Solutions to Floating Fiber Defects of Glass Fiber Reinforced Molded Parts
2026-05-27 10:49:46
Injection Molds
Floating fiber is a common defect of glass fiber reinforced injection molded parts, manifested as exposed glass fibers, surface whitening, frosting and disordered texture. It not only damages appearance, but also reduces surface hardness, scratch resistance and coating adhesion. This problem can be systematically solved by optimizing raw materials, mold structure, injection process, equipment and product design.
Optimization of Raw Materials and CompoundingPriority shall be given to finished pellets with surface-modified glass fibers. Fibers treated with coupling agents and impregnants have stronger bonding force with resin matrix, which greatly reduces fiber exposure. Do not mix raw materials from different manufacturers or batches. The proportion of regrind materials shall be controlled within 10%, since excessive regrind will cause fiber breakage and poor resin encapsulation and worsen floating fiber. Regrinds must be uniformly crushed and kept free of impurities. Special dispersants, compatibilizers and lubricants can be added to improve fiber dispersion and inhibit fiber floating. Note that excessive lubricant will weaken the combination between resin and fibers and aggravate the defect. A small amount of elastomer can also be blended to enhance the overall encapsulation performance of melt.
Glass fiber reinforced plastics are hygroscopic. Moisture will cause bubbles and unstable melt flow and force fibers to expose on surfaces. Strict drying is required according to material specifications. For glass fiber reinforced PP, ABS and PA, the drying temperature is 80~110 ℃ with a duration of 2~4 hours. Hopper dryers are installed to prevent secondary moisture absorption.
Adjustment of Injection Molding Process ParametersTemperature SettingProperly increase the temperature of the middle and rear sections of the barrel to improve melt fluidity and encapsulation capacity, so that resin can fully wrap glass fibers. The nozzle temperature shall not be too high to avoid local material degradation and sharp viscosity drop which lead to loss of constraint on fibers. Keep a stable temperature range for uniform plasticization. Raising mold temperature is a critical solution to floating fiber. Higher mold temperature prolongs the flow time of surface melt and delays surface solidification, providing sufficient time for resin to cover exposed fibers. The mold temperature for general glass fiber reinforced materials can be set at 60~90 ℃, and the value shall be increased correspondingly for high-temperature engineering plastics. Low mold temperature will form a solidified layer rapidly and push fibers to the surface.
Injection Speed and PressureAdopt multi-stage low-speed injection. Use medium speed for runners and gates, and low speed at the initial stage of cavity filling to reduce shear rate. Severe shear will break the resin coating on fibers and cause exposure. Moderately increase speed in the middle and late filling stages to ensure complete cavity filling. Full high-speed injection is prohibited. Appropriately raise injection pressure and packing pressure, and extend packing time. Pressure compacts melt and promotes surface resin migration to cover exposed fibers. Insufficient packing will result in loose surface structure and obvious floating fiber. Multi-stage pressure setting is adopted to avoid sudden pressure change.
Back Pressure and Screw SpeedIncrease screw back pressure and reduce screw rotating speed. High back pressure enhances mixing and shearing of materials to realize uniform fiber dispersion and reduce agglomeration. Excessively high screw speed will generate fierce shear, break fibers and damage the coating layer. Adjust back pressure gradually to prevent material degradation caused by overheating.
Optimization of Mold StructureDesign runners with large cross-section and short flow path to reduce flow resistance and shear force. Wide and thin gates such as fan gates and side gates are preferred to expand feeding area and lower instantaneous shear at gates. Tiny pinpoint gates and horn gates are not recommended due to strong shear effect. Balance runners in multi-cavity molds to ensure consistent feeding in all cavities.
Poor venting will cause air trapping, local overheating and disordered surface melt, which indirectly induce floating fiber. Machining venting grooves at parting lines, flow ends, ribs and sharp corners. The depth is controlled at 0.015~0.03 mm to guarantee smooth ventilation without flash.
Fine polish cavity surfaces to reduce melt flow resistance. Optimize draft angles and use internally lubricated raw materials instead of external release agents. External release agents will penetrate into the surface layer and destroy the bonding between resin and fibers.
Optimization of Product Structure and Post-treatmentAvoid large-area thin walls, sharp corners and drastic wall thickness changes on products, which lead to abrupt flow velocity change and uneven shear. Keep uniform wall thickness, reasonable rib thickness and fillet radius to stabilize melt flow and reduce fiber exposure. Slight floating fiber can be eliminated by surface grinding, polishing and flame treatment. For products to be sprayed or electroplated, apply primer to cover surface defects.
On-site Troubleshooting TipsFollow the optimization sequence: adjust process parameters first, then improve raw materials, and finally modify mold structure. For slight floating fiber, raise mold temperature, increase back pressure and reduce initial injection speed preferentially. If the effect is unsatisfactory, replace modified materials or add special additives. Optimize gates and venting systems as the last resort. Regularly inspect barrel and screw wear. Severe wear will cause uneven plasticization and abnormal shear and trigger floating fiber, so worn parts shall be replaced timely.
