GF reinforced plastic products floating fiber problems and solutions
Preface
The image on the left shows an automotive part made of polyamide 66 resin reinforced by 30% glass fiber (GF30 PA 66) . Floating fibers are clearly visible in the lower left corner.
Floating fibers in glass fiber-reinforced plastics not only affect the plastic's appearance, causing a mottled surface (particularly noticeable in dark-colored parts, where the floating fiber areas appear significantly lighter in color than other areas) and uneven textures, but also significantly degrade the component's structural strength and mechanical properties, directly impacting its durability.
Cause Analysis:
- The difference in fluidity between glass fiber and melt. Due to the difference in fluidity and mass density between glass fiber and resin, they tend to separate during the melt flow process. In polyamide products, the high-fluidity polyamide will accumulate in the flow direction, while the low-fluidity glass fiber will be entrained and float on the surface of the product.
- Frictional shearing forces lead to local variations in viscosity. During the plastic melt flow process, the plastic melt is affected by the frictional shearing forces of the screw, nozzle, runner, and gate, which results in local viscosity differences in the plastic product, damaging the interface layer on the surface of the glass fiber. The lower the melt viscosity, the more severe the damage to the interface layer, and the weaker the bonding force between the glass fiber and the resin, causing the glass fiber to break away from the constraints of the resin matrix, and thus float to the surface and accumulate.
- The fountain effect occurs when the melt is injected into the mold cavity. When the glass fiber contacts the mold wall surface, due to the lower mold surface temperature, the glass fiber cools faster than the resin particles. If it cannot be quickly surrounded by the resin melt, the glass fiber will float out.
Cause Investigation and Solution
Customers mixing glass fiber and resin themselves can result in poor material uniformity. Insufficient dispersion of the glass fiber in the resin matrix can lead to localized fiber accumulation. In comparison, the glass fiber reinforced plastic material pre-processed in the factory is more stable. Through twin-screw extrusion granulation, the fiber dispersion is more uniform. Additives such as silane coupling agent, maleic anhydride grafting compatibilizer, silicone powder, fatty acid lubricant, etc. are also added to improve the interface compatibility between glass fiber and the value, improve the uniformity of the dispersed phase and continuous phase, increase the interface bonding strength, and reduce the separation from the resin.
| Material Classification | Grade | Glass fiber Content |
| Polyphenylene sulfide (PPS) | GF40 PPS | |
| GF40 PPS | ||
| GF40 PPS | ||
| Liquid crystal polymer (LCP) | GF40 LCP | |
| GF30 LCP | ||
| GF30 LCP | ||
| Polyamide 66 (PA66) | GF30 PA66 | |
| Polyamide 6 (PA 6) | GF30 PA 6 | |
| Polyetheretherketone (PEEK) | GF30 PEEK |
Because glass fiber reinforced plastics have a lower melt index and poorer flow properties than unreinforced plastics, processing temperature, especially barrel temperature, is crucial for glass fiber reinforced plastics. Generally, the barrel temperature for glass fiber reinforced plastics should be 10-30°C higher than for unreinforced plastics. This increase in barrel temperature significantly increases the melt viscosity of fiber reinforced plastics, improves material flow, avoids glass fiber accumulation caused by glass fiber orientation problems, and reduces surface roughness of plastic products.
The second important factor is mold temperature. A large temperature difference between the mold and the melt should be avoided to prevent glass fiber accumulation due to preferential cooling of the glass fibers as the melt fills the mold cavity. However, higher mold temperatures increase the cooling time of the plastic product, reducing production efficiency and significantly increasing the shrinkage and deformation of the finished product. Therefore, comprehensive considerations must be made regarding the resin type, mold structure, and glass fiber content.
The left picture shows the defects of 30% glass fiber reinforced polyamide 66 (GF30 PA 66) material caused by processing at a temperature lower than the predetermined mold temperature. Due to insufficient mold temperature, some glass fibers accumulate on the surface.
Injection pressure has a significant impact on the molding of glass fiber reinforced plastics. Higher injection pressure helps the material fill the injection mold quickly, improves the dispersion of the glass fiber, and reduces the shrinkage of the plastic product. However, excessive injection pressure increases the shear stress of the material and the orientation of the glass fiber, which can easily cause deformation of the plastic product and difficulty in demolding. In severe cases, it can even cause the material to overflow the mold, resulting in a noticeable parting line. Therefore, improving the performance of glass fiber reinforced plastics only requires a slight increase in injection pressure.
The left picture shows the surface defects of TPU material during high injection pressure processing. Due to the high pressure, some resin material gushes out from the joint of the injection mold, resulting in the so-called resin material overflow phenomenon.
Using a faster injection speed can effectively reduce fiber floating in glass fiber reinforced plastics. A higher injection speed allows the glass fiber reinforced plastic to quickly fill the mold wall, enhancing the dispersion of the material and reducing the orientation of the glass fibers. However, it should be noted that excessively fast injection speeds can easily cause jetting at the pouring gate, resulting in ripple-like defects near the gate. Therefore, it is necessary to adjust the injection speed appropriately to avoid this phenomenon.
The left picture shows a product made of 30% glass fiber reinforced polyamide 6 material (GF30 PA6) at a high injection speed.
Due to the slow melt flow rate and fast injection speed, the finished product will have a serpentine material accumulation phenomenon near the gate
5.Screw Speed
During the processing of glass fiber reinforced plastics, the screw speed should not be too high to avoid excessive frictional shear forces that can damage the glass fiber interface, reduce the bond strength between the glass fiber and the resin material, and increase the phenomenon of floating fibers. This is especially true when the glass fiber is long. This can easily lead to uneven glass fiber lengths due to breakage, resulting in uneven strength in different parts of the plastic product and unstable mechanical properties.
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