MACHINERY | PROCESSING
Processing with alternative technologies
The twin-screw extruder is the workhorse of the compounding industry, but single-screw extruders and other technologies can be more suited to certain materials and processes. By Chris Saunders
Conventional twin-screw extrusion remains the industry standard for most compounders, but issues such as rising raw material and energy costs, and the increasing use of recyclate, are leading to developments in single-screw extruders, co-knead- ers and other alternative technolo- gies. Developing these technologies represents a critical step toward addressing sustainability challenges while enabling enhanced perfor- mance and ensuring product quality. According to US extrusion equipment supplier
Entek, the reason the co-rotating twin screw extruder is considered so superior for compound- ing recycled plastics is because improved venting capabilities help remove volatiles, contaminants, and moisture. A screen changer offers added versatility and through Entek’s patented VFT technology, air can be extracted from fluffy fillers inside the extruder to increase throughput rates. An early atmospheric vent is especially useful to mitigate hydrolysis with plastics like polyamides, polyesters, polyurethanes, and polycarbonates that are susceptible to hydroly- sis, and a vacuum vent is employed later in the process after the plastic is melted to devolatilise the materials and remove moisture and air. In a co-rotating extruder, both screws rotate in the same direction so the material is pushed along between the screws, spreading particles through- out the melt and creating a uniform blend. But in a counter-rotating extruder, the screws rotate towards each other. The material is trapped in C-shaped chambers and is pushed forward more like a positive-displacement pump so it is better at dispersive mixing. While co-rotating twin-screws have become the default for many compounding tasks, co-kneaders, low-shear rotating and reciprocating single-screw
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extruders, retain an edge in the low-temperature compounding of highly filled or temperature-sensi- tive systems (such as HFFR and certain PVC grades), where the mixing mechanism needs to achieve effective dispersion without excessive melt tem- perature or fibre/filler damage. For these reasons, they occupy a sweet spot between traditional single-screw extruders and co-rotating twin-screws, and remain a mainstay in cable compounds, PVC, and other demanding applications. Chinese company Xinda, which began as a mas-
terbatch producer in 1988, now manufactures both co-rotating and counter-rotating twin screw extruders, but is best known for its co-kneaders, saying they are the preferred choice for many compounders as they often offer lower energy input and melt temperatures, facilitating more homogenous mixing with no shear spikes and equal treatment of the polymer matrix. They also present a high degree of self-wiping and disper- sive mixing without destroying delicate fillers, and enable high volumetric loadings of fillers and additives. Explaining why they are so consistently
October 2025 | COMPOUNDING WORLD 53
Main image: CPM’s Ring Extruder Technology offers
enhancement in elongational flow through the multiple intermeshing apex regions
IMAGE: CPM
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