A water bottler asks you to take its 500 ml preform from 24 g to 19 g: same neck, same mould base, new cores and cavities. The bottles blow well, but within a week the lab flags higher acetaldehyde, and the cycle has dropped by less than you hoped. That is a typical result of PET preform lightweighting done on the old settings. A thinner wall saves cooling time, but a lighter shot on the same barrel spends longer in the melt. Plan wall, cycle and machine settings together, before the new cavities are cut.
Key Takeaways
- Cooling time rises with the square of the wall, so a wall 20% thinner needs about 36% less cooling.
- Cycle time follows wall thickness, not weight; a lighter preform that keeps its old wall saves very little time.
- A lighter shot on the same barrel raises residence time, and acetaldehyde rises with residence time and steeply with temperature.
- Re-tune injection rate, transfer, hold, cooling and screw recovery after any weight change, and confirm the new shot still sits inside the barrel window.
What does PET preform lightweighting change in the process?
It takes PET out of the body wall, base or neck, and each route shifts something different. A thinner wall cuts cooling time, a lighter shot raises residence time in the barrel, and a shorter neck needs new neck tooling and closures. Check the machine for all three.
The body wall carries the bulk of the material, which makes it the obvious first target. It is also the flow channel. A frozen boundary layer of about 0.25 mm forms against the steel, so a 2.3 mm wall leaves only about 1.8 mm for the melt to pass. Thin the wall further and that channel narrows again, so fill behaviour and gate design deserve a fresh look.
Neck lightweighting is a separate route. A shorter neck finish can save more than a gram per preform, but it needs new neck inserts, new closures and the bottler's agreement, as the neck finish article in this series explains.
How much cycle time does a thinner wall save?
Cooling time scales with the square of wall thickness. A wall 20% thinner needs about 36% less cooling, and a 2.5 mm wall needs about 31% less than a 3.0 mm wall. Only the cooling share of the cycle shrinks; mould movement, take-out and screw recovery stay as they were.
In preform moulding, in-mould cooling is typically about 1 to 3 s, depending on wall, and the take-out tubes finish the job after ejection. A thinner wall therefore shortens the in-mould part of the cycle, and the gain on the total cycle is smaller than the cooling ratio suggests.
Weight is a poor guide to cycle. Two preforms with the same wall, one twice as heavy as the other, may differ by only 1 to 2 s. If the new preform keeps the old wall and only gets shorter, expect almost no cycle gain at all.
Relative cooling time by wall thickness, compared with a 3.0 mm wall (calculated, square law)
| Body wall (mm) | Relative cooling time | Cooling saved vs 3.0 mm |
|---|---|---|
| 3.0 | 1.00 | none |
| 2.7 | 0.81 | about 19% |
| 2.5 | 0.69 | about 31% |
| 2.3 | 0.59 | about 41% |
| 2.0 | 0.44 | about 56% |
Why can a lighter preform raise acetaldehyde?
Acetaldehyde, or AA, forms when PET is held at melt temperature. It rises in a straight line with residence time and exponentially with temperature. A lighter shot on the same barrel, at a similar cycle, stays longer in the melt, so AA can climb even though nothing else has changed.
AA matters most in water bottles. Still water shows its taste at small AA levels, while the sugar in carbonated drinks and juices masks it. Indian still-water grades are made with low AA for this reason; Relpet G5761, for example, lists 1.0 ppm maximum in the resin. The moulder's task is to add as little as possible on top.
As a rough guide, residence is the barrel's swept volume divided by the shot volume, times the cycle. Halve the shot on the same barrel and cycle, and residence doubles. Moisture makes it worse, because water in the resin both breaks chains and creates AA. A resin temperature of about 165 °C at the extruder throat has proven a good point for low AA.
Our view: do not lightweight on the old barrel until you have checked the new shot against the barrel window. If the shot falls near 20% of capacity, a smaller screw costs less than a quality complaint.
