PET screw design is the part of a preform project that gets the least attention during machine selection and causes the most trouble afterwards. The screw and barrel are treated as a commodity item inside a machine that is bought on tonnage and cycle time, yet they determine whether the melt arrives at the hot runner with the right temperature uniformity, the right intrinsic viscosity and an acceptable acetaldehyde level. Get the screw wrong and every other investment in the line is working against a defect that cannot be fixed downstream.
The failure is quiet. Nobody sees a screw problem on the factory floor the way they see a short shot or a flash. What they see is a gradual rise in acetaldehyde complaints on still water, a creeping loss of top load strength in lightweight bottles, inconsistent preform weight from cavity to cavity, and black specks that appear suddenly after months of clean running. By then the plant is chasing the symptom at the mould, the hot runner or the cooling stage — the three places the defect is not.
Sailwin builds injection molding machines for preform production on a dedicated PET screw and barrel configuration with far-infrared nano heating coils, across a SW-P range of 14 models from 170 kN to 5,500 kN clamping force, with support for 64-cavity valve-gate hot runners and EUROMAP 67 robot interface. Machines are manufactured under ISO 9001:2015 with CE marking, with Siemens or Mitsubishi PLC control and Schneider electrical components. This guide explains what makes a PET screw different from a general-purpose screw, which design variables decide melt quality, and how to tell when a screw is the cause of a quality problem.
Key Takeaways
- PET is processed in a narrow melt window, and the screw defines how tightly that window can be held. A general-purpose screw built for polyolefins generates more shear heat than PET tolerates.
- Acetaldehyde and IV drop are both consequences of excessive thermal and shear history. They are decided in the screw, not corrected at the mould.
- Corrosion protection is a lifetime cost decision. PET processing produces acidic by-products that attack unprotected steel; the barrel that looks cheapest at purchase is often the most expensive over ten years.
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1. Why a General-Purpose Screw Is the Wrong Tool for PET
The default screw shipped with most general injection molding machines is designed for a broad family of commodity polymers — polypropylene, polyethylene, polystyrene, ABS. These materials tolerate a wide melt range, they are not especially sensitive to hydrolytic degradation, and they do not produce aggressive by-products at processing temperature. A screw optimised for them favours high output and thorough mixing, which usually means a relatively deep feed section, a moderately high compression ratio and aggressive shear in the transition zone.
PET is a different proposition on all three counts. It must be dried to a low moisture level before processing, because moisture drives hydrolysis and destroys molecular weight. It has a comparatively narrow processing window. And it degrades through two separate mechanisms that both worsen with heat and shear exposure: thermal degradation that reduces intrinsic viscosity, and acetaldehyde generation that affects taste and odour in the finished pack. Every additional unit of shear energy the screw puts into the melt is energy the plant has to manage somewhere else.
| Requirement | General-purpose screw | Dedicated PET screw |
|---|---|---|
| Shear input | High shear used deliberately to achieve mixing and output on shear-tolerant polymers | Shear kept as low as melt quality allows, with gentler transitions and controlled compression |
| Melt temperature stability | Wide window acceptable; small fluctuations tolerated by the material | Uniform melt essential; temperature spread across the shot directly affects preform weight and AA |
| Residence time | Long residence time tolerable in most commodity materials | Must be minimised; long dwell at temperature accelerates both IV loss and AA build-up |
| Surface condition | Standard nitrided or bimetallic surface, adequate for neutral polymers | Corrosion-resistant surface required because processing by-products are acidic |
2. The Design Variables That Decide Melt Quality
A screw specification is usually described with a handful of numbers, and each one has a direct consequence for preform quality. The ranges below represent typical industry practice for PET processing; the optimum for a specific project depends on resin grade, shot weight, machine size and cycle time.
| Variable | Typical PET practice | What it controls | Symptom if wrong |
|---|---|---|---|
| Compression ratio | Lower than for polyolefins, in a narrow band that favours gentle compression over aggressive working of the melt | How much the material is worked and how much shear heat is generated in the transition zone | Too high: AA rise and IV loss. Too low: unmelted particles and weight variation between shots |
| Length-to-diameter ratio | Longer than the shortest available option, to allow melting at lower screw speed rather than by shear intensity | Melting capacity per revolution; effectively the trade between screw speed and shear energy | Too short: forced to run high screw speed, which raises shear heat and AA |
| Feed and metering depth | Set against shot weight so that residence time stays short relative to the cycle | Volume of material held in the barrel and therefore how long it stays hot | Oversized barrel on a small shot: long residence time, yellowing, AA build-up |
| Screw tip and non-return valve | Low-shear, streamlined design without dead zones where material can stagnate | Shot repeatability and cleanliness; stagnation zones are where black specks originate | Weight drift between shots, and black specks appearing after clean running |
| Barrel heating | Zoned control providing uniform temperature along the barrel rather than a hot spot near the feed throat | The thermal history of the melt before it ever reaches the screw tip | Localised overheating that raises AA even when average temperature looks correct |
Sailwin preform machines use far-infrared nano heating coils on the barrel, largely because uniform heating reduces the need to compensate with screw work. The principle is worth stating plainly: heat added through the barrel wall is controllable and evenly distributed; heat generated by shear inside the melt is neither. A screw designed to melt primarily through barrel heat rather than through mechanical working gives the operator a process that responds to setpoint changes in a predictable way.
