In the plastic recycling industry, controlling moisture during the washing of film and woven bags is critical for producing high-quality pellets. Effective drying not only protects the integrity of the final product but also optimizes the entire recycling operation. This guide explains practical techniques and technologies for reaching target moisture levels, the equipment choices behind them, and how to keep a drying stage running reliably.
Why Moisture Control Decides Pellet Quality
Moisture content directly impacts the quality of recycled plastic pellets. Excess moisture causes bubbling during extrusion, poor pellet formation, and reduced material strength. Industry practice targets moisture below 7% after washing, but pushing below 3% noticeably improves granulation quality, pellet density, and end-product performance.
There is also an energy angle. Every kilogram of water carried into the extruder must be evaporated by the extruder’s own heaters, which wastes energy and forces lower screw speeds. Removing that water in a dedicated drying stage — where mechanical squeezing does most of the work — is far cheaper per kilogram than boiling it off inside the extruder barrel.
The reason is straightforward: water trapped in or on the film turns to steam inside the extruder. Steam creates voids in the melt, which show up as bubbles, rough pellet surfaces, and inconsistent sizing. Dryer feedstock also runs more stably, with fewer surges and less die drool, so the pelletizer can hold tighter tolerances.
Mechanical Dewatering Options for Film Lines
Before any thermal stage, mechanical dewatering removes the bulk of surface water at a fraction of the energy cost. Two approaches dominate film and woven-bag lines:
- Centrifugal dryers – High-speed rotation flings surface water off the material. Well-tuned units typically bring film down to 5–7% moisture. They are simple, robust, and handle high throughputs, but they leave the material short of the sub-3% levels premium pelletizing wants.
- Screw-press / squeeze dryers – A screw conveys the film while mechanical pressure squeezes water out through drainage ports. This achieves deeper dewatering in a single stage and doubles as a densifying step, which improves feeding into the extruder.
Many PP/PE film washing lines combine both: a centrifugal dryer for bulk water removal followed by a squeeze extrusion dryer for the final push below 3%. For woven bag washing lines, the squeeze dryer is especially valuable because woven PP holds water in its weave structure where centrifuges struggle.
How the Integrated Squeeze Extrusion Dryer Works
ASG’s integrated extrusion dryer for film and woven bags is engineered around the way these materials behave: light, bulky, and prone to wrapping. After initial crushing and cleaning, material is fed evenly into the machine, where a screw applies consistent pressure and forces water out through drainage ports in the barrel. A heating zone at the discharge then plasticizes the film surface, flashing off residual moisture, while the pelletizing unit cuts the output to size.
Key Features and Benefits
- Screw-type squeezing – Built-in barrel drainage holes let pressed-out water escape continuously instead of being reabsorbed.
- Discharge heating – Integrated heating at the discharge section instantly plasticizes the film, driving off the last residual moisture.
- Frequency-controlled pelletizing – Cutting speed adjusts with pressure, producing uniform agglomerates sized for stable extruder feeding.
- Moisture down to 3% – Reliably reaches levels centrifugal-only setups cannot, with lower energy use than full thermal drying.
The technology suits LDPE film, LLDPE stretch film, PP woven bags, and PP ton bags, and it integrates into film shredding and densifying lines as well as complete washing plants.
Measuring and Monitoring Moisture
You cannot control what you do not measure. Practical moisture monitoring for a film washing line includes:
- Discharge sampling – Take grab samples at the dryer outlet and check them with a halogen moisture analyzer. Test at startup, after material changes, and at least once per shift.
- Inline sensors – Microwave or NIR sensors on the dryer discharge give continuous readings and can feed back into screw speed or heating control.
- Pelletizer feedback – Bubbles, rough pellet surfaces, or surging at the die are lagging indicators that moisture has crept up. Treat them as alarms, not as normal variation.
Set a hard ceiling (for example 5%) that triggers an immediate check, and a target (below 3%) for normal operation. Log readings against material type and throughput so drifts become visible before they affect quality.
Maintenance Tips
Drying performance degrades gradually, which makes scheduled maintenance the cheapest insurance:
- Keep drainage ports clear – Fines and labels can blind the barrel drainage holes. Inspect and clean them on a fixed schedule; blocked ports are the most common cause of rising moisture.
- Watch screw and barrel wear – A worn screw loses compression, and dewatering efficiency falls with it. Check clearances during planned stops.
- Verify heating elements – Failed heater bands at the discharge leave residual moisture in place. A quick current check per shift catches failures early.
- Match feed rate to capacity – Overfeeding is the fastest way to push moisture up. Keep the feed steady and within the dryer’s rated throughput.
- Tailor settings to the material – Wet agricultural film and dry post-industrial offcuts need different screw speeds and temperatures. Record proven settings per material for fast changeovers.
Frequently Asked Questions
What is the ideal moisture content for recycled plastic pellets?
Below 7% is generally acceptable after washing, but getting below 3% significantly improves pellet density, surface finish, and extrusion stability.
How does a squeeze extrusion dryer differ from a centrifugal dryer?
A centrifugal dryer uses high-speed rotation to remove surface water, typically reaching 5–7% moisture. A squeeze extrusion dryer adds mechanical screw pressure plus discharge heating, reliably reaching as low as 3%.
Which materials can a film extrusion dryer handle?
It is designed for LDPE film, LLDPE stretch film, PP woven bags, and PP ton bags — the light, bulky materials that are hardest to dewater by rotation alone.
Where should the dryer sit in the washing line?
After washing and rinsing, before pelletizing. The film should be crushed and cleaned first so the dryer works on contaminant-free material and the drainage ports stay clear.
How do you measure moisture in washed film?
Take discharge samples and test with a halogen moisture analyzer, or install an inline microwave/NIR sensor for continuous readings tied into the line controls.
Why is my dried film still coming out too wet?
The usual causes are overfeeding beyond rated throughput, clogged barrel drainage ports, a worn screw losing compression, or failed heater bands at the discharge. Check them in that order.
Does lower moisture really improve pelletizing?
Yes. Drier feedstock means less steam in the melt, fewer bubbles, denser and more uniform pellets, and a more stable extrusion process with less downtime.
What maintenance does a squeeze dryer need?
Regular inspection of the screw, heating elements, and drainage ports. Keep ports clear of fines, verify heater current, and check screw-to-barrel clearance during planned stops.
Can a squeeze dryer be added to an existing washing line?
Yes. It is designed for integration into existing lines — the main requirement is matching its throughput to the upstream washing capacity so it is neither starved nor overfed.
Conclusion
Mastering moisture control in film and woven-bag washing comes down to three things: removing bulk water mechanically, finishing with a squeeze extrusion stage to break below 3%, and measuring continuously so drifts are caught early. Plants that treat drying as a controlled process — rather than a black box before the extruder — see better pellet quality, steadier output, and fewer rejected batches.
To discuss drying equipment for your material and throughput, contact the ASG team or explore our film washing line solutions.