How Does a Two-Stage Extruder Solve the XLPE Extrusion Problem?

The quality and performance of XLPE cables are directly related to the safe and stable operation of power systems. However, the extrusion processing of XLPE is a challenging technology.

Compared with ordinary polyethylene, XLPE material is more prone to problems such as early crosslinking, poor plasticization, and residual air bubbles during extrusion, which seriously restricts production efficiency and product quality. These are specific manifestations of the “XLPE extrusion difficulties.”

This article will deeply analyze the “core difficulties of XLPE extrusion” and provide a detailed interpretation of how the two-stage extruder solves these difficulties, aiming to help cable compound manufacturers fully understand this advanced technology and provide technical references for enhancing the competitiveness of XLPE cable compounds.

XLPE Extrusion Difficulties

XLPE, or cross-linked polyethylene, is a high-performance material that forms a three-dimensional network structure between polyethylene molecular chains through a specific process. This crosslinked structure gives XLPE excellent heat resistance, environmental stress cracking resistance, and electrical insulation properties, but also makes its extrusion processing more complex.

1. Sensitive to Temperature, High Risk of Early Crosslinking

XLPE material is extremely sensitive to temperature in the molten state. A slight lapse in temperature control can trigger early crosslinking. Once early crosslinking occurs, the fluidity of the XLPE melt deteriorates rapidly, leading to:

Unstable extrusion process: Severe fluctuations in extrusion pressure, uneven extrusion volume, and even die blockage, seriously affecting the continuity and stability of production.
Frequent product quality defects: Early crosslinking leads to poor material plasticization, resulting in defects such as gels and scorching, directly reducing the performance and reliability of the cable insulation layer.
Increased equipment maintenance costs: Early crosslinked material adheres to the inner walls of the screw and barrel, making cleaning difficult, increasing equipment maintenance workload, and even potentially damaging equipment.

Accurate and stable temperature control is the top priority in XLPE extrusion processing.

2. High Melt Viscosity Demands Plasticization and Mixing

The crosslinked network structure of XLPE molecular chains significantly increases the melt viscosity, making it much higher than ordinary polyethylene. High viscosity means:

Increased plasticization difficulty: The material is more difficult to melt and plasticize evenly, requiring the extruder to have stronger shear and mixing capabilities.
Increased extrusion pressure: The melt flow resistance increases, requiring higher extrusion pressure to push the material forward, increasing equipment energy consumption and mechanical load.
Increased difficulty in temperature rise control: High-viscosity materials are more prone to generating shear heat under shear action, further aggravating the difficulty of temperature control.

Efficient plasticization and uniform mixing are the keys to overcoming the high viscosity characteristics of XLPE.

3. Volatile By-Products, Difficult to Remove Bubble Defects

During the chemical crosslinking process of XLPE, especially silane crosslinking, a small amount of low molecular by-products are inevitably generated. These by-products easily volatilize into gas at the extrusion temperature. If they cannot be effectively discharged, they will form bubbles inside the cable insulation layer.

Bubbles are one of the most common quality defects in XLPE cable insulation layers, which can lead to:

Reduced insulation strength: The presence of bubbles disrupts the continuity of the insulation layer, significantly reducing the withstand voltage strength and insulation performance of the cable.
Increased risk of partial discharge: The air medium inside the bubbles has a large difference in dielectric constant from the XLPE medium, which easily causes partial discharge and accelerates insulation aging.
Shortened product life: Under long-term operation, bubbles tend to expand and rupture, further accelerating insulation layer degradation and shortening cable service life.

Efficient and thorough vacuum degassing is a necessary means to eliminate bubble defects in XLPE cables.

The Inadequacy of Traditional Processing Methods

Faced with the three major challenges of XLPE extrusion mentioned above, traditional single-screw extruders appear to be somewhat inadequate in terms of technology:

Limited temperature control accuracy: Single-screw extruders have fewer temperature control zones, and the screw’s own temperature is difficult to effectively control. This makes it difficult to achieve precise temperature management of the XLPE extrusion process. Local overheating and temperature fluctuations are difficult to avoid, and the risk of early crosslinking is high.
Difficult to meet degassing efficiency requirements: Single-screw extruders have simple degassing structures and insufficient degassing power. For XLPE melts with high viscosity and difficult volatile component escape, the degassing effect is often poor, making it difficult to eliminate bubble hazards.
Difficult to balance plasticization quality and shear heat control: Single-screw extruders have an inherent contradiction between improving plasticization ability and controlling shear heat. It is difficult to simultaneously achieve sufficient plasticization and low-temperature extrusion of XLPE, and product performance improvement encounters bottlenecks.

