A moulder running 2 litre preforms takes an order for 5 litre water bottles. The new preform weighs several times more, its wall is thicker, and the first trial shows a longer cycle than the buyer's price assumed. Heavy preforms for large-volume bottles change three things at once on a 180 t machine: the shot takes a larger share of the barrel, the wall sets a longer cooling time, and the melt's time in the barrel decides acetaldehyde.
This article covers how to check the shot in PS terms, why the wall drives the cycle, and how residence time and resin grade affect taste-sensitive water bottles.
Key Takeaways
- Machine shot ratings are in PS; PET melt is about 1.22 times as dense as PS melt, and a good shot uses about 20% to 65% of the barrel.
- Cooling time rises with the square of wall thickness, so a 4.5 mm wall needs about 2.25 times the cooling of a 3.0 mm wall.
- Acetaldehyde rises linearly with residence time and exponentially with temperature, so match the screw to the preform weight.
- Reliance's Relpet G5761 still water grade lists acetaldehyde of 1.0 ppm max and IV of 0.76 ± 0.02 dl/g.
What Makes Heavy Preforms for Large-Volume Bottles Different?
A bigger shot, a thicker wall and a longer, wider core. The shot uses more of the barrel, the wall sets the cooling time and the core needs more stroke to clear. Large water, edible oil and beverage bottles all start from preforms like these.
Cavity count tends to fall as weight rises. Each cavity needs more melt, more cooling and more steel around it, so a heavy preform mould can run fewer cavities than a light one on the same machine, even when the clamp would allow more.
Length matters too. A large bottle preform is long, so the mould needs enough opening stroke to pull the core clear and open the neck splits, and the mould must still fit the shut height range. The stroke and mould-fit checks are set out in the article in this series on when 180 t is enough.
Our view: on heavy preforms, fix the wall thickness with the bottle designer before anything else. Every extra tenth of a millimetre costs cycle time for the life of the mould, while a little extra weight in the neck or base often costs far less.
How Do You Check the Shot Against the Barrel?
Convert the PET shot to the PS rating used on machine data. PET melt is about 1.15 g/cm³ against 0.945 for PS, so a barrel rated R grams of PS holds about 1.22 × R grams of PET melt. Add 10% and keep the shot between about 20% and 65%.
The figures come from two sources. John Bozzelli's Plastics Technology article gives PS melt at 0.945 g/cm³, advises working in volume and adding 10% for check-valve leakage, and puts the shot within 20% to 65% of barrel capacity. Ottmar Brandau's practical guide to preform moulding in PETplanet Insider gives PET melt at about 1.15 g/cm³.
Barrel rating window (g of PS) for a given PET shot, using a 20% to 65% shot share plus 10%
| PET shot (g) | Smallest barrel rating (g PS) | Largest barrel rating (g PS) |
|---|---|---|
| 150 | about 209 | about 678 |
| 210 | about 292 | about 949 |
| 300 | about 417 | about 1,356 |
| 400 | about 556 | about 1,808 |
Bozzelli warns that above 65% you risk unmelt and long recovery, and below 20% the melt sits too long. For heavy preforms the top of the window is the usual problem, so check the largest shot in your plan, not the average.
Why Does Wall Thickness Set the Cycle?
Because cooling time follows the square of wall thickness, a result of how heat conducts out of the wall in the standard cooling-time equation. A wall 1.5 times as thick needs about 2.25 times the cooling, and on a thick preform cooling is the largest part of the cycle.
Brandau makes the same point for preforms: cycle time follows wall thickness, not weight. Two preforms with the same wall, one twice as heavy, may differ by only 1 to 2 seconds. A heavy preform with a thick wall is slow because of the wall, not the weight.
A preform does not have to be fully cooled in the mould. It only needs a frozen skin strong enough to eject without marking. Preform lines use this by ejecting warm preforms into cooled receivers, so the mould does not hold the full cooling time on every shot.
How Do Residence Time and Acetaldehyde Change With Weight?
In opposite ways to what you might expect. A heavy preform empties more of the barrel each shot, so its melt spends less time hot. The risk sits with light preforms on a large screw. Brandau reports acetaldehyde rising linearly with residence time and exponentially with temperature.
Acetaldehyde matters most for water. Brandau notes that still water taste reacts to small levels, while the sugar in carbonated drinks and juices masks it. Moisture in the resin also breaks chains and creates more, which is why drying comes first, as the output planning article in this series explains.
Choose the grade for the bottle. Reliance's data sheet for Relpet G5761, a copolymer bottle grade for still water, lists acetaldehyde of 1.0 ppm max, IV of 0.76 ± 0.02 dl/g and low preform acetaldehyde. Ask your supplier for the current data sheet before you set the process.
What Should You Send for a Heavy Preform Check?
Send the preform drawing with weight, wall thickness, length and neck finish, the bottle it will be blown into, the resin grade and the cavity count you want. With those, shot, cooling and stroke can be checked together. See the 180 ton PET injection moulding machine page.
Frequently Asked Questions About Heavy Preforms
Is there a fixed weight above which a preform counts as heavy?
No. What matters is the shot against the barrel and the wall against the cycle. A preform is heavy for a given machine when the full shot nears the top of the barrel's good range, or when its wall makes cooling, not mould movement, the longest part of the cycle.
Does a heavy preform need a higher IV resin?
Not because of weight alone. IV is chosen for the bottle: strength, carbonation, hot fill and stretch ratio. Large still water bottles may run a standard water grade, while aerated drinks use grades made for pressure. Ask the resin supplier and the blow moulder together.
Can I shorten the cycle by cooling the mould water further?
Only up to a point. Preform moulds already run chilled water, and colder water can bring condensation on the mould in humid weather. Thinner walls, better cooling channels and cooled receivers after ejection usually give more than a lower water temperature.
Will a heavy preform blow into a good bottle on any blow moulder?
No. Large bottles need a blow moulder sized for the preform weight, length and neck, with enough heating to bring a thick wall to stretch temperature evenly. Agree the preform design with the blow moulder before cutting steel, since a heavy wall can be hard to reheat.
Should heavy preforms run on a hot runner mould?
Usually yes. A hot runner avoids a large cold runner that would add regrind, drying load and IV loss on every shot. Each cavity is gated near the preform base. Check that the hot-runner zones and the machine's controls match, and keep spare heaters and thermocouples on site.
Related Articles
- What PET Products Can You Make on a 180 Ton Machine?
- When the PET 180 Ton Is Enough and When to Move to 220 or 260 Ton
- Planning Preform Output: Cavities, Cycle Time and Drying Capacity
Sources & References
- Plastics Technology, J. Bozzelli: A Simpler Way to Calculate Shot Size vs. Barrel Capacity
- PETplanet Insider, O. Brandau: Practical Guide to Injection Moulding of Preforms: Injection parameters, part 17 (Vol. 16, 01+02/2015)
- PETplanet Insider, O. Brandau: Bottles, Preforms and Closures extract: Acetaldehyde in PET Bottles (Vol. 16, 03/2015)
- Reliance Industries: Relpet G5761 TDS (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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