Frequently Asked Questions About Very Long Fiber Composites


What are long fiber composites?

Long fiber thermoplastics (LFT) are bulk molding materials that feature continuous fiber filaments running the full length of the pellet allowing these materials to exhibit simultaneous improvements in strength, stiffness, and impact resistance over a wide temperature range.

Heralded as an exciting growth area in thermoplastics, LFT composites are widely accepted as a viable alternative to traditional reinforced thermoplastics and metals for a vast array of applications.

What process is used to make long fiber composites?

RTP Company uses a pultrusion process to manufacture long fiber thermoplastics (LFT), which entails pulling continuous fiber rovings through a polymer melt in a specialized processing die. The resulting composite strands are cooled and chopped into pellets.

Pultrusion is dramatically different than the process used to manufacture short fiber thermoplastics (SFT) where chopped fiber is melt-blended with plastic resin using conventional extrusion compounding methods. The fiber length in a SFT pellets is typically less than 1 mm while in LFT pellets fiber length is up to 1/2 inch.

What is “wet-out” and why is it important?

Wet-out refers to the impregnation of the fiber rovings with thermoplastic resin during the pultrusion manufacturing process. Successful wet-out completely coats the individual filaments and results in pellets that are free of loose fibers or other debris which can cause problems during conveying.

How long are long fiber composite pellets?

Standard pellet length for long fiber composites at RTP Company is 7/16 inch, or 11mm, although other lengths can be specified in the 6 to 12 mm range. Experience has shown that this length provides optimal properties while still allowing processing in standard injection molding equipment.

Why is fiber length important in reinforced plastics?

The fiber is the structural component of long fiber composites. The longer the fiber, the more effectively the polymer is able to “grab on” and transfer stress to the fiber. At lengths around 1/2 inch the strength of the fiber is fully attained, and longer lengths provide no significant improvement in physical properties while making the material more difficult to process.

Are fibers other than glass available?

Long fiber thermoplastic (LFT) composites are available with glass, aramid, stainless steel, or carbon fiber reinforcement at loadings up to 60% by weight.

  • Glass fiber is the most popular and cost effective reinforcement used for LFT composites
  • Aramid fiber is utilized for its excellent wear resistant properties
  • Sainless steel fiber for its electrostatic dissipation (ESD) and electromagnetic interference (EMI) shielding properties
  • Carbon fiber provides additional enhancements in flexural modulus and potential weight reductions while also providing ESD properties


What are typical long fiber loading levels?

Available fiber loading levels for most thermoplastic resins are 20, 30, 40, 50, and 60% by weight. Balancing performance, cost, and weight are critical criteria in selecting a correct polymer to fiber ratio. In general, 40 and 50% loading levels are most common.

What polymers are long fiber composites available in?

RTP Company has commercialized long fiber thermoplastics (LFT) in: polypropylene (PP), nylon 6 (polyamide 6), nylon 6/6 (polyimide 6/6), PPA, PBT, polyurethane elastomer (TPUR), PC/ABS alloy, PPS, and PEEK. Other semi-crystalline engineering resin systems offer potential for development if appropriate end-use applications present themselves.

Where does RTP Company manufacture long fiber composites?

Our network of production facilities is located on the three continents (Asia, North America, and Europe) where major manufacturing enterprises are concentrated.

To ensure timely and efficient delivery our long fiber thermoplastic (LFT) composites are produced at two sites in North America (Winona, MN and Indianapolis, IN); at our European hub in Beaune, France; and at our new facility in Suzhou, China. RTP Company is committed to adding additional manufacturing capacity as the market for LFT composites grows.


What performance characteristics make long fiber composites unique?

Long fiber thermoplastic (LFT) composites bridge the price-to-performance gap between short fiber thermoplastic (SFT) materials and advanced composite materials while still being processable via efficient injection molding methods.

Successful end-uses often require greater performance than is attainable with SFT products and where metal is not a desirable option due to weight or per part costs. LFT composites have gained rapid acceptance as a viable alternative to die castings, metal assemblies, and traditional plastic materials in a vast array of markets, including: industrial equipment, automotive, consumer goods, sports and leisure, and information technology.

What are the typical properties and benefits of long fiber composites?

PyrolysisThe benefits of long fiber thermoplastic (LFT) composites can be easily understood by examining the entangled long fiber matrix formed in a finished part. Exposing a molded part to pyrolysis conditions, commonly referred to as burn-off, removes the polymer and leaves behind the glass reinforcing fibers allowing visual examination. The fiber network that is formed by an LFT material retains the shape of the molded part and resembles a fiber preform with well and evenly distributed fibers.

