Injection molded surface floating fiber
Surface fiber removal refers to the phenomenon of fibers being exposed on the surface of a fiber-reinforced plastic part during the injection molding process. This phenomenon manifests as white or gray fiber spots or streaks, severely affecting the part’s appearance and potentially degrading its surface and mechanical properties. Surface fiber removal primarily occurs in parts reinforced with materials like glass fiber and carbon fiber. Its causes are complex, involving multiple factors, including material selection, process parameters, and mold design, requiring analysis and resolution from multiple perspectives.
Material factors are one of the root causes of surface fiber floating. The compatibility of the fiber and the matrix resin is key. If the bonding between the fiber and the resin is weak, the fiber will easily separate from the resin during the injection molding process and migrate to the surface of the molded part. To improve compatibility, the fiber surface treatment is usually required, such as treating the glass fiber with a coupling agent to enhance its interfacial bonding with the resin. In addition, the fiber length and content also affect the fiber floating phenomenon. Excessive fiber length increases its flow resistance in the melt, making it easy for it to aggregate and float to the surface during flow. Excessive fiber content increases the melt viscosity and uneven fiber dispersion, increasing the possibility of fiber floating. Resin viscosity is also a key factor. When the resin viscosity is too low, the fiber’s ability to suspend in the melt is poor, making it easy for it to sink or float. When the viscosity is too high, it increases the difficulty of fiber dispersion and may also cause fiber floating.
Improper setting of injection molding process parameters is the main cause of surface fiber floating. One of the common causes is a melt temperature that is too low. Too low a temperature can lead to poor resin fluidity, reduced fiber and resin wettability, and the fibers are easily separated from the resin during the flow process, and then float to the surface. Properly increasing the melt temperature can reduce the viscosity of the resin, improve the compatibility between the fiber and the resin, and reduce the phenomenon of fiber floating. The selection of injection speed is also crucial. If the injection speed is too slow, the melt will flow slowly in the mold cavity, and the fibers will easily settle or accumulate on the surface under the action of gravity and flow resistance. If the injection speed is too fast, the melt may cause turbulence in the mold cavity, causing the fibers to be thrown to the surface. At the same time, high-speed flow increases the friction between the fibers and the mold surface, resulting in fiber exposure. Therefore, it is necessary to select an appropriate injection speed based on the structure and material properties of the plastic part so that the melt fills the mold cavity in a smooth laminar state.
Mold design significantly influences the occurrence of surface fiber floating. The location and form of the gate are key factors. Improper gate placement can create dead zones or eddies during the melt filling process, causing fibers to accumulate in these areas and float to the surface. For example, placing the gate away from thick-walled areas or complex structures can slow the melt flow rate upon reaching these areas, leading to fiber sedimentation. The gate form also affects the melt flow. For example, using a point gate results in a rapid melt flow through the gate, which can easily generate turbulence and increase the likelihood of fiber floating. Conversely, using a fan gate or side gate allows the melt to enter the mold cavity more smoothly, reducing fiber floating. Mold temperature is also a crucial parameter. Excessively low mold temperature can cause the melt to cool rapidly near the mold surface, causing the resin to solidify before the fibers, forcing the fibers to the surface. Properly increasing the mold temperature can prolong the melt’s flow time on the mold surface, allowing the resin to better wrap around the fibers and reduce fiber floating.
The dispersion of the fibers and the flow state during the molding process have a direct impact on the floating fiber phenomenon. During the raw material preparation process, if the fibers are unevenly dispersed and there is a phenomenon of fiber agglomeration, these agglomerated fibers can easily be brought to the surface during the melt flow process during injection molding, forming obvious floating fibers. Therefore, during the raw material mixing stage, it is necessary to ensure that the fibers can be evenly dispersed in the resin matrix. The dispersion of the fibers can be improved by optimizing the mixing process, using dispersants, etc. During the process of melt filling the mold cavity, the morphology of the flow front plays a key role in the distribution of the fibers. Laminar flow helps the fibers to be evenly distributed along the flow direction, while turbulent flow will cause fiber disorder and increase the possibility of floating fibers. By optimizing the flow channel design of the mold, reducing flow resistance, and avoiding turbulence at the flow front, the occurrence of floating fibers can be effectively reduced.
Comprehensive measures are needed to solve the problem of surface floating fibers. In terms of materials, fibers with good compatibility with the resin should be selected, appropriate surface treatment should be performed, the length and content of the fibers should be controlled, and resins with appropriate viscosity should be selected. In terms of process parameters, the melt temperature and mold temperature should be reasonably increased, the injection speed should be optimized, and the melt should be ensured to fill the cavity in a smooth laminar state. In terms of mold design, the position and form of the gate should be optimized, and reasonable runners should be designed to ensure smooth melt flow and avoid eddies and dead corners. In addition, additives such as compatibilizers and lubricants can be added to improve the interfacial bonding between the fiber and the resin and the fluidity of the melt, thereby reducing the floating fiber phenomenon. In actual production, it is necessary to continuously adjust parameters and optimize the design through experiments based on the specific structure and material properties of the plastic part in order to effectively solve the problem of surface floating fibers and improve the appearance quality and performance of the plastic part.
