Why Do Your LFT Fibers Come Out Shorter Than They Go In?
Three processing factors that determine whether your part performs like LFT - or like a short-fiber compound.
This is the most common frustration in LFT processing. The material is right. The machine is running. But the fiber length in the finished part tells a different story - and that story shows up in failed impact tests, unexpected creep, and parts that don't meet the performance specified on the data sheet.
Fiber length is not just a number. It's the physical foundation of everything LFT promises: impact resistance, creep resistance, fatigue life, and dimensional stability. When fibers break during processing, those properties degrade - sometimes by 40% or more.
The good news? Fiber breakage is not inevitable. It's a processing problem with a processing solution. Three factors determine whether your fibers survive the journey from pellet to part.
Screw and Nozzle: The First Cutting Zone
The screw is where fibers first encounter shear. A standard short-fiber screw is designed to mix aggressively - and it does exactly that to your long fibers.
The key parameter is the metering section's compression ratio. Standard short-fiber screws typically run at 2.5:1 to 3.0:1. For LFT, this ratio should be reduced to approximately 2:1. Lower compression means less mechanical working of the fibers as they travel through the screw.
Equally important is the nozzle. Use a 100% free-flow design - no reverse-taper nozzles, no restrictive flow paths. The nozzle and gate orifice should be 5.5mm or larger, with no sharp edges. Every sharp edge is a fiber-breaking point.
Avoid: Standard short-fiber screws · High compression ratios · Reverse-taper nozzles · Nozzle orifices below 5.5mm · Sharp edges anywhere in the flow path.
Reverse Temperature Profile: A Counterintuitive Fix
Conventional injection molding uses a rising temperature profile - cool at the hopper, hot at the nozzle. For LFT, this is exactly backwards.
The problem with the conventional profile: pellets stay solid longer, so they enter the high-shear compression zone as hard, brittle solids. The fibers inside get crushed and snapped before the polymer has a chance to soften.
The solution is a reverse temperature profile. Heat the feed zone more aggressively so the polymer softens before it reaches the compression section. When the fiber-filled pellets are already surrounded by softened resin, the fibers can slide and orient rather than fracture.
This is not a subtle adjustment. In practice, reversing the temperature profile - warmer at the feed, cooler toward the nozzle - can be one of the single most effective changes for improving fiber length retention.
Key principle: Soften the matrix before the fibers reach the high-shear zone. The resin protects the fibers - but only if it's soft enough to flow around them.
Mold Design: Where Fibers Make Their Final Stand
The mold is the last place fibers can break - and often the place where the most damage occurs. Three design rules matter most.
Wall thickness
Aim for uniform walls around 3mm, with a minimum of 2mm. Thick sections allow fibers to flow and orient; thin sections force them through tight gaps where breakage is unavoidable.
Gate location
Gate into the thickest section and let the melt flow toward thinner areas. This gives fibers the maximum opportunity to distribute and orient before reaching restrictive geometry.
Radius every corner
Sharp internal corners around ribs, bosses, and mounting features act as fiber "guillotines." Every sharp edge in the flow path forces fibers to bend and break. Generous radii allow fibers to flow around features without fracturing.
The practical rule: If a feature would be a stress concentrator in a metal part, it's a fiber breaker in an LFT part. Flow design, not just for strength.
Processing Factors That Make or Break Fiber Length
LFT performance is not determined by the pellet alone. It's determined by what happens to the fibers between the pellet and the finished part. Screw design, temperature profile, and mold geometry all play a role - and all three can be optimized.
The most common mistake is treating LFT like a drop-in replacement for short-fiber compounds. It isn't. The processing window is different, the screw needs different geometry, and the mold needs different design rules.
But when those adjustments are made, the payoff is substantial: impact strength, creep resistance, and fatigue life that actually match the data sheet - and the reason you chose LFT in the first place.
LFT-G® materials are engineered to retain fiber length through processing. Our technical team can help you optimize screw configuration, temperature profile, and mold design to get the most out of every pellet. Reach out for processing support.
LFT team: ready to optimize your LFT processing
Xiamen LFT Composite Plastic Co., Ltd. is a global supplier of long-fiber-reinforced thermoplastics with 20+ years of innovation. We specialize in LFT‑PP, LFT‑PA, LFT‑PPS, and LFT‑Elastomers – offering high‑strength, lightweight alternatives to metal for automotive, aerospace, industrial, and consumer applications. Our engineering team provides material selection, mold design review, process optimization, and on-site technical support for customers worldwide. Reach out for a free consultation.