The extensive fiber network in an LFT part serves to improve most all performance characteristics and also inhibit crack propagation. Often referred to as “stiff and tough,” LFT composites exhibit a wide range of performance advantages:

  • Better impact resistance and rigidity at elevated and sub-zero temperature ranges (up to 5 times that achieved with short fiber thermoplastics)
  • Modulus retention at elevated temperatures
  • Creep resistance under constant load in severe environments
  • Dimensional stability and resistance to warpage resulting from reduced shrinkage — which occurs as thermoplastic materials solidify during molding

How much do long fiber composites cost, relative to other reinforced materials?

Relative to short fiber materials long fiber thermoplastic (LFT) composites are slightly more expensive, but yield price-to-performance advantages that make them attractive for end-use applications that need an engineered material solution. Due to the efficiencies of the injection molding manufacturing process LFT composites result in a lower per piece cost when replacing metal and provide a significant reduction in weight.

Are long fiber composites really better than short fiber?

Each class of material has unique benefits. Polymers reinforced with long fiber offer elevated performance at levels unattainable by similarly reinforced short fiber materials. The physical requirements of individual applications should dictate which material is best positioned to provide a solution. Modulus is nearly equivalent between short and long fiber products, remaining more dependent on filler content than fiber length.

What custom formulating capabilities does RTP Company have?

RTP Company has multiple engineers dedicated to long fiber composite product development. You can draw on their expertise during the application assessment and material selection process, or at anytime during your product development or manufacturing cycle. In total, RTP Company has over 30 R&D engineers with expertise across a variety of disciplines — ranging from color, conductive/antistatic, wear resistance, and flame retardants — who can assist with any application requirement you may have.

Which automotive specifications do RTP Company materials meet?

RTP Company has many specialty compounds certified to the automotive specifications of major OEMs. Please contact RTP Company to inquire about a specific standard to determine which materials are suitable.

How does RTP Company handle special requests?

Customizing materials to end-use requirements is a hallmark of RTP Company. Our experienced team of over 30 product development engineers are highly accessible and distributed worldwide, allowing them to answer your inquiries personally. Depending on usage volume, additional additives can be incorporated directly into our long fiber composites to meet your requirements or combined as a cube blended long-cut masterbatch.

Are long fiber composites colorable?

Yes, RTP Company offers color masterbatches that can be combined with our long fiber products during molding; these masterbatches are optimized to preserve fiber length. Choose from our portfolio of standard colors or have our specialists custom match a color to your precise standard.

Application Development

Why should I use long fiber composites?

Reasons for using long fiber thermoplastics (LFT) vary, but a perennial conclusion is that the application would not be feasible using any other plastic material. LFT composites have opened possibilities for plastics to displace traditional materials and gain improved performance, design freedom, corrosion resistance, and weight reduction.

How do I choose the right long fiber composite?

Ask our engineers for a material recommendation! RTP Company has a streamlined application assessment process that considers many criteria, such as operating temperature, static or kinetic loads, impact, cost targets, and environmental considerations to identify a long fiber composite that will meet your applications’ demands. In-house design engineers can also be tapped ensure a smooth transition from concept through production.

Who can help me determine if long fiber composites are right for my application?

Any of our product development engineers, regardless of their area of expertise, can help you identify your applications’ unique requirements and recommend a plastic compound that will meet your precise needs. Our engineers are polymer experts renowned for their accessibility and ability to solve your application problem.

Can plastics replace metals?

Reinforced plastic compounds have replaced metal in many applications by providing better aesthetics, corrosion resistance, and opportunities for part consolidation. When choosing long fiber composite materials designers can now exploit these benefits when confronting demanding load requirements of structural components.

Is metal replacement common with long fiber composites?

Die castings and metal assemblies are opportunities to apply the advantages of long fiber thermoplastic (LFT) composites to new applications. To bring new designs to reality, while avoiding typical metal concerns (corrosion, painting, recycling, etc.), designers regularly choose reinforced plastic compounds. In instances where other reinforced plastics do not fulfill performance requirements of complex parts, LFT composites are the logical choice.

Can long fiber composites replace short fiber reinforced thermoplastics?

Use of longer reinforcing fibers elevate the performance of thermoplastics to such a high degree that cheaper resins begin to provide the properties of more expensive engineering polymers. In many instances, long fiber composites have proven themselves to be an economically viable alternative to other reinforced plastics. Contact any of our sales engineers for assistance in examining the differences between available reinforcing technologies.

What application development support is available?

