Fiber-Reinforced Injection Molding – Glass Fiber (GF), Carbon Fiber (CF) and Long Fiber (LFT)

Introduction to Fiber-Reinforced Plastics

Fiber-reinforced plastic injection molding is a critical technology for engineers producing high-strength, rigid parts for automotive, electronics and industrial applications. Glass fiber (GF), carbon fiber (CF) and long-fiber (LFT) thermoplastics dramatically increase tensile strength and stiffness compared to unreinforced plastics, but introduce significant technical challenges related to fiber orientation, fiber reduction, porosity and anisotropic material properties.

This guide covers the physics of fiber injection molding, process parameter optimization, flow simulation and practical strategies for minimizing defects in fiber-reinforced part production.

Fiber Types: GF, CF, LFT

1. Glass Fiber (GF)

Glass fiber is the most commonly used reinforcement in plastic injection molding. It is characterized by:

• Fiber content: typically 10-40% by weight (% wt)
• Fiber length in pellets: 3-5 mm (or shorter in low-content grades)
• Single fiber diameter: 10-20 μm
• Density: 2.55 g/cm³
• Fiber tensile strength: 1500-3500 MPa
• Fiber Young's modulus: 70-80 GPa

Applications: Automotive parts (suspensions, connectors), electronics (housings, connectors), industrial (pumps, fittings).

2. Carbon Fiber (CF)

Carbon fiber offers higher strength and stiffness than glass fiber, but is more expensive:

• Fiber content: typically 10-30% by weight
• Fiber length in pellets: 3-5 mm (or shorter)
• Fiber diameter: 5-10 μm (thinner than glass)
• Density: 1.6 g/cm³ (lighter than glass)
• Fiber tensile strength: 3500-7000 MPa
• Fiber Young's modulus: 230-600 GPa (significantly higher)

Applications: High-performance automotive parts (engine covers, EV battery housings), aerospace, sports equipment.

3. Long Fiber (LFT)

LFT is a category of reinforced materials where fibers retain greater length during injection molding (rather than fragmenting):

• Fiber length after molding: 5-20 mm (compared to 0.5-2 mm in typical GF30)
• Process: LFT-PP, LFT-PA, LFT-PBT (polypropylene, polyamide, PBT-based)
• Manufacturers: SABIC, LyondellBasell, RTP Company, Hanwha, Quadrant
• Strength and stiffness: between GF30 and CF10
• Cost: intermediate between GF and CF

Applications: Structural automotive parts (door frames, suspensions), appliances (housings, frames).

Properties of Reinforced Materials

Effect of Fiber Content on Properties

Fiber content (% wt) drastically affects part properties:

• 0% (unreinforced): low stiffness, high elasticity, low tensile stress
• 10-15% GF: stiffness increases 50-100%, tensile strength increases 30-50%
• 20-30% GF: stiffness increases 150-200%, tensile strength increases 60-80%
• 30-40% GF: maximum reinforcement effect before drop-off from fiber reduction

Material Anisotropy

Fiber-reinforced parts exhibit anisotropy – different properties in different directions:

• Flow direction (MD): fibers oriented along flow direction, maximum reinforcement
• Transverse direction (TD): fibers less oriented, lesser reinforcement
• Thickness direction (ZD): fibers primarily in plane, weak properties in normal direction

The MD to TD property ratio is typically 1.5:1 to 3:1, meaning parts are significantly stronger along the flow direction than transversely.

Fiber Content and Its Impact

Optimal Fiber Content

There is a balance point between fiber content and part properties:

• Insufficient content (< 20%): weak reinforcement, minimal stiffness improvement
• Optimal content (20-30%): best strength-to-stiffness ratio
• Excessive content (> 35%): fibers irritate machine, fiber reduction, porosity

Fiber Reduction During Injection

Fibers fragment during injection molding due to:

• Shear in screw: shear forces in injection screw fragment fibers
• Turbulence in channels: turbulent material flow in cavity channels causes fragmentation
• Impact on mold walls: high-velocity fiber impact against walls causes shortening
• Impurities in material: sharp impurities in pellets are fracture points

Typical reduction: 4.5 mm fibers in pellets may be shortened to 0.5-1.5 mm in final parts.

Injection Process Parameters for Fibers

Material Temperature

Temperature must be sufficient for flowability, but not so high as to degrade fibers:

• GF-PP 30%: 230-260°C (typically 240-250°C)
• GF-PA 30%: 260-290°C (typically 270-280°C) – higher than unreinforced PA
• GF-PBT 30%: 250-280°C
• CF-PA 20%: 280-310°C

Note: Fibers reduce material viscosity, but may impede flow in narrow sections.

Injection Pressure

Fibers increase flow resistance, requiring higher pressures:

• Unreinforced PP: 50-100 MPa
• GF30-PP: 80-150 MPa (higher due to fiber flow resistance)
• GF30-PA: 100-180 MPa
• CF-PA: 120-200 MPa

Injection Time

Slower injection can reduce fiber fragmentation:

• Fast injection: rapid pressure rise, increased shear, more fiber reduction
• Slow injection: lower shear, reduced fragmentation, better fiber retention in part
• Optimal strategy: slow injection to 50-70% fill, then fast to completion

Hold Time (Packing Pressure)

Hold time should be adjusted for fiber content:

• Shorter hold (2-4 s): if preserving fiber orientation is priority
• Longer hold (5-10 s): typically necessary for reinforced parts

Fiber Orientation and Anisotropy

Orientation Layers in Parts

Fiber-reinforced parts have typical layered orientation structure:

• Outer layer (skin layer): fibers primarily oriented along flow direction (MD)
• Transition layer: mixed orientation
• Core layer: fibers may be oriented transverse (TD) or randomly

The thickness of these layers depends on part thickness and mold temperature.

