PET has to be dried below about 50 ppm of water before it is melted. Otherwise moisture breaks the polymer chains and lowers the intrinsic viscosity (IV) of every preform. PET moisture and drying decide preform quality because the damage is invisible at the machine: it shows up later as weaker bottles and a process that drifts. In practice that means dry air at a dew point of -40 °C or lower, pellets at 149 to 166 °C, and four to six hours in the hopper.
Those figures come from a resin maker's drying guidance and a preform troubleshooting guide, both cited below. Where your own resin supplier's data sheet differs, follow the data sheet.
Key Takeaways
- Dry PET below 50 ppm of water; an IV drop of more than 0.04 dl/g between resin and preform signals a drying or melt problem.
- Keep the drying air at a dew point of -40 °C or lower and the pellets at 149 to 166 °C at the dryer exit.
- Plan four hours of hopper residence as the minimum and six as the preferred figure, with the hopper kept full.
- Bagged resin arrives far wetter than the moulding target, so no lot goes to the machine undried.
Why Does Moisture Damage PET Preforms?
PET is hygroscopic: it absorbs water from humid air into the whole pellet, not only the surface. When wet PET melts, the water breaks polymer chains by hydrolysis, and molecular weight, process control and mechanical properties all fall. The preform can look normal and still be weaker.
Alpek Polyester describes this in its drying guidance for bottle-grade PET: moisture diffuses through the pellet and is held by molecular attraction, so it must be driven out with heat and dry air before processing. Skipping that step leads to hydrolytic degradation in the melt.
The loss is measured as IV. Rick Unterlander and Ottmar Brandau, in their preform guide for PETplanet Insider, treat a drop of more than 0.04 dl/g between resin and preform as a fault. It cannot be seen on the preform; it needs a lab IV tester or melt viscometer.
Indian resin shows the size of the job. Reliance lists its Relpet G5761 still-water grade with moisture below 0.2% when packed. That is up to 2,000 ppm, or 40 times the 50 ppm moulding target, so even a fresh bag needs full drying.
What Conditions Does PET Moisture and Drying Need?
Dehumidified air at a dew point of -40 °C or lower, inlet air up to 171 °C, pellets reaching 149 to 166 °C at the dryer exit, and at least four hours in the hopper, preferably six. Those figures come from Alpek Polyester's guidance for its bottle-grade PET.
PET drying settings from two published sources
| Setting | Alpek Polyester drying guidance | Unterlander and Brandau preform guide |
|---|---|---|
| Dew point of drying air | Not above -37 °C; preferably -40 °C or lower | Above -40 °C must be corrected |
| Air temperature into hopper | Up to 171 °C; absolute maximum 180 °C | Not stated |
| Pellet temperature | 149 to 166 °C at the dryer exit | Not stated |
| Time in hopper | At least 4 h, preferably 6 h | About 4 h at 175 °C, up to 6 h at 160 °C |
| Dry airflow | About 28.3 L/min per 0.45 kg/h of PET, minimum | Not stated |
| Moisture target | Not stated on the sheet | Below 50 ppm |
| Long stoppage | Drop air to 116 °C or lower, keep dry air flowing | Not stated |
The two sources differ a little at the edges, which is normal; your resin's own data sheet settles the numbers. Alpek sets the 180 °C ceiling to prevent discoloration, so extra heat is not a shortcut to drier resin. Longer high-temperature exposure, it adds, can also upset processing.
Dew point is the setting that slips without anyone noticing. A dryer can show the right air temperature while its desiccant is saturated, so check that regeneration temperatures and frequency are correct, as Alpek advises.
How Do You Size a Dryer for a 100 t Preform Machine?
Start from the resin the machine uses each hour. Multiply it by the residence time to get the hopper's working capacity, and use Alpek's airflow figure for the minimum dry air. A machine using 25 kg of PET an hour needs about 150 kg in the hopper at six hours.
Alpek defines residence time as dryer capacity divided by throughput. That holds only while the hopper stays full: let it run half empty and the resin spends half the time drying. A level alarm on the hopper is a cheap way to protect that drying time.
Moisture can also get back in after drying. The Unterlander and Brandau guide lists silos, the dryer hopper cone and the resin feed line among the places to check when IV drops.
Energy matters too. The ExxonMobil, AMI and Tangram energy guide for moulders puts drying at up to 15% of process energy, so an insulated, correctly sized dryer pays back in power as well as in preform quality.
What Should Happen During Stoppages and Start-Up?
During a long stop, keep dry air flowing but lower its temperature, which Alpek puts at 116 °C or below. At start-up, check the dryer for alarms and confirm the dew point before the first shot. Purge resin that sat in the throat or barrel while the line was down.
Our view: buy the dryer together with the machine, size it for the heaviest preform job you plan, and fit a dew point reading where operators see it every shift. A preform line without a dew point reading is guessing at quality it cannot see.
What Should You Check Before the First PET Preform Run?
Send the preform weights, cavity counts and cycle times for each mould, and the resin grades you plan to run. From those, the hourly resin use, hopper size and airflow can be worked out alongside the machine. See the 100 ton PET injection moulding machine page.
Frequently Asked Questions About PET Moisture and Drying
Can I dry PET in a hot-air oven instead of a dehumidifying dryer?
Not for preforms. Hot shop air carries its own moisture, so it cannot reach the -40 °C dew point the drying guidance asks for. Heating PET in humid air removes some surface water but leaves the pellet far above the 50 ppm target, and the IV loss appears in the preforms.
How do I know the resin is dry enough?
Measure it. A moisture analyser on samples from the hopper outlet shows if the resin is below 50 ppm, and an IV test on preforms against the resin's certificate shows whether drying and melt conditions together are holding. Run both checks at start-up and after any dryer alarm.
Does recycled PET need different drying?
It needs the same target, often with more care. Recycled flakes vary in size, moisture and history from lot to lot, and amorphous material can stick if heated before it is crystallised. Ask the supplier for drying conditions and test moisture on each lot rather than assuming virgin settings will do.
What happens if PET is dried too hot or too long?
Alpek caps the air at 180 °C to prevent discoloration and warns that long periods at high temperature can upset processing. If the line stops for a long time, it advises dropping the air to 116 °C or lower while keeping dry air flowing, rather than holding full drying heat.
Why does the dryer hopper cone or feed line matter?
Dry resin can pick up moisture again on its way to the screw. The Unterlander and Brandau guide lists the hopper cone, the resin feed line and storage silos among the parts to inspect when preform IV drops, because a leak or damp air there undoes good drying upstream.
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
- Choosing Between the 32 mm and 35 mm Screw for PET on a 100 Ton Machine
Sources & References
- Alpek Polyester: Laser+ C (F65A) Product Information, Material Drying (PDS-F65A 230601MO)
- 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)
- ExxonMobil, AMI and Tangram Technology: An Energy Saving Guide for Injection Moulders
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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