glass fiber-rich surface in injection molding
During the mold testing, there were occasional appearance problems as radial white marks would appear on the surface of the product. It is noted that the appearance quality problems became more serious with the increased content of glass fiber.
A surface defect, “glass emergence,” is most commonly found on glass fiber plastic products, a surface defects not appropriate for automotive plastic parts with high appearance requirements.
First of all, due to the difference between the fluidity of the resin and the glass fiber and the difference in mass density, the two tend to separate during the filling of the plastic melt. High-density resin sinks, while low-density fiber floats. It leads to the formation of the glass fiber-rich surface;
Secondly, due to the friction and shear forces produced by the screw, nozzle, runner, and gate during the flow, different viscosity is generated. At the same time, the interface layer within the glass fiber is destroyed.
A lower melt viscosity results in severe damage to the interface layer, which will result in a smaller viscosity between the glass fiber and the resin. The bond between the resin and fiber breaks when the viscosity is low enough, causing the fiber to accumulate at the surface and become exposed.
Upon injection of the plastic melt into the cavity, a “fountain” effect takes place, in which glass fibers flow from the inside out of the resin and contact the cavity surface. Since the mold surface is at a lower temperature, the lightweight, fast-condensing glass fiber freezes instantly. If the melt does not fully surround it in time, the glass fibers are exposed and develop into a “glass fiber-rich surface.”
This means that glass fiber-rich surfaces aren’t just about the plastic material. They’re also about the molding process, which is complex and uncertain. Various production methods can prevent glass fiber-rich surfaces.
To modify the material, it is popular to add additives to resin, such as silane coupling agents, maleic anhydride graft compatibility agents, silicone powders, fatty acid lubricants, etc.
Through these additives, they could
- Make the glass fibers and resins more compatible,
- Make the dispersed phase and continuous phase more uniform.
- Increase the interface bonding strength,
- reduce the separation of glass fiber and resin,
Mold feed system is closely related to the development of glass fiber-rich surfaces.
Because the fluidity of glass fiber reinforced plastics is poor and the fluidity of glass fiber and resin is inconsistent, the flowing distance cannot be too long.
The melt should fill the cavity quickly to ensure the uniform dispersion of glass fiber and prevent the glass fiber-rich surface. Therefore, the principle of feed system design is that the runner cross-section should be large, and the flow should be straight and short.
Big cross-section and short main runner, sub runner, and gate should be adopted. The gate style can be of the tab, fan, and ring, or multi-gate design can be adopted to make material flow turbulence, glass fiber diffuse and reduce orientation.
Moreover, good venting is required, which can timely exhaust the gas generated by the volatilization of glass fiber surface treatment agent to avoid defects such as poor welding, short shot, and burns. In addition, the glass fiber-rich surface is easy to appear in the parts with large wall thickness.
Because the melt flow velocity change greatly, and its central velocity is high when the melt flows, while the velocity near the wall surface of the cavity is low, which makes the tendency of glass fiber-rich surface.
Therefore, the wall thickness should be made as uniform as possible, and sharp corners should be avoided to ensure smooth melt flow.
It is very important to get a suitable molding process set up to improve the glass fiber-rich surface. Each setting-up of the injection molding process has different effects on glass fiber reinforced plastic products. The following are some basic rules :
The temperature of the cylinder
As the melt index of glass fiber reinforced plastics is 30% ~ 70% lower than that of non-reinforced plastics, and its fluidity is poor, the temperature of the barrel should be 10~30℃ higher than that of the general case.
Increasing the temperature of the barrel can reduce the viscosity of the melt, improve the fluidity, avoid poor filling and welding, increase the dispersibility of glass fiber and reduce the orientation.
However, the too high temperature of the barrel has a negative side.
Excessive temperature will increase the tendency of oxidation and degradation of nylon polymer.
A slight color change will be seen.
When setting the temperature of the barrel, the temperature should be slightly higher than the conventional requirement and slightly lower than the compression section to utilize the preheating effect to reduce the shearing effect of the screw on the glass fiber, reduce the local different viscosity and damage to the glass fiber surface, and 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 it can select the barrel temperature within this range.
- The mold temperature
The temperature difference between the mold and the melt should not be too large to prevent glass fiber from depositing on the surface when the melt is filled.
Therefore, a higher mold temperature is required, which is also beneficial to improve the mold filling, increase the strength of weld lines, improve the surface finish of products, and reduce orientation and deformation.
However, the higher the mold temperature is, the longer the cooling time will be.
The longer the molding cycle is, the lower the productivity will be, and the larger the molding shrinkage will be.
The setting of mold temperature should also consider the resin type, mold structure, glass fiber content, etc.
If the cavity is complex, glass fiber content is high, and mold filling is difficult, it should increase the mold temperature appropriately. Likely, an automobile handle cover is made of PA66+33% GF, the mold temperature we selected is 110℃.
Injection pressure has a great influence on the molding of glass fiber reinforced plastics. Higher injection pressure is good for filling. It will improve the dispersibility of glass fiber and reduce the shrinkage rate of products. Still, it will increase shear stress and orientation, which will easily lead to warping deformation, difficult demolding, and even flash.
Therefore, it is necessary to appropriately increase to a higher injection pressure than that of non-reinforced plastics. The choice of injection pressure is related to the wall thickness and gate size of the product and related to the glass fiber content and morphology.
Generally, the higher the glass fiber content is and the longer the glass fiber length is, the greater the injection pressure should be.
The screw-back pressure influences the uniform dispersion of glass fiber in the melt, the fluidity of the melt, the appearance of the product, and the mechanical and physical properties.
Generally, a slightly higher back pressure is more favorable and helps to prevent the rich glass fabric surface. However, too high backpressure will produce large shearing on long fibers, making melt easy to degrade due to overheating, resulting in discoloration and deterioration of mechanical properties.
Therefore, it is enough to set the back pressure slightly higher than that of non-reinforced plastic.
A faster injection speed can improve the rich glass fabric surface.
Increasing the injection speed enables the glass-fiber-reinforced plastic to quickly fill the mold cavity. The rapid movement of the glass fiber along the flow direction helps increase the dispersibility of the glass fiber. It will reduce the orientation and improve the strength of the weld line and the product’s appearance.
When plasticizing glass fiber reinforced plastics, the rotating speed of the screw should not be too high to avoid damage to glass fiber caused by excessive friction shear force, destroy the interface state of the glass fiber surface, reduce the bonding strength between glass fiber and resin.
Especially when the glass fiber is long, the uneven length will occur due to the breakage of some glass fibers, which will result in unequal strength of plastic parts and unstable mechanical properties of products.
The above shows that injection molding setting with high material temperature, high mold temperature, high injection pressure, high speed, and low screw speed is beneficial to prevent the rich glass fabric surface.