Nature of friction between macromolecules when molten plastic flows is called viscosity of plastic. Coefficient of this viscosity is called viscosity, so viscosity is a reflection of fluidity of molten plastic. The greater viscosity, the stronger melt viscosity, the worse fluidity, and the more difficult processing.
In industrial applications, comparing fluidity of a plastic is not by its viscosity value, but by its melt flow index (called MFI): so-called MFI is weight of melt that passes through standard die in a unit time (usually 10 minutes) under action of a rated pressure at a certain melting temperature. Expressed in g/10min, such as injection-grade PP materials, with different grades, value of MFI can vary from 2.5 to 30. Viscosity of plastics is not static. Changes in characteristics of plastics, external temperature, pressure and other conditions can all contribute to changes in viscosity.

The larger molecular weight, the narrower molecular weight distribution, and the greater viscosity reflected.

Addition of low molecules can reduce force between macromolecules. Therefore, viscosity is reduced. Some plastics are formed by adding solvents or plasticizers to reduce viscosity and make them easy to mold.

Temperature has a great influence on viscosity of most molten plastics. Generally, the higher temperature, the lower reflected viscosity, but magnitude of decrease in viscosity of various plastic melts is different:
For PE/PP plastics, increasing temperature has little effect on improving fluidity and reducing melt viscosity. Temperature is too high and consumption increases, but gain is not worth loss.
For PMMA/PC/PA and other plastics, viscosity will decrease significantly when temperature rises, increase in temperature of PS ABS also has great benefits for reducing viscosity and molding.

Effectively increasing shear speed of plastic can reduce viscosity of plastic, but there are exceptions to some plastics, such as PC, whose viscosity is hardly affected by screw speed.

Effect of pressure on viscosity is more complicated. Generally, viscosity of PP&PE is not greatly affected by pressure, but effect on PS is quite significant. In actual production, on machines with better equipment, attention should be paid to high-speed injection, that is, high shear speed, and pressure should not be blindly increased.

So-called barrel temperature control refers to how plastic is uniformly heated from raw material particles into plastic viscous fluid in barrel, that is, how to configure barrel baking temperature.
2.1. Temperature of barrel should be adjusted to ensure that plastic is well plasticized, injection and filling can be performed smoothly without causing decomposition.
This requires us not to consciously lower plasticizing temperature due to sensitivity of plastic to temperature, and use injection pressure or injection speed to forcibly fill mold.
2.2. Melting temperature of plastics mainly affects processing performance, but also affects surface quality and color.
2.3. Control of material temperature is related to part mold. For large and simple parts, weight of part is close to injection volume, a higher baking temperature and thin wall are required. High baking temperature should also be used for complex shapes. On the contrary, for thick-walled parts, some additional operations, such as inserts, can be used at a low baking temperature to identify whether temperature of plastic melt is appropriate. You can use jog action to observe air injection at low pressure and speed. Appropriate material temperature should make sprayed material stiff and powerful, without bubbles, no curling, and continuous light.
2.4. Configuration of material temperature is generally increased from feeding section to discharging section in order, but in order to prevent overripe decomposition of plastic and change of color of part, it can also be slightly lower than middle section. Improper configuration of material temperature may sometimes cause jamming screw failure-screw does not rotate or idling, it may also be caused by excessive injection pressure or failure of screw check ring (meson), which caused thin melt at the front of barrel to flow back in the direction of feed zone.
When these backflowing materials are poured into tiny gap between threaded end surface and inner wall of barrel, are cooled at a lower temperature, cold solidified into a film tightly clamped between two wall surfaces, so that screw cannot rotate or slip, thus affecting feeding.
At this time, do not forcibly loosen or inject. It is recommended that cooling water at feeding port be temporarily closed, temperature of feeding section is increased to 30-50 degrees Celsius higher than melting point of plastic, temperature of discharging section is reduced to near melting temperature at the same time. After 10-20 minutes, carefully turn screw, restart it when it can turn, and then slowly feed.

3.1. Actual applied pressure should be higher than pressure of full cavity. During injection process, mold control pressure rises sharply, and finally reaches a peak. This peak is usually referred to as injection pressure. Injection pressure is obviously higher than pressure to fill cavity.
3.2. Role of pressure holding pressure: After mold cavity is filled with plastic until gate is completely cooled and closed for a period of time, plastic in mold cavity still needs a relatively high pressure support, that is, pressure holding. Its specific functions are:
A: Supplement amount of material close to gate, prevent unhardened plastic in cavity from flowing back to source of gate under action of residual pressure before gate is condensed and closed.
B: Prevent shrinkage of parts and reduce vacuum bubbles.
C: Reduce phenomenon of sticking mold bursting or bending deformation due to excessive injection pressure of part. Therefore, holding pressure is usually 50% to 60% of injection pressure. If holding pressure or time is too long, it is possible to extrude cold material on gate and runner into parts, so that cold material is added to position near gate, and cycle is extended without any benefit.
3.3. Selection of injection pressure
A. Choose according to shape and thickness of part. B. Choose according to different plastic materials.
When production conditions and product quality standards permit, it is recommended to adopt process conditions of low temperature and low pressure.
3.4. Adjustment of back pressure
Back pressure represents pressure experienced by plasticization process. Sometimes it is also called plasticizing pressure.
A. Color mixing effect is affected by back pressure, back pressure increases, and mixing effect is strengthened.
B. Back pressure helps to eliminate various gases from plastic parts, reduce silver streaks and bubbles.
C. Proper back pressure can avoid local stagnation in barrel, so back pressure is often increased when cleaning barrel.

Effect of speed: advantage of low-speed filling is that flow rate is stable, size of part is relatively stable, fluctuation is small, internal stress of part is low, internal and external stress is better. Disadvantage is that part is prone to poor melting point marks, water marks, etc. Low injection pressure can be used for high-speed filling, which can improve gloss and smoothness of product, eliminate phenomenon of seam lines and delamination, reduce shrinkage and depression, and make color more uniform.
Disadvantage is that it is easy to produce "free jet", that is, stagnation or eddy current. Excessive temperature rise, yellow color, poor exhaust and sometimes difficult demolding. Plastics with high viscosity may cause melt fracture and fog on the surface of part. At the same time, it also increases tendency of wing curvature and thick parts to crack along seam line caused by internal stress.