RTP Company’s global Technical Service team is available to provide expertise regarding design analysis, mold construction, pre-production trials and trouble-shooting. A wide variety of Computer Aided Design analysis and metal replacement tutorial services are also available.


Can I convert a die cast tool to evaluate long fiber composites?

Many die cast tools can easily be modified to allow them to be used for plastic injection molding. Often, this is a convenient first step in creating a meaningful prototype to measure success potential for changing materials.

How can I get a sample?

Samples of our long fiber composites, or any other RTP Company product, can be obtained by contacting a sales engineer or business manager who servers your geographic area or market segment. Use this website’s People Finder feature to locate an RTP Company representative or call us at 1-800-433-4787.

Processing & Handling

How are long fiber composites delivered?

Typically, specialty compounds manufactured by RTP Company, including long fiber composites, are packaged in 1,100 pound gaylord bulk material boxes. When ordered in 40,000 pound quantities long fiber composites can be delivered via bulk truck and directly conveyed into your material silo.

Are long fiber composites molded similar to other thermoplastics?

Yes, care should be taken to maximize fiber length in the molded article to maintain physical properties. Typical accommodations include: use of free flowing check-rings and general purpose screws; generous mold gates and runners; and radiused corners and direction changes. A detailed Long Fiber Molding Guide is available to reference.

Are long fiber composites abrasive to tools and molding equipment?

All reinforced plastics, particularly those with fiber ends, are more abrasive to molding machinery than unfilled resins. Since long fiber composites have fewer fiber ends than an equal loading by weight of short fiber material, it wears less on equipment.

Is special equipment needed to mold long fiber composites?

No special equipment is required to injection mold long fiber thermoplastic (LFT) composites. However, to produce quality parts care should be taken to minimize fiber breakage and maximize fiber distribution and packing. A detailed Long Fiber Molding Guide is available to reference which discusses these and other issues related to molding LFT composites.

What are the most important things to remember when molding long fiber composites?

Use low shear conditions to minimize unnecessary fiber filament breakage and maintain maximum physical properties in finished parts. Low shear conditions commonly include: proper barrel temperatures, slower screw rotation, minimal back pressure, and moderate injection speeds.

What is typical fiber breakage during molding and how does it affect properties?

Even with the utmost care, some fiber breakage will occur during the molding process – this is to be expected. Successful molding practices minimize broken fibers and result in a normal bell-curve distribution of complete long and broken fibers.

If excessive fiber breakage occurs molded articles will not have the high stiffness and toughness associated with well made long fiber composite parts. Consult our Long Fiber Molding Guide for details on maintaining optimum physical properties during molding.

How do I check if fiber length is maintained?

Look for evidence of long fibers protruding from the sprue entry. Conducting a pyrolysis, where the polymer is burned away leaving the fiber matrix intact, allows fiber distribution, packing, and breakage to be visually examined.

Can long fiber composites be molded in very thin walls?

Long fiber thermoplastic (LFT) composites exhibit excellent flow characteristics, even at high fiber loadings of 60% by weight. Not only can LFT composites flow long distances, they routinely fill intricate and thin wall sections with remarkable ease. Evaluation of any material in its end-use design should always be conducted to ensure appropriate properties are maintained.

Are long fiber composites suitable for molding large parts?

Long fiber thermoplastic (LFT) composites are regularly molded into parts weighing in excess of 10 pounds and with surface areas comparable to doorways. The limiting factor is not LFT composites, but the availability of large injection or compression molding equipment.

Why should I buy long fiber composite pellets instead of using direct in-line compounding (D-ILC or D-LFT)?

To be economically viable, direct in-line compounding demands large numbers of large parts to be molded on a single machine. Deploying a direct in-line compounding system also requires a significant investment in specialized equipment that must be kept continuously in operation along with the development of in-house compounding expertise. For many applications and molders purchasing long fiber composites as precompounded pellets is a better option because of its flexibility and reduced capital investment.

Why should I buy long fiber composites from RTP Company?

Supplier selection criteria vary, but most customers rely on RTP Company because of our broad portfolio of specialty compound products and the engineering expertise to support them during development and production. All RTP Company manufacturing facilities are registered to ISO 9001:2000 quality standards and are located across the globe allowing consistent and timely delivery no matter where you do business.

Can long fiber composites be air conveyed?

Yes, the 1/2 inch size of long fiber compoiste pellets can be easily accommodated by most air conveyance systems. Full “wet-out” of long fiber composites minimizes loose fibers and other fines that cause problems in air conveying systems. Use of abrasion resistant elbows is recommended.


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