Why and how to prevent glass fiber rich surface in injection molding

When the mold was tested, sometimes the product had a serious appearance quality problem with radial white marks appeared on the surface.

The white marks tended to be serious with the increase of the glass fiber content.

This  is  known as ” glass emergence on the surface(also glass fiber rich surface )” which is a surface defect most likely on the  glass fiber plastic products, which is unacceptable for automotive plastic parts with high appearance requirements.

glass fibre rich surface

glass fibre rich surface

  • Cause

The glass fiber rich surface is caused by the exposure of glass fiber.

The white glass fiber appears on the surface during the plastic melt filling and molding. After condensation and molding, radial white marks are formed on the surface of the plastic part.

When the plastic part is black ,it will become more obvious as the color difference increases.

There are several reasons behind:

First of all, during the plastic melt filling, due to the difference in fluidity between glass fiber and resin, and the difference in mass density, the two tend to separate.

The low-density glass fiber floats to the surface, and the high-density resin sinks.

The glass fiber rich surface was formed;

Secondly, due to the friction and shear force caused by the screw, nozzle, runner and gate during the flow , it will cause the different viscosity, and it will also destroy the interface layer on the surface of the glass fiber at the same time .

The lower the melt viscosity is, The more severe the interface layer damage will be, which will lead to the smaller viscosity between the glass fiber and the resin.

When the viscosity is small enough, the bondage between the resin and glass fiber will break , and the glass fiber will gradually accumulate to the surface and be exposed;

In addition, when the plastic melt is injected into the cavity, a “fountain” effect is formed, that is, the glass fiber flows from the inside to the outside of soften resin and contacts the cavity surface.

Due to the lower temperature mold surface , the light-weight and fast-condensing glass fiber freezes instantly, if it is not fully surrounded by the melt in time, it will be exposed and develop to  “glass fiber rich surface”.

Therefore, the glass fiber rich surface are not only related to the composition and characteristics of plastic materials, but also to the molding process, which is of complexity and uncertainty.

In production, there are various ways to prevent the glass fiber rich surface.

The traditional method is to add additives such as compatibilizers, dispersants, and lubricants to the resin material, including silane coupling agents, maleic anhydride graft compatibility agents, silicone powders, fatty acid lubricants, and some other  agent, etc.,

Through these additives, they could 

  • improve the interface compatibility between glass fiber and resin,
  • improve the uniformity of the dispersed phase and continuous phase,
  • increase the interface bonding strength,
  • reduce the separation of glass fiber and resin,

They function well, but most of them are expensive, which increase production costs and affect the mechanical properties of materials.

In recent years, a method of adding short fiber or hollow glass microbeads has also been adopted. The resin with short fiber or hollow glass microbeads has good fluidity and dispersibility, and is easy to form a stable interface compatibility with the resin.

Solution

feed system

Mold feed system is closely related to the development of  glass fiber rich surface.

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 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 volatilization of glass fiber surface treatment agent, so as to avoid defects such as poor welding, short shot and burns.

In addition, the glass fiber rich surface are 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.

B, injection molding setting-up

It is very important to get suitable molding process setting-up to improve the glass fiber rich surface. Each setting-up of injection molding process has different effects on glass fiber reinforced plastic products. The following are some basic rules :

 Temperature

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,  too high temperature of the barrel have negative side .

Excessive temperature will increase the tendency of oxidation and degradation of nylon polymer.

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, so as 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 the barrel temperature can be selected 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, the mold temperature should be appropriately increased. Likely, a automobile handle cover is made of PA66+33% GF, the mold temperature we selected is 110℃. 

pressure

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, but 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 not only related to the wall thickness and gate size of the product, but also 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 has the influence on 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 back pressure 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.

injection speed

The rich glass fabric surface can be improved by a faster injection speed.

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 to increasing the dispersibility of the glass fiber, it will reduce the orientation, improve the strength of the weld line and the apparent of the product.

screw speed

When plasticizing glass fiber reinforced plastics, the rotating speed of the screw should not be too high, so as to avoid damage to glass fiber caused by excessive friction shear force, destroy the interface state of glass fiber surface, reduce the bonding strength between glass fiber and resin.

Especially when the glass fiber is long, 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.

Through the above , it can be found 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.

 

 

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