Benefits of Long Fiber Reinforced Thermoplastic Composites

Long fiber reinforced thermoplastic composites possess many unique capabilities that translate into desirable product benefits.  Alone or in combination, these benefits provide meaningful motivation to utilize long fiber composites in a broad spectrum of applications.

Balance of Properties

Long fiber composites are the pinnacle of thermoplastic performance.  They combine high levels of stiffness, strength, and toughness together in a single material.  No other method of reinforcing thermoplastics is able to match their performance trifecta of crucial properties.

The high mechanical performance characteristics of long fiber composites is the reason they are often chosen as substitutes for metals, as a replacements for under-performing plastics, or as alternatives to higher cost engineering polymers through up-engineering of lower cost plastics.

Stiffness

Adding fiber reinforcement to ductile thermoplastic polymers provides a substantial boost in modulus.  This modulus gain combined with appropriate component design that includes uniform walls combined with underlying ribs and gussets, instead of thick wall cross-sections, significantly increase stiffness in molded articles.

The type and amount of reinforcing fiber included in the composite contributes to increased stiffness.  Carbon fiber increases modulus more than glass or natural fibers and a composite with 50% fiber will be stiffer than one containing 30%.  Utilizing composites that offer more stiffness increases load carrying ability or allows designing with thinner wall sections to decrease material use and lower cost.

Stiffness gains through fiber reinforcement also translate into increased performance at elevated temperatures. Heat deflection temperatures (HDT), which provide an indication of short-term load carrying ability, increase significantly in fiber-reinforced materials over those of unmodified polymers.

Effects of Fiber Reinforcement in Thermplastic Composites

Strength

Longer length, or higher aspect ratio, of reinforcing fibers provide long fiber composites with increased strength, which translates into the ability to resist deformation or creep under loads and fatigue endurance with minimal compression.  More fiber surface area provides the ductile polymer with more ability to grab onto and transfer stress to the stronger internal fiber skeleton formed during component molding.

It is important to maintain maximum fiber length through careful control of processing parameters.  Fiber attrition can occur from shear in the injection molding press or from tight runner radii and improper gating in the mold.  Significant reduction in median fiber length will reduce performance.

Orientation of reinforcing fibers within injection-molded components also significantly influences composite strength.  Although long fibers intertwine to form an internal structural skeleton, they also align in the direction of polymer flow as molds fill.  To obtain maximum performance, design molds so fibers align perpendicular to the direction of stress forces during component use.

Toughness or Durability

Long Fiber Composites Internal Fiber SkeletonTypically, stiffer plastics are more brittle.  However, with long fiber composites the longer length of reinforcing fibers inverts that analogy.  The higher aspect ratio of the reinforcement in long fiber composites facilitates more efficient energy transfer between the polymer and fiber upon impact and dissipates those forces throughout the composite structure instead of localizing them in one area.

The higher toughness of long fiber composites makes them among the most durable of plastic materials with structural characteristics and ideal for applications that experience repeated impact forces but need to retain their shape without permanent deformation or deterioration of function.  Superior energy dissipation also increases their sound and vibration dampening capabilities.

The inclusion of long fiber reinforcement helps composites resist cracking and impedes crack propagation by forming an internal fiber skeleton.  Although long fiber composites offer excellent impact resistance, designs can permit failure at specified loads to prevent damaging forces from transferring to adjacent systems.  Longer fiber length also minimizes fragmentation during failure.

Additionally, long fiber composites retain a significant amount of their durability at elevated and low temperatures making them desirable for devices exposed to varying climates.

Weight Reduction

The high strength-to-weight ratio of long fiber composites makes them a suitable metal replacement medium for those seeking to reduce weight.  In conjunction with proper component design, long fiber composites can provide the same level of mechanical performance as many metals.

Weight Reduction using Long Fiber CompositesLightweighting or mass reduction is a core focus in aerospace and automotive markets where weight reduction facilitates increased fuel economy and decreased emissions.  In this context, long fiber composites are eco-friendly alternatives to heavier materials.  Using less of a higher performing material is not only a tactic to reduce weight but can also lower overall material costs, especially when fiber reinforcement is used to up-engineer the structural properties of a lower cost polymer.

For many consumer items, weight reduction promotes in-demand portability along with increased functionality and improved ergonomics.  Portability extends to tools and sporting goods where lighter weight components ease handling fatigue.  With carbon fiber reinforced composites there can be additional perceived value added to products by consumers for utilizing “high tech” materials.  This can provide a competitive marketing advantage over products made from simpler plastics.

In metal replacement applications, long fiber composites are most successful when components take advantage of plastics unique characteristics through redesign instead of attempting to use them as drop-in replacements.  Design analysis using computer simulations are especially helpful in attempting to duplicate structural performance attributes of metals in reinforced plastics as fiber alignment becomes a crucial design criteria.

Design Freedom

In regard to plastics, the concept of design freedom incorporates several sub-topics:

  • Complex Geometry – Switching from methods of metal forming to injection molding long fiber composites allows production of components with more complex geometries.  From sweeping curves to intricate details, designs that would be cost prohibitive to machine or difficult to cast in metals are easy and inexpensive to repeatedly produce using injection molding processing.
  • Parts Consolidation – The ability produce more complex 3-D shapes leads to consolidation of parts and the elimination of corresponding production, secondary operations, and assembly steps.  Injection molding can repeatedly produce net shapes that meet finished specifications.  Easily join components using laser, sonic, or thermal welding techniques.
  • Improved Functionality – More organic and aesthetically pleasing designs allow for integration of functions and ergonomics that would not be practical in other material mediums. These characteristics differentiate products produced using long fiber composites from competitors. Rigid thermoplastics are overmoldable with softer thermoplastic elastomers to improve ergonomics or add additional impact protection.

