December 30, 2025
The issue of fiber floating on the surface of glass fiber-reinforced Polycarbonate (PC) plastics can significantly detract from both the aesthetic appeal and the mechanical performance of the final product. KEYUAN Plastics offers comprehensive solutions to mitigate this challenge by addressing key factors across raw material selection, processing techniques, and mold design.
I. Raw Material Optimization
(1) PC Resin Selection
Molecular Weight: Utilizing a PC resin with a higher molecular weight enhances melt strength and improves the encapsulation of glass fibers. Selecting a resin with a viscosity-average molecular weight within the range of 28,000-35,000 is highly effective in reducing the occurrence of floating fibers.
Purity: High-purity PC resin is essential. Impurities can interfere with the interfacial bonding between the glass fibers and the resin matrix, thereby increasing the propensity for fibers to float to the surface.
(2) Use of Compatibilizers
Incorporating a suitable compatibilizer, such as maleic anhydride-grafted PC (PC-g-MAH), is a proven method to enhance the interfacial adhesion between the inherently dissimilar glass fibers and PC resin. This promotes superior dispersion of the fibers within the matrix and minimizes their separation and migration to the surface.
II. Processing Parameter Adjustments
(1) Injection Temperature
Barrel Temperature: A controlled increase in the barrel temperature lowers the viscosity of the PC melt, facilitating better wetting and infiltration of the glass fiber bundles.
Mold Temperature: Maintaining a sufficiently high mold temperature aids the flow of the melt and allows for more controlled fiber orientation, reducing surface fiber exposure.
(2) Injection Pressure and Speed
Injection Pressure: Excessively high pressure can cause glass fiber breakage and increase floating. Employing a moderate injection pressure ensures proper mold filling while minimizing shear-induced damage to the fibers.
Injection Speed: Implementing a faster injection speed initially helps the melt fill the cavity quickly, reducing erratic fiber orientation. However, to prevent fiber breakage near the end of fill, a multi-stage speed profile is recommended—starting high and then reducing as the cavity nears complete fill.
(3) Holding Pressure and Cooling
Holding Pressure: Applying an appropriate holding pressure compensates for material shrinkage during cooling, preventing sink marks and surface defects that can exacerbate the appearance of floating fibers.
Cooling Strategy: A gradual, uniform cooling process allows for better consolidation between the fibers and resin. Optimizing cooling channel design and slightly extending cooling time can be beneficial.
III. Mold Design Considerations
(1) Gate Design
Gate Type: Pin-point or submarine gates are preferred as they enable the melt to enter the cavity with higher velocity and shear, improving fiber dispersion. Compared to side gates, they offer better control over flow direction, lowering the risk of fiber accumulation on visible surfaces.
Gate Location: The gate should be positioned at a thicker section of the part wall to ensure smooth melt progression. This avoids premature freezing in thin sections where fibers can accumulate. Its location should also be planned considering the predominant flow path to align with, not oppose, the desired fiber orientation.
(2) Runner System Design
Runner Size: A runner with ample cross-sectional dimensions (typically not less than 6mm in diameter, larger for big parts) reduces flow resistance, allowing fibers to be carried smoothly into the cavity without excessive separation.
Runner Surface Finish: A smooth inner runner surface (with a surface roughness Ra controlled below 0.2μm) minimizes friction and mechanical damage to the glass fibers during transit.
(3) Venting System
An efficient venting system is crucial to evacuate air and volatiles from the cavity promptly. Trapped gases can push fibers to the surface. Vents should be placed on parting lines, ejector pins, and cores, with depths typically between 0.02-0.05mm to allow gas escape without permitting melt flash.
