Put a 92 g PET shot, four 23 g preforms, on two screws with the same 150 mm stroke. The 32 mm screw sweeps about 121 cm³ and the 35 mm about 144 cm³. With PET melt at 1.15 g/cm³ and a 10% allowance, the shot needs 88 cm³: 73% of the smaller screw's capacity and 61% of the larger one's. Choosing between the 32 mm and 35 mm screw for PET comes down to that share.
The smaller screw gives about 20% more melt pressure on the same unit. For PET preforms, our view is that where the shot sits in the barrel matters more than that pressure, and the sections below explain why.
Key Takeaways
- A 35 mm screw has about 20% more area than a 32 mm screw: more melt per millimetre of stroke and about 16% less melt pressure.
- PET melt is about 1.15 g/cm³ against 0.945 for PS, so a barrel rated in PS grams holds about 22% more PET by weight.
- Keep each shot between about 20% and 65% of the screw's capacity, after adding 10% to the calculated melt volume.
- PET loses IV with heat and time, so a screw too large for the shot costs melt quality, not only money.
How Do the 32 mm and 35 mm Screw for PET Differ?
On the same injection unit the hydraulic force is fixed, so screw area decides both pressure and volume. The 35 mm screw has (35 ÷ 32)², about 1.20 times the area: about 20% more melt per millimetre of stroke and about 84% of the smaller screw's pressure.
John Bozzelli makes the same point in Plastics Technology: a larger screw generally delivers lower plastic pressure, and he warns against defaulting to the biggest screw on offer. Compare the two options on melt pressure and swept volume, not on the hydraulic pressure shown on the screen.
32 mm and 35 mm screws on the same unit, 150 mm stroke assumed for illustration
| Point | 32 mm screw | 35 mm screw |
|---|---|---|
| Relative screw area | 1.00 | About 1.20 |
| Melt pressure available | Higher | About 84% of the 32 mm screw |
| Swept volume | 120.6 cm³ | 144.3 cm³ |
| PET at full stroke (1.15 g/cm³) | 138.7 g | 166.0 g |
| Same volume as PS (0.945 g/cm³) | 114.0 g | 136.4 g |
| PET shot range, 20% to 65%, with 10% allowance | About 25 to 82 g | About 30 to 98 g |
Why Does PET Melt Density Change the Shot Check?
Barrel ratings are usually quoted in grams of polystyrene, but your shot is PET. Ottmar Brandau gives PET melt density as about 1.15 g/cm³, against 0.945 for PS melt in Bozzelli's figures, so the same barrel volume holds about 22% more PET by weight. Work in volume to compare.
Brandau, writing in PETplanet Insider, also gives PET's solid amorphous density as 1.335 g/cm³ and about 1.3 at a demoulding temperature near 100 °C. The 13% gap between melt and demoulded preform is why he sets the transition to hold after about 87% of the screw's travel.
Bozzelli adds about 10% to the calculated shot volume to allow for check-valve leakage and melt uniformity, then sizes the barrel so the shot uses no more than 65%. The table and the worked example both use that allowance.
How Much of the Barrel Should a PET Shot Use?
Bozzelli warns that shots below about 20% of barrel capacity sit too long and degrade, while shots above 65% bring unmelt, uneven melt and slow recovery. PET loses IV when its melt is held hot too long, so the low end of that window deserves extra care with preforms.
Rick Unterlander and Ottmar Brandau's preform guide in PETplanet Insider puts PET melt at about 280 °C, plus or minus 10 °C, and typical back pressure at 15 to 20 bar. Reliance lists its Relpet G5761 still-water grade with a melting point of 247 ± 3 °C, so that melt runs about 30 °C above it.
The same guide asks for screw speed as low as possible while still recovering inside the cycle, with about 2 seconds of stopped time at the end of recovery. A screw working near the top of its capacity struggles to leave that margin, which is the other side of the window.
You can read the share on a running job without weighing anything. Divide the metering stroke by the screw's maximum stroke: a preform job metering 110 mm on a 150 mm stroke is using about 73% of the barrel.
Which Screw Suits a Mix of Preform Jobs?
List your regular preform shots in cm³, add 10%, and divide each by both swept volumes. Pick the screw that keeps the most shots between 20% and 65%. Where one screw cannot cover both your heavy and light preforms, a second barrel can, if planned at purchase.
Bozzelli notes that a second barrel ordered with a new machine can be low cost, and that a skilled operator can swap one in under 30 minutes, even hot. For a preform shop with one heavy and one light job, that can cost less than running either job outside its window.
Our position: for PET preforms, choose the screw that keeps your everyday shot nearest the middle of the window. Time in a hot barrel costs PET more than a few per cent of melt pressure, and the extra pressure of the smaller screw only earns its place where fill tests show the mould needs it.
What Should You Send to Settle the Screw Choice?
Send the preform weights and cavity counts for every mould you plan to run, the resin grade, the cycle times you expect and any future preforms already on the drawing board. Both screws can then be checked shot by shot. See the 100 ton PET injection moulding machine page.
Frequently Asked Questions About the 32 mm and 35 mm Screw for PET
How do I convert a barrel rating in PS grams to PET?
Divide the PS rating by 0.945 to get the swept volume in cm³, then multiply by 1.15 for PET melt. At full stroke that is about 1.22 times the PS figure. Then apply the 20% to 65% window and the 10% allowance to find the PET shots the barrel suits.
Does a PET screw differ from a general-purpose screw?
It can. PET melts at a high temperature and is sensitive to shear heating and time in the barrel, so screw design, check valve and barrel heating all matter. Ask whether the screw offered was designed and tested for PET, and what melt temperature it holds at your shot size.
Why does my preform weight drift from shot to shot?
Look at screw recovery first. If the screw barely finishes recovering before injection, the shot varies. A worn check valve lets melt slip back, which is one reason for the 10% allowance. A shot near the top of the barrel also melts less evenly, so check the share of capacity too.
What back pressure suits PET preforms?
The preform guide by Unterlander and Brandau gives 15 to 20 bar as a typical range. Back pressure should mix the melt well without adding excess shear heat, which can lower IV. Start in that range and adjust while watching melt temperature, recovery time and preform IV.
Can I change from a 32 mm to a 35 mm screw later?
Usually as a screw and barrel set, not a screw alone, because the barrel bore must match. Ordering the second set with the machine can cost less than buying it later. Before changing, recheck every mould's shot share, because light preforms may fall near the bottom of the larger barrel.
Related Articles
- What PET Products Can You Make on a 100 Ton Machine?
- Water Bottle Preforms on a 100 Ton Machine: What to Check First
- PET Moisture and Drying: Why It Decides Preform Quality
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, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, 16. IV Loss (Vol. 11, 11/2010)
- 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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