When the injection molding factory tested the mold before production, all the mechanisms were running basically normally, but the product had serious appearance quality problems, with radial white marks on the surface.
And these white marks became more serious as the glass fiber content increased, This phenomenon is commonly known as “floating fiber”, which is a surface defect that is prone to occur in fiberglass plastic products.
Cause Analysis
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
First of all, during the plastic melt flow process, due to the difference in fluidity and mass 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, the glass fiber is exposed;
Secondly, because 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;
Furthermore, 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.
Because the temperature of the mold surface is low, the light-weight glass fiber that condenses quickly is If frozen instantly, if it is not 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.
First of all, during the plastic melt flow process, due to the difference in fluidity and mass 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, the glass fiber is exposed;
Secondly, because 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;
Furthermore, 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. Because the temperature of the mold surface is low, the light-weight glass fiber that condenses quickly is If frozen instantly, if it is not fully surrounded by melt in time, it will be exposed and form “floating fibers”.
Measures to improve the phenomenon of “floating fibers”
The more traditional method is to add additives such as compatibilizers, dispersants and lubricants to the molding material, including silane coupling agents, maleic anhydride graft compatibilizers, silicone powder, fatty acid lubricants and some domestic or imported Anti-glass fiber exposure agent, etc.
These additives are used to improve the interfacial compatibility between glass fiber and resin, improve the uniformity of the dispersed phase and the continuous phase, increase the interface bonding strength, reduce the separation of glass fiber and resin, thereby improving the phenomenon of glass fiber exposure, including The use effect is better.
However, most of them are expensive, increase the production cost, and also have an impact on the mechanical properties of the material. For example, the more commonly used liquid silane coupling agent is difficult to disperse after being added, and the plastic is easy to agglomerate, which will cause The equipment feeds unevenly and the glass fiber content is unevenly distributed, which leads to uneven mechanical properties of the product.
In recent years, methods have also been adopted to add short fibers or hollow glass beads. The small-sized short fibers or hollow glass beads have good fluidity and dispersion, and are easy to form stable interface compatibility with the resin.
To achieve the purpose of improving “floating fibers”, 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 have a lower price, but the disadvantage is to make the material impact resistant Performance degrades.
Solution
Adjustment of mold pouring system
The mold pouring 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 cannot be too long, and the melt must quickly fill the cavity to ensure that the glass fibers are evenly dispersed and no siltation occurs. layers to form “floating fibers”.
Therefore, the basic principle of gating system design is that the flow channel section should be large and the process should be straight and short. Thick and short main channels, shunt channels and thick gates should be used. The gates can be lamellar, fan-shaped or ring-shaped, or multiple gates can be used to clutter the material flow, diffuse the glass fiber and reduce orientation.
It also requires a good exhaust function, which can promptly discharge the gas generated by the volatilization of the fiberglass surface treatment agent to avoid defects such as poor welding, material shortage, and burns.
For the pouring system of the handle cover mold, its long runner process is a factor causing the serious “floating fiber” phenomenon. However, this is the need of the mold structure and cannot be shortened.
Therefore, only the cross-sectional size of the runner and the Gate form and size are adjusted. The gate was changed to a fan-shaped gate, and the gate and runner dimensions were gradually increased during the mold trial process.
In addition, it should be noted that “floating fibers” tend to appear in parts with larger wall thickness of plastic parts. This is because the melt flow velocity gradient is large there. When the melt flows, its center velocity is high, and it is close to the cavity wall.
When the speed is low, the tendency of glass fiber exposure is intensified, and the relative speed is slower, causing retention and accumulation to form “floating fibers”. Therefore, the wall thickness of the plastic part should be as uniform as possible and sharp corners and gaps should be avoided to ensure smooth melt flow.
Optimization of injection molding process conditions
Establishing appropriate molding process conditions is crucial to improving the “floating fiber” phenomenon. 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.
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 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. If the temperature is too high, the nylon polymer will increase the tendency of oxidation and degradation. In mild cases, the color will change, and in severe cases, it will cause coking and blackening.
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. The melting temperature of PA66+33%GF is 275~280℃, and the maximum temperature should not exceed 300℃. 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 fibers from being cold and depositing on the surface during melt filling, 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. For the car handle cover made of PA66 + 33% GF, the mold temperature we selected is 110°C.
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.
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 mechanical and physical 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.
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 “jet” 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.
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.
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