When the mold is tested, the operation of each part is basically normal, but the product has a serious appearance quality problem, and the surface has a radial white mark, and this white mark tends to be serious with the increase of the glass fiber content. This phenomenon is commonly called “ "Floating fiber" is a kind of surface defect that is easy to appear in glass fiber plastic products, which is unacceptable for automotive 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 floats on the surface during the filling process of the plastic melt. After the condensation is formed, a radial white mark is formed on the surface of the plastic part. When the plastic part is black It will become more apparent as the difference in color is increased.
The reasons for its formation are mainly as follows:
1. During the flow of plastic melt, due to the difference in fluidity between glass fiber and resin, and the different mass densities, the two have a tendency to separate. The glass fiber with low density floats to the surface, and the resin with high density sinks. Inside, the glass fiber exposed phenomenon was formed;
2. Since the plastic melt is subjected to the frictional shearing force of the screw, the nozzle, the runner and the gate during the flow, the local viscosity will be different, and at the same time, the interface layer on the surface of the glass fiber will be destroyed, and the melt viscosity will be smaller. The more serious the damage of the interface layer, the smaller the adhesion between the glass fiber and the resin. When the adhesion is small to a certain extent, the glass fiber will get rid of the bond of the resin matrix and gradually accumulate and expose to the surface;
3. When the plastic melt is injected into the cavity, a “fountain” effect will be formed, that is, the glass fiber will flow from the inside to the outside, and contact with the surface of the cavity. Due to the low temperature of the mold profile, the glass fiber with light weight and fast condensation is instantaneous. Freezing, if not fully surrounded by the melt in time, will be exposed to form "floating fiber."
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, with great complexity and uncertainty.
In actual production, there are various measures for improving the phenomenon of "floating fiber":
The more traditional method is to add additives such as compatibilizer, dispersant and lubricant to the molding material, including silane coupling agent, maleic anhydride grafting compatibilizer, silicone powder, fatty acid lubricant and some domestic or imported. Anti-glass fiber exposed agent, etc., through these additives to improve the interfacial compatibility between the glass fiber and the resin, improve the uniformity of the dispersed phase and the continuous phase, increase the interfacial bonding strength, and reduce the separation of the glass fiber and the resin, thereby Improve the exposure of glass fiber;
Some of them have good effects, but most of them are expensive, which increases the production cost and also affects the mechanical properties of the materials. For example, the more commonly used liquid silane coupling agents are difficult to disperse after being added, and the plastics are easy to knot. The problem of block formation will result in uneven feeding of equipment and uneven distribution of glass fiber content, which will result in uneven mechanical properties of the product.
In recent years, there have also been methods of adding short-fiber or hollow glass microbeads, which utilize small-sized staple fibers or hollow glass microbeads to have better fluidity and dispersibility, and are easy to form stable interfacial compatibility with resins. To achieve the goal of improving "floating fiber", especially hollow glass microspheres can reduce the shrinkage deformation rate, avoid post-warpage of the product, increase the hardness and elastic modulus of the material, and the price is lower, but the disadvantage is to make the material impact resistant. Performance is degraded.
Solution:
1. Adjustment of the mold casting system
The mold casting system is closely related to the formation of the "floating fiber" phenomenon.
For the glass fiber reinforced plastics with poor fluidity, and the fluidity of the two components of the glass fiber and the resin are inconsistent, the flow distance cannot be too long, and the melt must be quickly filled into the cavity to ensure uniform dispersion of the glass fibers without fouling. Layers form "floating fibers."
Therefore, the basic principle of the design of the gating system is that the cross section of the flow channel should be large, and the process should be straight and short. Thick and short main channels, split runners and coarse gates should be used. The gates can be in the form of flakes, sectors and rings, or in the form of multi-gates, so that the flow is disordered, the glass fibers are diffused, and the orientation is reduced. Moreover, it is required to have a good exhaust function, and the gas generated by the volatilization of the surface treatment agent of the glass fiber can be discharged in time to avoid defects such as poor welding, lack of material and burns.
For the pouring system of the handle cover mold, the long flow path is a serious factor causing the phenomenon of "floating fiber", but this is a need of the mold structure, and it cannot be shortened, so only the cross-sectional dimension of the flow passage and The gate form and size are adjusted. The gate was changed to a fan gate, and the gate and runner dimensions were gradually increased during the trial process.
