Parallel Twin Screw and Barrel

Two screws, one process: stable feeding and controlled plasticizing.

MILON Endüstri determines the screw-barrel solution for parallel twin-screw extruders by evaluating the existing machine's dimensions, processed formulation and production targets together. For new production or refurbishment projects, geometry, material and surface treatment are selected according to the application.

Key message: A parallel twin screw-barrel is not simply two same-size screws placed side by side. The screw profile, the center distance between the two screws, the mating geometry, the barrel bores and the gearbox output arrangement all work together as a precision system.

Let's Determine the Right Parallel Screw-Barrel for Your Machine

What Is a Parallel Twin Screw and Barrel?

A parallel twin-screw system consists of two screws with parallel axes that maintain the same diameter along their length, together with a twin-bore barrel that carries them. This structure is suitable for extruding pipe, profile, sheet and micro-foam products in PVC, CPVC and some PE applications.

Parallel twin screw and twin-bore barrel structure

Technical Size Range and L/D Information

Various manufacturers give different production ranges for parallel twin screw-barrels; a diameter of approximately Ø45–350 mm and an L/D range of 18:1–40:1 is commonly seen in the industry.

• Screw diameter — approximately Ø45–350 mm (manufacturer ranges)
• L/D ratio (Length / Diameter) — approximately 18:1–40:1; varies by machine and product design
• Model notation — for example in "67/22", "90/28", "92/28" or "114/32", the first value indicates the screw diameter (mm) and the second the L/D ratio
• Center distance — the distance between the two screw axes; for a compatible set, the center distance and screw mating geometry must also match
• As L/D increases — a longer process section can be provided for plasticizing and mixing; however, torque, residence time, heat generation and the machine's mechanical design must be evaluated together
• L/D selection — evaluated together with the PVC/CPVC formulation, CaCO₃ and additive ratio, target capacity, screw speed, gearbox torque, venting needs and product type

Sizing note: for a parallel twin screw-barrel order, "Ø90, L/D 28" information alone is not sufficient. Center distance, rotation direction, phasing, gearbox connection, barrel bore geometry, total length and feed/degassing locations must also be verified.

Important distinction: the product on this page is the parallel twin screw-barrel set used in extruders. The segmented, element-based co-rotating twin screw systems seen in compounding lines are a separate product group and should not be evaluated with the same design logic.

Which Applications Is It Used In?

• PVC pipe and profile extrusion
• CPVC pipe and profile applications
• PVC sheet, board and film production
• Micro-foaming PVC products
• Some PE applications where machine and formulation design are suitable
• Screw-barrel refurbishments for existing parallel twin-screw extruders

How Does It Work?

• Feed Zone — raw material enters the extruder and is conveyed through the barrel by the movement generated by the two screws. A feed geometry suited to the formulation's flow behavior is important for stable output.
• Pre-plasticizing / Compression — the raw material is compressed as it advances; plasticizing begins through barrel heating and the mechanical energy generated by screw motion.
• Mixing / Melting — the profiles of the two screws working relative to each other support material distribution and homogenization.
• Metering / Pressurizing — the plasticized material is delivered to the die with more stable flow rate and pressure. The final product geometry is formed by the extrusion head and die.

Screw rotation direction, thread geometry, center distance and phasing can vary by machine. For new screw-barrel production, the existing machine's gearbox output arrangement and original screw mating must always be verified.

Real production examples of twin-bore barrel geometry for parallel twin screw-barrel sets

Difference Between Parallel and Conical Twin Screws

• Screw diameter — constant along the length in parallel twin screws; changes from inlet to outlet in conical twin screws
• Axis structure — the two screw axes are parallel in parallel twin screws; the axis/center distance changes along the structure in conical twin screws due to the geometry
• Barrel — two equal-diameter parallel bores in parallel twin screws; a variable cross-section matching the conical screw geometry in conical twin screws
• Selection — determined according to the machine's original design in both systems
• Interchangeability — the two systems are not a direct alternative to each other

The choice between parallel and conical is not made by looking at the raw material alone. The gearbox, screw centers, torque transmission, L/D and the machine's mechanical design determine the selection.

Why Are Two Screws Used?

