Accumulation Conveyor

Every packaging line is a chain of machines that do not run at exactly the same rate and do not stop at the same time. A labeler jams, a case packer waits for a carton blank, a palletizer changes a pallet. Without a buffer, each of those stops travels up and down the line. An accumulation conveyor holds product in the gap so the machines on either side can run more independently. This page is part of the packaging conveyor systems section. The sibling guides on bottle conveyors and case conveyors cover the container-specific hardware.

How it works

Accumulation turns a continuous flow into a queue, then back into a flow. When the machine downstream slows or stops, product still arriving from upstream has nowhere to go, so the accumulation section stores it. When the downstream machine resumes, the stored product feeds it and the section empties.

Three things have to work together. The section needs room, in length, width, or height, for the product that arrives during a stop. Product that sits in a queue must not be crushed, scuffed, or tipped, and this is the main way accumulation types differ. And something has to decide when to stop, when to release, and in what order. That can be a mechanical design, sensors on each zone, or a line controller that also tells machines to slow down.

Why lines need buffers

Short stops are a normal part of packaging. Carton or label supply changes, jams, sensor trips, cleaning, pallet changes, and film roll swaps each last seconds to minutes. If every machine stopped the moment its neighbor stopped, a short fault anywhere would stop the entire line, and the restart would be a ramp-up of every machine in turn. A buffer decouples neighboring machines. The upstream machine keeps working while the downstream one is stopped, as long as the buffer has room, and the downstream machine keeps working after an upstream stop, as long as the buffer has product.

A buffer also changes what a stop costs. It does not remove the stop. It moves the stop from a machine that is costly to interrupt, such as a filler, to a place where interruption is cheap. Fillers and ovens, for example, run best when they run steadily, so they are often placed behind a buffer that absorbs stops from slower machines further along.

Line-speed relationships around the bottleneck

The slowest machine in the line, in steady running, sets the line’s output. That machine is the bottleneck. Machines before it can produce faster than it takes product, and machines after it can handle more than it delivers, so the buffers around it behave differently. Upstream of the bottleneck, the buffer tends to fill, because product arrives faster than the bottleneck takes it. Downstream, the buffer tends to run low, because the machines after the bottleneck can process faster than product arrives.

That is why a buffer before the bottleneck keeps the bottleneck fed when upstream machines stop, and a buffer after it keeps downstream machines running when the bottleneck stops. Placing a buffer on the wrong side, or sizing it for the wrong direction, is a common error.

Illustrative worked example

The numbers below are in arbitrary units and are only for explaining the logic. They do not describe any real machine.

Suppose three machines run in sequence. Machine A can run at 100 units per time period. Machine B, the bottleneck, runs at 90. Machine C can run at 120.

1. In steady running, the line output is limited by B, at 90 units per period. A is blocked or slowed for part of the time, since it can make more than B takes. The buffer between A and B fills until it is full, and then A must slow down or wait.
2. If B stops for 3 periods, A keeps making product at 100 per period. The buffer between A and B must hold 3 times 100, or 300 units, to avoid blocking A. Without that space, A stops too.
3. After B restarts, it runs at 90, while A makes 100, so the buffer does not empty at all. The buffer stays full, and the next stop of B stops A again after the buffer fills. The buffer helped only once.
4. Now suppose B had spare capacity and could run at 120 for a short recovery period. The buffer drains at 120 minus 100, or 20 units per period, so 300 units take 15 periods to empty. During those 15 periods the line runs above its normal rate, and the buffer is ready again afterward.
5. Between B and C, C is faster than B, so the buffer there stays near empty in steady running. When C stops for 2 periods, B keeps making 90 per period, and that buffer must hold 2 times 90, or 180 units.

The lesson is general. A buffer absorbs a stop only if it has room for the stop’s volume, and it recovers only if some machine can run faster than the one feeding it. A line in which every machine is rated at exactly the same speed has no way to empty a full buffer. In real lines the speeds also change with product and format, so the supplier needs the actual rated and sustained rates of each machine, not nameplate speeds alone.

Types and configurations

Zero-pressure accumulation

In zero-pressure accumulation, a conveyor is divided into zones, and each zone has a sensor, most often a photo-eye. When a case or container reaches the last zone, it stops and blocks the sensor, which tells the zone behind it to stop when the next item arrives. Items queue with gaps between them. One supplier states that, because products never touch, a large package cannot crush a smaller one, which matters for mixed weights and sizes. When the downstream blockage clears, the lead zone starts, then the next, and the release passes upstream, giving first-in first-out order. Zones can be built on motor-driven rollers, on line-shaft or belt-driven rollers with zone clutches or lifters, or on chain-driven rollers. Supplier descriptions also note that photo-eye zero-pressure designs have replaced older sensor rollers, which needed a minimum weight to trigger.