It should also be noted that “floating fiber” is easy to appear in the part where the wall thickness of the plastic part is large, because the flow velocity gradient of the melt is large at this point, and the center speed of the melt is high, and close to the cavity wall surface. The low speed makes the glass fiber appear to be inflated, the relative speed is slower, and the accumulation is accumulated to form a "floating fiber". Therefore, the wall thickness of the plastic parts should be made as uniform as possible, and sharp corners and notches should be avoided to ensure smooth melt flow.
2. Optimization of injection molding process conditions
Establishing appropriate molding process conditions is critical to improving the “floating fiber” phenomenon. The effects of various factors of the injection molding process on glass fiber reinforced plastic products are different. The following are some basic rules that can be followed:
temperature
1 The first is the barrel temperature.
Since the melt index 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 10~30 °C higher than normal. Increasing the temperature of the barrel can reduce the viscosity of the melt, improve the fluidity, avoid the filling and welding failure, and help to increase the dispersion of the glass fiber and reduce the orientation, and obtain a lower surface roughness of the product.
However, the barrel temperature is not as high as possible. If the temperature is too high, the tendency of the nylon polymer to oxidize and degrade will increase. When it is slight, color changes will occur, and in severe cases, coking and blackening will occur.
When setting the temperature of the barrel, the temperature of the feeding section should be slightly higher than the conventional requirement, which is slightly lower than the compression section, so as to use the 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 bond strength between the glass fiber and the resin. The melting temperature of PA66+33%GF is 275~280°C, the maximum temperature should not exceed 300°C, and the barrel temperature can be selected within this range.
2 Next is the mold temperature.
The temperature difference between the mold and the melt should not be too large to prevent the glass fiber from being deposited on the surface when the melt is filled, forming a "floating fiber", so a higher mold temperature is required, which improves the melt filling performance and increases It is also advantageous to weld the weld strength, improve the surface finish of the article, 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 rate, so the higher the better. The mold temperature setting should also consider the resin type, mold structure, glass fiber content, etc. When the cavity is complicated, the glass fiber content is high, and the mold filling is difficult, the mold temperature should be appropriately increased. For automotive handle covers with PA66+33% GF, we chose a mold temperature of 110 °C.
pressure
1 injection pressure
The injection pressure has a great influence on the molding of glass fiber reinforced plastic. The higher injection pressure is beneficial to filling, improve the dispersion of glass fiber and reduce the shrinkage rate of the product, but it will increase the shear stress and orientation, easily cause warping deformation and release. Difficulties, and even lead to overflow problems, so to improve the "floating fiber" phenomenon, it is necessary to increase the pressure on the basis of slightly higher than the non-reinforced plastic injection pressure.
The choice of injection pressure is related to the thickness of the product, the size of the gate, etc. It is also related to the content and shape of the glass fiber. Generally, the higher the glass fiber content, the longer the length of the glass fiber, and the larger the injection pressure should be.
2 back pressure
The size of the screw back pressure has an important influence on the uniform dispersion of the glass fiber in the melt, the fluidity of the melt, the solidity of the melt, the appearance quality of the product and the mechanical and physical properties, and it is generally advantageous to use a slightly higher back pressure. It helps to improve the phenomenon of "floating fiber". However, excessive back pressure will cause a large shearing effect on the long fibers, so that the melt is easily degraded by overheating, resulting in discoloration and deterioration of mechanical properties. Therefore, the back pressure can be set slightly higher than the non-reinforced plastic.
Injection speed
The use of faster injection speeds can improve the "floating fiber" phenomenon. Increasing the injection speed, so that the glass fiber reinforced plastic quickly fills the cavity, and the glass fiber moves rapidly in the axial direction, which is beneficial to increase the dispersibility of the glass fiber, reduce the orientation, improve the weld line strength and the surface appearance of the product, but Care should be taken to avoid the phenomenon of "spraying" at the nozzle opening or gate due to the excessive injection speed, which forms a serpentine defect and affects the appearance of the plastic part.
Screw speed
When glass fiber reinforced plastic is plasticized, the screw rotation speed should not be too high, avoiding excessive friction shearing force and causing damage to the glass fiber, destroying the interface state of the glass fiber surface, reducing the bonding strength between the glass fiber and the resin, and intensifying the "floating fiber" "Phenomenon, especially when the glass fiber is long, will be uneven due to the breakage of some glass fibers, resulting in different strengths of the plastic parts, and the mechanical properties of the products are unstable.
