Screw options for PET on a 220 t machine, where one injection unit takes three diameters, come down to a trade between shot and pressure. The small screw gives the most injection pressure and the smallest shot. The large screw gives the biggest shot at the lowest pressure. The middle screw sits between them. Choose the one that keeps your usual preform shot inside the barrel's healthy range, because in PET a barrel that is too large raises acetaldehyde.
This article explains the trade between shot and pressure, why residence time matters more for PET than for most resins, and how to choose.
Key Takeaways
- On the same injection unit, swept volume rises with the square of screw diameter and injection pressure falls by the same factor.
- Acetaldehyde in PET rises linearly with residence time and exponentially with temperature, so an oversized barrel costs you taste in water bottles.
- Keep the shot between about 20% and 65% of barrel capacity, after adding about 10% for check-valve leakage and melt uniformity.
- Pick the screw for the preforms you will run most of the year, and keep throughput in kg/h below the screw maker's rating.
How Do the Three Screw Options for PET Differ?
They share the same hydraulic or electric drive, so the force behind the screw is fixed. A larger diameter spreads that force over more area: more shot volume, less melt pressure. A smaller diameter does the reverse. The middle screw is the usual compromise for a mixed preform range.
The scaling is plain geometry. Swept volume follows the screw area, which goes with diameter squared. Melt pressure follows the inverse. Going from 60 mm to 65 mm adds about 17% to swept volume and takes about 15% off the available pressure.
Screw options for PET on one injection unit: what changes with diameter
| Item | Small screw | Middle screw | Large screw |
|---|---|---|---|
| Shot volume | Smallest | Medium | Largest |
| Injection pressure available | Highest | Medium | Lowest |
| Residence time on a light preform | Shortest | Medium | Longest |
| Suits | Light preforms, thin walls, long flow | A mixed preform range | Heavy preforms and high cavity counts |
Why Does Residence Time Matter So Much in PET?
Because acetaldehyde, a by-product of heating PET, builds up the longer the melt sits hot. In O. Brandau's book extract in PETplanet Insider, acetaldehyde rises linearly with residence time and exponentially with temperature. A screw that is too large for the shot keeps melt in the barrel longer.
It matters most for still water. The same source notes that water reacts to small amounts of acetaldehyde, while sugar in soft drinks and juices masks it. It advises matching the extruder size to the preform weight, and notes that worn screws raise shear and acetaldehyde too.
Lightweighting makes this worse. Cycle time follows wall thickness, not weight, so a lighter preform with the same wall runs at almost the same cycle. Brandau gives an example where the lighter of two such preforms spends almost twice as long in the barrel.
Start from the resin certificate. Reliance's data sheet for Relpet G5761, a still water grade, gives an IV of 0.76 dl/g, plus or minus 0.02, and acetaldehyde of 1.0 ppm maximum in the chips. Anything above that in the preform came from your process.
How Do You Choose Between the Three Screws?
Size for the shot you will run most often. John Bozzelli, writing in Plastics Technology, puts the healthy range between 20% and 65% of barrel capacity. Below 20% brings long residence and degradation, above 65% brings melt-quality problems. He also warns against buying the biggest screw by default.
Bozzelli makes two further points worth weighing. Bigger screws usually mean lower plastic pressure, and a second barrel ordered with a new machine can cost little, with a skilled operator able to swap one in under 30 minutes. If your range spans light and heavy preforms, a second screw and barrel can beat one compromise.
Check throughput as well as shot. The PETplanet Rapid Guide to Perfect Preforms, in its section on unmelts, says processed kg/h must stay below the screw maker's rating. Above it, you must raise barrel temperatures or lengthen the cycle, and both work against low acetaldehyde.
Our view: pick the screw for the preforms you run most of the year, not for the heaviest preform you might one day quote.
When Is the Small or Large Screw the Better Choice?
Choose the small screw when your preforms are light or your mould needs high pressure to fill thin walls along a long flow path. Choose the large screw when shots are heavy or cavities many. In both cases, check that the shot stays within the barrel's healthy range.
A pressure shortfall shows up as short shots or weight variation at the far end of the preform, even with injection pressure at its maximum. Before you blame the mould, compare the pressure the mould needs with what the screw can deliver. A smaller screw on the same unit gives more.
A shot shortfall shows up differently: long recovery, unmelts or a cushion that drifts toward zero. That points to a shot near the top of the barrel's range, and a larger screw or a smaller cavity count fixes it. Settings alone will not.
What Should You Tell the Machine Supplier About Your Preforms?
List each preform you expect to run with its weight, wall thickness, cavity count and yearly volume, plus the resin grade. With that list, each screw's swept volume and pressure can be matched to your mix and a second barrel weighed up. See the 220 ton PET injection moulding machine page.
Frequently Asked Questions About Screw Options for PET
Can I use a general-purpose screw for PET preforms?
You can mould PET with it, but preform makers judge a screw by acetaldehyde, IV loss and unmelts. Ask the supplier which screw design is meant for PET, its rated throughput in kg/h, and its barrel and screw materials. Then test preform acetaldehyde on the first production runs.
Does the small screw always give better preforms?
No. It gives more pressure and a shorter residence on light preforms, but a heavy shot can push it past 65% of its capacity. Then you risk unmelts and long recovery. The right screw keeps your usual shot in range and recovers within the cycle.
How do I know if screw wear is raising acetaldehyde?
Track preform acetaldehyde on the same preform, resin lot and settings over time. A steady rise with nothing else changed points toward wear, since worn flights raise shear. Have the screw and barrel measured at planned shutdowns and keep the records for each machine.
What back pressure suits PET on a preform screw?
The PETplanet Rapid Guide, in its section on unmelts, gives back pressure of typically 15 to 20 bar. Start there and change one setting at a time. Higher back pressure and screw speed add shear heat, which helps melt the resin but also adds acetaldehyde, so test preforms after each change.
Is it worth buying a second screw and barrel?
It can be, if your preform weights vary widely. One barrel cannot keep both a light and a heavy shot within the healthy range. A spare set also covers wear. Weigh the cost against the reject rate and acetaldehyde risk of running one compromise screw.
Related Articles
- What PET Products Can You Make on a 220 Ton Machine?
- Higher-Cavity Preform Moulds on a 220 Ton Machine
- Hot Runner Preform Moulds: What the Machine Must Provide
Sources & References
- PETplanet Insider, O. Brandau: Bottles, Preforms and Closures, extract: 3.6 Acetaldehyde in PET Bottles
- Plastics Technology, J. Bozzelli: A Simpler Way to Calculate Shot Size vs. Barrel Capacity
- PETplanet Insider, R. Unterlander and O. Brandau: The Rapid Guide to Perfect Preforms, part 32: Unmelts
- Reliance Industries: Relpet G5761 typical properties
Technical Disclaimer
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