Process of Plastic Film Pelletizing Machine
|
|
|
|
|
Feeding and Pre-treatment
The film pelletizing system integrates a cutter-compactor unit at the feeding stage. It can cut large pieces of film scrap, preheat lightweight film, and compact it into a denser form suitable for extrusion.
The goal of this stage is to provide a consistent material flow into the extruder screw.
|
Melting and Plasticizing
The extrusion system is the core of the pelletizing machine. As the screw conveys the plastic forward, heat and pressure melt the plastic film.
If the pelletizer is a single-stage extruder, all melting and degassing occur within one screw barrel. If it is a two-stage extruder, the material passes through two screws in series (commonly referred to as "main" and "sub" extruders). In either case, the plastic is transformed into a hot, viscous melt.
|
Vacuum Degassing
Plastic film, especially post-consumer or agricultural waste film, often contains residual moisture or volatile substances. Trapped moisture can evaporate and form voids or bubbles in the melt, resulting in weak or porous pellets.
To prevent this, the pelletizing machine uses one or more degassing vents equipped with vacuum pumps to extract moisture and volatiles. Removing these gases is critical to ensuring the melt is pure and bubble-free before pellet formation.
|
Filtration (Melt Filtration)
After the plastic is melted and homogenized, it passes through a filtration system (screen changer). This is essentially a metal mesh or sieve (or a series of screens with decreasing mesh sizes) that captures any remaining contaminants or unmelted particles before the plastic is formed into pellets.
Pelletizing machines use automatic or continuous screen changers, allowing clogged screens to be replaced without stopping production—an important efficiency advantage. For example, piston-type or plate-type continuous screen changers enable filters to slide in and out without interrupting the flow, and some advanced systems feature self-cleaning (scraper) filters.
|
|
|
|
|
|
Extrusion and Pellet Cutting
There are several cutting methods used in pelletizers, but the most common for film recycling is hot die-face cutting, typically implemented as a water-ring pelletizer. In this system, rotating blades are mounted flush with the die face and cut the extruded plastic immediately as it exits the die holes, preventing it from forming long strands.
|
Vibratory Screening System
In water-ring and underwater systems, cooling occurs instantly through water. The pellets then enter a vibrating screen along with the water to remove any fine or oversized particles.
|
Final Product Collection System
Collects finished pellets after cooling and screening, ensuring clean discharge, efficient packaging, and easy downstream handling.
|
|
Types of Plastic Film Pelletizing Machines
Plastic film pelletizers can be classified based on pelletizing methods and machine configurations. The choice of pelletizing system affects pellet shape, cooling method, and overall ease of operation. Additionally, recycling extruders can be configured as single-stage or multi-stage.
Here, we explore the main type of film pelletizing system—water-ring (hot die-face) pelletizers—and the distinction between single-stage and two-stage extruders.
1. Water-Ring (Hot Die-Face) Pelletizing System:
This is the most widely used system for recycling polyolefin films (PE, PP) and similar materials. In a water-ring pelletizer, the melt exits the extruder through a perforated die, and rotating blades immediately cut the molten plastic at the die face into pellets. Once cut, the pellets are flung outward into a circular chamber where water flows (hence the name “water ring”). The water instantly cools the pellet surface and carries them away from the die area.
Water-ring systems are very popular in film recycling due to several advantages:
-
Easy start-up and operation: There’s no need to manually pull out strands or worry about them breaking. Once the extruder starts and polymer begins to flow, it is immediately cut and carried away by the water, reducing start-up stress.
-
Stable production: There’s no risk of strand breakage, so the process can run continuously. Cutting is automatic and adjusts to the extruder output.
-
Simple maintenance: Replacing or sharpening the rotating blades is relatively easy and low-cost, and there’s no need to maintain a long water bath.
The only major limitation is if the material has extremely high melt flow or low viscosity (meaning it smears easily when hot or doesn’t form pellets well)—but typical films like LDPE, LLDPE, HDPE, and PP perform very well in water-ring systems. Also, water-ring pelletizers usually require a minimum throughput to maintain balance in the water system; very small lab setups may use different methods, but for industrial scale, this is a preferred solution.
2. Strand Pelletizing System:
In a strand pelletizer, the plastic melt is extruded through a perforated die into long “spaghetti-like” strands. These strands are then cooled—typically by water or air—before being solidified and cut into pellets by a rotating cutter.
Usually, the strands fall directly into a water trough after exiting the die (cooling bath), then are pulled by a set of feed rollers into the pelletizing unit, where a rotating cutting rotor chops them into pellets of a set length.
Strand pelletizing is one of the oldest methods and is still used today, especially for materials that are difficult to cut while molten or for certain polymers like PET.
3. Underwater Pelletizing System:
Underwater pelletizing is another form of die-face cutting, where the cutting assembly is completely submerged in a water chamber attached to the die head. Essentially, the die holes open directly into a water "cutting chamber," and molten plastic is cut underwater by rotating blades. The pellets are instantly cooled as they form, then flushed away by circulating water and sent to a dryer, similar to water-ring systems.
