When mold was tested, each mechanism was basically operating normally, but there were serious appearance quality problems on the surface of product, radial white marks were produced on the surface, and white marks became more serious with increase of glass fiber content. This phenomenon is commonly known as "Floating fiber", which is a kind of surface defects that glass fiber plastic products are prone to appear, which is unacceptable for automotive plastic parts with high appearance requirements.

 

Phenomenon of "floating fiber" is caused by exposure of glass fiber. White glass fiber is exposed on the surface during plastic melt filling and flow process. After condensation and molding, it will form radial white marks on the surface of plastic part. When plastic part is black, it will be more obvious due to increased color difference.
Main reasons for its formation are as follows:
(1) In process of plastic melt flow, due to difference in fluidity of glass fiber and resin, difference in mass density, the two have a tendency to separate. Glass fiber with low density floats to surface, and resin with high density sinks into it, so glass fiber is exposed.
(2) Because plastic melt is subjected to frictional shear force of screw, nozzle, runner and gate during the flow process, it will cause difference in local viscosity, and at the same time, it will destroy interface layer on the surface of glass fiber. The smaller melt viscosity, the more severe interface layer will be damaged, the smaller bonding force between glass fiber and resin. When bonding force is small to a certain level, glass fiber will get rid of bondage of resin matrix, gradually accumulate and expose to surface.
(3) When plastic melt is injected into cavity, it will form a "fountain" effect, that is, glass fiber will flow from inside to outside and contact surface of cavity. Due to low mold surface temperature, light-weight and fast-condensing glass fiber is instantly frozen, and if it cannot be fully surrounded by melt in time, it will be exposed and form "floating fibers".
Therefore, formation of "floating fiber" phenomenon is not only related to composition and characteristics of plastic materials, but also related to molding process, which has greater complexity and uncertainty.

In actual production, there are various measures to improve phenomenon of "floating fiber":
More traditional method is to add compatibilizers, dispersants and lubricants to molding materials, including silane coupling agents, maleic anhydride graft compatibilizers, silicone powder, fatty acid lubricants, some domestic or imported anti-glass fiber exposure agents, etc., Use these additives to improve interface compatibility between glass fiber and resin, improve uniformity of dispersed phase and continuous phase, increase interface bonding strength, reduce separation of glass fiber and resin, thereby improving exposure of glass fiber. Some of them have good effects, but most of them are expensive, increase production costs, and also affect mechanical properties of materials. For example, the more commonly used liquid silane coupling agents are difficult to disperse after being added, and plastics are easy to agglomerate and form agglomerates, which will cause uneven feeding of equipment and uneven distribution of glass fiber content, which will lead to uneven mechanical properties of product.
In recent years, method of adding short fibers or hollow glass microspheres has also been adopted. Use of small-sized staple fibers or hollow glass beads has better fluidity and dispersibility, and is easy to form stable interface compatibility with resins to achieve purpose of improving "floating fibers". In particular, hollow glass beads can also reduce shrinkage deformation rate, avoid post-warpage of product, increase hardness and elastic modulus of material, have a lower price, but disadvantage is that impact resistance of material is reduced.

Mold casting system is closely related to formation of "floating fiber" phenomenon.
In view of poor fluidity of glass fiber reinforced plastics and inconsistent fluidity of two components of glass fiber and resin, flow distance should not be too long, melt must fill cavity quickly to ensure that glass fibers are uniformly dispersed, and silt integration layer does not occur to form "floating fibers".
Therefore, basic principle of design of gating system is that cross section of runner should be large, flow should be straight and short. Stubby main runners, runners, and thick gates should be used. Gates can be thin, fan-shaped, or ring-shaped, can also be multi-gate to make material flow chaotic, glass fiber diffuse, and reduce orientation. It also requires a good exhaust function, which can promptly exhaust gas generated by volatilization of glass fiber surface treatment agent, so as to avoid defects such as poor welding, lack of material, and burns.
For automobile handle cover mold gating system, long flow channel process is a factor that causes serious "floating fiber" phenomenon, but this is need of mold structure and cannot be shortened. Therefore, only flow channel cross-sectional size, gate form and size can be adjusted.. Gate is changed to a fan gate, size of gate and runner are gradually increased during mold trial process.
In addition, it should be noted that "floating fiber" tends to appear in part with large wall thickness of plastic part. This is because flow velocity gradient of melt is large there. When melt flows, its center velocity is high, and velocity near cavity wall is low, which intensifies tendency of glass fiber to float, and relative velocity is slower, resulting in retention and accumulation to form "floating fibers". Therefore, wall thickness of plastic parts should be made as uniform as possible, sharp corners and gaps should be avoided to ensure smooth flow of melt.