The cost is more sensors and more controls. In return, product is protected, and according to one supplier a motor-driven roller design runs only the zones that have product, which lowers energy use and wear. The case-handling details are on the case conveyors page.

Minimum-pressure accumulation

Minimum-pressure accumulation is usually a belt-driven roller conveyor with a mechanism that lets the rollers slip under queued product. A supplier describes it as a low-tech way to reduce back pressure on the load. Product still touches, but the force on the first item is far less than on a plain driven conveyor. It is simpler and often less expensive to install. Because the drive keeps rubbing under the stopped product, it can use more energy and wear belts and rollers faster, and it is less suited to fragile or lightweight packs.

Mass accumulation tables

A mass accumulation table is a wide, flat surface, usually a plastic belt or chain, on which bottles, cans, jars, or packs sit many abreast. Product enters at one side, spreads out, and leaves at the other. When the machine downstream slows, the table fills. The table is simple and has no sensors per zone. The design questions are the width and length of the table, how product leaves it, and how it controls back pressure when full. Mass tables suit stable containers and are common between a filler and a labeler or between a labeler and a packer. The bottle conveyors guide covers the single-file sections that feed and empty them.

Bi-directional and re-flow tables

A bi-directional table has belts that run in opposite directions in neighboring sections, so product moves back and forth across the table instead of straight through. One supplier describes the purpose as shifting production continuously from upstream or downstream until containers are routed through filers or rails to the next destination. Product usually enters in groups, and the same description notes that product is reintroduced in last-in first-out order. A re-flow or loop table keeps product circulating so it never builds pressure on a stopped front. Another supplier describes loop designs that single-file product with zero back pressure, no breakage, and no label damage. These tables are compact for the volume they hold, and they handle round bottles, cans, and jars well, and some handle unstable shapes. Check the product order if lots or dates need to stay in sequence.

Spiral, vertical, and alpine accumulators

When floor space is short, accumulation can go up. A spiral conveyor carries product around a helical path, and a supplier explains that a belt or modular belt is guided around a central tower, with rails keeping the belt in shape. Because the path is long in a small footprint, a spiral can hold many cases or containers, and it can serve as a buffer or give product dwell time to cool. One supplier describes vertical spiral accumulators that suit bottles, cans, and other cylindrical items, with bi-directional versions that both accumulate and de-accumulate in one unit. Alpine conveyors, which carry product up and over on a belt, are another way to build a buffer in a small floor area. The vertical conveyors guide compares spirals with lifts and inclines.

FIFO and dwell accumulators

Some products must leave in the same order they arrived, because of dwell time, lot tracking, or date codes. Serpentine and flow-through designs, spiral accumulators, and zero-pressure roller zones all keep order. A last-in first-out design, such as a bi-directional table, does not. A line that needs dwell time, such as cooling before packing, can use a FIFO accumulator as both buffer and process step, and then the sizing logic includes the minimum dwell time, not only the stop duration.

Quick comparison

Specifications to evaluate

Sizing logic

Sizing a buffer starts with a simple relationship.

Buffer capacity (units) = stop duration (time) x rate of the machine that keeps running (units / time)
Buffer length            = buffer capacity x product pitch / units per row
Recovery time            = buffer capacity / (recovering machine rate - feeding machine rate)

Product pitch is the length that each unit takes along the conveyor, including the gap. Units per row is the number of units across the width, which is one for single file and more for a table. If the recovery rate is not higher than the feeding rate, the recovery time has no solution, which means the buffer never empties. Add a margin to the capacity, and apply it to both the stop duration and the rate.

For a real line, work through these steps:

1. Identify the bottleneck and the rated and sustained rate of each machine. Use sustained rates under real product and format, not only nameplate speeds.
2. Collect stop data: the typical short stops, how often they happen, and the longest stop you want the line to ride through. The supplier cannot invent this data, and the plant usually has records from machine controls or maintenance logs.
3. Choose which stops to cover. A buffer sized for every possible stop becomes very large, so many lines size for the frequent short stops and let rare long stops stop the line.
4. Compute the capacity with the first formula, then convert it to length or table area using pitch and rows.
5. Check recovery. Confirm that the machine after the buffer can run faster than the one feeding it, or that the line can slow the feeder, so the buffer empties between stops.
6. Check dwell and order if the product needs time or sequence.
7. Check the building. Compare the length or area with the space available, and consider vertical accumulation if floor area is short.

A line with many machines needs one calculation per buffer, each with the right stop and rate. The supplier should work from actual data, and the figures on this page only illustrate the method.

Product compatibility

Zero-pressure zones are the usual choice for cases and cartons with printed or fragile faces, since they stop without touching. Minimum-pressure designs suit sturdy corrugated cases. See case conveyors for detail.

For bottles, jars, and cans, mass tables, bi-directional tables, and loop tables are typical. The container’s tolerance to back pressure and to friction on a plastic belt sets the design, and light or thin-walled containers need loop or pressureless designs. Flexible and soft packs such as trays, bags, and pouches may need belts and gentle zone transitions, and bags often do poorly on rollers. Pallet conveyors can accumulate unit loads between a palletizer and a wrapper, with zones or stops sized for the pallet length.

