Downstream Packing and Palletising for PET Bottle Lines

The blower gets the capital approval, the specification meeting and the factory visit. The packer is bought later, from whatever budget is left, and the mismatch only becomes visible in the third month. A line built around a 12,000 BPH blower feeding a packer that handles 9,000 BPH is not a 12,000 BPH line; it is a 9,000 BPH line with an expensive machine idling at the front of it.

The cost of that mismatch is not one number. When accumulation tables fill, the blower has to stop, and every stop restarts the thermal argument inside the machine: preform reheating has to re-stabilise, blow pressure has to rebuild, and bottle weight drifts until the process settles. Operators respond by slowing the blower to a rate the packer can absorb, and that quiet number becomes the plant’s real capacity without ever being written down.

Sailwin has built PET blow molding machines for 15+ years, with 500+ machines in 60+ countries, CE marking, ISO 9001:2015 manufacturing and a 2-year whole-machine warranty. The automatic range runs from the SW-F2-650 at 2,800 BPH for containers up to 650 ml to the SW-F8H-800 at 16,000 BPH for containers up to 800 ml, blowing at 25–40 bar with a PLC logging 40+ parameters in real time. This article covers how to size downstream packing against the blower’s real output, which pack format to choose, and where the buffer belongs.

1. Why Packing and Palletising Decide Whether the PET Line Hits Its Rate

A stretch blow molding machine is a fixed-rate device. Once the mould is in, the preform is qualified and the heating profile is set, it produces bottles at the rate its cavity count and cycle determine, and it does so most efficiently when it runs continuously. Everything downstream exists to protect that continuity.

Packing equipment, by contrast, is usually quoted in cases per minute, and the word "efficiency" rarely comes with a definition attached. One supplier means the mechanical maximum when nothing jams. Another means the rate with film splices and box changes included. When the two definitions meet on your factory floor, the difference between them is measured in stopped blowers.

There are four places where PET bottle packing palletising arrangements typically steal rate from the blow molding machine, and only the first is a genuine capacity problem:

• Rate mismatch. The packer’s sustainable rate is below the blower’s output, so the blower is permanently throttled or permanently stopping.
• Format changeover. Switching pack sizes stops the packing hall for longer than the blower needs to change a mould, so the two ends cannot change over on one schedule.
• Consumable starvation. Film rolls, blank cartons and trays run out mid-shift because replenishment was planned on paper output.
• Palletising starvation. The palletiser waits for a layer that never completes because a case was rejected upstream, and the line waits with it.

Only the first item needs new equipment. The other three are specification and control problems, and they explain why two plants with identical machinery report capacity figures that differ by more than 10%. Sailwin machines support that diagnosis because the PLC records 40+ parameters in real time.

2. Matching PET Bottle Packing Palletising Speed to Blower Output

Most specifications go wrong at the conversion from bottles per hour to cases per minute, done once at the start of the project using the wrong bottles-per-case figure. Do it explicitly and write both numbers down.

Read that last column as the number you hand to a packing supplier. Change the pack format and the target moves with it: the same SW-F8H-800 feeding a 12-bottle pack needs about 22.2 packs per minute, and a 6-bottle club pack about 44.4.

3. Shrink Film, Carton or Tray: Choosing the Pack Format

The three formats in common use for PET beverage bottles are not interchangeable versions of the same thing. Each shifts cost and risk to a different part of the process.

The interaction that catches most projects is between pack format and bottle design. A bottle light-weighted for material saving has less wall stiffness, and shrink film pulls the pack inward as it cools. Decide the packing format while the preform and mould are still being designed. Sailwin handles the standard neck finishes — PCO 28, 30 and 38 mm, with wide-mouth support up to 130 mm — and containers from 0.1 L to 20 L.

Changeover time deserves the same scrutiny as rate. A format change on a shrink line is a reel change plus tunnel settings; on a carton line it is a magazine reload plus erector adjustment; on a tray line it is both. If the packing hall cannot be changed over in the same window as the blow molding machine — and Sailwin machines are designed for mould changes in under 30 minutes — then the plant will end up scheduling production around the packer rather than around demand.

4. Accumulation, Buffer and Line Control Between Blower and Packer

The buffer between the blow molding machine and the packing machine has one job, and it is worth stating precisely before any conveyor is ordered: to let the packing machine stop briefly without forcing the blower to stop with it. Its size should be expressed in seconds of blower output, because that is the only unit in which it means anything operationally.

A buffer buys time; it does not create capacity. If the packer needs five minutes of buffer to survive a two-minute jam, the answer is the jam, not a bigger table.

Practical sizing starts from the jam distribution rather than from a rule of thumb. Record every packing stop over two weeks with its duration, then size the accumulation for the duration that covers the large majority of stops. On most PET beverage lines that lands between two and four minutes of blower output, which for a 12,000 BPH machine is 400 to 800 bottles of capacity. It is a design calculation, not a catalogue number.

Three control details matter as much as the size. Use accumulation that moves bottles without pressure, because bottles pushed against a dead stop will scuff. Run the take-away conveyor at roughly 110% of the blower’s rated output so the machine is never waiting on conveyor speed. And let the blower act as pacemaker rather than running both ends on independent speed references.

Palletising has its own failure mode. A palletiser starved of one case to complete a layer stops, and that stop propagates backwards within seconds. Interlayer sheets, slip sheets and pallet dispensers are consumed in discrete events rather than continuously, so they are the components worth monitoring with a sensor rather than a person.

5. Case Study: A Packing Bottleneck on a High-Speed Water Line

A beverage producer running 500 ml still water on a six-cavity machine could not hold the line’s planned rate. The blower was blamed, although its cycle matched the machine card.

Scenario based on a Sailwin customer project; site-specific figures available on request during engineering review.

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