PET Resin Drying: Temperature, Time and Dew Point

PET resin drying in injection molding is treated in most plants as a service function: the dryer runs, the hopper is full, the preform machine runs. The problem with that view is where the damage actually happens. By the time molten PET reaches the screw, moisture has already reacted with the polymer, and the molecular weight loss is permanent. The barrel is not where hydrolysis starts. It is where hydrolysis finishes.

This matters commercially because the symptoms appear in places nobody connects to the dryer. Preforms that are marginally brittle, thread profiles that crack under capping torque, acetaldehyde levels that drift upward, and a stretch blow process that needs hotter settings to achieve acceptable wall distribution — all of these can trace back to residual moisture in granulate that was loaded a shift earlier.

Sailwin has delivered injection moulding machines into 500+ installations across 60+ countries over 15+ years, manufacturing under ISO 9001:2015 with CE marking. Machines are FAT tested at full load before shipment, installation and commissioning on site takes 3–7 days, common wear parts ship within 48 hours, remote support runs 7×24, and the machine warranty is 2 years. This guide covers drying parameters for preform moulding, what the injection machine needs from correctly dried melt, how to separate dryer faults from barrel faults, and how to verify performance from the moulding side rather than from a setpoint display.

1. Hydrolysis Starts in the Dryer, Not the Barrel

PET is hygroscopic: granulate absorbs atmospheric moisture during storage, conveying and any period a hopper sits partly empty. That water is dissolved in the polymer, not sitting on the surface, so it cannot be blown off. When the material is heated above its melting point, water molecules attack the ester linkages in the polymer chain. Each attack shortens a chain, reducing molecular weight and intrinsic viscosity.

The chemistry is what makes drying non-negotiable. The reaction is fast at melt temperature, and it is irreversible — there is no downstream step that restores the chain length. A preform moulded from under-dried resin is a structurally different part from one moulded from correctly dried resin, even though both come off the same machine with the same settings and look identical on the visual inspection bench.

There is a second consequence that traps operators. Degraded PET flows differently. Viscosity is lower, so fill behaviour changes, and the intuitive response is to raise melt temperature to restore flow. That accelerates hydrolysis further and pushes acetaldehyde formation up, so the process spirals: hotter melt, more degradation, more AA, and a stretch blow department compensating with its own temperature changes. Reconciling the two departments usually ends with somebody finally measuring the granulate.

2. Drying Parameters for Preform Moulding

Three parameters work together: temperature, residence time and process air dew point. Setting two of the three correctly still produces under-dried material. The figures below are industry-typical starting points, not a Sailwin specification — the resin supplier’s data sheet governs, and thick-walled or high-IV grades are usually dried tighter and longer.

One practical point about thick-walled preforms, which are common for carbonated and large-format containers: thicker sections need longer to reach equilibrium, and their centre is the last place to dry. A setting that produces acceptable thin preforms can leave the centre of a heavy preform wet, which is why the defect appears on one product family and not another on the same machine.

3. What the Injection Machine Needs From Correctly Dried Melt

Drying is upstream, but the injection machine’s design assumes it has been done. PET is a demanding material to plasticise because of its narrow processing window and its sensitivity to shear and residence time. Several design and setting choices matter more than others.

• A screw designed for PET, not a general-purpose screw. PET needs a compression ratio and geometry chosen for its viscosity behaviour, so that melting is complete without excessive shear heating. Sailwin preform machines use a dedicated PET screw geometry for this reason.
• Even barrel heating. Far-infrared nano heating coils on Sailwin PET barrels are used to keep zone temperatures stable and repeatable, which matters because melt temperature offsets are one of the easiest ways to push acetaldehyde up.
• Short, controlled residence time. Melt that sits in the barrel degrades regardless of how well it was dried. Barrel capacity should be matched to shot size so the melt is neither starved nor over-resident, and cycle time must stay within the grade’s window.
• Consistent decompression and back pressure. Both affect shot-to-shot repeatability. If shot weight scatters, wall distribution scatters with it, and the stretch blow department pays for it in heating adjustments.
• A balanced hot runner. Valve-gate hot runner systems — Sailwin supports up to 64 cavities — must be balanced thermally and hydraulically, or individual cavities fill slightly differently. Drying problems and hot runner imbalance both produce cavity-to-cavity variation, which is why the two get confused.
• Automation that keeps the cycle identical. EUROMAP 67 robot interface on Sailwin machines allows consistent part removal and cooling, removing operator variation from the cycle. Cycle consistency is what makes a drying problem diagnosable, because it removes one moving variable.

4. Separating Dryer Faults From Barrel Faults

The two produce overlapping symptoms, and the fastest way to tell them apart is to look for the pattern rather than the defect. Moisture does not know where the cavities are.

