If you’re building a recycling pelletizing line, you’ll eventually face a key decision: single-screw or twin-screw extrusion. Both are proven technologies, but they solve different problems.
In many recycling projects, single-screw systems are chosen because they can build steady melt pressure for filtration and pelletizing with a simpler mechanical layout. Twin-screw systems are often chosen when mixing and dispersion define pellet quality, or when the process is closer to compounding than recycling.
This guide explains the trade-offs in practical terms so you can choose based on your feedstock and your customer’s acceptance tests.
Quick Takeaways
- Single-screw extruders are often chosen for stable melt pressure, filtration, and pelletizing.
- Twin-screw extruders are often chosen when mixing and dispersion drive product performance.
- Start from the output spec (contamination window, additives, MFR/IV target), then select the extruder type.
How a Single-Screw Recycling Pelletizer Works (High Level)
In a typical recycling pelletizer, the screw has functional zones:
– a feed/conveying zone to accept regrind or densified feedstock
– a melting/compression zone where solids transition into melt (heat + shear)
– a metering zone that stabilizes output and builds pressure for filtration and die flow
Most recycling projects package these functions with:
– stable feeding (especially for film)
– melt filtration (screen changer)
– degassing as needed
– pelletizing system matched to the polymer and end market
For typical line configurations and options, see Energycle’s plastic pelletizer machines.
Single-Screw vs Twin-Screw: The Buyer Comparison Table
| Decision factor | Single-screw tends to fit when… | Twin-screw tends to fit when… |
|---|---|---|
| Main job | Melt + build pressure + filter + pelletize | Mix/compound + disperse additives + controlled devolatilization |
| Melt pressure stability | You need steady pressure for filtration and consistent pellet size | You can justify more complex screw elements and controls to reach the mixing target |
| Mixing requirement | You mainly need homogenization (regrind + virgin, small additive package) | You need stronger dispersive/distributive mixing for dispersion-sensitive products |
| Feedstock variability | You can keep moisture and contamination tighter | You need more processing headroom for variability and mixing demands |
| Maintenance model | You want simpler mechanical service routines | You accept higher mechanical complexity for the processing capability |
Why Single-Screw Is Common in Recycling Lines
1) It builds stable pressure for filtration and pelletizing
Recycling melt often carries fine contamination (paper fibers, fines, aluminum specks, degraded polymer). Filtration and pelletizing need stable melt pressure and predictable flow. If pressure swings, you typically see more screen-change downtime, more restart scrap, and wider pellet size variation.
2) It keeps the mechanical layout simpler
Single-screw systems are mechanically simpler than most twin-screw compounding lines. That can matter if you want predictable wear-part management and faster service routines—especially in plants that run a lean maintenance team.
3) It works well when the line stabilizes feedstock upstream
Many recycling lines stabilize feedstock before the extruder (densification for film, washing/drying, metal removal) so the screw sees a steadier feed. That can make a single-screw line perform well without paying for compounding-level mixing capability.
Where Twin-Screw Makes Sense
Twin-screw extrusion is commonly considered when mixing and dispersion define success, such as:
– compounding with fillers, reinforcements, or larger additive packages
– blending polymers where dispersion quality drives mechanical properties
– processes that need more complex screw element layouts for controlled mixing behavior
Plastics Technology explains why twin-screw mixing behaves differently: intermeshing screws can transfer polymer between screws multiple times for full-channel mixing, which can support more complete mixing without the same output loss and melt-temperature penalty that high-shear mixing sections can create in single-screw designs. (Source: Plastics Technology — Twin-screw mixing differences)
How Extruder Choice Affects Filtration and Pelletizing
Pelletizing is downstream, but extruder choice changes the operating window. Underwater pelletizing systems, for example, depend on coordinated specification of extruders, pumps, filters, water systems, and dryers. Plastics Technology notes that the underwater pelletizer only performs well when those systems are specified together to achieve consistent pellet quality. (Source: Plastics Technology — The Path to Pellet Perfection)
For a practical overview of pelletizing methods and what they require from the melt, see Energycle’s pelletizing methods guide.
What to Ask Suppliers (So You Don’t Buy Assumptions)
Use these questions to force “apples-to-apples” quotes:
1) What is the target polymer and end market (film, injection, sheet, strapping, fiber)?
2) How variable is the input quality (moisture, paper/labels, fines, other polymers)?
3) Do you need mixing/compounding, or mainly melt + filter + pelletize?
