Water Strand Pelletizing Line — SJ Series

Overview

The SJ Series Water Strand Pelletizing Line is a single-screw extrusion system purpose-built for recycling both conventional thermoplastics and challenging low-bulk-density foamed materials. Covering an output range of 20–700 kg/h across six models, this plastic pelletizer machine handles PE, PP, PS, ABS alongside foamed PET, PS, PP, and EVA — materials that typically demand pre-densification before extrusion. The line integrates a forced-feeding single-screw extruder with an enlarged feed throat, a 33:1 L/D ratio high-compression barrel, inter-stage vacuum venting for volatile extraction, a slide-plate non-stop screen changer, temperature-controlled water cooling tank, high-velocity blow dryer, and rotary strand cutter. The entire process operates at ambient granulation temperatures with no pre-drying step, delivering dense, pore-free cylindrical granules from post-industrial scrap, edge trim, reject parts, and foamed packaging waste in a single pass.

Technical Parameters

Output rates measured with standard recycled PE/PP flakes; foamed feedstock throughput varies with bulk density. Dual power ratings indicate optional motor configurations.

System Configuration

Core Features

1. Multi-Material Recycling Capability

One line processes both conventional and foamed plastics: rigid PE, PP, PS, ABS for standard recycling and foamed PET, PS, PP, EVA — materials that are notoriously difficult to feed due to bulk densities as low as 20–80 kg/m³. Chenxing provides material-specific screw configurations with tailored compression ratios, mixing element placement, and vent positioning. Unlike a shredder-extruder recycling setup that often requires a separate densification stage, this line accepts foamed scrap directly, eliminating one process step and its associated capital investment.

2. L/D 33:1 High-Compression Single-Screw Design

The 33:1 length-to-diameter ratio provides an extended processing window essential for foamed feedstock. The enlarged feed zone accommodates incoming low-density material without bridging, while the compression section delivers a large volumetric compression ratio — typically in the range of 3.0:1 to 4.0:1 — sufficient to collapse foam cell structures and consolidate the melt. This architecture is directly comparable to a dedicated single-screw extruder optimized for high-compression duties, but with feed-zone geometry specifically modified for recalcitrant lightweight scrap.

3. Vacuum Venting for Pore-Free Granules

A vacuum pump connected at the inter-stage barrel connection maintains a target absolute pressure of –0.06 to –0.08 MPa, actively extracting residual moisture, monomer volatiles, and entrapped air before the melt reaches the metering zone. This two-stage venting architecture — decompression at the inter-stage joint followed by vacuum-assisted extraction — is the key to producing dense, pore-free granules from foamed feedstock without a separate drying or degassing operation. The condenser trap captures condensable volatiles, preventing vacuum pump oil contamination and maintaining consistent suction over extended runs.

4. Non-Stop Production with Fast Screen Changer

The slide-plate fast screen changer enables operator replacement of contaminated filter mesh in under 30 seconds while the line continues extruding at full throughput. Melt pressure — maintained throughout the changeover cycle via the sliding seal design — does not drop below extrusion setpoint, ensuring strand diameter consistency at the die exit. Typical screen mesh specifications range from 60 mesh for clean post-industrial scrap to 40 mesh for mixed recycled feedstock with higher particulate loading. Eliminating the 5–15 minutes of downtime per manual screen change directly improves line OEE by 8–12% in high-contamination applications.

5. Water Cooling and Blow Drying System

Extruded strands pass through a temperature-controlled water bath maintained at 15–30 °C (adjustable via chilled water supply or an optional SML series chiller). Multi-strand separation rollers prevent adjacent strands from fusing during quenching. Immediately after exiting the water tank, strands enter the blow dryer, where high-velocity air knives strip surface moisture to achieve a residual moisture content of ≤0.1% — sufficient for direct downstream processing without an additional drying step. For applications with stringent moisture limits, an inline hopper dryer can be integrated post-cutting.