Controlling Orientation

Engineers can influence fiber orientation through:

• Gate design: gate positioned in part center promotes uniform orientation
• Flow direction: fibers orient along flow path
• MFT simulation: Moldex3D, Autodesk Simulation can predict fiber orientation
• Mold temperature: higher mold temperature allows fibers greater orientation capability

Machine Equipment for Fiber Molding

Injection Screw

Standard screws may cause excessive fiber reduction. Specialized fiber screws have:

• Lower compression ratio: reduces fragmentation
• Optimal transitions: smaller temperature gradients
• Surface-hardened material: reduces wear from fiber abrasion

Injection System (Injection Unit)

System must be capable of generating sufficient pressure for fibers and have good temperature control.

Cavity Channels (Sprue, Runners, Gates)

Channels should be designed to minimize turbulence:

• Rounded edges in channels (instead of sharp)
• Gradual diameter transitions (instead of abrupt steps)
• Larger channel dimensions for fiber materials (reduces flow resistance)

Common Defects in Fiber Injection Molding

1. Porosity and Voids

Cause: gas entrapment during process, especially at high temperatures.

Solution: lower temperature, increase injection time, add mold venting.

2. Cracks and Fractures

Cause: high internal stress from fiber orientation and rapid cooling.

Solution: increase mold temperature, reduce cooling rate, increase fillet radii.

3. Short Shots (Incomplete Fill)

Cause: fibers increase flow resistance, insufficient pressure or temperature.

Solution: increase injection pressure, increase material temperature, optimize channel design.

4. Fiber Flow Lines

Cause: visible lines on surface where fibers are poorly oriented or show flow traces.

Solution: optimize temperature, increase mold temperature, change gate design.

5. Dull Surface

Cause: rapid cooling, fiber protrusion at surface.

Solution: increase mold temperature, reduce injection pressure.

6. Insufficient Part Strength

Cause: excessive fiber reduction, poor orientation, low fiber content.

Solution: optimize temperature and pressure, use specialized fiber screw, increase fiber content.

Flow Simulation and Orientation

Simulation Tools

Modern CAD tools can predict fiber orientation:

• Moldex3D: comprehensive simulation for GF, CF, LFT
• Autodesk Simulation: Moldflow with fiber orientation
• ANSYS: detailed flow and orientation analysis

These tools are invaluable for optimizing mold design and predicting part anisotropy before production.

Material Grades and Specifications

Common Combinations

• GF30-PP: polypropylene with 30% glass fibers (most common)
• GF30-PA6: polyamide 6 with 30% glass fibers (high performance)
• GF15-PBT: polybutylene terephthalate with 15% glass fibers (electronics)
• CF10-PA12: polyamide 12 with 10% carbon fibers (lightweight, high performance)
• LFT-PP: polypropylene with long fibers

Material Manufacturers

Major manufacturers of reinforced thermoplastics:

• SABIC: leader in GF and CF, portfolio includes Noryl, Lexan, Udel
• LyondellBasell: Hostalen, Lupolen, Pro-fax (GF PP)
• Dupont: Zytel PA reinforced with fibers
• BASF: Ultramid PA, LFT solutions
• RTP Company: custom fiber-reinforced materials

Best Practices for Fiber Injection Molding

1. Choose the Right Fiber Type

Selection between GF, CF and LFT depends on performance requirements and budget:

• GF: lowest cost, good reinforcement, most common
• CF: high performance, more expensive, for premium applications
• LFT: balance between GF and CF, better properties than GF

2. Use Flow Simulation

Simulate fiber orientation before mold design to optimize part properties.

3. Specialized Fiber Screws

Consider specialist screws designed for fiber-reinforced materials to minimize fiber reduction.

4. Optimize Process Parameters

Test temperature, pressure and times to find optimal balance between part fill and fiber reduction.

5. Control Material Moisture

Fibers can absorb moisture – dry material before injection (especially PA and PBT).

6. Monitor Material Degradation

Fiber-reinforced materials can degrade under certain conditions – monitor fumes and sprue color.

Summary

Fiber-reinforced injection molding (GF, CF, LFT) is an advanced technology that significantly improves part strength and stiffness. Key takeaways:

• Glass fiber (GF) is the most popular and economical
• Carbon fiber (CF) offers higher performance, but is more expensive
• Long fiber (LFT) is a compromise between performance and cost
• Fiber content typically 10-40% by weight, optimal is 20-30%
• Fiber orientation affects material anisotropy (MD vs TD have different properties)
• Fiber reduction is inevitable – minimize through optimal temperature and pressure
• Process parameters: higher temperature, higher pressure, specialized screws
• Flow simulation is invaluable for mold design and optimization
• Defects such as porosity, cracks and short shots are typical – resolve through parameter optimization
• Moisture content and drying are important for fiber-reinforced PA and PBT

Mastering fiber injection molding opens possibilities for manufacturing high-performance parts for automotive, electronics and industrial applications. Combining technical knowledge, good simulation tools and careful process management leads to parts of highest quality and durability.