The Two-Stage Extruder Becomes a New Solution to XLPE Extrusion Challenges

To break through the technical bottlenecks of traditional single-screw extruders, extruder manufacturers have innovatively developed two-stage extruders. This two-stage extrusion system ingeniously combines the efficient plasticization capability of a twin-screw extruder with the low-temperature precision extrusion characteristics of a single-screw extruder, achieving complementary advantages and perfectly solving various problems of XLPE extrusion.

The Essence of a Two-Stage Extruder

As the name suggests, a two-stage extruder consists of two extruders connected in series:

First Stage: Twin-Screw Extruder (Main Extruder)

The core tasks are to complete the forced conveying, efficient plasticization, uniform mixing, and initial pressure boost of XLPE material. The twin-screw extruder, with its powerful mixing capability and efficient conveying characteristics, prepares fully for subsequent low-temperature precision extrusion. Usually, a co-rotating parallel intermeshing twin-screw extruder is used to ensure the best mixing effect and stability.

Second Stage: Single-Screw Extruder (Secondary Extruder)

The core tasks are to achieve melt pressure stabilization metering, homogenization, low-temperature precision extrusion molding, and final die shaping. The single-screw extruder utilizes its simple structure to achieve precise control of the melt, especially excelling in low-temperature extrusion and eliminating melt pulsation. Usually, a specially designed low-shear single screw is used to further reduce the melt temperature and ensure the stability of the extrusion process.

Intermediate Connection Section and Vacuum Degassing System

The intermediate section connecting the twin-screw and single-screw extruders is a crucial part of the entire system. Usually, a highly efficient vacuum degassing system is set up here. Utilizing the conveying capacity of the twin-screw and the pressure stabilizing characteristics of the single-screw, a strong negative pressure environment is created to thoroughly extract volatile by-products from the XLPE melt, minimizing bubble generation. The intermediate connection section also has functions such as melt diversion, pressure stabilization, and temperature regulation, ensuring a smooth transition of the melt to the single-screw extrusion stage.

How Does Two-Stage Extrusion Solve XLPE Extrusion Challenges?

The two-stage extrusion system is not a simple accumulation of equipment, but a comprehensive solution carefully designed and optimized for the challenges of XLPE extrusion. It fundamentally solves the inherent defects of traditional single-screw extruders and brings a qualitative leap to XLPE cable production.

1. Precise Temperature Control, Completely Eliminating Early Crosslinking Troubles

Through precise temperature control in two stages, refined management of the XLPE melt temperature is achieved. The twin-screw stage can use a higher temperature for rapid plasticization, while the single-screw stage strictly controls low-temperature extrusion, effectively avoiding local overheating and temperature fluctuations. This eliminates the occurrence of early crosslinking from the source, ensuring a stable and reliable extrusion process and consistent product quality.

2. Efficient Degassing, Fundamentally Eliminating Bubble Defects

By setting up multi-stage venting ports in the intermediate connection section and the single-screw extruder, and using high-performance vacuum pumps, the system can establish an ultra-high vacuum degree, “uprooting” volatile by-products from the XLPE melt. This ensures that the final product contains almost no bubbles, significantly improving insulation performance and significantly extending cable life.

3. Gentle Plasticization, Fully Releasing Material Performance

The twin-screw extruder is responsible for providing powerful shear force to efficiently complete the plasticization task; the single-screw extruder is responsible for using low-shear extrusion to gently treat the XLPE melt, avoiding excessive shear and material degradation. This gentle plasticization method maximizes the protection of the inherent properties of the XLPE material, making the cable product excellent in mechanical strength, electrical insulation performance, and environmental resistance.

4. Stable and Efficient, Significantly Reducing Production Costs

While ensuring product quality, it also significantly improves production efficiency and effectively reduces production costs:

More stable extrusion process: Precise temperature control and efficient degassing system reduce process fluctuations, reduce downtime risk, and ensure long-term stable operation of the production line.
Higher first-pass yield: Excellent extrusion quality significantly reduces scrap rate, improves product first-pass yield, and reduces material waste and rework costs.
Higher degree of automation, lower labor costs: Advanced two-stage extrusion production lines are usually equipped with highly automated control systems and auxiliary equipment, reducing manual operation links and reducing labor costs.

Conclusion

We believe that through this in-depth analysis, the answer is clear. The two-stage extruder is not a simple equipment upgrade, but a “1+1 > 2” combination advantage that completely breaks through the technical bottlenecks of traditional single-screw extruders, opening up a new path for high-quality, high-efficiency, and low-cost production of XLPE cables.

Faced with the increasing market demand for XLPE cables and the increasingly fierce market competition, embracing two-stage extrusion technology is undoubtedly a key move for XLPE cable manufacturers to enhance their core competitiveness and gain the initiative for future development.

Contact us now to learn more about our two-stage extrusion solutions and improve the quality of your XLPE cable compounds!