In the plastic recycling industry, moisture is the silent killer of pellet quality. Whether you are recycling post-consumer LDPE films, PET bottles, or PP woven bags, failing to hit the correct moisture target before extrusion can lead to catastrophic results: porous pellets, hydrolyzed polymer chains, and a significant loss in market value.
Choosing the right drying system is not about finding the “best” machine, but finding the right combination for your specific material and moisture goals.
This guide compares the three industry standards—Centrifugal Dryers, Squeezers, and Thermal Dryers—to help you engineer the perfect drying line.
Why Moisture Control is Non-Negotiable
Before diving into the machinery, it is crucial to understand the “why.” Excess moisture in the extruder barrel leads to:
- Surface Defects: Bubbles and foaming in the final pellets.
- Structural Weakness: Hydrolytic degradation (especially in PET), which breaks down polymer chains and reduces viscosity.
- Process Instability: Steam generation that disrupts material feeding.
Generally, rigid plastics require moisture levels below 1% (or lower for PET), while films can sometimes tolerate slightly higher levels depending on the extruder’s degassing capabilities, though 3-5% is the standard target for film densification.
1. Centrifugal Dryers: The High-Speed Workhorse
Best For: Rigid flakes (PET, HDPE, PP) and initial dewatering of films.
Centrifugal drying is a mechanical process. It uses high-speed rotation (centrifugal force) to throw water off the surface of the plastic. It is typically the first step after the washing line.
How It Works
Wet material enters a spinning rotor surrounded by a screen. Paddles impact the plastic, forcing water out through the screen mesh while conveying the plastic forward.
Pros & Cons
- Pros: Extremely energy-efficient compared to thermal methods; excellent for removing surface water from rigid flakes.
- Cons: Cannot remove bound moisture or dry effectively inside the folds of thin films on its own.
Expert Insight
For film recycling lines, a standard centrifuge often leaves too much moisture (10-15%). However, advanced models like the High-Speed Plastic Film Centrifugal Dewatering Machine are specifically engineered with optimized paddle geometry to handle the difficult aerodynamics of light films, significantly reducing the load on downstream drying equipment.
2. Plastic Squeezers: The Game Changer for Films
Best For: Soft plastics (LDPE/LLDPE films, PP woven bags, PP raffia).
When dealing with washed films, water gets trapped inside the folds and surface tension makes it hard to remove. A “Squeezer” (or Squeezing Dryer) is a mechanical dewatering solution that uses high pressure and friction.
How It Works
A specialized screw compresses the wet film flakes against a perforated barrel / hydraulic gate. The immense pressure physically squeezes water out. Furthermore, the friction generates heat, causing the plastic to semi-plasticize (sinter), which evaporates residual moisture.
Pros & Cons
- Pros: Drastically reduces moisture from ~30% down to 3-5% in a single pass; increases bulk density of the material (making it easier to feed into the extruder).
- Cons: Not suitable for rigid plastics; higher power consumption than a simple centrifuge (but often lower than thermal drying for the same result).
Expert Insight
For film recyclers, the Plastic Film Squeezing Machine is often the most ROI-positive investment. It effectively replaces the need for a separate agglomerator and thermal drying line, delivering material that is ready for the extruder immediately.
3. Thermal Dryers: The Final Polish
Best For: Achieving precise, ultra-low moisture targets (PET flakes, final stage for films).
When mechanical methods (spinning or squeezing) aren’t enough, you need thermodynamics. Thermal dryers (often called hot air pipe dryers or spiral dryers) use heated air to evaporate the remaining surface moisture.
How It Works
Plastic flakes are suspended in a stream of hot air within a long, coiled pipeline. The turbulence ensures maximum contact between the hot air and the plastic surface, flashing off the remaining water.
Pros & Cons
- Pros: Can achieve very low moisture levels (<1%); gentle handling of material.
- Cons: Higher energy cost (requires heating elements or gas burners); requires careful temperature control to avoid melting the plastic.
Expert Insight
In a PET bottle washing line, a centrifugal dryer alone is rarely enough. A Thermal Dryer for Plastic Recycling is typically installed as the final stage to ensure the flakes meet the stringent dryness requirements for high-grade polyester fiber or sheet production.
Summary: Which Solution fits Your Line?
To hit your moisture targets, you rarely choose just one. It is about the combination.
| Material Type | Typical Process Flow | Target Moisture |
|---|---|---|
| Rigid Flakes (HDPE/PP) | High-Speed Centrifuge → Thermal Dryer (Optional) | < 1% |
| PET Flakes | High-Speed Centrifuge → Thermal Dryer | < 100 ppm (with crystallizer) |
| Soft Films (LDPE/PP) | Centrifugal Dewatering → Squeezing Machine | 3-5% |
The Verdict
- Use a High-Speed Plastic Film Centrifugal Dewatering Machine for the initial removal of bulk water from both films and rigids.
- Upgrade to a Plastic Film Squeezing Machine if you are recycling soft films and want to densify material while drying.
- Add a Thermal Dryer for Plastic Recycling if you are processing rigid flakes requiring less than 1% moisture.
Optimizing your drying process not only improves your pellet quality but also reduces energy consumption per ton. If you are unsure which combination suits your plant’s capacity and material type, consult with drying technology specialists to simulate your specific scenario.