The key difference from standard water-ring systems is that in underwater pelletizers, the entire pellet formation process occurs underwater, typically under slight pressure. This method is commonly used in virgin polymer production—such as in polymer plants making PP, PE, or polystyrene pellets—and is capable of handling very high throughputs (tons per hour).
For most film recycling applications (PE/PP), underwater pelletizing is unnecessary, as water-ring systems can efficiently handle the task at typical production scales (several hundred to 1000 kg/h). However, if pellet quality requirements are extremely stringent or if the production volume is exceptionally high, underwater pelletizing may be used in recycling operations.
Single-Stage vs. Two-Stage Extruders:
Apart from the cutting method, another key “type” distinction is whether the pelletizing line uses a single extruder or a tandem (two-stage) extruder setup. Both single-stage and two-stage machines can use any of the pelletizing methods mentioned above (water-ring, strand, etc.), but the internal configurations differ:
A single-stage pelletizer has one extruder barrel and screw. It takes in plastic, melts it, degasses (usually through one vent), and goes directly to pelletizing. This simpler setup is generally sufficient for relatively clean, dry plastic film (for example, factory film scrap or low-ink, washed film).
The limitation of single-stage machines lies in their degassing and filtration capacity: there is only one melt filtration point and one vacuum vent (or at most two vents on one barrel), which may be insufficient for heavily printed or severely contaminated materials. Therefore, single-stage extruders struggle to process materials with residual moisture or a high amount of ink/volatiles, which, if not fully removed, can result in pellets with small bubbles or defects.
A two-stage pelletizer (also known as a two-stage extruder, or as some say, a “mother-baby” extruder) uses two consecutive extruders. The first stage (usually with a larger diameter screw) primarily handles melting and aggressive degassing, and sometimes initial filtration. The melt then enters the second-stage extruder, which builds pressure and performs a second filtration before pelletizing.
This configuration is well-suited for materials with high contamination, high moisture, or heavy printing (ink). For example, recycled agricultural film with dirt and moisture or printed packaging film is often run on a two-stage extruder. The first stage can have one large or even two large vents to pull out moisture and ink volatiles, followed by a coarse screen filter; the second stage provides an additional opportunity to filter fine particles (using a finer mesh) and stabilize melt pressure for pellet cutting. The result is more uniform and defect-free pellets—even from challenging raw materials.
While single-stage machines may be cheaper, two-stage machines can handle lower-cost feedstock (such as washed film that still contains contaminants) and produce higher-value pellets, offering a faster return on investment despite the higher initial cost.
Most plastic film pelletizers use single-screw extruders—they are robust, cost-effective, and provide sufficient mixing for homogeneous materials such as a single type of polymer.
However, in some cases, recyclers may want to add fillers or colorants to the recycled pellets, in which case a twin-screw pelletizer can be used.
But for direct film-to-pellet recycling, twin-screw extruders are generally unnecessary unless you're specifically blending materials, as they are more expensive and more complex to maintain.
FAQ & Expert Tips
Straight Answers to the Questions You Might Be Hesitant to Ask
1. What Types of Plastic Film Can a Pelletizing Machine Recycle?
Plastic film pelletizers can process most thermoplastic films, including polyethylene (LDPE, LLDPE, HDPE) films (such as agricultural film, packaging film, and shopping bags), polypropylene (PP) films (such as BOPP packaging film and woven PP sacks), and blends of PE and PP films. These machines can handle both clean industrial film scrap and washed post-consumer film waste.
2. What’s the Difference Between a Single-Stage and Two-Stage Pelletizer, and Which One Do I Need?
If you're dealing with relatively clean, dry material with minimal ink or contaminants, a single-stage pelletizer is simpler and typically more cost-effective.
If your material has higher moisture content or residual impurities, use a two-stage system. While it has a higher upfront cost, it usually delivers a better return by allowing you to process lower-cost input material and produce higher-value pellets.
3. Which Pelletizing System Is Best—Water-Ring, Strand, or Underwater?
For most PP/PE film recycling, the water-ring (hot die-face) pelletizing system is the best and most commonly used option.
4. How Much Does a Plastic Film Pelletizing Machine Cost?
Small-scale pelletizers (100–200 kg/h): around $20,000–$40,000
Mid-size lines (300–500 kg/h): around $50,000–$80,000
Large industrial lines (800–1000+ kg/h, two-stage, fully equipped): can cost $100,000 or more
5. How to Maintain a Plastic Film Pelletizer for Optimal Performance:
Regular cleaning, lubrication, and inspection
Verify all heating zones are functioning correctly, with no unusual noise or vibration
Lubricate bearings and gearboxes as recommended
Regularly sharpen or replace cutter blades in the compactor and pelletizer
Keep the vacuum degassing system clear and functional
Occasionally calibrate and inspect sensors (temperature and pressure) to ensure accurate control
6. How to Ensure the Quality of Recycled Pellets?
High-quality pellets come from a combination of clean input material, proper machine configuration, and correct operating settings.
Remember, pellet quality directly depends on feedstock quality and process control, so invest in these areas for the best results.
EN 
English
简体中文
Русский
España
عرب .