6. 38CrMoAlA Nitrided Screw and Barrel

Both screw and barrel are manufactured from 38CrMoAlA alloy steel with a gas nitriding treatment that yields a surface hardness of HV 900–1000 and a case depth of 0.4–0.7 mm. This hardness level — equivalent to approximately 67–70 HRC — provides excellent wear resistance against the moderately abrasive nature of recycled feedstock, which often contains trace inorganic fillers, pigment residues, and processing-aid degradation products. In typical post-industrial recycling service, nitrided 38CrMoAlA screws deliver 15,000–25,000 running hours before requiring reconditioning, depending on contamination levels.

7. No Pre-Drying — Direct Extrusion from Waste

Most pelletizing processes for foamed materials require a pre-drying stage to remove surface and absorbed moisture, adding capital equipment, floor space, and energy consumption. The SJ series eliminates this step entirely: the combination of the enlarged forced-feed hopper, 33:1 L/D barrel providing extended residence time for gradual moisture evolution, and active vacuum venting at –0.06 to –0.08 MPa allows waste plastic and foamed scrap to be fed directly without pre-treatment. The result is a one-step process — feed waste in, get uniform granules out — at ambient granulation temperature, yielding both lower capital cost and reduced per-kilogram operating expense compared to multi-stage recycling systems.

Production Process

Step 1 — Feedstock Loading: Waste plastic scrap, edge trim, reject parts, or foamed packaging (PET, PS, PP, EVA) is loaded into the enlarged forced-feeding hopper. The oversized feed throat geometry prevents bridging of low-bulk-density materials. No pre-drying or densification is required.

Step 2 — Melting and Compression: Material is conveyed through the 33:1 L/D single-screw barrel under multi-zone PID-controlled heating. The enlarged feed zone transitions into a high-compression section where the large volumetric compression ratio collapses foamed cell structures and consolidates the melt into a homogeneous mass.

Step 3 — Vacuum Venting: At the inter-stage connection between barrel sections, a vacuum pump maintains –0.06 to –0.08 MPa absolute pressure, actively extracting residual moisture, monomer volatiles, and entrapped air. A condenser trap captures condensable species before they reach the vacuum pump. This step ensures a pore-free melt stream downstream.

Step 4 — Melt Filtration: The homogenized, degassed melt passes through the slide-plate fast screen changer. Contaminants and unmelted particles are captured on the filter mesh. When differential pressure signals the need for a mesh change, the operator actuates the slide plate — clean mesh enters the melt stream while the fouled screen is withdrawn, all without stopping extrusion.

Step 5 — Strand Extrusion and Water Cooling: Melt exits through a multi-strand die head, forming continuous strands that are drawn through a temperature-controlled stainless steel water bath. Adjustable strand guides and separation rollers maintain consistent strand positioning. Water temperature is maintained at 15–30 °C for rapid quenching without thermal shock.

Step 6 — Blow Drying and Granule Cutting: Cooled strands pass through high-velocity air knives that strip surface moisture to ≤0.1% residual. The dried strands enter the rotary cutter, where hardened alloy blades cut them into uniform cylindrical granules. An optional pellet vacuum loader transfers finished granules to packaging or a plastic heating mixer for downstream compounding.

Frequently Asked Questions

Q1: Why is L/D 33:1 critical for recycling foamed PET/PS/EVA?

Foamed materials present two compounding processing challenges: extremely low bulk density (typically 20–80 kg/m³ for expanded EVA or PS foam) and the need to collapse millions of closed cells into a homogeneous melt. An L/D of 33:1 provides the extended residence time necessary to accomplish both without resorting to a separate densification stage. The first 8–10 L/D of the barrel constitute the enlarged feed zone, allowing the fluffy feedstock to be progressively compacted without bridging at the hopper throat — a common failure mode with standard 25:1 or 28:1 extruders. The subsequent compression zone, spanning approximately 12–15 L/D, delivers the high volumetric compression ratio (typically 3.0:1 to 4.0:1) required to collapse foam cell walls. The remaining barrel length provides metering and homogenization. Critically, the inter-stage vent is positioned at approximately 55–65% of barrel length from the feed end — far enough downstream that the melt is consolidated and conveying stably, but early enough that volatiles are extracted before entering the high-pressure metering zone where they could cause surging at the die.

