A filling line running 60,000 bottles per hour has roughly 1,000 bottles in transit between the depalletizer and the palletizer at any moment. If those bottles are pushed together under back-pressure — the accumulation method where containers are packed tightly, each one pushing against the next, with the line pressure transmitted all the way back to the filler discharge — the glass-to-glass and PET-to-PET contact creates scuff rings, label damage, and in the worst case, bottle breakage that stops the line for 15 minutes of glass-cleanup. Pressure-free conveying spaces the containers apart, eliminating contact damage at the cost of more conveyor length and more sophisticated controls. This guide explains how each strategy works and where the trade-off breaks in each direction.
How Back-Pressure Conveying Works
Back-pressure conveying is the traditional accumulation method for high-speed container lines. The conveyor runs at a constant speed — typically 15–30 m/min — and containers are packed tightly together, each one pushing against the one ahead of it. The back-pressure is transmitted along the entire accumulation length: a container at the end of a 30-meter accumulation table experiences 30 meters of upstream bottles pushing against it. Each container exerts roughly 0.1–0.3 N of forward force, multiplied by hundreds of containers in contact, producing line pressure of 20–50 N at the discharge end — enough to crush a thin-wall PET bottle or scuff a glass bottle's labeling surface through sustained contact friction.
The advantage is maximum density accumulation: every centimeter of conveyor chain is occupied by a container, and the accumulation capacity per linear meter is the container diameter divided into the conveyor length — for 70 mm diameter bottles, approximately 14 bottles per meter of conveyor. The disadvantage is container damage that increases linearly with accumulation length and contact time. A glass bottle that spends 3 minutes in a back-pressure queue rubbing against its neighbors will show visible scuff rings on the shoulder and heel; a PET bottle may ovalize under sustained axial load.
How Pressure-Free Conveying Works
Pressure-free conveying eliminates container-to-container contact by dividing the conveyor into independently driven zones — typically 0.5–2 meters each — with a photoelectric or ultrasonic sensor at the discharge end of each zone. When a zone's sensor detects a container, the zone's motor stops or slows, creating a gap between the detected container and the next upstream container. The gap — typically 5–20 mm — ensures that containers never touch each other during accumulation. When the downstream container clears the sensor, the zone restarts to advance the next container.
The control architecture is a state machine across the accumulation table. The filler discharge is the master: as long as the filler is running, the conveyor zones downstream of the filler run at line speed. When the filler stops (for a downstream jam, a label change, or a quality check), the zones immediately downstream of the filler begin stopping sequentially from the filler outward, creating a growing accumulation queue of spaced containers. When the filler restarts, the zones restart in reverse order — the zone closest to the filler starts first, and the accumulated queue is depleted from the discharge end without ever creating container-to-container contact.
The trade-off is reduced accumulation density: each container occupies its diameter plus the gap — 70 mm + 10 mm = 80 mm per container, versus 70 mm under back-pressure. The same 30-meter accumulation table holds 375 containers under pressure-free control versus 428 under back-pressure — a 12% capacity reduction. For a line that needs 60 seconds of accumulation buffer (1,000 containers at 60,000 bottles per hour), the conveyor must be 12% longer, or the accumulation strategy must combine pressure-free accumulation with a multi-lane mass-flow table that uses table-top chain to provide high-density buffering without back-pressure.
Drive and Control Requirements
Pressure-free conveying requires individual motor control per zone — typically a distributed drive on each zone's motor roller or a compact VFD per zone motor. A 30-meter accumulation table with 1-meter zones requires 30 motors and 30 drives, each one responding to its zone sensor within 10–20 ms to maintain the gap. The control logic — typically handled by a dedicated conveyor controller or a high-speed PLC — implements the zone-state machine across all zones, with zone-to-zone handshakes that prevent a zone from advancing a container into an occupied downstream zone.
Back-pressure conveying requires one drive for the entire accumulation table — simpler, cheaper, and easier to maintain — but at the cost of the container damage described above. The economics are straightforward: if the annual cost of container damage (scuffed rejects, broken bottles, label damage) exceeds the additional capital and maintenance cost of a pressure-free system, pressure-free conveying is the correct investment.
The Siemens SINAMICS G115D distributed drive provides 1.1 kW of decentralized conveyor motor control with integrated PROFINET communication — a representative drive for pressure-free conveyor zone control where each zone needs its own compact, networked motor drive. For the complete conveyor control system, the PLC catalog includes high-speed controllers capable of managing dozens of conveyor zones with sub-20-ms zone-to-zone response.
Pressure-free conveying trades conveyor length and control complexity for container quality. If your line runs glass bottles at more than 30,000 per hour, the payback from eliminating scuffing and breakage alone usually justifies the additional conveyor and drive investment within the first year of operation.
