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Advancements in Long Fiber Reinforced Thermoplastics

What is Long-fiber Reinforced Thermoplastic?

In recent years, there has been rapid development in fiber-reinforced thermoplastic composites based on thermoplastic resins. Pure thermoplastic materials typically lack sufficient strength and stiffness to meet the requirements of demanding applications, making them prone to fracture or failure under impact loads, thus limiting their suitability for applications requiring toughness and durability.Short-fiber Reinforced Thermoplastic (SFRT) provides a solution to these challenges by introducing short fibers into the thermoplastic matrix.

The fibers act as reinforcement materials, providing additional strength, stiffness, and impact resistance to the composite material. Subsequently, to further enhance mechanical performance, such as strength, stiffness, and impact resistance, long fibers are introduced into the thermoplastic matrix because short fibers dispersed in the matrix cannot provide as much reinforcement as continuous long fibers. The development of long fiber reinforced thermoplastic materials (LFRT or LFT) overcomes the limitations of traditional short fiber reinforced thermoplastic materials.

The main differences between long fibers and short fibers lie in their size and orientation. Long fibers typically reach the millimeter level, are relatively long and arranged in a more orderly manner, effectively increasing the material’s strength and stiffness. Short fibers, on the other hand, are generally at the micron level, relatively short in length, and have a more random distribution.

Initially, researchers and engineers began experimenting with incorporating continuous fibers such as glass or carbon into thermoplastic matrices to enhance the mechanical properties of materials. From the late 20th century to the 21st century, driven by advances in fiber reinforcement, resin formulations, manufacturing processes, and applications, LFRT technology has continued to develop. Today, LFRT is widely used in various industries due to its excellent mechanical properties, lightweight characteristics, and cost-effectiveness compared to traditional materials.

LFT (Long-Fiber Reinforce Thermoplastic)

By implanting relatively long fibers into the matrix, the material’s strength, stiffness, and load-bearing capacity are increased, making it more durable and reliable. The orientation of long fibers can also determine the material’s performance. For example, placing fibers perpendicular to the direction of force can increase bending stiffness, while placing them parallel can increase strength and tensile stiffness.

Applications of LFRT

LFRT is widely used in industries such as aerospace, automotive, and construction due to its enhanced mechanical properties, lightweight characteristics, and design flexibility.

For example:

1.LFRT materials are used in automotive seat structures and body panels, offering a high strength-to-weight ratio and impact resistance.
2.LFRT materials are used in aerospace applications for lightweight structural components, interior panels, and aircraft interiors. They meet the stringent requirements of aerospace applications while reducing overall weight and improving fuel efficiency.

Manufacturing Challenges

Uneven dispersion of long fibers in the thermoplastic matrix can lead to weak or inconsistent mechanical properties in local areas.

During processing, long fibers may break or damage, reducing their reinforcing effect and compromising the overall performance of composite materials.

The high processing temperature required to melt thermoplastic resins can lead to thermal degradation, affecting material performance and posing processing challenges.

Insufficient wetting of fibers by the thermoplastic matrix can lead to poor interfacial bonding and reduced composite strength.

Enhancing LFRT Production with Twin Screw Extruders

Twin-screw extruders allow precise control of processing parameters such as temperature, screw speed, and residence time, enabling flexibility in material formulation.

Due to their excellent mixing capability, twin-screw extruders ensure uniform dispersion of long fibers in the thermoplastic matrix. This results in improved mechanical properties and performance of LFRT materials.

The strong mixing action and advanced processing technology of twin-screw extruders facilitate better wetting of fibers by the thermoplastic matrix, improved fiber dispersion, thereby enhancing interfacial adhesion and composite strength.

Twin-screw extruders can continuously produce LFRT materials, providing high productivity and consistent product quality. This makes them highly suitable for large-scale manufacturing operations.

Manufacturing Process of Long Fiber Reinforced Thermoplastic

Common fiber-reinforced composite materials include glass fiber-reinforced composite materials (GFRP), carbon fiber-reinforced composite materials (CFRP), and aramid fiber-reinforced composite materials (AFRP).

Long fiber-reinforced composite materials are typically prepared by cutting continuous fibers soaked in resin into certain lengths. The common processing method is the pultrusion process, which involves stretching continuous roving mixed with thermoplastic resin through special molding dies to produce continuous bulky yarns. Currently, long fiber-reinforced PEEK thermoplastic composite materials can achieve structural properties of over 200MPa through FDM printing, with a modulus of over 20GPa, and will perform better through injection molding.

The fibers in continuous fiber-reinforced composite materials are “continuous,” ranging in length from meters to kilometers. Taking GFRP as an example, the length of glass fibers exceeds 3mm, while commercial LFRT is generally 6-25mm. Continuous fiber composite materials mainly provide laminates, prepregs, or fabrics, formed by impregnating continuous fibers with the desired thermoplastic matrix.

LFT-G-Impregnation-&-Extrusion3

Process Flow:

Briefly divided into the following steps:

Material Preparation
Preparation of raw materials, carefully selecting resin particles and fibers according to the performance required for the final LFRT product.

Composite Preparation
Thermoplastic resin is mixed in a twin-screw extruder to form a homogeneous composite. This is crucial for achieving the required mechanical properties.

Impregnation
After fully impregnating continuous fibers with molten thermoplastic resin in the impregnation mold, the LFRT compound is pulled out into uniform and consistent cross-sectional strip-shaped compounds through a shaping die.

Cooling, Cutting
Then, after cooling through a cooling system and cutting, the compound is cut into uniform and consistent granules in length. Finally, the LFRT granules needed are formed.

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