Take three screws on one injection unit: 36, 39 and 42 mm. If the 36 mm screw reaches 2,400 kg/cm², the 42 mm screw reaches only about 1,763 kg/cm², but it sweeps 36% more volume. That trade, set by (D2/D1)², is the whole logic of screw options. The right PET preform screw is the one that puts your shot inside the 20% to 65% window of barrel capacity, recovers within the cycle without overheating the melt, and still has the pressure and injection rate to fill every cavity.
Key Takeaways
- A larger screw gives more shot volume and injection rate but less injection pressure, in proportion to diameter squared.
- Keep the PET shot between 20% and 65% of barrel capacity; a shot that is too small leaves melt in the barrel too long.
- Acetaldehyde rises with residence time and much faster with temperature, so an oversized screw can hurt water preforms.
- Check the mould's injection rate need, about 10 to 12 g/s per cavity, before settling on the smallest screw.
What changes when you move to a larger PET preform screw?
Swept volume and shot capacity rise with diameter squared, and available injection pressure falls by the same ratio, because the hydraulic force behind the screw stays the same. Where the screws share one length, L/D also falls as diameter rises, which means fewer flights to melt and mix the PET.
The table runs these ratios for three assumed screws with a 200 mm stroke. PET melt density is about 1.15 g/cm³, and the good shot window allows 10% extra for check-valve leakage and melt uniformity, following Plastics Technology's method.
Three assumed screws on one unit (200 mm stroke; 36 mm screw assumed at 2,400 kg/cm²)
| Screw | Swept volume | Injection pressure | Good PET shot (20% to 65%, +10%) |
|---|---|---|---|
| 36 mm | 203.6 cm³ | 2,400 kg/cm² | 42.6 to 138.3 g |
| 39 mm | 238.9 cm³ | 2,045 kg/cm² | 50.0 to 162.4 g |
| 42 mm | 277.1 cm³ | 1,763 kg/cm² | 57.9 to 188.3 g |
Plastics Technology's Jim Bozzelli calls 20:1 his recommended minimum L/D for moulding. If the larger screw options on your machine sit well below that, ask how the screw design makes up for it before choosing them for PET.
Why can a screw that is too big raise acetaldehyde?
A big screw making a small shot keeps each pellet in the barrel for many cycles. Acetaldehyde in PET rises linearly with residence time and exponentially with temperature, so melt that sits too long, or runs hot to make up for poor mixing, carries more acetaldehyde into the preform.
PETplanet Insider explains why this bites on water bottles: still water shows small acetaldehyde levels in taste, while the sugar in carbonated drinks and juices masks them. It also notes that cycle time follows wall thickness rather than weight, so a lightweighted preform with the same wall runs on almost the same cycle with nearly double the residence time.
Resin grades for water set a tight starting point. Reliance's Relpet G5761, a still-water grade, specifies acetaldehyde of 1.0 ppm maximum in the resin itself, so the moulding step decides how much is added on top. A rough residence check is barrel swept volume divided by shot volume, times the cycle; halving the shot on the same barrel and cycle doubles it.
Choose the smallest screw that meets your shot and injection rate with margin. Buying the biggest screw "for flexibility" is an easy way to spoil a water preform.
How do injection rate and recovery limit the choice?
Each cavity needs about 10 to 12 g/s of PET during injection, so the screw and unit must deliver cavities times that rate. The screw must also recover within the cycle at modest speed and back pressure, leaving about 2 s stopped at the end of recovery.
PETplanet Insider's preform guide gives back pressure of typically 15 to 20 bar, kept to the minimum, and screw speed as low as allows recovery within the cycle. It also warns that processed kg/h must stay below the screw maker's rating, or unmelts can appear. A smaller screw delivers less per revolution, so a high-output mould can push it to high rpm and shear heat.
That is the balance: the small screw protects melt quality on light shots, the large screw protects output and recovery on heavy ones, and the middle screw is the compromise between the two.
Which preforms suit the small, middle and large screw?
Match the screw to the shot, not to the product name. The small screw suits light preforms, where shot volume is low and residence time matters most. The middle screw suits typical edible oil and soft drink shots. The large screw suits heavy jar preforms and heavier multi-cavity shots.
A machine may run more than one preform family over its life, so think about the mix you expect over the next few years rather than the first mould alone. If light water preforms and heavy jar preforms will share the machine, a single screw may sit at the wrong end of its window for one of them. That is the case for ordering a second barrel and screw with the machine, as the FAQs below explain.
Heavy jar preforms bring their own clamp and cooling questions, covered in the wide-mouth jar preform article in this series.
What should you send to choose a PET preform screw?
Send the preform weight, cavity count, target cycle and the bottle's end use, since water, soft drink and edible oil preforms carry different acetaldehyde sensitivity. Add the hot runner volume if known. With those, a supplier can place your shot in each screw's window and check injection rate and recovery. See the screw options on the 150 ton PET injection moulding machine page.
Frequently Asked Questions About PET Preform Screws
What is L/D ratio and why does it matter for PET?
L/D is the screw's flighted length divided by its diameter. A longer screw relative to its diameter gives more flights to melt and mix the PET evenly. Plastics Technology's Jim Bozzelli recommends 20:1 as a minimum for moulding. On one barrel length, a larger diameter screw has a lower L/D.
Can I change to a different screw later?
Yes. A second barrel and screw ordered with a new machine can cost little extra, and Plastics Technology notes that a skilled operator can swap a barrel in under 30 minutes, even hot. That makes a second screw size a practical option if your preform mix will change between light and heavy work.
How do I measure actual residence time in the barrel?
With the machine cycling, drop a few granules of a different colour of the same resin into the feed throat, count the shots until the colour disappears and multiply by the cycle time. This catches resin trapped at the base of screw flights, which barrel-capacity formulas miss. Swirls in the colour show poor mixing.
What back pressure should I use for PET preforms?
Preform guidance from PETplanet Insider puts back pressure at typically 15 to 20 bar, kept to the minimum that gives a consistent shot. Higher back pressure adds shear heat, which raises acetaldehyde and can degrade the melt. Raise it only to fix mixing or unmelt problems, and recheck AA afterwards.
Why do unmelted pellets appear in my preforms?
Unmelts can come from wet resin, a dryer dew point above -40 °C, resin entering the throat below 160 °C, or a screw pushed beyond its kg/h rating. Check drying first, then melt temperature around 280 ± 10 °C, back pressure and screw rpm. Degraded pellets can also have a higher melting point.
Related Articles
- What PET Products Can You Make on a 150 Ton Machine?
- Wide-Mouth Jar Preforms on a 150 Ton Machine
- Thick-Wall PETG Cosmetic Jars: Cooling Time and Sink Control
Sources & References
- Plastics Technology, J. Bozzelli: A Simpler Way to Calculate Shot Size vs. Barrel Capacity (2017, updated 2022)
- PETplanet Insider, O. Brandau: Bottles, Preforms and Closures, 3.6 Acetaldehyde in PET Bottles (Vol. 16, 03/2015)
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, part 32: Unmelts (Vol. 13, 09/2012)
- 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.
Limitation of Liability: Pratishna Engineers makes no representation or warranty, express or implied, regarding the completeness, accuracy or suitability of the information for any particular application. To the extent permitted by applicable law, Pratishna Engineers shall not be liable for any loss, damage, cost or consequence arising from reliance on or use of the information contained in this document. Final machine selection, configuration and application suitability should be confirmed with Pratishna Engineers based on complete and accurate project requirements.