Formulation of suitable molding process conditions is essential to improve "floating fiber" phenomenon. Various elements of injection molding process have different effects on glass fiber reinforced plastic products. Here are some basic rules that can be followed:

Since melt index of glass fiber reinforced plastic is 30% to 70% lower than that of non-reinforced plastic, and its fluidity is poor, temperature of barrel should be 10 to 30℃ higher than normal. Increasing barrel temperature can reduce melt viscosity, improve fluidity, avoid poor filling and welding, help increase dispersion of glass fiber and reduce orientation, resulting in lower product surface roughness.
But barrel temperature is not as high as possible. Too high a temperature will increase tendency of nylon polymer to oxidize and degrade. Color will change when it is slight, and it will become black when it is severe.
When setting barrel temperature, temperature of feeding section should be slightly higher than conventional requirement, slightly lower than compression section, so as to use its preheating effect to reduce shearing effect of screw on glass fiber, reduce difference in local viscosity and damage to glass fiber surface, ensure bonding strength between glass fiber and resin. Melting temperature of PA66+33% GF is 275~280℃, the highest temperature should not exceed 300℃, and barrel temperature can be selected within this range.

Temperature difference between mold and melt should not be too large to prevent glass fiber from silting on the surface when melt is cold, forming "floating fibers". Therefore, a higher mold temperature is required, which is useful for improving melt filling performance, increasing strength of weld line, improving surface finish of product, reducing orientation and deformation.
However, the higher mold temperature, the longer cooling time, the longer molding cycle, the lower productivity, and the higher molding shrinkage, so the higher is not better. When setting mold temperature, resin variety, mold structure, glass fiber content, etc. should also be considered. When cavity is complex, glass fiber content is high, and mold filling is difficult, mold temperature should be appropriately increased. For car handle cover made of PA66+33% GF, suitable mold temperature is 110℃.

Injection pressure has a great influence on molding of glass fiber reinforced plastics. Higher injection pressure is conducive to filling, improving glass fiber dispersion and reducing product shrinkage, but it will increase shear stress and orientation, which will easily cause warpage and deformation, difficult demoulding, and even overflow problems. Therefore, to improve "floating fiber" phenomenon, it is necessary to increase injection pressure slightly higher than injection pressure of non-reinforced plastic according to specific situation.
Choice of injection pressure is not only related to product wall thickness, gate size and other factors, but also related to glass fiber content and shape. Generally, the higher glass fiber content, the longer glass fiber length, the greater injection pressure should be.

Size of screw back pressure has an important effect on uniform dispersion of glass fiber in melt, fluidity of melt, density of melt, appearance quality of product, mechanical and physical properties. It is usually advantageous to use a slightly higher back pressure, which helps to improve "floating fiber" phenomenon. However, excessively high back pressure will have a greater shearing effect on long fibers, making melt easily degraded due to overheating, resulting in discoloration and deterioration of mechanical properties. Therefore, back pressure can be set slightly higher than that of non-reinforced plastic.

Using a faster injection speed can improve "floating fiber" phenomenon. Increasing injection speed makes glass fiber reinforced plastic fill the mold cavity quickly, glass fiber makes rapid axial movement along flow direction, which is beneficial to increase dispersion of glass fiber, reduce orientation, improve strength of weld line and surface cleanness of product. However, attention should be paid to avoid "spraying" phenomenon at nozzle or gate due to excessively fast injection speed

When glass fiber reinforced plastic is plasticized, screw speed should not be too high to avoid excessive friction and shearing force that will damage glass fiber, destroy interface state of glass fiber surface, reduce bonding strength between glass fiber and resin, and aggravate "floating fiber" phenomenon, especially when glass fiber is longer, there will be uneven length due to part of glass fiber fracture, resulting in unequal strength of plastic parts and unstable mechanical properties of product.
Through above analysis, it can be seen that use of high material temperature, high mold temperature, high pressure, high speed, and low screw speed injection is beneficial to improve "floating fiber" phenomenon.