Counter-Rotating Parallel Twin Screw Based Process

Time:2026-04-14

Recycling plastic films, jumbo bags, and PP fiber waste poses unique challenges due to their bulk densities as low as 0.03–0.12 g/cm³, making feeding into conventional single-screw extruders extremely difficult. Operators are forced to run at speeds above 120 rpm, which generates excessive shear heat and drives energy consumption to 350–500 kWh per ton. Agglomerator-based solutions, though they compact the feed, convert 80–90% of electrical energy into useless frictional heat—becoming notorious power hogs on the production line. To overcome these limitations, the Counter-Rotating Parallel Twin Screw three-stage series process provides an efficient alternative.

In this solution, a counter-rotating parallel twin screw is used as the first-stage extruder. The two screws rotate in opposite directions, forming closed pockets in the intermeshing zone that enable positive displacement conveying with volumetric efficiency exceeding 95% (compared to only 60–80% for single-screw drag flow). The main screw speed is only 40–50 rpm, yet it achieves the same throughput as a single screw at 120 rpm. Moreover, the first stage operates without a screen pack and at a pressure below 2 MPa—all energy is used for feeding and melting, not wasted on overcoming back pressure.

Counter-Rotating Parallel Twin Screw Based Process

The system adopts a three-stage series architecture, dividing the work:

Stage	Function	Technology	Operating Pressure
1	Feeding, compacting, melting	Counter-rotating twin screw (L/D=23-25)	<2 MPa
2	Building pressure, coarse filtration (60-80 mesh)	Short single screw (L/D=12)	8-12 MPa
3	Fine filtration (120-200 mesh), pelletizing	Short single screw (L/D=10-12)	15-20 MPa

This division avoids the inefficiency of “one machine does everything”. Compared to conventional “agglomerator + single screw” lines, installed power drops from 272 kW to 165 kW, and specific energy consumption decreases by 30–40%.

Material Scope

LDPE Agricultural Film (bulk density 0.08–0.12 g/cm³)	Flaky, high moisture content, requires venting capacity. Uses 300×300 mm standard feed opening, channel depth 18–20 mm, speed 40–50 rpm.
PP Jumbo Bags (bulk density 0.05–0.08 g/cm³)	Mixture of fiber bundles and fabric pieces, prone to wrapping, high risk of metal contaminants. Uses 350×350 mm enlarged feed opening, channel depth 20–22 mm, speed 35–45 rpm.
PP Fiber Waste (bulk density 0.03–0.06 g/cm³)	Extremely fluffy, the most difficult to feed. Uses 350×350 mm side feed, channel depth 22–25 mm (1.3–1.5 times standard depth), speed 30–40 rpm.
Drip Irrigation Pipe Scrap (bulk density 0.15–0.25 g/cm³)	Irregular fragments, may contain copper fittings. Easier feeding but requires metal contamination protection.

Supporting Technologies

Conventional asynchronous motors have efficiency of only 78% at the 40% load typical for counter-rotating twin screws, power factor 0.75–0.85, and severe torque drop at low speeds. Our standard is the permanent magnet synchronous servo motor:

1.Full-operating efficiency ≥90%, reaches 93% at 40% load

2.Power factor ≥0.95, very low reactive loss

3.100% rated torque even at 1 rpm – perfectly matches the low-speed high-torque demand of counter-rotating twin screws

4.Speed accuracy ±0.01%, precise coordination with the feeder

Electromagnetic Induction Heating

Resistance heating bands have thermal efficiency of only 60–70%, slow warm-up, and poor temperature control. Our barrel sections use electromagnetic induction heating:

1.Thermal efficiency ≥90%, saves 30–35% energy compared to resistance heating

2.Reaches 200°C in 5–10 minutes (resistance bands take 20–30 minutes)

3.Temperature control accuracy ±1°C (resistance bands only ±5°C), reducing thermal degradation

For die and screen changer sections, we use infrared radiant heating with 88% thermal efficiency, still far superior to resistance heating.

Key Advantages

Efficient Conveying	Positive displacement conveying with volumetric efficiency >95%. High throughput at low speeds, independent of material friction coefficient – completely eliminates slip losses.
Maximum Energy Savings	Zero back-pressure first stage + permanent magnet servo motors + electromagnetic induction heating reduces specific energy consumption by 30–40% compared to industry averages. Start-up time cut by 50%, saving 15 kWh per start-up.
Three-Stage Division of Labor	Counter-rotating twin screw handles low-pressure melting; two short single-screw stages manage staged pressure build-up and filtration. Each stage does its job without compromising others.
Material-Specific EngineeringMaterial-Specific	Dedicated feed opening sizes, channel depths, and speed ranges for agricultural film, jumbo bags, and fiber waste – prevents wrapping, ensures stable feeding.
Superior Pellet Quality	Low-shear processing with ±1°C temperature control delivers MFI variation ±10%, moisture <0.5%, zero bubbles, and minimal thermal degradation.
Comprehensive Metal Protection	Three-stage magnetic separation (suspended magnet before shredder, magnetic roller at feeder inlet, magnetic grid at extruder feed opening) protects the screws and gearbox.
Proven Reliability	Low-speed operation reduces mechanical wear. Every system passes a 72-hour continuous running test before delivery with fewer than 2 unscheduled stops.

Performance Data Comparison

LDPE Agricultural Film LDPE	0.08–0.12 g/cm³	≤220 kWh/ton	350–400 kWh/ton
Material	Bulk Density	Our Consumption	Typical Industry Value
PP Jumbo Bags PP	0.05–0.08 g/cm³	≤240 kWh/ton	380–450 kWh/ton
PP Fiber Waste PP	0.03–0.06 g/cm³	≤260 kWh/ton	400–500 kWh/ton
Drip Irrigation Pipe Scrap	0.15–0.25 g/cm³	≤200 kWh/ton	300–350 kWh/ton

Taking a 300 kg/h LDPE agricultural film line running 24 hours per day, with electricity at $0.11/kWh (approx. 0.8 RMB/kWh):

1.Conventional line (asynchronous motor + resistance heating): actual active power 220 kW → annual electricity cost ≈ $169,000

2.Our line (servo + electromagnetic): actual active power 145 kW → annual electricity cost ≈ $112,000

3.Annual electricity saving: $57,000

The premium cost of the permanent magnet servo motor (about $7,000–11,000) is recovered in half a year.