4) What QC property defines success (MFR, contamination count, color, odor, IV for PET)?
5) What melt pressure range and fluctuation do you expect at the target throughput?
6) What screen-change interval and restart scrap should we expect at our contamination level?
7) What service model do you assume (in-house maintenance vs supplier service), and what are the rebuild intervals?
If you want Energycle to recommend a line concept and supporting modules, use the contact form.
FAQ (Real Procurement Questions)
If I’m mainly filtering and pelletizing, is twin-screw overkill?
Sometimes. If your success metric is stable melt pressure through filtration and consistent pellet size, a well-specified single-screw line can be the more practical choice. Twin-screw systems add value when you need mixing intensity or dispersion quality that changes pellet properties, but that capability comes with cost and mechanical complexity. A better way to decide is to define what “bad pellets” look like for your customer—gels, specks, odor, MFR drift, IV loss for PET—then map each failure mode to a process need (mixing, drying, degassing, filtration). If mixing is not your limiting factor, prioritize feeding stability and filtration strategy instead.
What’s the clearest sign I actually need twin-screw mixing?
You need to add or disperse a meaningful package of additives or fillers and the pellet quality depends on dispersion. Plastics Technology notes that intensive mixing with high shear is required to fully disperse additives or other polymers, and explains that twin-screw designs can transfer polymer between screws multiple times for full-channel mixing. If your customer tests show that poor dispersion causes failures (strength loss, inconsistent MFR, visible defects), a twin-screw process becomes easier to justify. If your issues are mainly contamination and pressure instability, focus on feeding, drying, and filtration first. (Source: Plastics Technology — Twin-screw mixing differences)
Can single-screw still handle odor and volatile removal in recycling?
Yes, but you need to specify the degassing concept correctly. If your feedstock carries moisture or volatiles (inks, residual organics), venting and vacuum system selection matter. Confirm how the design prevents melt carryover into the vacuum line and how maintenance is handled when deposits form. For PET, a vacuum supplier’s extruder degassing overview describes that PET degassing can extract vapors such as water, solvents, monomers/oligomers, plus entrained particles into the vacuum system, which explains why traps and maintenance planning matter in real plants. (Source: Leybold extruder degassing overview)
How should I compare single-screw and twin-screw quotes without getting lost in specs?
Force both suppliers to quote the same outcome: saleable pellets per hour at a defined feedstock condition. Provide moisture and contamination ranges, target pellet spec, and the acceptance tests your buyers use. Then ask each supplier to list the modules that make that outcome possible: feeding/conditioning, degassing, filtration type and screen area, and pelletizing method. A quote that does not include a filtration plan (expected screen-change interval, pressure trend expectations, restart scrap) is not comparable. Treat screw L/D ratios and motor kW as secondary. Outcome definition is what prevents surprises after commissioning.
If I plan to use underwater pelletizing, what should I demand in the proposal?
Demand a coordinated system proposal, not an “underwater pelletizer price.” Plastics Technology notes that underwater pelletizing success depends on properly specified extruders, pumps, filters, water systems, and dryers working together. Ask for the water system and dryer specs, expected pellet fines level, and how pellet size is controlled as cutters and dies wear. Also require melt pressure and temperature trend targets at your throughput. If the supplier can’t show how filtration and melt pressure stability support the pelletizer, you’ll likely fight pellet quality issues and fines after startup. (Source: Plastics Technology — The Path to Pellet Perfection)
References
- ISO — Plastics recycling guideline (ISO 15270 overview): https://www.iso.org/standard/15270.html
- ASTM — Melt flow rate test method (ASTM D1238 overview): https://store.astm.org/d1238-23.html
- Plastics Technology — Why mixing differs in twin-screw vs single-screw: https://www.ptonline.com/articles/single-vs-twin-screw-extruders-why-mixing-is-different
- Plastics Technology — Underwater pelletizing system specification considerations: https://www.ptonline.com/articles/the-path-to-pellet-perfection
- Leybold — Extruder degassing overview: https://www.leybold.com/en/products/systems-and-solutions/customized-vacuum-systems/extruder-degassing
- Energycle — Plastic pelletizer machines: https://www.energycle.com/plastic-pelletizers/
- Energycle — Pelletizing methods guide: https://www.energycle.com/pelletizing-methods-in-plastic-pelletizer-a-comprehensive-guide/
- Energycle — Contact: https://www.energycle.com/contact-us/
