The reason and effect of floating fibers in injection molding defects
Analysis of Causes:
Floating fiber phenomenon is caused by exposure of glass fiber. White glass fiber floats on the surface of plastic melt in the process of mold filling flow, and forms radial white traces on the surface of plastic parts after condensation. When the plastic parts are black, the difference in color will be more obvious due to the increase of color difference.
First of all, in the process of plastic melt flow, glass fiber and resin have a tendency of separation due to their different fluidity and mass density. The glass fiber with small density floats to the surface, while the resin with large density sinks into the interior, thus forming the phenomenon of glass fiber exposure.
Secondly, the plastic melt is subjected to the screw nozzle during the flow Runner and gate of the friction shear force, can cause the difference of local viscosity, will destroy the interface layer on the surface of the glass fiber again at the same time, the smaller the melt viscosity, interfacial layer is damaged more serious, the bond between glass fiber and resin is the less, when bond small to a certain degree, glass fiber will cast off the yoke of the resin matrix, to the exposed surface accumulates gradually;
In addition, when plastic melt is injected into the mold cavity, fountain effect will be formed, that is, glass fiber will flow from the inside to the outside and contact with the surface of the mold cavity. Due to the low temperature of the mold surface, the glass fiber with light weight and fast condensation will be frozen instantly. If it cannot be fully surrounded by the melt in time, it will be exposed and form floating fiber. The formation of floating fiber is not only related to the composition and characteristics of plastic materials, but also to the forming process, which has great complexity and uncertainty.
solution:
A. Adjustment of mold pouring system.
Mold pouring system is closely related to the formation of floating fiber phenomenon. In view of the poor fluidity of glass fiber reinforced plastics and the inconsistent fluidity of glass fiber and resin components, the flow distance should not be too long, and the melt should quickly fill the cavity to ensure the uniform dispersion of glass fiber and the formation of floating fiber without deposition stratification.
Therefore, the basic principle of gating system design is that the flow channel section should be large, the flow should be straight and short, and the coarse and short main channel shunt channel and the coarse gate should be adopted. The gate can be a thin section fan and ring, or the form of multiple gate can be adopted, so as to make the material flow chaotic and reduce the orientation of glass fiber diffusion. Moreover, it is required to have a good exhaust function, which can timely discharge the gas generated by the volatilization of the glass fiber surface treatment agent, so as to avoid the defects such as poor fusion connection, lack of material and burns.
For the pouring system of the hand cover mold, its long flow path causes floating fiber The phenomenon is a serious factor, but this is the need of the mold structure, can not shorten it, so only to adjust the size of the runner section and the form and size of the gate to the fan gate, gate and runner size is gradually increased in the process of mold testing another thing to note is that, Floating fiber is easy to appear at the site of the plastic parts wall thickness is larger, this is because the melt flow velocity gradient is larger, at the center its melt flow rate is high, and low speed near the cavity wall, made of fiberglass fulu trend intensified, relative speed is more slow, stranded accumulation and forming floating fiber Therefore, we should try to make the plastic parts wall thickness uniform throughout, and avoid sharp corners Notch to ensure smooth melt flow.
B. Temperature
The first is the barrel temperature because the melting index of glass fiber reinforced plastic is 30%~70% lower than that of non-reinforced plastic, and the fluidity is poor, so the barrel temperature should be 10~30 higher than the general situation Increasing barrel temperature can reduce melt viscosity, improve fluidity, avoid bad filling and fusion, increase glass fiber dispersion and reduce orientation, and obtain lower product surface roughness.
However, the higher the barrel temperature is not the better, too high temperature will increase the trend of oxidation and degradation of nylon polymer, slight color change, serious coking blackening When setting the temperature of the barrel, the temperature of the feeding section should be slightly higher than the conventional requirements, slightly lower than the compression section, so as to use its preheating effect, reduce the shear effect of the screw on the glass fiber, reduce the difference in local viscosity and damage to the glass fiber surface, ensure that the bonding strength between the glass fiber and the resin PA66+33%GF melting temperature of 275~280 , the highest temperature should not exceed 300, the barrel temperature can be selected within this range.
Secondly, the temperature difference between the mold and the melt should not be too large to prevent the glass fiber from silting on the surface and forming floating fibers when the melt is filled. Therefore, a higher mold temperature is needed, which is beneficial to improve the melt filling performance, increase the strength of fusion marks, improve the surface finish of products, reduce orientation and deformation . However, the higher the mold temperature is, the longer the cooling time is, the longer the molding cycle is prolonged, the productivity is reduced, and the shrinkage rate of molding is increased, so it is not the higher the better the mold temperature setting, but also consider the glass fiber content of the resin mold structure, etc., when the glass fiber content is high in the complex cavity filling difficulty, the mold temperature should be appropriately increased For the material of PA66+33%GF car handleface cover, we choose the mold temperature of 110.
C: Pressure
Shrinkage rate of products, but it will increase shear stress and orientation, which is easy to cause warping deformation, demoulding difficulty and even overflow problem. Therefore, it is necessary to improve the floating fiber On the basis of slightly higher injection pressure of non-reinforced plastic, the selection of injection pressure should be appropriately increased according to the specific situation. In addition to factors such as the size of the round gate of the product, it is 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.
The size of the screw back pressure has an important influence on the fluidity of the uniformly dispersed glass fiber in the melt, the compacting degree of the melt, the appearance quality and mechanical and physical properties of the product. Usually, a slightly higher back pressure is more favorable to improve the floating fiber phenomenon. However, high back pressure will produce a larger shear effect on the long fiber, so that the melt is easy to degrade due to overheating, resulting in discoloration and poor mechanical properties. Therefore, the back pressure can be set slightly higher than the non-reinforced plastic.
E. Injection speed
With a faster injection speed, the floating fiber phenomenon can be improved, the injection speed can be increased, the glass fiber reinforced plastic can fill the cavity quickly, and the glass fiber moves along the flow direction rapidly, which is beneficial to increase the dispersion of the glass fiber and reduce the orientation of the glass fiber Improve the strength of the fusion mark and the apparent finish of the product, but pay attention to avoid the injection speed is too fast, in the nozzle mouth or gate "spray" phenomenon, the formation of serpentine defect, affect the appearance of plastic parts.
F. Screw speed
Glass fiber reinforced plastic plasticizing, the screw rotation speed is unfavorable and exorbitant, avoid friction shear force is too large and cause harm to the glass fiber, glass fiber surface interface state, reduce the intensity of adhesion between glass fiber and resin, intensifying "floating fiber" phenomenon, especially when the glass fiber is longer, will appear with the part of the glass fiber fracture length uneven phenomenon, caused by plastic everywhere intensity range, The mechanical properties of the products are unstable.
Through the above analysis, it can be seen that the injection with high material temperature, high mold temperature, high pressure, high speed and low screw speed is more favorable to improve the "floating fiber" phenomenon.
XIAMEN LFT COMPOSITE PLASTIC CO.,LTD is a manfuacturer professional in product long fiber thermoplastic in xiamen,China. Xiamen LFT company have provide the best material and solution for meet your inquirement.
candyhu@lfrtplastic.com