Compared to a single-screw system, a twin-screw structure can increase control over conveying and mixing, especially for multi-component formulations or those with more difficult flow behavior. For heat-sensitive, potentially highly filled formulations such as PVC, screw geometry is critical for running the process in a controlled way.

Material and Surface Treatment Options

• Nitrided — an economical and common solution for standard / low-to-medium abrasive formulations.
• Bimetallic Barrel — for CaCO₃, mineral filler, recycled material or higher-wear conditions; an alloy layer can be applied to the barrel inner surface to increase wear and corrosion resistance.
• Bimetal / Hardfacing Screw — for applications with high wear on the screw flights; special alloy / hardfacing solutions can be used on the flights and contact zones to extend service life.
• Special Alloy — for high wear, corrosion or special process conditions; a special material is selected according to formulation and operating temperature.

Bimetal Solution for Parallel Twin Screw-Barrels

Bimetal application is evaluated to protect the critical contact surfaces of the screw-barrel, especially in PVC/CPVC formulations with high mineral filler content and in operating conditions where wear accelerates. There is no single "standard bimetal" structure; the alloy and application method should be selected according to the formulation, temperature, wear type and existing machine geometry.

• Bimetallic barrel — an alloyed layer can be applied to the inner working surface of the twin-bore barrel to increase wear resistance and, where needed, corrosion resistance.
• Bimetal / hardfacing screw — wear-resistant hardfacing or alloy applications can be preferred on the crown and contact zones of the screw flights.
• High CaCO₃, mineral filler, recycled content ratio or continuous heavy-load operation make evaluating the bimetal option more important.
• Bimetal is not mandatory for every application. For low- and medium-abrasive formulations, a correctly nitrided material can be an economical and sufficient solution.

Why Is Wear Critical?

• Calcium carbonate (CaCO₃) and other mineral fillers can accelerate wear.
• Wear may not be equal on the two screws; the screw pair should be checked together.
• Rather than visual inspection alone, screw diameter, barrel inner diameter and critical mating zones should be measured.
• Material selection should be evaluated together with the filler ratio in the formulation and the target service life.

In a parallel twin-screw system, not only the screw-barrel working clearance but also the mating of the two screws relative to each other is important. Wear on the screw outer diameter, side surfaces or barrel bores can lead to backflow, capacity loss, uneven plasticizing and process instability.

Information Needed to Determine the Right Screw-Barrel

• Machine brand, model and year of manufacture
• Screw diameter, total screw length and, if available, L/D ratio
• Center distance between the two screws
• Screw rotation direction and gearbox output arrangement
• Existing screw and barrel technical drawings
• Barrel total length, twin-bore dimensions, flange and connection details
• Location of the feed opening, venting/degassing port and heater zones
• Raw material processed: PVC, CPVC, PE, etc.
• CaCO₃, pigment, additive and recycled content ratios in the formulation
• Current and target production capacity
• Problem experienced: wear, low capacity, high amperage, unstable pressure or plasticizing
• Photos of the old screw-barrel and measurement report, if available

In parallel twin screw-barrel production, center distance and screw mating are critical dimensions. Having the "same diameter and length" alone is not sufficient for compatibility.

Frequently Asked Questions

Is a single screw or a parallel twin screw more suitable?

The machine's design and process need are decisive. Single screw is very common in general PE/PP extrusion, while parallel or conical twin-screw systems can be preferred for formulations such as PVC/CPVC depending on the machine design.

Can a conical screw be fitted in place of a parallel screw?

No, they are not directly interchangeable products. Screw geometry, center distances, barrel and gearbox design are different.

Can only one screw be replaced?

While mechanically possible in some cases, the wear, profile and mating condition of both screws should be evaluated together. A mismatched new-old screw pair can create performance and reliability issues.

Is bimetal always necessary?

No. A nitrided or bimetal solution should be selected according to the abrasiveness of the formulation, the CaCO₃/mineral filler ratio and the expected service life.

Can production be done without an existing technical drawing?

Reverse engineering can be carried out by taking measurements from the existing screw-barrel. Critical dimensions and screw mating must be reliably extracted.

Share your machine brand/model, existing screw-barrel drawings and formulation information; let's evaluate the right geometry and material selection for your parallel twin screw-barrel set together.

Share Your Information