Fifteen minutes of measurement saves days of tuning. Before adjusting a barrel profile, take two readings: process air dew point at the dryer, and granulate temperature at the machine feed throat. Then compare a suspected preform’s intrinsic viscosity against a reference preform produced from verified-dry resin.

If the dew point is out of specification, stop there. Nothing downstream can be tuned into correctness while the material arriving at the screw is wet.

5. Verifying Dryer Performance From the Moulding Side

Dew point and hopper temperature tell you what the dryer is doing. They do not prove what the moulded part received, because material can regain moisture in conveying or sit too long in the hopper. Four checks close that gap.

• Intrinsic viscosity on finished preforms. This is the most direct evidence. Compare a suspect preform against a reference produced from verified-dry resin of the same grade. A measurable drop indicates hydrolysis, and therefore a drying failure, regardless of what the dryer display shows.
• Acetaldehyde testing. AA rises with thermal history. If AA is trending up while melt settings are unchanged, look at whether drying has drifted and melt temperature has been raised to compensate.
• Shot weight consistency. Under-dried resin flows differently, so shot-to-shot weight scatter often increases before visual defects appear. A weight series is a cheap early indicator that the melt has changed.
• Residual moisture on the granulate. Sample at the machine feed throat, not the dryer outlet. Comparing the two readings locates losses in the transfer path — a blocked filter, a leaking hopper seal or humid conveying air.
• Thread and neck performance. Neck finish dimensions and capping-torque behaviour are sensitive to material condition. A preform that cracks at the thread under standard capping torque is a strong indicator of embrittlement rather than a mould problem.
• Downstream blow settings. If the stretch blow process needs progressively hotter preform heating to achieve acceptable wall distribution, that is a signal from the next department about material quality, and it usually predates any complaint about the preforms themselves.

6. Drying Discipline Through Grade and Colour Changes

Changeovers are where drying control is most often lost, because the dryer is treated as common equipment while the parameters belong to the material. A grade change without a drying change is a quality escape waiting to happen.

• Purge the hopper, do not top it up. Topping up a hopper of the old grade with a new one delivers a blend whose drying history is unknown. Empty and refill.
• Check drying parameters before the first shot. Different grades and IV levels have different drying windows. Add the drying row to the changeover checklist so it is signed off with the mould and barrel settings.
• Re-baseline after any supply change. Even the same grade from a different lot can behave differently. Take a residual moisture reading and a preform IV reference at the start of each new lot, then hold that as the baseline.
• Handle masterbatch and additives deliberately. Colour and additive masterbatches have their own drying requirements and are often lower in proportion, so a small amount of undried masterbatch can introduce moisture into an otherwise dry melt. Follow the masterbatch supplier’s instructions rather than assuming the base resin setting covers it.
• Plan stoppages. Short stops, weekend shutdowns and maintenance windows leave material in the hopper. Define how long material may remain before it must be purged, and write the rule where operators will find it.
• Maintain the dryer as equipment, not as furniture. Filters, desiccant, gaskets and heater elements have finite service lives. Schedule their replacement from the dew point trend so the work happens before a quality event rather than after one.

7. Where a Sailwin Injection Machine Fits the Process

These are Sailwin’s confirmed specifications for PET preform injection moulding machines.

• Model range. The SW-P series covers 14 models from 170 kN to 5,500 kN clamping force, which spans small single-cavity preform tools through to high-cavity production moulds.
• High-cavity capability. Support for 64-cavity valve-gate hot runner systems, with the thermal and hydraulic balance that high-cavity preform production demands.
• PET-specific plasticising. A dedicated PET screw design combined with far-infrared nano heating coils, aimed at complete melting with controlled thermal history rather than aggressive shear heating.
• Automation interface. EUROMAP 67 robot interface for consistent part removal, so the cycle does not vary with operator technique.
• Control and monitoring. Siemens or Mitsubishi PLC, Schneider electrical and ABB components, with PID control holding ±1 °C and 40+ parameters monitored in real time. Temperature stability is what allows a drying problem to be detected at all, because it removes thermal noise from the process record.
• Delivery and support. Machines are FAT tested at full load before shipment, standard delivery is 30–45 days (45–60 days for custom builds), installation and commissioning takes 3–7 days, common wear parts ship within 48 hours, remote support is available 7×24, and the machine warranty is 2 years.

8. Case Study: A Cavity-Specific Defect That Was Not the Mould

A preform producer was rejecting parts from one side of a high-cavity tool for hazy patches, and had already stripped and cleaned the tool twice without improvement. Visual defects were attributed to hot runner imbalance, and the drying system had not been examined for several months.

Composite scenario from preform injection process support work, with no customer-identifying detail. Drying parameters, IV references and acceptance limits are confirmed against each resin supplier’s data sheet.

9. Frequently Asked Questions About PET Resin Drying