The influence of viscosity and condition of plastic on viscosity
The friction between macromolecules in molten plastics is called the viscosity of plastics. The coefficient of viscosity is called viscosity. Therefore, viscosity is the reflection of the fluidity of molten plastics. The larger the viscosity, the stronger the melt viscosity, the worse the fluidity, the more difficult the processing
In industrial application, comparing the fluidity of a plastic is not to see its viscosity value, but to see its melt flow index (MFI): MFI is the melt weight passing through the standard die in unit time (generally 10 minutes) under the action of rated pressure at a certain melting temperature. In g/10min, for example, PP materials of injection grade, different grades, MFI values can vary from 2.5 to 30, The viscosity of plastics is not constant. The change of plastic properties, the influence of external temperature, pressure and other conditions can promote the change of viscosity
1.1 influence of molecular weight
The larger the molecular weight, the narrower the molecular weight distribution, the greater the viscosity reflected
1.2 effect of low molecular additivity
The low molecular addition can reduce the force between macromolecules, so the viscosity is reduced. Some plastics are added with solution or plasticizer in order to reduce viscosity and make it easy to mold
1.3 influence of temperature viscosity
The temperature has a great influence on the viscosity of most melt plastics. The general temperature increases, and the lower the viscosity reflected, but the decrease of viscosity of various plastics melts varies in magnitude
Pe/pp plastics, the effect of increasing temperature on improving fluidity and reducing melt viscosity is very small, the temperature is too high and consumption is increased, but it is not worth losing
The temperature rise viscosity of pmma/pc/pa plastics decreases significantly. The increase of PS ABS temperature is also beneficial to reduce viscosity and molding
1.4 influence of shear velocity
The viscosity of plastics can be reduced by increasing the shear speed of plastics effectively, but some plastics, such as PC, have the exception, and the viscosity is almost independent of screw speed
1.5 pressure impact
The pressure has a complex influence on viscosity. Generally, PP PE viscosity is not affected by pressure, but it has a significant influence on PS. in actual production, in the machine with perfect equipment, it is necessary to pay attention to the effect of high-speed injection, that is, high shear speed, rather than blindly increase the pressure
The influence of injection temperature control on Molding
The so-called barrel temperature control refers to how plastic is heated uniformly from raw material to plastic viscous fluid in the barrel, that is, how to configure the baking temperature of the barrel
2.1. The temperature of the barrel shall be adjusted to ensure that the plastic is plasticized well, and the injection mold can be injected smoothly without causing decomposition
This requires that we can not consciously reduce the plasticizing temperature due to the sensitivity of plastic to temperature, and force the mold filling by injection pressure or injection speed
2.2. The melting temperature of plastics mainly affects the processing performance, and also affects the surface quality and color
2.3. The control of material temperature is related to the mold of the parts. For large and simple parts, if the weight of the parts is close to the injection quantity, the high baking temperature, thin wall and complex shape shall be used. Otherwise, for thick wall parts, some additional operations, such as those with embedded parts, can be used with low baking temperature, To identify whether the temperature of plastic solution is appropriate, it can be observed by point action under low pressure speed. The appropriate material temperature should make the sprayed material strong and strong, without bubble, curl and bright and continuous
2.4. The material temperature is generally increased from the feed section to the discharge section. However, in order to prevent the over ripening decomposition of plastic and the change of the color of the parts, it can also be slightly lower than the middle section. Improper material temperature configuration sometimes causes the screw to fail -- the screw does not rotate or idling, which may also be caused by the excessive injection pressure or the failure of the screw check ring (meson) which causes the thin melt at the front of the barrel to flow back to the feed area
When these backflow materials are poured into the small gap between the thread end face and the inner wall of the barrel and cooled at a lower temperature, the cold film is tightly clamped between the two walls so that the screw can not rotate or slip, which will affect the feeding
At this time, do not force release or inject. It is recommended that the cooling water at the feeding port be temporarily closed, and the temperature of the feeding section should be increased until 30-50 ℃ higher than the melting point of plastic. Meanwhile, the temperature of the discharge section should be reduced to the melting temperature. After 10-20 minutes, carefully turn the screw, and restart the machine when it can be turned on, and then slowly feed
Pressure control during injection cycle
3.1. The actual application pressure should be higher than that of the full cavity. During the injection process, the pressure of mold control rises sharply, and finally reaches a peak value, which is generally referred to as injection pressure. Obviously, the injection pressure is higher than that of the full cavity
3.2. Pressure retaining effect: after the mold cavity is filled with plastic until the gate is completely cooled, the plastic in the mold cavity still needs a relatively high pressure support, namely, pressure retaining. Its specific role is:
A: The amount of material near the gate position shall be supplemented, and the plastic in the mold cavity which has not hardened before the gate condensation is closed shall be stopped from flowing back to the gate material source under the residual pressure
B: Prevent the contraction of the parts and reduce the vacuum bubble
C: Reduce the phenomenon of sticking die burst or bending deformation due to the excessive injection pressure of the parts. Therefore, the pressure maintaining pressure is usually 50% - 60% of the injection pressure. If the pressure holding pressure or the time is too long, it is possible to squeeze the cold material on the gate and the runner into the parts, so that the cold material bright spots are added near the gate, and the cycle is not beneficial
3.3. Selection of injection pressure
A. According to the shape of the parts. Thickness selection. B. for different plastic materials
Under the condition of production conditions and the quality standard of parts, it is recommended to adopt the process conditions of temperature and low pressure
3.4. Adjustment of back pressure
Back pressure is the pressure that plastic plastic process bears. Progressive is also called plasticizing pressure
A. The mixing effect of color is affected by back pressure, back pressure increases, mixing effect is strengthened
B. Back pressure helps to remove all kinds of gases in plastic parts and reduce the phenomenon of silver grain and bubble
C. Proper back pressure can avoid the phenomenon of local material stagnation in the barrel, so the back pressure is often increased when cleaning the barrel
Control of injection speed
The influence of speed: the advantages of low-speed mold filling are steady velocity, stable size of parts, small fluctuation, low internal stress of parts, good consistency of internal and external anisotropic stress. The disadvantage is that the parts are prone to poor melting point marks and water lines. The high-speed filling mold can adopt lower injection pressure to improve the gloss and smoothness of the products, eliminate the phenomenon of seam line and lamination, and reduce shrinkage depression, The color is more uniform
The disadvantage is that it is easy to produce "free jet", that is, stagnant flow or eddy current. The temperature rise is too high, the color is yellow, the exhaust is poor and sometimes demoulding is difficult. The plastic with high viscosity may produce melt rupture, and the surface of the parts will produce fog spots, and at the same time, the tendency of wing bending and thick parts cracking along the joint suture caused by internal stress is increased
