Net Weigh Filler

Overview

A Net Weigh Filler fills a container by weighing the product before it goes in, not by counting it or by measuring its volume. Product flows from a Supply Hopper into a Weigh Hopper that sits on a Load Cell. The control system watches the weight climb, slows the feed as it nears the target, and stops at the set weight. Only then does the charge drop into the container. Because the weight is verified before discharge, the filler holds tolerance even when product density, moisture, or particle size drifts from batch to batch.

This makes net weighing the standard choice for granular and free-flowing products where the value is in the weight on the label: coffee beans, nuts, rice, dried fruit, fasteners, pet food, granular fertilizer, and many powders. Volumetric fillers are faster and cheaper, but they fill to a volume, so any change in bulk density shows up as a giveaway or an underweight. A net weigh filler trades some speed for the assurance that every container holds the declared weight.

A typical machine runs 2 to 8 weigh heads in parallel. Each head is an independent unit with its own feeder, weigh hopper, and discharge flap, so the heads cycle on a staggered schedule and the Indexing Conveyor never has to wait on a single slow fill. Four heads filling 1 kg targets will reach 60 to 80 containers per minute.

Weighing

The heart of the machine is the Weigh Station. The Weigh Hopper is a stainless bucket that hangs on a single-point Load Cell. The cell is a metal flexure with strain gauges bonded to it: as product loads the bucket, the flexure bends a few microns and the gauges change resistance. A Load Cell Mount holds the cell rigidly to the deck and isolates it from frame flex, because any side load or twist would show up as a false weight.

The bridge signal from the cell is tiny, a few millivolts, so it goes straight to a Weigh Board mounted close to the head. A 24-bit Load Cell ADC digitizes it at a resolution near 1 part in 30,000, which is what lets the machine resolve a single gram on a multi-kilogram target. The board filters out conveyor vibration and the shock of product landing, then reports a settled weight to the PLC many times per second.

When the weighed charge reaches target, the controller fires the Flap Cylinder and the Discharge Flap drops the product into the Fill Head below. A Flap Seal keeps fines from leaking past the flap between cycles. Each weigh result is logged, so an out-of-tolerance fill is flagged and the container can be diverted.

Two-speed feeding

Accuracy and speed pull in opposite directions, so the Feed System uses two feeders per head. The Bulk Feeder throws most of the charge fast on a wide Vibratory Tray, getting the bucket to roughly 90 percent of target in the first part of the cycle. As the weight closes in, the bulk feeder cuts off and the Dribble Feeder trickles the last grams on a narrow tray so the target lands inside tolerance.

Both feeders are driven by a Vibratory Drive: an Electromagnet Coil pulses against an Armature, and the Tray Leaf Spring leaf springs set the tray's natural frequency. The controller varies the drive amplitude to set feed rate, and it learns the in-flight weight (the product already airborne when the feeder stops) so it can shut off early and hit the number. Product reaches the feeders from the Feed Pan and drops to the bucket through the Feed Chute.

Container handling and discharge

Empty containers ride the Conveyor Belt into position. A Container Stop blade rises to hold each container under a fill head while a Container Sensor photoeye confirms it is present before the flap opens. The belt is driven by a Conveyor Gearmotor through a Drive Belt, and the rollers turn on Ball Bearing supports.

At each station the Spout Cylinder lowers a telescoping Fill Spout into the container mouth, the Fill Funnel guides the dumped charge down, and a Dust Shroud catches the puff of fines that comes off a falling charge. Once filled, the stop drops and the conveyor indexes the container forward.

Pneumatics and controls

Flaps, stops, and spouts all run on air. The Pneumatic System takes plant air through a Filter-Regulator, distributes it across a Valve Manifold of Solenoid Valve units, and drives each Air Cylinder through the Air Tubing Set. A Pressure Sensor watches supply pressure so the machine faults instead of mis-firing on low air.

The Control System lives in a washdown Control Enclosure. A PLC sequences every head, reads the weigh boards, and switches the valves through a bank of Relay outputs fed by a 24 V Power Supply. The operator works from the Operator Panel touch panel, setting target weight and tolerance and reading live fill data per head. A Emergency Stop on each side cuts motion at once.

Structure, dust, and use

Everything mounts on the Machine Frame, a stainless Base Weldment on adjustable Leveling Leg sets, enclosed by Guard Panel sections with interlocked Guard Door access. Dusty products feed the Dust Collection system: a Dust Hood over the fill zone, a Extraction Blower that pulls the air, and a Cartridge Filter cartridge that traps the fines. Pulling dust off the fill point keeps the area clean and keeps stray product from settling on the weigh buckets and corrupting a reading.

Net weigh fillers are sold in inline and rotary layouts, in open-frame versions for dry product and full washdown 316L builds for food. They pair with upstream filling lines and downstream cappers, seamers, and checkweighers, with the same weigh data passed along for traceability.