Lower System Cost

When deploying long fiber composites as a substitute for other materials one needs to look beyond material cost alone. Long fiber composites are typically processed via injection molding which is a highly efficient method to repeatedly produce high quality components in large quantities. Plastics also require fewer finishing operations than other materials and their increased design freedom can reduce component count eliminating assembly steps.

When all the costs of producing a component are accounted for, using a material that is easier to work with can result in lower overall costs even if it is more expensive on a per pound or kilogram basis.  Lighter weight materials also produce more components on a cubic volume basis.

Processing plastics requires low energy input producing favorable life cycle analysis (LCA) benefits and doesn’t create any toxic effluents. As a melt processable material, plastics are reformable and recyclable.

Corrosion Resistance

Steel can rust, wood can rot, and dissimilar metals used in tandem can lead to corrosion, but the anti-corrosive nature of thermoplastic polymers lends themselves to products with an indefinite lifecycle.

Some polymers are susceptible to degradation when exposed to certain classes of chemicals and types of radiation. When selecting an appropriate long fiber composite, verify that the host resin is compatible with the intended usage environment and operating conditions of your application to ensure compatibility with lifecycle requirements.

Dimensional Stability

Computer Aided Design of Long Fiber Reinforced Thermoplastic ComponentLong fiber composites are the most dimensionally stable of injection moldable reinforced thermoplastic materials. All plastics are susceptible to some level of shrink as they cool and harden as this aids in their removal from the injection molding tooling. The amount polymers shrink is consistent and accounted for in component designs.

As reinforcements align in the direction of polymer flow as molds fill, they can lead to anisotropic properties.  Anisotropic shrink can cause warping of components.  Because of longer fiber length in long fiber composites, they are prone to bending, tumbling, and intertwining to form an internal structural skeleton within injection molded components.  This random fiber alignment with long fiber composites results in improved dimensional stability with nearly isotropic shrink characteristics in comparison to other reinforcements.

Long fiber reinforced composites also exhibit reduced thermal expansion due to their internal network of reinforcing fibers.

Heat Resistance

The modulus increase that long fiber reinforcement provides also increases heat resistance of the composite.  Fiber reinforcement allows composites to retain mechanical function closer to the polymers melting point than an unmodified polymer.

Heat resistance for long fiber composites measured through deflection temperature under load (DTUL) or heat deflection temperature (HDT) provides an indication of the materials short-term load carrying ability at elevated temperatures.

Thermoplastics are viscoelastic materials; their mechanical properties are subject to time and temperature influences present in usage environments as well as morphologic characteristics as they near their melting point.  Composites using a semi-crystalline polymer matrix will retain some mechanical performance closer to the polymers melting point than an amorphous polymer.

Low & Elevated Temperatures

Long Fiber Reinforced Composite Cold Impact ResistanceThe ductile nature of polymers combined with the longer fiber length in long fiber composites allows them to retain more of their durability at low and elevated temperatures than other types of reinforced plastics.

Long fiber composites typically have similar impact performance at -60°F (-50°C) as they do at room temperature.  Higher elevated temperature performance corresponds to the morphology of the polymer and its melting point.

Recyclability / Life Cycle Analysis

Melt processable plastics are fully reformable and recyclable.  Lower melting points than metals require less energy input to fabricate components and processing or reprocessing doesn’t create any toxic effluents producing favorable life cycle analysis (LCA) benefits.

Thermal Conductivity

Thermoplastics have a lower thermal conductivity than metals; this provides a consistent and more comfortable tactile feel during handling at low and elevated temperatures.  Incorporating special additives into long fiber composites can facilitate thermal transfer if heat dissipation is required.

radiolucency

Plastic articles are transparent to short wavelength radiation and appear nearly invisible on x-rays or during fluoroscopy procedures. Inclusion of additional additives into long fiber composites can render molded articles radiopaque for medical diagnostic procedures.

They are also safe to use in proximity with magnetic resonance imaging (MRI) and computed tomography (CT or CAT) scan diagnostic equipment because of their non-magnetic characteristics.

Radio transparency and high dielectric properties makes reinforced thermoplastic an ideal material for consumer electronic items that utilize Bluetooth or other wireless transmission technologies.

Functional Performance

Custom Color Long Fiber CompositesThe inclusion of additives in addition to fiber reinforcement can provide thermoplastic composites with valuable performance enhancements that can expand their functionality and allow the creation of more effective products.

  • Add wear (surface abrasion and friction) resistance by incorporating internal lubricants that migrates to the surface to reduce maintenance and increase reliability or minimize noise between connected moving parts.
  • Modify the inherent electrical insulation nature of plastics with additive technologies to render them antistatic, static dissipative, conductive, or to provide EMI shielding characteristics.
  • Increase product safety by including flame retardants to provide halogen-free fire resistance/smoke density/smoke toxicity (FST) and UL94 compliance.
  • Custom colors modify appearance to increase appeal and ease identification. No longer do you need to compromise and receive only black parts to obtain structural performance.  Polymer flow enhancers produce smooth, resin-rich surface finishes free of visible fibers.

Additional Resources

Recorded Webinar

Technical White Paper

Molding Guide and Processing Conditions