Which machine settings need re-tuning after a weight change?
Injection rate, transfer position, hold pressure and time, cooling time and screw recovery all need a fresh look. A new weight changes both the shot stroke and the wall, so settings carried over from the heavier preform can overpack, underpack or overheat the lighter one.
Start with the shot. Set the injection rate per cavity again; about 10 to 12 g/s per cavity is a working figure for preforms. Move the transfer position to suit the shorter stroke. Hold pressure must not be too low, or the body can turn wavy as it pulls away from the cavity wall.
Keep mould water at 7 to 10 °C at the mould maker's flow, and trim cooling time in small steps while you check wall, dimensions and AA. Then look at recovery. With a shorter shot the screw finishes early, so lower the screw speed until recovery ends close to the end of cooling instead of leaving melt waiting.
What should you check before approving a lighter preform?
Ask for the new preform drawing with wall at the body, base and gate, the target weight, and the barrel and cycle you run today. Work out the shot share of the barrel, the expected cooling saving and the change in residence time before the cavities are cut. If the numbers point to a new screw or a new press, see the PET injection moulding machine range.
Frequently Asked Questions About Lightweight PET Preforms
How light can a 500 ml water bottle preform go?
There is no single figure. It depends on bottle shape, top-load and filling needs, the neck finish and the stretch the resin can take. Short necks such as 26/22 have allowed much lighter designs, as the neck finish article in this series shows. Run blow trials and bottle tests on any new weight before committing to cavities.
Does lightweighting affect the blow moulding side?
Yes. A thinner, lighter preform reheats differently, and its stretch depends on wall and IV. A preform that has lost IV inflates at lower pressure and gives a weaker bottle, so hold IV loss within limits and expect to retune the blow moulder heating profile for the new preform before production.
Should I lightweight the neck or the body first?
Body changes need only new cores and cavities, while neck changes also need new neck inserts, new closures and the bottler's agreement on capping. That makes the body the simpler first step. If both are planned, confirm them together in one blow trial so that weight, stretch and capping are tested as a set.
Will a thinner preform always run at higher output?
Only if cooling was the bottleneck. A thinner wall cuts in-mould cooling, but mould movement, take-out and screw recovery stay the same. If the robot cooling tubes or the dryer were already at their limit, output will not rise. Check each one before you quote a new hourly figure to the customer.
How do I know if AA has risen after lightweighting?
Test it. Preform AA is measured in a lab, for example by headspace gas chromatography, and compared with the bottler's limit. Test the first lots after any weight change, after a screw or barrel change, and whenever melt temperature or cycle time moves, so a rise is caught before bottles reach the filler.
Related Articles
- PET Preform Making Machine: How to Choose Tonnage, Cavities and Output
- Moulding With rPET: Meeting India's Recycled Content Rules for PET Bottles
- PET Preform Neck Finishes Explained: PCO 1881, 26/22 and Wide Mouth
Sources & References
- PETplanet Insider, O. Brandau: Bottles, Preforms and Closures extract, 3.6 Acetaldehyde in PET Bottles (Vol. 16, 03/2015)
- RJG Inc.: How to Determine Injection Molding Cooling Time (Mar 2021)
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, part 34: Wavy Body (Nov 2012)
- Reliance Industries: Relpet G5761 Technical Data Sheet (Aug 2022)
Technical Disclaimer
Disclaimer: The information provided in this document is for general informational and educational purposes only. It is based on technical references and industry information available at the time of preparation and may be subject to change. It does not constitute engineering, technical, commercial or professional advice, nor does it constitute a quotation, specification, warranty or contractual commitment by Pratishna Engineers.
Actual injection moulding machine and process requirements vary according to the component design, material, mould, production volume, machine configuration and operating conditions. The information, examples, calculations and recommendations provided should be independently evaluated and verified for the specific application before making any equipment, process or production decision. Information obtained from third-party sources remains subject to the accuracy and limitations of those sources.
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