Floating fibers on injection molding surfaces and solutions
Surface floating is a common defect in injection-molded fiber-reinforced plastics (FRPs). It manifests as white or dark fiber strands protruding from the surface of the molded part, affecting not only the appearance but also potentially degrading the material’s mechanical properties. The root cause of floating is insufficient compatibility between the fiber and the matrix resin, as well as differences in their flow behavior during the molding process. In FRPs, the density of the fiber typically differs from that of the resin. For example, the density of glass fiber is 2.5 g/cm³, while that of PA6 resin is 1.14 g/cm³. This density difference facilitates separation of the fibers from the resin during melt flow. As the melt flows through the mold cavity, the fibers tend to accumulate at the melt front due to inertia. If this front cools too rapidly, the fibers cannot be covered by the subsequent melt, resulting in exposed floating fibers. Fiber length and content also affect the degree of floating. Long fibers (5-10 mm in length) are more prone to uneven distribution than short fibers (0.2-0.5 mm in length). When the fiber content exceeds 30%, intertwined fibers become more difficult to encapsulate within the resin, significantly exacerbating the floating phenomenon.
Optimizing injection molding process parameters is a key approach to resolving fiber floating issues. Adjusting the melt temperature can effectively improve the compatibility of fibers with the resin. Properly increasing the melt temperature can reduce resin viscosity, enhance its ability to wet the fibers, and ensure a more even dispersion of the fibers within the matrix. For example, the typical melt temperature for glass-fiber-reinforced PP is 180-200°C. Raising the temperature to 210-220°C increases the fluidity of the PP resin, enhancing its fiber encapsulation and significantly reducing fiber floating. However, it is important to avoid excessively high temperatures, as this can lead to resin degradation and exacerbate fiber floating. Controlling the injection speed is equally important. High-speed injection reduces the cooling time of the melt in the mold cavity, allowing the fibers to be quickly filled before they are fully encapsulated by the resin. Experiments have shown that increasing the injection speed from 50 mm/s to 100 mm/s for long-fiber-reinforced PA66 parts can reduce the surface area of fiber floating by over 60%. However, high-speed injection requires appropriate holding pressure to prevent uneven fiber distribution caused by turbulence.
Optimizing mold structure design can fundamentally reduce the occurrence of floating fibers. The location and form of the gate significantly influence fiber distribution. Using multi-point or fan-shaped gates allows the melt to evenly fill the cavity from multiple directions, reducing fiber accumulation in a single direction. For example, in the molding of large, flat plastic parts, using four symmetrically distributed point gates can shorten the melt flow path, achieve more uniform fiber distribution, and reduce the incidence of floating fibers by 50%. Runner design should avoid sharp bends or sudden changes in cross-section. Using a gradual runner can reduce shear force differences during melt flow, preventing fiber accumulation at the runner edges due to shear. Furthermore, the mold’s venting system must be efficient to ensure that gases generated during filling are discharged promptly. Otherwise, trapped gases can hinder melt flow, leading to fiber accumulation at the edges of the venting area, forming floating fibers. The venting groove should be 0.02-0.05mm deep and at least 5mm wide to effectively vent gases and promote even fiber distribution.
Modification of raw materials is a long-term solution to the problem of floating fibers. By improving the interfacial bonding between the fiber and the resin, fiber dispersion can be significantly enhanced. Surface treatment of fibers is a common method. For example, glass fibers can be treated with silane coupling agents. The organic groups of the coupling agent bind to the resin molecules, while the inorganic groups react with the hydroxyl groups on the glass fiber surface, forming chemical bonds and enhancing compatibility between the two. Treated glass fiber-reinforced PA6 can reduce the number of surface floating fibers by over 70%, while also improving mechanical properties. Adding a compatibilizer to the raw materials can also be effective. For example, in a PP/glass fiber system, maleic anhydride-grafted PP is added. The polar groups in the compatibilizer molecules interact with the fiber surface, while the non-polar portion is compatible with the PP resin, effectively promoting uniform fiber dispersion and reducing floating fibers. For extremely demanding plastic parts, specially treated “low-floating fiber materials” can be used. These raw materials have optimized fiber surface treatment and formulation during production, mitigating the risk of floating fibers from the source.
Detailed control during the production process is equally important for reducing floating fibers. Operators must strictly adhere to process specifications to ensure stability at all stages. First, the raw materials must be mixed evenly. The premixing process for the fiber and resin should prevent fiber agglomeration. A high-speed mixer can be used to stir at a low speed for 5-10 minutes to ensure even dispersion before adding the fibers to the barrel. Second, the barrel must be kept clean. If not thoroughly cleaned when changing raw materials, residual resin or impurities can affect the bonding between the fiber and the matrix, leading to localized floating fibers. Furthermore, precise mold temperature control is crucial. Properly increasing the mold temperature can extend the flow time of the melt in the mold cavity, facilitating resin encapsulation of the fibers. For example, increasing the mold temperature from 50°C to 80°C can increase surface coverage of floating fibers by 30% for PA66/glass fiber parts. Furthermore, the mold cavity surface should be regularly inspected to ensure it is free of scratches and oil stains to prevent fibers from becoming snagged or exposed due to uneven surfaces. These detailed controls together form a comprehensive system for addressing floating fiber issues.
