What Is Section Weight Analysis? A Guide to Cutting and Weighing PET Bottles
Section weight analysis cuts a PET bottle into predetermined sections and weighs each one, to show how the material of the preform has been distributed by blow molding. Bottle makers and bottlers use it to set up and monitor blow molding, and to check that a lighter bottle still has enough material where it needs it.
How does section weight analysis work?
The bottle is cut at fixed heights into sections, for example the finish and shoulder, the label panel and the base. Each section is weighed on a precision balance, and its weight is compared with the target weight and the limits set for that section.
Two cutting methods are used:
Hot wire cutting: heated wires melt through the PET and give a clean section without deformation. The AT2E HWBC-1 and HWBC-2 cut a bottle into up to 5 sections, and the ACWD-1 into up to 6 sections, which it weighs automatically.
Blade cutting: a knife is pressed into the bottle at the cut position and the bottle is turned by hand. The AT2E SSC-1 works this way, with no heating and no fumes.
Main applications
Blow molding set-up: adjusting the blowing settings until each section reaches its target weight.
Lightweighting: checking the material distribution when the preform weight is reduced.
Production monitoring: comparing bottles from different molds, machines or shifts.
Preforms: cutting preforms vertically or horizontally to check the material distribution, especially in multi-layer preforms.
Cosmetic tubes: cutting plastic or metal tubes into sections of a predetermined size.
Key concepts: material distribution, cut position and weighing resolution
Material distribution: the total weight of a bottle is the weight of its preform. What blow molding changes is where that material ends up. Section weights show it directly, and they complement the wall thickness measurements made at single points.
Cut position: if a cut moves by a few millimeters, part of one section ends up in the next one, and both weights change. Repeatable positioning is therefore as important as the balance. The HWBC models have graduated rulers to position the wires, and the ACWD-1 positions its cuts automatically.
Weighing resolution: differences between sections can be small, so the balance must read finely. The ACWD-1 weighs sections from 0 to 500 g with a resolution of 0.01 g.
Efficiency: on the ACWD-2S model, sections are weighed incrementally and pushed out together at the end of the cycle, which increases weighing efficiency by about 8 to 15 % compared with the ACWD-1, depending on the number of sections.
A loaf of bread, sliced and weighed
Imagine cutting a loaf of bread at marked positions and weighing each slice. The total weight tells you how much dough there was; the slices tell you where it rose. Two loaves of the same weight can be very different inside.
A PET bottle is the same. Two bottles blown from identical preforms weigh the same, but one may have a heavy base and a thin shoulder, the other an even wall. Cutting at the same marks every time, with clean cuts, and weighing each section shows the difference.
Calibration of a section weight system
Two things must be right: the weights and the cut positions.
Check the balance with certified reference weights across its range.
Check the cut positions against the bottle drawing, with a calibrated rule or a reference bottle.
Record the deviations, and adjust the balance or the cut positions if they exceed the acceptable limit.
Hot wires also need attention. On the HWBC models, the heating power and, on the HWBC-2, the cutting and return speeds can be adjusted, which helps to obtain a clean cut and to extend the life of the wires.
AT2E cutting and weighing equipment
AT2E covers section weight analysis from a hand tool to a fully automatic system:
ACWD-1: automatic bottle cutting and weighing device for PET bottles, up to 6 sections, hot wire cutting, weighing from 0 to 500 g with a resolution of 0.01 g; ACWD-2S and ACWD-A versions available.
HWBC-2: automatic hot wire bottle cutter, up to 5 sections, for bottles up to 2 L, with safety door and exhaust fan.
HWBC-1: hot wire bottle cutter, up to 5 sections, for bottles up to 410 mm high and 125 mm in diameter.
SSC-1: simple section cutter for plastic bottles, portable and with no heating.
Precision scale: precision weighing scale for section weight and fill weight control.
PPS-1: fully mechanical preform profile saw for vertical and horizontal profiles of preforms up to 125 mm high.
PC: preform cutter.
CTC: cosmetic tube cutter for plastic or metal tubes 25 to 50 mm in diameter.
Conclusion
The weight of a bottle tells how much material was used; its section weights tell whether it was used well. With repeatable cut positions, a calibrated balance and a target weight for each section, section weight analysis gives blow molding teams a direct, numerical view of their process.
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AT2E is an ISO 9001 certified company (AFNOR Certification).
Request a quote: info@at2e.com
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