The nature of friction between macromolecules when molten plastic flows is called the viscosity of plastics, and the coefficient of viscosity is called viscosity, so the viscosity is a reflection of the fluidity of molten plastics. The higher the viscosity is, the stronger the viscosity of the melt is. The worse the fluidity, the more difficult it is to process.
Comparing the fluidity of a plastic does not depend on its viscosity value, but on its melt flow index (called MFI): The so-called MFI is the melt under a certain pressure at a certain melting temperature. (usually 10 minutes) The weight of the melt passing through the standard die. Expressed in g/10min, the viscosity of the plastic is not static. The changes in the characteristics of the plastic itself, external temperature, pressure and other conditions can all contribute to the viscosity change.
Effect of molecular weight
The larger the molecular weight, the narrower the molecular weight distribution and the greater the reflected viscosity.
Low Molecular Weight Addition
The addition of low-molecular-weight additives can reduce the interaction between macromolecules. As a result, the viscosity can be reduced. In some plastic molding processes, the addition of a solvent or plasticizer is to reduce the viscosity and make it easier to mold.
Effect of temperature viscosity
The effect of temperature on the viscosity of most of the molten plastics is very large, generally the temperature increases, the lower the reflected viscosity, but the magnitude of the decrease in the viscosity of various plastic melts is different.
In PE/PP plastics, raising the temperature has little effect on improving the fluidity, reducing the melt viscosity, the temperature is too high, and the consumption is increased, but it is not worth the candle.
PMMA/PC/PA plastics and other plastics, the temperature rises the viscosity to drop obviously, the PS ABS rises the temperature to reduce the viscosity in the molding also has the big advantage.
Effect of shear rate
Effectively increasing the shear rate of the plastic can make the plastic viscosity drop, but there are some plastics, such as PC, with the exception of its viscosity is almost not affected by the screw speed.
The impact of pressure
The effect of pressure on viscosity is more complicated. In general, the viscosity of PP and PE is not greatly affected by pressure, but the effect on PS is quite significant. In practical production, on machines with perfect equipment, high speed injection should be used, ie high shear. The role of cutting speed, but should not blindly increase the pressure.
Effect of injection temperature control on molding process
The so-called barrel temperature control is how the plastic in the barrel is uniformly heated from the raw material particles to a plastic viscous fluid, that is, the problem of how to configure the barrel temperature.
The adjustment of the barrel temperature should ensure that the plastic is well plasticized and can be injected smoothly without causing decomposition.
This requires that we must not consciously reduce the plastification temperature due to the sensitivity of plastics to temperature, and forcibly fill molds with injection pressure or injection speed.
Plastic melting temperature mainly affects the processing performance, but also affects the surface quality and color.
The control of material temperature is related to the part molds. Large and simple parts, parts weight and injection volume are close to each other, need to use higher baking temperature, thin wall, complex shape, but also use high baking temperature. For thick-walled parts, some of which require additional operations, such as inserts, can be used with low temperature, identify whether the temperature of the plastic solution is appropriate or not. Use the jog action to observe the air at low pressure, the appropriate temperature The material should be sprayed vigorously and vigorously, without foaming, no curling, and continuous light.
The configuration of material temperature is generally from the feed section to the discharge section in ascending order, but in order to prevent the over-exposure of plastic decomposition and the change of the color of the parts can also be slightly lower than the middle section, improperly configured material temperature may sometimes cause the screw failure of the card - - The screw does not turn or idling. This may also be caused by excessive injection pressure or failure of the screw check ring (meson) to cause the thin molten material at the front end of the cartridge to flow back toward the feed zone.
Pressure control during injection cycle
The actual pressure applied should be higher than the full cavity pressure. During the injection process, the mold pressure rises sharply and eventually reaches a peak. This peak is known as the injection pressure. The injection pressure is obviously higher than the pressure of the full cavity. .
The role of pressure holding pressure: After the cavity is filled with plastic until the gate is completely cooled and closed for a period of time, the plastic in the cavity still needs a relatively high pressure support, that is, maintaining pressure, and its specific role is:
The amount of material near the gate is supplemented, and before the gate condensation closes, the uncured plastic in the mold cavity is controlled to flow back toward the gate material source under residual pressure.
Prevent shrinkage of parts and reduce vacuum bubbles.
Reduce the phenomenon of sticking or bending deformation caused by excessive injection pressure of the parts. Therefore, the holding pressure is usually 50%~60% of the injection pressure. The holding pressure or the time is too long, the gate may be too large. The cold material on the flow path slams into the workpiece, adding cold spots near the gate, and extending the cycle without any benefit.
Selection of injection pressure:
According to the shape of the part, the thickness is selected.
Choose for different plastic materials.
In the case of production conditions and quality standards permitting of parts, it is recommended to use the process conditions of temperature and pressure.
Back pressure regulation:
Back pressure is represented by the plastic plasticizing process pressure. There is also called plasticizing pressure.
The color mixing effect is affected by the back pressure, the back pressure is increased, and the mixing effect is enhanced.
Back pressure helps to eliminate all kinds of gas from plastic parts and reduce silver lines and bubbles.
Proper back pressure can avoid local hysteresis in the barrel, so the back pressure is often increased when the barrel is cleaned.
Injection speed control
Effect of speed: The advantages of low-speed filling are smooth flow, stable size of parts, less fluctuations, low internal stress of parts, and good uniformity of internal and external isotropic stress. The disadvantage is that the parts are susceptible to layering and poor melting point. Traces, water lines, etc., high-speed filling can be used to lower the injection pressure, improve product gloss and smoothness, eliminating the phenomenon of seam lines and delamination, shrinking depression, color more uniform.
Disadvantages are easy to generate 'free jet', ie stagnation or turbulence. Excessive temperature rise, yellowish color, poor exhaust and sometimes difficulty in stripping. High viscosity plastic may cause melt fracture and fog on the surface of parts. Spots, at the same time, also increase the tendency of the fins caused by internal stresses and the cracking of thick parts along the seam line.