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What are the floating fiber phenomena in injection molding defect analysis?

When the mold was tested, the various mechanisms were basically operating normally, but the product had serious appearance quality problems, with radial white marks on the surface, and this white mark became more serious with the increase of the glass fiber content. This phenomenon is commonly known as " "Floating fiber" is a kind of glass fiber plastic products that are prone to surface defects, 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 is exposed on the surface during the plastic melt filling and flow process. After condensation and molding, 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.

First of all, in the plastic melt flow process, due to the difference in the fluidity of the glass fiber and the resin, and the different mass density, the two have a tendency to separate. The low-density glass fiber floats to the surface, and the denser resin sinks. Inside, the glass fiber is exposed;

Secondly, 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;

Furthermore, 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 with light weight and fast condensation will be 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.

In actual production, there are various measures to improve the phenomenon of "floating fiber". 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, uneven distribution of glass fiber content, and uneven mechanical properties of products.

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.

Solution:

A. Adjustment of mold casting system

The mold casting system is closely related to the formation of the "floating fiber" phenomenon. In view of the poor fluidity of glass fiber reinforced plastics and the inconsistent fluidity of the two components of glass fiber and resin, the flow distance should not be too long, and the melt must fill the cavity quickly to ensure uniform dispersion of the glass fiber without sludge integration. Layer to form a "floating fiber".

Therefore, the basic principle of the design of the gating system is that the cross section of the runner should be large, and the flow should be straight and short. Stubby main runners, runners, and thick gates should be used. The gates can be thin, fan-shaped, or ring-shaped, and 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 the gas generated by the volatilization of the glass fiber surface treatment agent, so as to avoid defects such as poor welding, lack of material, and burns.

For the gating system of the handle cover mold, the longer flow channel process is a factor causing the serious "floating fiber" phenomenon, but this is the need of the mold structure and cannot be shortened, so only the flow channel cross-sectional size and The gate form and size are adjusted. The gate is changed to a fan gate, and the size of the gate and runner are gradually increased during the mold trial process.

In addition, it should be noted that "floating fiber" tends to appear in the part with large wall thickness of the plastic part. This is because the flow velocity gradient of the melt is large there, and the center velocity of the melt is high when the melt flows, and it is close to the cavity wall. The low speed at the location intensifies the tendency of the glass fiber to float, and the relative speed is slower, resulting in stagnation and accumulation to form "floating fibers". Therefore, the wall thickness of the plastic parts should be made as uniform as possible, and sharp corners and gaps should be avoided to ensure the smooth flow of the melt.

B. Optimization of injection molding process conditions

The formulation of suitable molding process conditions is essential to improve the "floating fiber" phenomenon. 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.

C, temperature

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, and its fluidity is poor, the temperature of the barrel 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 lower product surface roughness.

But the barrel temperature is not as high as possible. Too high a temperature will increase the tendency of nylon polymer to oxidize and degrade, and 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. The melting temperature of PA66+33% GF is 275~280℃, the highest temperature should not exceed 300℃, and the barrel temperature can be selected within this range.

The second is the 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. When setting the mold temperature, the resin variety, mold structure, glass fiber content, etc. should also be considered. When the cavity is complex, the glass fiber content is high, and the mold filling is difficult, the mold temperature should be appropriately increased. For the car handle cover made of PA66+33% GF, the mold temperature we choose is 110°C.

D, 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, so to improve the "floating fiber" phenomenon, it is necessary to increase the injection pressure slightly higher than the injection pressure of non-reinforced plastics 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.

The size of the screw back pressure has an important effect 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 mechanical and physical properties. It is usually advantageous 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 deterioration of mechanical properties. Therefore, the back pressure can be set slightly higher than that of the non-reinforced plastic.

E. Injection 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.

F. 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.

Through the above analysis, it can be seen that the use of high material temperature, high mold temperature, high pressure, high speed, and low screw speed injection is beneficial to improve the "floating fiber" phenomenon.More about:Horizontal Injection Molding Machine

 

Horizontal Injection Molding Machine