Glass Fiber Protrusion in LFT Molding: Causes and Solutions
Glass fiber protrusion occurs when glass fibers become visible on the surface of a molded LFT part. Instead of being fully encapsulated by the resin matrix, the fibers sit partially exposed, creating a rough texture, poor aesthetics, and in some cases, accelerated wear or environmental degradation.
This is not a material defect - it is a processing defect. And in most cases, it can be fixed without changing the material or modifying the mold.
The Mechanism: Why Does It Happen?
Glass fibers and polymer resin have fundamentally different flow behaviors. During injection molding:
- The resin is a viscous liquid that flows readily under shear and temperature
- The glass fibers are solid rods that do not flow - they only move when carried by the resin
As the melt flows through the cavity, the resin near the mold wall experiences the highest shear and moves faster. The molten resin tends to flow toward the cooler mold surface, while the glass fibers - being less mobile - lag. The result: resin migrates to the surface, but fibers are left behind or pushed toward the surface by the advancing flow front. When the part cools and solidifies, the fibers remain exposed at the surface.
The core challenge: The resin wants to flow to the cold mold wall. The fibers cannot follow. The result is separation - and that separation manifests as surface protrusion.
Root Causes: Material, Process, and Mold
Material-Related Causes
- Excessive glass fiber content: Above 40%, the risk of protrusion increases significantly - the resin cannot encapsulate all fibers
- Poor fiber-matrix adhesion: Inadequate coupling agent or degraded sizing means fibers separate more easily from the resin during flow
- High moisture content: Moisture turns to steam at the melt front, carrying fibers to the surface as it escapes
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Chart 1: Higher glass fiber content increases the risk of surface protrusion. The data shows that parts with >40% glass content require more aggressive process controls to achieve acceptable surface quality.
Process-Related Causes
- Excessive injection speed: High velocity creates high shear, accelerating resin toward the mold wall and leaving fibers behind
- Low mold temperature: The resin freezes too quickly, before it has time to encapsulate the surface fibers
- Low back pressure: Results in non-uniform melt - fibers are not evenly distributed in the melt
- High screw speed: Excessive shear during plastication degrades the fiber-matrix interface before the melt even enters the cavity
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Chart 2: Higher mold temperatures improve resin flow and surface coverage. The data shows the relationship between mold temperature and the severity of glass protrusion across different LFT base resins.
Mold-Related Causes
- Undersized gate: High shear at the gate causes fiber-resin separation before the melt reaches the cavity
- Poor venting: Trapped gas pushes fibers to the surface, creating localized protrusion clusters
- Excessive wall thickness: Uneven cooling patterns can cause resin to shrink away from fibers at the surface
Solutions: How to Eliminate Glass Protrusion
Most glass protrusion problems can be solved by adjusting processing conditions. Start with these five approaches in order:
1. Increase Mold Temperature
Raise mold temperature to the upper end of the recommended range. For PP-LFT, target 80–90°C; for PA-LFT, 90–120°C. Warmer molds keep resin mobile longer, allowing it to flow over and encapsulate surface fibers before freezing.
2. Reduce Injection Speed
Use multi-stage injection: slow initial fill, then medium speed through the gate, and reduce speed in the final 10–20% of fill. This reduces shear at the flow front and gives resin time to cover fibers.
3. Lower Screw Speed
Reduce screw rotation speed to 50–80 rpm (versus 100+ rpm for short-fiber materials). Lower speed reduces shear during plastication, maintaining the fiber-matrix interface for better encapsulation during filling.
4. Optimize Back Pressure
Increase back pressure moderately (3–7 bar) to improve melt homogeneity. Better mixing means more uniform fiber distribution, reducing the likelihood of fiber-rich zones migrating to the surface.
5. Improve Venting
Add or deepen vents at the end of fill and at weld line locations. Trapped gas is a major carrier that pushes fibers to the surface - if gas escapes cleanly, fibers stay embedded.
Quick Troubleshooting Checklist
If you see glass protrusion on your LFT parts, run through this checklist before assuming it's a material or mold problem:
- ⭐ Mold temperature - Is it at the upper end of the recommended range for your material?
- ⭐ Injection speed - Are you using multi-stage speed control? Is the final fill stage slow?
- ⭐ Screw speed - Is it below 80 rpm? Could you reduce it further?
- ⭐ Back pressure - Is it sufficient to ensure melt homogeneity?
- ⭐ Venting - Are vents clear and located at all potential gas-trapping points?
- ⭐ Gate size - Is the gate at least 1.5 mm thick? Would a larger gate help?
- ⭐ Material drying - Is the material properly dried before processing?
Key takeaway: Most glass protrusion issues can be solved through process adjustments - mold temperature, injection speed, screw speed, and back pressure. Only after exhausting these options should you consider mold modifications or material changes.
Summary: Process Control Is the Answer
Glass fiber protrusion is not an inevitable consequence of using LFT materials. It is a processing defect caused by fiber-resin separation during flow - and it is fixable.
By systematically adjusting mold temperature, injection speed, screw speed, back pressure, and venting, you can achieve a surface that is smooth, fiber-free, and representative of the material's true performance potential.
The goal is not just to eliminate a visual defect - it's to ensure that the reinforcing network you paid for stays intact and delivers the mechanical performance your design requires.
LFT team: ready to solve your molding challenges
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, and industrial applications. Our engineering team provides on-site troubleshooting, process optimization, and material selection support for customers worldwide - from floating glass fibers to warpage and dimensional stability, we help you identify root causes and implement effective solutions. Reach out for technical consultation.