Test the product in a full queue. The first item in a stopped queue sees the worst conditions, and a long dwell can mark labels or soften packs.

Integration upstream and downstream

Accumulation sits at joints between machines, so placement matters. On bottle lines it usually goes between the filler and labeler or the labeler and case packer, using mass or bi-directional tables. On case lines it goes between the case packer and palletizer, so pallet changes and wrapper stops do not stall the case packer. The palletizing hub and the case palletizers guide describe the palletizer’s feed needs, and the palletizer selection guide lists the questions to settle before the buffer is sized. A buffer after a depalletizer smooths the supply of containers, and the depalletizing hub describes how containers enter the line. Around a bottleneck, one buffer upstream keeps it fed and one downstream keeps other machines running.

Controls should let the buffer talk to the machines. A line controller can slow an upstream machine when the buffer is nearly full and speed it up when the buffer empties, so the machine runs steadily instead of starting and stopping. Interlocks stop feeding a faulted machine. Ask how the line restarts after a long stop, since a full buffer must empty in an orderly manner, and whether a restart needs manual clearing.

For how buffering fits a beverage line, see beverage packaging automation, and for the case-level view see case handling applications.

Footprint, utilities, controls, and safety

Buffers are space hungry. A zero-pressure conveyor is as long as the number of zones times the zone length, and a table occupies a rectangle. Spirals and vertical units reduce floor area but add height and need structure and clearance. A drawing with columns, aisles, and fire exits avoids rework.

Zone conveyors need power and sometimes low-voltage supplies, and pneumatic zone designs need compressed air. Tables need drive power, and may need air for sweep arms or gates. Control level ranges from stand-alone zone cards to PLC logic with recipes. Define the signals between the buffer and each machine, including fault, full, and empty states.

Accumulation conveyors start and stop automatically, and a section that looks idle can move when a downstream zone clears. ASME B20.1 is the U.S. conveyor safety standard, covering design, construction, installation, maintenance, inspection, and operation in relation to hazards. OSHA’s 29 CFR 1910.147 covers lockout and tagout during servicing, and it requires energy isolation before maintenance in the covered cases, which matters here because automatic restarts are normal. Guard pinch points at drives, transfers, and spiral entries, mark access for clearing jams, and agree emergency stops with the plant safety team.

Cost factors

This page gives no prices. Capacity comes first: the number of units sets the length, area, or height. Zero-pressure zones, minimum-pressure rollers, tables, and spirals differ in hardware and controls, and more zones mean more sensors and controller cards. Delicate product needs gentler and more expensive handling. Vertical or compact designs cost more per unit of storage but save floor area.

Stainless frames, drainage, and open designs add cost for sanitation, and line controls, recipes, and links to machine controls add cost for integration. Operating cost covers energy, belt and roller wear, and cleaning time. See the packaging conveyor cost guide for a wider view.

Selection checklist

1. Identify the bottleneck and the rated and sustained rate of every machine.
2. Gather stop data for the machines on both sides of the buffer.
3. Decide the longest stop you want to cover and why.
4. Calculate capacity and recovery time with the formula, adding a margin.
5. Confirm the recovering machine can run faster than the feeder.
6. Choose the accumulation type from the product’s tolerance to pressure and contact.
7. Decide whether FIFO order or dwell time is required.
8. Check the available floor area and height.
9. Define the controls link between the buffer and the machines.
10. Test the product in a full queue and during release.
11. Review guarding, stops, and lockout points with the plant safety team.

The commissioning and acceptance checklist lists questions that apply to every machine.

Common mistakes

• Sizing for the average stop. Stops vary, so size the buffer to a deliberate target.
• Ignoring recovery. A buffer that cannot empty is only a delay.
• Placing the buffer on the wrong side of the bottleneck.
• Using minimum pressure for fragile packs.
• Treating buffer length as capacity. Capacity is in product units, and it depends on pitch.
• Letting the buffer hide a chronic fault. If a buffer is constantly full or empty, find out why.
• Forgetting the restart. A full buffer feeding a restarted machine needs a controlled release.
• Leaving out FIFO. Product that needs order or dwell may not suit a LIFO table.

Alternatives

Sometimes a buffer is not the right answer. Improving the reliability of the machine that stops most often can be better than a larger buffer. Adding speed to the machine after the bottleneck gives recovery. Linking machines directly, with a block or monobloc design, removes the need for buffers between them, though it also links their stops. In the other direction, an operator-fed staging table or a robotic pick that works from a pile gives a simple temporary store. For product that must change level and wait, a spiral or vertical unit combines buffering and elevation. When the line moves pallets, a robotic palletizer with a case buffer can absorb some of the same stops.