Long Fiber Reinforced Polymers (LFRPs) – Principles & Best Practices

This article introduces fundamental principles and best practices for processing long fiber reinforced polymers (LFRPs), including practical tips for preserving fiber length and maximizing performance in demanding applications.

Fiber Reinforcement Overview

Adding fibers—glass or carbon, long or short—enhances the mechanical and structural performance of thermoplastics. The main difference between short and long fibers lies in how much the fibers are processed:

Short fibers: Easier to process, but provide lower strength-to-weight ratios.
Long fibers: Require careful handling but significantly improve strength, toughness, fatigue resistance, and dimensional stability.

Preserve Fiber Length, Don’t Expect Perfection

Fiber length is critical for optimizing composite performance. Breakage reduces strength and toughness, potentially negating the advantages of long fibers. Causes of fiber breakage include:

Improper handling
Poorly designed tooling or molds
Unoptimized processing equipment and conditions

Long fiber materials are typically produced via pultrusion:

Continuous fiber rovings are pulled through an impregnation die.
Resin fully coats and bonds with fibers.
Fibers are chopped into pellets (~12 mm) with unidirectional reinforcement.

Left: short fibers after resin removal | Center: injection-molded part | Right: long fiber structure

Advantages of Long Fiber Reinforced Composites

Higher strength-to-weight ratio
Improved impact resistance
Longer fatigue life
Broader thermal stability
Superior dimensional stability

Carbon fiber composites can replace metals:

70% lighter than steel
40% lighter than aluminum

Common base resins: PA/Nylon, PP, ETPU, PEEK, PPA, PEI

Fiber content: Up to 70% for glass, 60% for carbon (PP, PA, TPU)

Semi-crystalline resins are better suited for fiber reinforcement than amorphous resins.

Key Considerations for Processing LFRPs

1. Mold Material & Design

Standard mold steels (P20) are suitable; for higher durability, use H13 or A9 steel.
Avoid small-diameter gates to reduce fiber shear. Wide fan-shaped or full-round gates are recommended.
Avoid sharp angles in runners; larger vents are acceptable.

2. Molding Equipment

Standard injection machines can process LFRPs with minor adjustments.
Recommended: low-pressure/general-purpose screw, minimum 5.6 mm nozzle, avoid tapered/nylon nozzles.
Fill only 60–70% of shot capacity to prevent excessive shear or residence time.

3. Processing Conditions

Warpage & Creep: Long fibers reduce warpage and creep compared with short fibers. Optimize gate placement and part design to minimize distortion.
Drying: Use dehumidifying dryer at –40 °C dew point.
Feeding: Pneumatic conveying with filters; avoid sharp bends.
Molding: Minimize screw shear and back pressure; avoid crushed regrind.
Injection Speed & Pressure: Slow speeds (25.4–50.8 mm/s) for high fiber contents; adjust within supplier-recommended ranges.

Part Design Guidelines

Promote fiber alignment along flow direction.
Uniform wall thickness: min 1.524 mm, optimal 3.175 mm, >5.08 mm reduces alignment.
Avoid long, flat sections without ribs.
Place weld lines strategically; fibers should reinforce weld lines.

Preserving Long Fiber Advantages

Adjust standard resin/short-fiber processing guidelines. Improper handling, mold design, or equipment setup can reduce or eliminate the high strength and toughness benefits. Following best practices justifies the higher cost of long fiber materials and ensures performance.