Property of friction between macromolecules when molten plastic flows is called plastic viscosity. Coefficient of this viscosity is called viscosity, so viscosity is a reflection of fluidity of molten plastic. The greater viscosity, the stronger viscosity of melt, the worse fluidity and the more difficult processing.
In industrial applications, comparing fluidity of a plastic is not based on its viscosity value, but on its melt flow index (called MFI): so-called MFI means that at a certain melting temperature, melt is subjected to a rated pressure, weight of melt passing through standard die per unit time (usually 10 minutes). It is expressed in g/10min, such as injection molding grade PP material, grade is different, MFI value can vary from 2.5 to 30. Viscosity of plastic is not static. Change of characteristics of plastic itself, influence of external temperature, pressure and other conditions can all contribute to change of viscosity.

 

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

Low-molecular additives can reduce force between macromolecular chains. Therefore, viscosity is reduced, and some plastics are added with solvents or plasticizers during molding to reduce viscosity and make it easy to mold.

Temperature has a great influence on viscosity of most molten plastics. Generally, the higher temperature, the lower reflected viscosity, but degree of viscosity reduction of various plastic melts varies:
For PE/PP plastics, increasing temperature has little effect on improving fluidity and reducing melt viscosity. If temperature is too high, consumption will increase, but gain outweighs loss.
For plastics such as PMMA/PC/PA, viscosity will decrease significantly when temperature rises, increase of temperature for PS ABS will also have great benefits in reducing viscosity and molding.

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

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

So-called barrel temperature control refers to how plastic is uniformly heated from raw material particles to a plastic viscous fluid in barrel, that is, how to configure barrel baking temperature.

It should be ensured that plastic is well plasticized, can be injected and filled smoothly without causing decomposition.
This requires us not to consciously reduce plasticizing temperature due to sensitivity of plastic to temperature, and to forcibly fill the mold with injection pressure or injection speed.

It mainly affects processing performance, but also affects surface quality and color.

It is related to mold of part. For large and simple parts, weight of part is closer to injection volume, a higher baking temperature is required, and wall is thinner. Complex shapes should also be baked at high temperatures.
Conversely, for thick-walled parts that require additional operations, such as inserts, you can use 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. Temperature of material should be such that sprayed material is firm and powerful, without bubbles, curls, and continuous brightness.

Generally, it ascends successively from feed section to discharge section, but in order to prevent the overcooked decomposition of plastic and change of color of product, it can also be slightly lower than middle section. Improper configuration of material temperature may sometimes cause stuck screw faults - screw does not rotate or runs idly. This may also be caused by excessive injection pressure or failure of screw check ring (meson), which causes thin molten material at the front end of barrel to flow back to direction of feed area.
When reflowed material is poured into tiny gap between threaded end surface and inner wall of barrel and cooled at a lower temperature, it will freeze and solidify into a thin film tightly stuck between two walls, so that the screw cannot rotate or slip, thus affecting feeding.
At this time, do not forcibly loosen or inject. It is recommended to temporarily close cooling water at feeding port, increase temperature of feeding section until it is 30~50 degrees Celsius higher than melting point of plastic, and at the same time reduce temperature of discharging section to near melting temperature. After 10 to 20 minutes, turn screw carefully, restart machine when it can turn, and then slowly add materials.

 

During injection process, mold control pressure rises sharply and finally reaches a peak value, which is commonly referred to as injection pressure. Injection pressure is obviously higher than cavity filling 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, holding pressure. Its specific functions are:
Supplement amount of material close to gate, and prevent unhardened plastic in mold cavity from flowing backward to source of the gate under action of residual pressure before gate is condensed and closed.
Prevent shrinkage of parts and reduce vacuum bubbles.
Reduce phenomenon of mold sticking burst or bending deformation due to excessive injection pressure of part. So holding pressure is usually 50%~60% of injection pressure.
If holding pressure or time is too long, it is possible to squeeze cold material on gate and runner into part, adding bright spots of cold material near gate, and prolonging cycle without benefit.

Choose according to shape and thickness of workpiece
Choose from different plastic raw materials.
When production conditions and product quality standards permit, it is recommended to adopt process conditions of low temperature and low pressure.

Back pressure represents pressure that plastic is subjected to during plasticization process. It is also called plasticizing pressure.
Color mixing effect is affected by back pressure, back pressure increases, and mixing effect is strengthened.
Back pressure helps to remove various gases from plastic parts, reducing silver streaks and air bubbles.
Appropriate back pressure can avoid local material stagnation in barrel, so back pressure is often increased when cleaning barrel.

Advantages of low-speed mold filling are that flow rate is stable, size of part is relatively stable, fluctuation is small, internal stress of part is low, consistency of internal and external stress is good. Disadvantage is that parts are prone to lamination and poor melting point marks, water lines, etc. High-speed mold filling can use lower injection pressure, improve gloss and smoothness of product, eliminate seam line phenomenon and delamination phenomenon, shrinkage depression is small, and color is more uniform.
Disadvantage is that it is easy to produce "free jet", that is, stagnation or vortex. Excessive temperature rise, yellow color, poor exhaust and sometimes difficult demoulding. Plastics with high viscosity may cause melt fracture and fog spots on the surface of part, and also increase tendency of wing warping caused by internal stress and cracking of thick parts along seam line.

High plastic viscosity, fast cooling speed, long process parts
Parts with too thin wall thickness
glass fiber reinforced plastic

Due to different geometric conditions of runner system and various parts, different parts have requirements for flow (especially speed) of filling melt, which leads to multi-stage injection. According to shape of product, we can quickly fill relatively thin-walled and complex-shaped parts;
For water inlet and places that are easy to burn, use low or medium speed to fill mold. Most products can adopt low-speed-high-speed-medium-speed mold filling process, so as to achieve purpose of changing appearance and internal quality of product.