Q2: How does vacuum venting eliminate porosity in recycled granules — and what vacuum level is needed?

Porosity in recycled granules originates from three sources: (1) residual moisture absorbed by hygroscopic materials during storage or washing; (2) monomer and additive volatiles released as the melt temperature exceeds their boiling points; and (3) entrapped air carried in with low-bulk-density foamed feedstock. The vacuum venting system addresses all three. The vent is positioned at the inter-stage connection where barrel pressure drops to near-atmospheric (the decompression zone), creating a large surface-to-volume ratio in the melt for efficient devolatilization. A vacuum pump then draws the barrel annulus down to –0.06 to –0.08 MPa absolute pressure (approximately 200–400 mbar absolute or 60–80 kPa below atmospheric). At this vacuum level, the boiling point of water drops to approximately 60–70 °C — well below typical melt temperatures of 180–230 °C for polyolefins — ensuring moisture flashes off completely. The condenser trap between the vent port and vacuum pump captures condensable volatiles (plasticizers, residual monomers), protecting pump oil and maintaining consistent suction. Without active vacuum venting, recycled granules typically exhibit 3–8% internal void volume visible on cut cross-sections; with properly maintained vacuum, this drops to below 0.5% .

Q3: What is the water consumption and cooling system requirement across model sizes?

Cooling water demand scales approximately linearly with throughput. For the mid-range SJ-90/33 operating at 300 kg/h, the water cooling tank requires a circulation flow rate of approximately 2–4 m³/h with a supply temperature of 15–25 °C. The tank itself — typically 3–4 meters in length — provides sufficient residence time (approximately 8–15 seconds depending on line speed) for strand surface solidification. An open-loop cooling circuit using municipal water is viable for low-utilization operations, but continuous production demands a closed-loop system with a chiller. An SML series chiller rated at 15–25 kW cooling capacity adequately serves the SJ-65 through SJ-90 models. For the largest SJ-150/33 at 700 kg/h, cooling demand rises to 6–8 m³/h and a 30–40 kW chiller is recommended. Water quality matters: total hardness should be kept below 150 ppm CaCO₃ to prevent scale buildup on strand guides and tank walls. All models include adjustable water flow control valves and temperature monitoring; the PLC can optionally control a proportional valve for closed-loop bath temperature regulation to within ±1 °C .

Q4: Can one screw configuration handle both foamed EVA and rigid PP effectively?

In principle, a single screw with a compression ratio of approximately 3.0:1 can process both materials, but with significant throughput and quality trade-offs — and this approach is not recommended for quality-critical production. Foamed EVA demands a deeper feed-zone channel depth (to accept low bulk density without bridging) and a compression ratio of 3.5:1 to 4.0:1 to aggressively collapse foam cells. Rigid PP, by contrast, benefits from a compression ratio of 2.5:1 to 3.0:1; exceeding this can generate excessive shear heating that degrades PP at the molecular level, manifesting as reduced melt strength and off-color granules. The practical solution is to maintain two dedicated screw-and-barrel sets — one optimized for foamed feedstock (aggressive compression, deep feed zone) and one for rigid recyclate (moderate compression, mixing elements for homogenization). Screw changeover on the SJ series takes approximately 3–4 hours using the quick-disconnect coupling design. For operations frequently switching between material families, the payback period on a second screw set is typically under 6 months when accounting for avoided off-spec production and reduced energy consumption per kilogram of good output.

