, it depends on a reliable refill source such as a bag tip station or bulk bag unloader. Downstream, it must discharge cleanly into a process that may be sealed, pressurised or under vacuum. Getting these interfaces right is as important to loss-in-weight feeding as the feeder itself.

Where the line handles combustible dusts, the design should consider earthing, dust extraction and explosion protection. Australian sites can refer to the AS/NZS 4745 code of practice for handling combustible dusts when assessing risks around feeders, hoppers and refill points.

Because Klausen Process Machinery designs and builds the surrounding equipment, we can supply the refill system, feeders, mixer and controls as one package, then handle installation and commissioning on site. Our guide to Duraflex auger feeders explains more about the feeder range.

Loss-in-weight feeding in continuous versus batch processes

In a continuous process, loss-in-weight feeding holds a steady rate for hours at a time, and several feeders can be linked so each ingredient follows a master rate. In batch work, the same weighing principle can be used to dispense a set quantity and then stop, which is useful when minor ingredients must be added accurately to a mixer.

Either way, loss-in-weight feeding gives the control system a live record of what was actually delivered, which supports quality checks and traceability. That data is often as valuable to the plant as the accuracy itself.

Maintenance and calibration

Load cells and controllers are reliable, but loss-in-weight feeding equipment still benefits from routine care. Check flexible connections for tears or stiffness, keep the area around the weigh frame clear of tools and debris, and inspect screws and tubes for wear. A periodic check weight test or catch test confirms that the feeder is delivering what the controller reports.

Easy disassembly helps here. On the Duraflex design the hopper slides out, the helix disconnects without tools and the agitation assembly can be removed, which makes inspection and cleaning straightforward during product changes.

Loss-in-weight feeding FAQs

Is loss-in-weight feeding always more accurate than volumetric feeding?

In most applications, yes, because it measures the actual mass discharged rather than assuming a constant density. The gain is greatest with powders whose density or flow changes during the run.

Can a loss-in-weight feeding system handle sticky or cohesive powders?

Often it can, provided the hopper keeps material moving to the screw. The flexible, massaged hopper on Duraflex feeders is designed for powders that tend to bridge or rat-hole.

Can I convert a volumetric feeder to gravimetric control?

Some feeders can be upgraded with load cells and a gravimetric controller. Whether it is practical depends on the feeder design and installation, so we assess each case.

Do you test powders before supplying a feeder?

Yes. Sending a sample for a trial at our Kings Park workshop is the best way to confirm screw selection, achievable rates and accuracy for your material.

Equipment that supports loss-in-weight feeding

Screw feedersBag tip stationsPowder mixers