Every degree of heat and every unit of shear the melt experiences is cumulative. The screw decides how much of the total budget is spent before the hot runner, the nozzle and the gate take their share.
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3. Shear, Intrinsic Viscosity and Acetaldehyde
Two quality parameters dominate PET preform production, and both are decided substantially by the screw.
Intrinsic viscosity
Intrinsic viscosity is a measure of the polymer’s molecular weight, and it falls when PET chains are broken. Two mechanisms break them: hydrolysis, driven by moisture in the resin, and thermal-mechanical degradation, driven by heat and shear. The drying system addresses the first. The screw addresses the second. A machine running a screw profile that is too aggressive will show a measurable IV drop between virgin resin and the finished preform, and that loss carries directly into the bottle: lower IV means less strain hardening during blow molding, which means lower top load strength and less resistance to creep in storage.
This is why a screw decision is also a bottle-strength decision. A plant that has accepted a slight IV loss to gain cycle time has effectively reduced the performance of every bottle it will make on that machine, and it will discover the consequence months later in a stack-load complaint rather than on the production floor.
Acetaldehyde
Acetaldehyde is a degradation by-product that alters taste and odour, and it is most critical in still water and other neutral-tasting products where there is nothing to mask it. AA forms when the melt is exposed to a combination of elevated temperature and extended residence time. A screw that melts by shear generates local hot spots well above the average melt temperature; those spots are where AA is created. So is any dead zone in the screw tip or non-return valve where a small volume of material sits hot across many cycles.
The practical consequence is that AA problems are usually not solved by lowering the barrel setpoint. If the screw is generating the heat, reducing barrel temperature simply shifts the burden further onto mechanical working. The correction has to happen in the screw geometry, the screw speed, the back pressure and the residence time — and it has to be verified on the finished preform rather than on the machine display. Sailwin machines monitor 40+ parameters in real time through the PLC, which makes it possible to see whether a change in AA correlates with a change in screw speed, back pressure or cycle time rather than guessing.
4. Wear, Corrosion and the Lifetime Cost of the Barrel
PET processing is chemically harder on a barrel than most commodity polymers. At processing temperature the material and its by-products are corrosive to ordinary nitrided steel, and glass-fibre or other filled grades are abrasive as well. The result is a wear pattern that develops slowly and then accelerates, and the symptoms are easy to attribute to the wrong cause.
- Increasing shot-to-shot weight variation. As the screw and barrel clearance opens through wear, the non-return valve no longer seals cleanly and material slips backwards during injection. Operators usually respond by raising hold pressure, which masks the symptom and adds stress to the mould.
- Rising AA on an unchanged process. A worn screw has to be run faster to achieve the same output, which increases shear and AA. A plant that has periodically tightened cycle time may find that AA and cycle time are drifting together for this reason.
- Black specks and discolouration. Corrosion pits and worn surfaces create stagnation points where material degrades over many cycles and is then released into the melt stream in bursts.
- Falling output at constant screw speed. Reduced pumping efficiency from increased clearance is the clearest single indicator that the screw and barrel are due for inspection.
Because these symptoms appear gradually, the useful discipline is to record a baseline when the machine is new and compare against it periodically. Preform weight, screw recovery time, melt temperature at a fixed setpoint and AA on a standard sample form a four-point record that will show wear before it becomes a quality incident. Sailwin machines are supplied with full-load FAT testing before shipment and commissioned on site over 3–7 days, which is the right point to capture that baseline, and common wear parts are dispatched within 48 hours with 7×24 remote support available for diagnosis.
5. Case Study: Weight Variation Traced to the Screw, Not the Mould
A preform producer running a high-cavity hot runner was investigating preform weight variation that appeared across all cavities at once, not in a cavity-specific pattern.
- Preform weight drifting across every cavity simultaneously rather than in a cavity-specific pattern
- Investigation initially directed at the hot runner and mould, where the symptom was visible but the cause was not
- Hold pressure gradually increased over months to compensate, adding stress and masking the real trend
- Uniform variation across all cavities identified as a melt-delivery problem, not a mould problem
- Screw recovery time recorded against the machine’s baseline to test for reduced pumping efficiency
- Screw and non-return valve inspected for clearance and stagnation points rather than replaced speculatively
- A four-point baseline record introduced — preform weight, recovery time, melt temperature and AA on a standard sample
- The mould was cleared without modification, avoiding an unnecessary tool intervention and the production loss that would have come with it
- Hold pressure returned to the original setting once melt delivery was corrected, removing stress that had been accumulating on the tool
- Wear became measurable rather than detectable, because the four-point baseline shows a trend before it becomes a quality incident
Scenario based on a Sailwin customer project; site-specific figures available on request during engineering review.
Frequently Asked Questions
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Related Reading:
• Injection Molding Machines — SW-P Series, 170–5,500 kN
• Selecting an Injection Molding Machine for PET Preforms
• Reducing Acetaldehyde in PET Preforms
• 64-Cavity Valve-Gate Hot Runner Preform Moulds
• Preform Cooling Time: How to Set It Correctly
• PET Blow Molding Machines for the Blowing Stage
