Viscosity is one of the most important basic concepts of plastic processability. It is a quantitative expression of fluidity. Factors that affect viscosity include melt temperature, pressure, shear rate, and relative molecular mass, which are described below.

It is known from previous analysis that viscosity of plastics is a function of shear rate, but viscosity of plastics is also affected by temperature. Therefore, only when shear rate is constant can it be of practical significance to study effect of temperature on viscosity. Generally speaking, plastic melt viscosity is more sensitive than shearing. Studies have shown that as temperature increases, viscosity of plastic melt decreases exponentially.
This is because increase in temperature will inevitably speed up movement between molecules and molecular chains, thereby reducing entanglement between plastic molecular chains and increasing distance between molecules, resulting in a decrease in viscosity. It is easy to form, but product has a large shrinkage rate, which will also cause decomposition, temperature is too low, melt viscosity is large, flow is difficult, moldability is poor, elasticity is large, and shape stability of product will also be poor.
But viscosity of different plastics has different degrees of temperature. Polyoxymethylene is the least sensitive to temperature changes, followed by polyethylene, polypropylene, and polystyrene. The most sensitive is cellulose acetate. Table 1 lists sensitivity of some commonly used plastics to temperature. Temperature control is very important for very sensitive plastics, otherwise viscosity will change greatly, which will make operation unstable and affect product quality.
Table 1. Viscosity of some plastics is affected by temperature

In practice, for melts with good temperature sensitivity, you can consider increasing molding temperature of plastic during molding process to improve flow properties of plastic, such as PMMA, PC, CA, and PA. But for plastics with poor sensitivity, increasing temperature is not obvious for improving flow properties, so generally method of increasing temperature is not used to improve its flow properties.
Such as POM, PE, PP and other non-polar plastics, even if temperature rises greatly, viscosity decreases very little. Also, increasing temperature must be restricted by certain conditions, that is, molding temperature must be within allowable molding temperature range of plastic, otherwise, plastic will degrade. Loss of molding equipment is large, working conditions are deteriorated, gains outweigh losses. Use of activation energy to express relationship between viscosity of material and temperature has a quantitative meaning. Table 2 shows activation energy of some plastics at low shear rates.
Table 2 Activation energy of some plastics kJ/mol

There are tiny spaces between molecules and molecular chains inside plastic melt, which is so-called free volume. Therefore plastics can be compressed. During injection process, maximum external pressure on plastic can reach tens or even hundreds of MPa. Under this pressure, distance between macromolecules is reduced, range of motion of chain segment is reduced, distance between molecules is reduced, and force between molecules is increased, which makes it more difficult to move between chains, which is manifested by an increase in the overall viscosity.
However, under same pressure, different plastics have different degrees of increase in viscosity. Polystyrene (PS) is the most sensitive to pressure, that is, when pressure is increased, viscosity increases rapidly. Compared with low-density polyethylene, high-density polyethylene has a smaller effect on viscosity, effect of pressure on polypropylene is equivalent to that of moderate polyethylene.
Fact that increasing pressure causes an increase in viscosity indicates that it is not appropriate to simply increase pressure to increase flow rate of plastic melt. Excessive pressure not only can not significantly improve filling of fluid, but also due to increase in viscosity, filling performance may sometimes decrease, which not only causes excessive power loss and excessive equipment wear, but also causes overflow and increase Disadvantages such as internal stress of product.
In addition, if pressure is too high, injection molding defects such as product deformation will occur, resulting in excessive power consumption. However, if pressure is too low, it will cause a shortage of materials.
Combined with effect of temperature on viscosity, it can be found that within normal processing parameters of plastics, effect of increasing pressure on viscosity of plastic melt is similar to that of decreasing temperature on plastic viscosity. For example, for many plastics, when pressure is increased to 100 MPa, change in viscosity is equivalent to reducing temperature by 30-50℃.
Sensitivity of several plastics to pressure is shown in Table 3.
Table 3 Influence of pressure on viscosity of plastic melt

As shear rate increases, viscosity of plastic generally decreases. But when shear rate is very low and very high, viscosity hardly changes with change of shear rate. Under certain conditions of temperature and pressure, different plastics have different degrees of viscosity reduction.
In other words, although viscosity of most plastic melts decreases as shear rate increases, sensitivity of different plastics to shear rate (shear stress) is different. Sensitivity of several commonly used plastics to shear rate is shown in Table 4.
Table 4 Sensitivity of plastic melt viscosity to shear rate

Enlightenment of this point for use is: within a certain range of shear rate, increasing shear rate will significantly reduce viscosity of plastic and improve its flow properties. Nevertheless, it is better to adjust process in a range where melt viscosity is not very sensitive to shear rate, otherwise fluctuation of shear rate will cause unstable processing and defects in quality of plastic products.

For plastics, at a given temperature, as relative average molecular mass increases, viscosity of plastic increases. The greater relative molecular mass, the stronger intermolecular force, and the higher viscosity.
The smaller relative molecular mass of plastic, the smaller dependence of viscosity on shear rate; the larger molecular weight, the greater dependence of viscosity on shear rate. Resins with broad molecular weight distribution and bimodal molecular weight distribution resins have low melt viscosity and excellent processability. Because low-molecular-weight chain part is beneficial to improve fluidity of resin melt.

Low molecules can reduce force between macromolecular chains, play a "lubricating" role and thus reduce viscosity of melt, and at the same time reduce viscous fluidization temperature. Such as adding plasticizers and solvents to make resin easy to mold filling.
Table 5 Methods of improving flow properties of commonly used plastics

In short, viscosity of polymer melt directly affects difficulty of injection molding process. If molding temperature of a certain plastic is controlled below its decomposition temperature, when shear rate is 103 sec-1, melt viscosity is 50-500 Pa-sec, and injection molding is easier. However, if viscosity is too high, a higher injection pressure is required, size of product is limited, and product is also prone to defects; if viscosity is too small, overflow phenomenon is serious, and product quality is not easy to guarantee. In this case, nozzle is required to have a self-locking device.