Q5: What post-granulation moisture content can be expected, and is additional drying needed?

After the blow dryer stage, surface moisture on cut granules consistently measures ≤0.1% by weight — a level suitable for direct feeding into most downstream processes including injection molding, profile extrusion, and compounding. This performance is achieved by the combination of two factors: the water bath temperature (maintained at 15–30 °C) ensures rapid surface solidification but limited water absorption into the polymer matrix, and the high-velocity air knives (airflow velocity typically 20–30 m/s at the nozzle exit) mechanically strip residual water droplets from the strand surface. For hygroscopic polymers — particularly ABS, PA (nylon), and PET — the ≤0.1% surface moisture figure still leaves absorbed moisture within the polymer itself. If the granules will sit in open storage before downstream use or if the downstream process is moisture-sensitive (e.g., PET fiber spinning requiring <0.005% or 50 ppm moisture), a secondary drying step using a hopper dryer or desiccant dryer is recommended. For non-hygroscopic materials (PE, PP, PS), the as-cut moisture level is fully sufficient and no additional drying is necessary.

Why Choose Chenxing

Proven SJ Single-Screw Platform. The SJ series extruder forms the backbone of hundreds of recycling and compounding lines deployed globally. Each pelletizing line is pre-configured and factory-tested with customer-supplied material samples before shipment, ensuring the screw geometry, vent positioning, and process parameters are validated against your actual feedstock — not a generic reference material.

Integrated Single-Source Supply. Chenxing manufactures the complete equipment chain: single-screw extruders, plastic heating mixers for upstream blending, hopper dryers for moisture control, pellet vacuum loaders for automated material transfer, chillers for process cooling, pipe coilers, and an extensive range of auxiliary machines. A single-source supplier means one point of contact for commissioning, one spare parts inventory to manage, and one warranty to track. Browse our full product catalog to see the complete scope.

Foamed Material Expertise. Processing foamed PET, PS, PP, and EVA without pre-densification is a specialized capability that Chenxing has refined through years of field experience. The enlarged feed throat geometry, 33:1 L/D barrel, inter-stage vacuum venting, and application-specific screw configurations are not catalog options chosen from a table — they are engineered per order based on your material characteristics.

Engineered-to-Order Configuration. Screw compression ratio, vent port position, screen changer mesh specification, water tank length, cutter blade count, and PLC feature set are all configured to your specific feedstock and throughput requirements. Visit our solutions page for application examples and configuration guidance.

Global Service and Support. Chenxing Machinery, headquartered in Zhangjiagang City, provides remote commissioning support via video link, optional on-site technician dispatch for installation and start-up, and lifetime after-sales technical support. Learn more about us and follow our latest news and case studies.

Start Your Inquiry in 4 Steps

Step 1 — Tell Us Your Material: Specify your feedstock type — post-industrial PE/PP/PS/ABS scrap, foamed PET/PS/PP/EVA, or mixed plastic waste. Include bulk density (if known), typical contamination level, and particle size range.

Step 2 — Define Your Output Target: Desired throughput in kg/h and preferred model range (SJ-50 through SJ-150). If you process multiple materials on the same line, describe your production schedule so we can evaluate screw interchangeability requirements.

Step 3 — Receive a Custom Proposal: Our engineering team reviews your material specifications and provides a tailored configuration — screw geometry, vent positioning, screen changer mesh grade, cooling system sizing, and auxiliary equipment recommendations — typically within 24 hours.

Step 4 — Validate with a Trial Run: Optionally send 50–100 kg of your actual feedstock for a production trial at our Zhangjiagang facility. We record throughput data, granule quality metrics (size distribution, moisture content, density), and energy consumption for your evaluation before any commitment.

Contact Nicole directly:

• Phone WeChat WhatsApp: +8615951187228

• Email: ceo@cxsljx.com

Or visit our contact page for the complete inquiry form. All inquiries receive a response within one business day.