What is "floating fiber" ?

The "floating fiber" phenomenon is caused by the exposure of glass fiber. The white glass fiber is exposed on the surface during the filling and flow process of the plastic melt. After condensation and molding, it forms radial white traces on the surface of the plastic part. When the plastic part is black The difference in color will become more obvious as the color difference increases.

The main reasons are as follows:

1. During the flow of plastic melt, due to the difference in fluidity and density between glass fiber and resin, the two have a tendency to separate. The glass fiber with lower density floats to the surface, and the resin with higher density sinks inside. , thus forming the phenomenon of glass fiber exposure;

2. Since the plastic melt is affected by the friction and shear force of the screw, nozzle, runner and gate during the flow process, it will cause local viscosity differences and at the same time destroy the interface layer on the glass fiber surface. The smaller the melt viscosity. , the more seriously the interface layer is damaged, the smaller the bonding force between the glass fiber and the resin. When the bonding force is small to a certain extent, the glass fiber will break away from the constraints of the resin matrix and gradually accumulate toward the surface and be exposed;

3. When the plastic melt is injected into the mold cavity, a "fountain" effect will be formed, that is, the glass fiber will flow from the inside to the outside and contact the surface of the mold cavity. Due to the low temperature of the mold surface, the light weight and fast condensing glass fiber will be If frozen instantaneously and cannot be fully surrounded by 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 to the molding process, which is highly complex and uncertain.

Let’s talk about how to improve the “floating fiber” phenomenon from the perspective of formula and process.

Process Optimization

In fact, the "floating fiber" problem can also be improved through the molding process. Each element of the injection molding process has different effects on glass fiber reinforced plastic products. Some basic rules that can be followed are introduced below.

1.Barrel temperature

Since the melt flow rate of glass fiber reinforced plastics is 30% to 70% lower than that of non-reinforced plastics, and the fluidity is poor, 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 glass fiber dispersion and reduce orientation, resulting in lower product surface roughness.

However, the higher the barrel temperature, the better. Excessive temperature will increase the tendency of polymer oxidation and degradation. In mild cases, color changes will occur, and in severe cases, coking and blackening will occur.

When setting the barrel temperature, the temperature of the feeding section should be slightly higher than the conventional requirements and slightly lower than the compression section, so as to take advantage of its preheating effect, reduce the shearing effect of the screw on the glass fiber, and reduce the local viscosity. Differences and damage to the surface of the glass fiber ensure the bonding strength between the glass fiber and the resin.

2.Mold temperature

The temperature difference between the mold and the melt should not be too large to prevent the glass fibers from settling on the surface when the melt is cold and forming "floating fibers". Therefore, a higher mold temperature is required, which is important for improving the melt filling performance and increasing the It is also beneficial to improve the strength of weld lines, improve the surface finish of the product, and reduce orientation and deformation.

However, the higher the mold temperature, the longer the cooling time, the molding cycle is extended, the productivity is reduced, and the molding shrinkage rate increases, so higher is not always better. The setting of the mold temperature should also consider the resin type, mold structure, glass fiber content, etc. When the cavity is complex, the glass fiber content is high, and mold filling is difficult, the mold temperature should be increased appropriately.

3.Injection pressure

Injection pressure has a great influence on the molding of glass fiber reinforced plastics. Higher injection pressure is beneficial to filling, improves glass fiber dispersion, and reduces product shrinkage, but it will increase shear stress and orientation, easily causing warping deformation and demoulding. It is difficult and may even lead to overflow problems. Therefore, in order to improve the "floating fiber" phenomenon, it is necessary to increase the injection pressure slightly higher than that of non-reinforced plastics according to the specific situation.

The choice of injection pressure is not only related to the thickness of the product, gate size and other factors, but also to the glass fiber content and shape. Generally, the higher the glass fiber content and the longer the glass fiber length, the greater the injection pressure should be.

4.Back pressure

The screw back pressure has an important impact on the uniform dispersion of glass fiber in the melt, the fluidity of the melt, the density of the melt, the appearance quality and physical and mechanical properties of the product. It is usually more advantageous to use a slightly higher back pressure. , helps to improve the "floating fiber" phenomenon. However, excessive back pressure will produce a large shearing effect on the long fibers, making the melt prone to degradation due to overheating, resulting in discoloration and poor mechanical properties. So just set the back pressure slightly higher than for non-reinforced plastics.

5.Injection speed

Using a faster injection speed can improve the "floating fiber" phenomenon. Increasing the injection speed allows the glass fiber reinforced plastic to quickly fill the mold cavity, and the glass fiber moves rapidly axially along the flow direction, which is beneficial to increasing the dispersion of the glass fiber, reducing the orientation, improving the strength of the weld line and the apparent smoothness of the product, but Care should be taken to avoid the 'jetting' phenomenon occurring at the nozzle or gate due to too fast injection speed, resulting in serpentine defects and affecting the appearance of the plastic parts.

6.Screw speed

When plasticizing glass fiber reinforced plastics, the screw speed should not be too high to avoid damage to the glass fiber due to excessive friction and shear force, damage to the surface interface state of the glass fiber, reduction of the bonding strength between the glass fiber and the resin, and aggravation of "floating fibers". " phenomenon, especially when the glass fiber is long, uneven lengths will occur due to part of the glass fiber breaking, resulting in uneven strength throughout the plastic part and unstable mechanical properties of the product.

Conclusion

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 improving the "floating fiber" phenomenon.