Chopped Fiber

Chopped fiber molding forms complex, near-net-shape composite parts using short fiber reinforcements for higher stiffness and strength than neat polymers at production volumes.

Overview

Chopped fiber molding, also called short fiber molding, uses discontinuous fiber-reinforced compounds (thermoplastic or thermoset) in injection or compression tools to create near-net-shape parts. Short glass or carbon fibers are premixed into the resin, then flowed into a closed mold to capture fine details, ribs, bosses, and undercuts with good repeatability.

Use chopped fiber when you need stronger, stiffer, higher-temperature plastic parts without the cost and cycle time of continuous fiber layups. It fits small to medium-sized parts, moderate wall thickness, and medium to very high production volumes. You trade off some mechanical performance and fatigue life versus continuous fiber composites, and properties will be anisotropic due to fiber orientation. Expect tighter tolerances than hand layup processes but looser than precision machining. Tooling cost is significant, so it’s not ideal for low-volume or frequently changing designs.

Common Materials

  • Glass fiber reinforced nylon 6/6
  • Glass fiber reinforced polypropylene
  • Carbon fiber reinforced PEEK
  • Glass fiber reinforced PBT
  • Glass fiber reinforced epoxy BMC

Tolerances

±0.005"–0.010"

Applications

  • Automotive under-hood brackets and housings
  • Electrical enclosures and connector bodies
  • Appliance structural frames and panels
  • Power tool and equipment housings
  • Aerospace interior clips and brackets
  • Pump, valve, and small structural hardware components

When to Choose Chopped Fiber

Choose chopped fiber molding for medium to high production of small to mid-size parts where you want a metal replacement or upgraded plastic with good stiffness and temperature resistance. It works best for parts with detailed geometry, integrated features, and reasonably uniform wall thickness where moderate composite performance is sufficient. Avoid it for very thick sections or when you need highly directional, maximum-strength laminate behavior.

vs Resin Transfer Molding

Pick chopped fiber molding over RTM when you need high-volume production of smaller, more detailed parts with integrated ribs, bosses, and consistent cycle times. RTM favors larger, more structural parts and longer fiber architectures; short fiber molding is better for complex 3D geometry and automated, repeatable output.

vs Vacuum-Assisted Resin Transfer (VARTM)

Choose chopped fiber molding instead of VARTM when you need shorter cycle times and true production rates rather than low-volume, labor-intensive builds. VARTM shines for large panels with tailored fabrics; chopped fiber molding is more efficient for smaller, highly featured components and tighter dimensional control.

vs Prepreg Layup with Autoclave

Use chopped fiber molding when cost, throughput, and geometry complexity matter more than absolute mechanical performance. Autoclave prepreg gives superior strength and fatigue properties but is slow and labor-heavy; chopped fiber molding delivers acceptable performance at far lower piece cost for many structural plastic replacements.

vs Filament Winding

Select chopped fiber molding over filament winding when you need complex 3D shapes, bosses, or non-axisymmetric parts. Filament winding is optimized for cylindrical or rotationally symmetric components; chopped fiber molding handles irregular geometries, mounting features, and integrated details in a single molding cycle.

vs Compression Molding (Composites)

Favor chopped fiber molding with pellet or BMC feedstock when you want higher automation and better filling of fine details than traditional sheet-based compression molding. Conventional composite compression molding is strong for simpler, flatter geometries; chopped fiber compounds excel at more intricate, net-shaped parts with moderate section thickness.

Design Considerations

  • Target uniform wall thickness and use ribs instead of solid mass to reduce sink, warpage, and cycle time
  • Add generous fillets and avoid sharp internal corners to reduce stress concentrations and improve material flow and fiber distribution
  • Provide adequate draft (typically 1–2° or more) on all molded faces to enable clean ejection and reduce tooling wear
  • Design robust gating and flow paths; avoid very thin, long flow lengths that can cause short shots, poor fiber orientation, and weak weld lines
  • Keep critical load paths aligned with dominant flow directions where possible to take advantage of fiber alignment in high-stress regions
  • Specify realistic tolerances and critical-to-function dimensions clearly so the molder can plan gate locations, packing, and potential secondary machining