Parallel Twin Screw Extruder: How Co-Rotating Compounding Works

By the Nanhaiya Technical Team — Extrusion Machinery, Zhoushan · Reviewed for factory accuracy · ~9 min read

If your job is turning ingredients into pellets — masterbatch, filled compounds, WPC, engineering plastics — the parallel twin screw extruder is almost certainly the machine you need. Its defining feature is not power but configurability: because both the screw and the barrel are built from modules, one machine can be rebuilt to suit recipe after recipe. This guide covers how it works, why co-rotating dominates compounding, how screw configuration is actually designed, and how to size a machine to your output.

How a parallel twin screw extruder works

Two screws of identical diameter run side by side in a figure-eight bore, intermeshing along their whole length. Material is metered in — usually by a gravimetric or volumetric feeder rather than a flood hopper — then conveyed, melted, mixed, vented and pumped to the die. Because the screws intermesh, material transfers positively from one to the other instead of being dragged along the wall, which gives the machine its steady output and self-wiping behaviour.

The sequence along the barrel:

1. Metered feeding — feeders dose resin, filler and additives at exact ratios into the feed section. Side feeders introduce glass fibre or high filler loads further downstream so they are not over-sheared.
2. Melting — kneading blocks convert mechanical energy into heat, melting the polymer quickly over a short length.
3. Mixing — distributive elements spread additives evenly; dispersive elements break down agglomerates. This is where compound quality is won or lost.
4. Devolatilisation — atmospheric and vacuum vents pull out moisture, monomer and volatiles.
5. Metering & pelletizing — the final conveying section builds pressure through the die, then a strand or die-face pelletizer cuts the melt into granules.

Why compounding uses co-rotating machines

A parallel twin screw extruder can be co-rotating or counter-rotating, and the choice defines the machine's character.

In practice, a co-rotating twin screw extruder is what most people mean by "compounding extruder": the self-wiping action prevents dead spots and material hang-up, and the high specific energy input melts and disperses quickly. Counter-rotating parallel machines survive where shear must stay low — mainly PVC. For direct PVC extrusion specifically, a conical twin screw extruder is often the better answer.

Screw configuration: the real specification

On a parallel machine, the screw is not a single part — it is a stack of elements keyed onto a splined shaft. The parallel twin screw and barrel is therefore specified by its element sequence, not just a diameter. The building blocks:

• Conveying elements — forward-pitch flights that transport material; long pitch for feeding, short pitch for pressure build.
• Kneading blocks — staggered discs that melt and mix. The stagger angle sets the character: 30° and 45° forward for distributive mixing, 90° neutral for intensive dispersion, reverse for maximum shear and full melt seal.
• Reverse (left-hand) elements — create a melt seal so a vacuum vent can work, and raise residence time locally.
• Mixing elements — toothed or slotted designs for gentle distributive mixing without excess shear.
• Barrel sections — feed, closed, side-feed, atmospheric-vent and vacuum-vent sections bolted in the order the recipe needs.

The extruder is only part of the line

A parallel twin screw extruder rarely works alone. A complete compounding line typically includes:

• Feeders — gravimetric loss-in-weight units for accurate recipe dosing.
• Side feeder — introduces glass fibre or heavy filler downstream to protect fibre length.
• Vacuum system — removes moisture and volatiles from the melt.
• Melt filter — screen changer to catch contamination, important on recycled feed.
• Pelletizer — strand, water-ring or die-face (underwater) cutting, chosen by material.
• Cooling & drying — water bath or air cooling plus a dryer before packing.

Nanhaiya supplies the extruder alone or the complete parallel twin screw pelletizing line, as well as other extruder machines for single screw and conical layouts.

Sizing and output

Parallel machines are named by screw diameter — a 65 mm machine has two 65 mm screws. Output rises with diameter, screw speed and torque, but the recipe sets the real ceiling: a highly filled compound that needs intensive dispersion will run slower than a simple colour masterbatch on the same machine.

The practical approach: tell us the compound, the filler level and the kg/h you need, and we will size the machine and design the element sequence around it — rather than selling you a diameter and hoping the recipe fits.

What parallel twin screw extruders make

• Masterbatch — colour, filler and additive concentrates.
• Filled compounds — CaCO₃, talc and mineral-filled PP and PE.
• Glass-fibre reinforced engineering plastics — PA, PBT and PP with side-fed fibre.
• WPC pellets — wood-fibre compounds for decking and panels.
• PS foaming — see our PS foaming parallel twin screw.
• Aluminum-plastic composite panel — see the aluminum-plastic plate parallel twin screw.
• Recycling — reprocessing and upgrading post-industrial material.

Parallel or conical machine?

Not sure which layout fits? Start from our broader twin screw extruder guide, which compares both families side by side.

Screw and barrel: the part that wears

Compounding is hard on steel. Glass fibre, mineral filler and high carbonate loadings abrade both the flight tips and the bore, and a worn element sequence loses mixing quality long before the machine stops running. That is why the parallel twin screw barrel and its elements should be specified for the abrasion your recipe actually creates.

Nanhaiya builds parallel screws and barrels from 38CrMoAlA nitrided steel, W6Mo5Cr4V2 or HIP powder steel, held to a tolerance of ±0.01 mm with straightness of 0.015 mm/m. For abrasive compounds we spray a tungsten-added hard alloy that reaches 62 HRC inside the bore — against a common 58 HRC — with a 2.0–3.0 mm alloy layer. The modular design also helps commercially: you replace only the worn elements, not the entire screw. See our comparison of a bimetallic vs nitrided screw barrel, and our spare parts range.

Frequently asked questions

Related products from our factory

Parallel Twin Screw Pelletizing Line Twin Screw Extruder PS Foaming Parallel Twin-Screw Aluminum-Plastic Plate Parallel Twin Screw Rubber And Plastic Parallel Twin Screw Single Screw Pelletizing Line