During the injection molding of glass fiber reinforced plastics, the operation of each mechanism is basically normal, but the product has serious appearance quality problems, and radial white marks are produced on the surface, and this white mark tends to be serious with the increase of glass fiber content. The phenomenon is commonly known as "floating fiber", which is unacceptable for plastic parts with high appearance requirements.
Cause Analysis
The phenomenon of "floating fiber" is caused by the exposure of glass fiber. The white glass fiber is exposed on the surface during the process of plastic melt filling and flow. After condensation, it will form radial white marks on the surface of the plastic part. When the plastic part is black When the color difference increases, it becomes more obvious.
The main reasons for its formation are as follows:
1. In the process of plastic melt flow, due to the difference in fluidity and density between glass fiber and resin, the two have a tendency to separate. The low-density glass fiber floats to the surface, and the denser resin sinks into it. , So the glass fiber exposed phenomenon is formed;
2. Because the plastic melt is subjected to the friction and shear force of the screw, nozzle, runner and gate during the flow process, it will cause the difference in local viscosity, and at the same time, it will destroy the interface layer on the surface of the glass fiber, and the melt viscosity will be smaller. , The more severe the damage to the interface layer, the smaller the bonding force between the glass fiber and the resin. When the bonding force is small to a certain level, the glass fiber will get rid of the bondage of the resin matrix and gradually accumulate to the surface and expose;
3. When the plastic melt is injected into the cavity, it will form a "fountain" effect, that is, the glass fiber will flow from the inside to the outside and contact the surface of the cavity. Because the mold surface temperature is low, the glass fiber is light and condenses quickly. It freezes instantly, and if it cannot be fully surrounded by the melt in time, it will be exposed and form "floating fibers".
Therefore, the formation of the "floating fiber" phenomenon is not only related to the composition and characteristics of plastic materials, but also related to the molding process, which has greater complexity and uncertainty.
Let's talk about how to improve the phenomenon of "floating fiber" from the perspective of formula and process.
Formula optimization
The more traditional method is to add compatibilizers, dispersants and lubricants to the molding materials, including silane coupling agents, maleic anhydride graft compatibilizers, silicone powder, fatty acid lubricants and some domestic or imported Use these additives to improve the interface compatibility between the glass fiber and the resin, improve the uniformity of the dispersed phase and the continuous phase, increase the interface bonding strength, and reduce the separation of the glass fiber and the resin. Improve the exposure of glass fiber. Some of them have good effects, but most of them are expensive, increase production costs, and also affect the 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 form. The problem of lump formation will cause uneven feeding of equipment and uneven distribution of glass fiber content, which in turn will lead to uneven mechanical properties of the product.
In recent years, the method of adding short fibers or hollow glass microspheres has also been adopted. The small-sized short fibers or hollow glass microspheres have the characteristics of good fluidity and dispersibility, and easy to form stable interface compatibility with the resin. To achieve the purpose of improving "floating fiber", especially hollow glass beads can also reduce the shrinkage deformation rate, avoid post-warping of the product, increase the hardness and elastic modulus of the material, and the price is lower, but the disadvantage is that the material is impact resistant Performance drops.
Process Optimization
In fact, the "floating fiber" problem can also be improved through the molding process. The various elements of the injection molding process have different effects on glass fiber reinforced plastic products. Here are some basic rules that can be followed.
01Cylinder temperature
Since the melt flow rate of glass fiber reinforced plastic is 30% to 70% lower than that of non-reinforced plastic, the fluidity is poor, so the barrel temperature should be 10 to 30°C higher than normal. Increasing the barrel temperature can reduce the melt viscosity, improve fluidity, avoid poor filling and welding, and help increase the dispersion of glass fiber and reduce the orientation, resulting in a lower surface roughness of the product.
But the barrel temperature is not as high as possible. Too high a temperature will increase the tendency of polymer oxidation and degradation. The color will change when it is slight, and it will cause coking and blackening when it is severe.
When setting the barrel temperature, the temperature of the feeding section should be slightly higher than the conventional requirement, and slightly lower than the compression section, so as to use its preheating effect to reduce the shearing effect of the screw on the glass fiber and reduce the local viscosity. The difference and damage to the surface of the glass fiber ensure the bonding strength between the glass fiber and the resin.
02 Mold temperature
The temperature difference between the mold and the melt should not be too large to prevent the glass fiber from silting on the surface when the melt is cold, forming "floating fibers". Therefore, a higher mold temperature is required, which is useful for improving the melt filling performance and increasing It is also beneficial to weld line strength, improve product surface finish, and reduce orientation and deformation.
However, the higher the mold temperature, the longer the cooling time, the longer the molding cycle, the lower the productivity, and the higher the molding shrinkage, so the higher is not the better. The setting of the mold temperature should also consider the resin variety, mold structure, glass fiber content, etc. When the cavity is complex, the glass fiber content is high, and the mold filling is difficult, the mold temperature should be appropriately increased.
03 injection pressure
Injection pressure has a great influence on the 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, easily causing warpage and deformation, and demoulding Difficulties, even leading to overflow problems. Therefore, to improve the "floating fiber" phenomenon, it is necessary to increase the injection pressure slightly higher than the injection pressure of the non-reinforced plastic according to the specific situation.
The choice of injection pressure is not only related to the product wall thickness, gate size and other factors, but also related to the glass fiber content and shape. Generally, the higher the glass fiber content, the longer the glass fiber length, the greater the injection pressure should be.
04 back pressure
The size of the screw back pressure has an important influence on the uniform dispersion of glass fiber in the melt, the fluidity of the melt, the density of the melt, the appearance quality of the product and the physical and mechanical properties. It is usually better to use a higher back pressure. , Help to improve the phenomenon of "floating fiber". However, excessively high back pressure will have a greater shearing effect on the long fibers, making the melt easily degraded due to overheating, resulting in discoloration and poor mechanical properties. Therefore, the back pressure can be set slightly higher than that of the non-reinforced plastic.
05Injection speed
Using a faster injection speed can improve the "floating fiber" phenomenon. Increase the injection speed, so that the glass fiber reinforced plastic quickly fills the mold cavity, and the glass fiber makes rapid axial movement along the flow direction, which is beneficial to increase the dispersion of the glass fiber, reduce the orientation, improve the strength of the weld line and the surface cleanness of the product, but Attention should be paid to avoid "spraying" at the nozzle or gate due to the excessively fast injection speed, forming serpentine defects and affecting the appearance of the plastic part.
06 screw speed
When plasticizing glass fiber reinforced plastics, the screw speed should not be too high to avoid excessive friction and shearing force that will damage the glass fiber, destroy the interface state of the glass fiber surface, reduce the bonding strength between the glass fiber and the resin, and aggravate the "floating fiber". "Phenomena, especially when the glass fiber is longer, there will be uneven length due to part of the glass fiber fracture, resulting in unequal strength of the plastic parts and unstable mechanical properties of the product.
Process summary
Through the above analysis, it can be seen that the use of high material temperature, high mold temperature, high injection pressure and back pressure, high injection speed, and low screw speed injection is more beneficial to improve the phenomenon of "floating fiber".
