A plant making 1 litre water preforms takes an order for 5 litre bottles. The new preform weighs several times more, its wall is thicker, and the first trial runs long and turns hazy near the gate. Heavy large-volume preforms run well on a 260 t machine when the shot sits inside the barrel window and the cycle is planned around the wall. Clamp is rarely what stops them.
This article covers what changes with heavy preforms, how to check the shot, why the wall sets the cycle, and the Indian rules for large water bottles.
Key Takeaways
- A good PET shot weighs about 22% to 72% of the barrel's PS rating in grams, so an eight-cavity mould of 90 g preforms needs a barrel rated roughly 1,000 to 3,250 g PS.
- Cooling time grows with roughly the square of the wall, so a 4.0 mm wall needs about 56% more cooling than a 3.2 mm wall.
- Acetaldehyde rises linearly with residence time and exponentially with melt temperature, so the screw should be matched to the preform weight.
- Under the FSSAI Packaging Regulations, packaged drinking water bottles need at least 85% transparency, and PET in food contact must conform to IS 12252.
What Makes Heavy Large-Volume Preforms Different?
More weight per cavity, a thicker wall and a longer core. The shot per cavity can be several times that of a water preform, cooling takes longer, and the mould needs more opening stroke. Clamp per cavity grows only with the ledge diameter, so it is seldom the first limit.
Large bottles are blown from long preforms, so the mould has to open far enough to clear the core and release the neck splits. Ask the mould maker for the stroke the tool needs and compare it with the machine before the cavity count is fixed.
Our view: design a heavy preform for the thinnest wall the blow moulder can stretch evenly, not the thickest one that is easy to fill. Every extra millimetre of wall costs cycle time on every shot for the life of the mould.
How Do You Check the Shot for Heavy Large-Volume Preforms?
Work in volume and keep the shot inside the barrel window. Jim Bozzelli of Plastics Technology advises shots between about 20% and 65% of barrel capacity, with 10% added for check-valve leakage and melt uniformity. Below that window the melt sits too long; above it, melt quality suffers.
Machine shot ratings are usually given in grams of polystyrene, so convert before you compare. Ottmar Brandau's practical guide to preform moulding in PETplanet Insider gives a typical PET melt density of 1.15 g/cm³, against 0.945 g/cm³ for PS melt in Bozzelli's article. A barrel rated R g PS therefore holds about 1.22 × R g of PET melt at full stroke.
Apply the margins and a good PET shot runs from about 0.22 × R to 0.72 × R. The worked example below turns that into a barrel range for an eight-cavity heavy preform mould.
Bozzelli also warns against buying the biggest screw by default, because big screws usually mean lower plastic pressure. For a heavy preform, aim for the lower half of the window rather than the edge of it.
How Does a Thick Wall Change the Cycle?
It sets the cooling time, a large part of any preform cycle. Heat leaves through the wall, so cooling time grows with roughly the square of the thickness. A 4.0 mm wall needs about 56% more cooling than a 3.2 mm wall, since 1.25² = 1.56.
Relative cooling time by preform wall (square of wall ratio, 3.2 mm = 1.00)
| Wall thickness | Wall ratio to 3.2 mm | Relative cooling time |
|---|---|---|
| 3.2 mm | 1.00 | 1.00 |
| 3.6 mm | 1.13 | 1.27 |
| 4.0 mm | 1.25 | 1.56 |
| 4.5 mm | 1.41 | 1.98 |
In his book extract on acetaldehyde, Brandau makes a related point: cycle time follows wall thickness, not weight. Two preforms with the same wall, one twice the weight of the other, may differ by only 1 to 2 s.
That has a cost on the melt side. Acetaldehyde, a by-product of heating PET, rises linearly with residence time and exponentially with temperature, according to the same extract. Screw and barrel wear raises shear and acetaldehyde as well, so Brandau advises matching the extruder size to the preform weight.
Still water shows acetaldehyde most, because its taste reacts to small amounts, while the sugar in carbonated drinks and juices masks it. Large water bottles are the case to watch on a heavy preform job.
Which Indian Rules Apply to Large Water Bottles?
If the bottle holds packaged drinking water, the FSSAI Packaging Regulations 2018 require at least 85% transparency, in plastics that conform to their listed Indian Standards. For PET, that standard is IS 12252. Your customer will ask for evidence with each supply of bottles made from your preforms.
Thick walls make the transparency target harder, because a wall that cools slowly can turn hazy or white. A milky preform blows into a milky bottle, so haze on a heavy preform is a cooling and design question as much as a resin one.
Large bottles for edible oil and other foods fall under the same regulations, which set an overall migration limit of 60 mg/kg or 10 mg/dm², tested to IS 9845.
What Should You Send to Check a Heavy Preform on 260 t?
Send the preform drawing with weight, wall thickness, length and neck finish, the bottle size and resin grade, the cavity count you want and the blow moulder you will use. With those, barrel, cycle and mould stroke can be checked together. See the 260 ton PET injection moulding machine page.
Frequently Asked Questions About Heavy Large-Volume Preforms
How heavy can a preform get before it needs a larger machine?
There is no fixed weight. The limits are the shot against the barrel window, the length against the opening stroke and the wall against the cycle you can accept. A heavy preform that fits the barrel at a low cavity count may run well on 260 t, while a higher count could need a larger screw.
Why do heavy preforms turn white near the gate?
White or milky areas point to crystallinity, where a thick wall or the gate area stays hot long enough for PET to crystallise. Check mould water temperature and flow, cooling time and the nozzle tip heat. A wall thicker than the bottle needs makes the problem harder to solve.
Do heavy preforms need longer drying?
Not per kilogram. The resin needs the same drying time and temperature for any preform, but a heavy job uses more resin per hour, so the hopper must hold more. Size it from throughput times drying time, and keep the dew point at minus 40 °C or lower.
Can 5 litre and 1 litre preforms share one barrel?
Possibly, if both shots sit inside the barrel window. The light job will spend longer in the melt, which raises acetaldehyde, so check it against your customer's limit. If the window does not cover both shots, a second barrel sized for the lighter job is worth pricing with the machine.
What wall thickness should a large bottle preform have?
The blow moulder and bottle design decide, through the stretch ratio and the bottle weight. From the moulding side, ask for the thinnest wall that blows evenly, because cooling time rises with roughly the square of the wall. Review the design with both suppliers before cutting steel.
Related Articles
- What PET Products Can You Make on a 260 Ton Machine?
- PET 260 Ton: Sizing a Machine for High-Cavity Preform Moulds
- Post-Mould Cooling and Handling for High-Output Preform Production
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)
- Food Safety and Standards Authority of India (FSSAI): Food Safety and Standards (Packaging) Regulations, 2018, compendium
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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