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

 

It has been known from previous analysis that viscosity of plastics is a function of shear rate, but viscosity of plastics is also affected by temperature. Therefore, it is only practical to study effect of temperature on viscosity when shear rate is constant. Generally speaking, sensitivity of plastic melt viscosity is stronger than that of shear. Studies have shown that as temperature increases, viscosity of plastic melt decreases exponentially.
This is because increase in temperature will inevitably accelerate 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 shrinkage rate of product is large, which can also cause decomposition, temperature is too low, melt viscosity is large, flow is difficult, formability is poor, and elasticity is large, which will also make shape stability of product poor.
But different plastics have different viscosities to temperature. Polyoxymethylene is least sensitive to temperature changes, followed by polyethylene, polypropylene, polystyrene, and the most sensitive is cellulose acetate. Table 1 lists sensitivity of some commonly used plastics to temperature. Very sensitive plastic, temperature control is very important, otherwise viscosity will change greatly, which will make operation unstable and affect product quality.
Table 1. Degree to which viscosity of some plastics is affected by temperature

In practice, for melts with good temperature sensitivity, it can be considered to increase molding temperature of plastic during molding process to improve flow properties of plastic, such as PMMA, PC, CA, PA. However, for plastics with poor sensitivity, it is not obvious to improve flow properties by increasing temperature, so method of increasing temperature is generally not used to improve flow properties.
Non-polar plastics such as POM and PE, PP, even if temperature rises greatly, viscosity decreases very little. Also, increase in temperature must be limited 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, and gains outweigh losses. Using magnitude of activation energy to express relationship between viscosity and temperature of material has quantitative significance. Table 2 shows activation energies of some plastics at low shear rates.
Table 2 Activation energies of some plastics        kJ/mol

There are tiny spaces between molecules and molecular chains inside plastic melt, so-called free volume. So plastic is compressible. During injection process, maximum external pressure on plastic can reach tens or even hundreds of MPa. Under action of this pressure, distance between macromolecules decreases, range of activity of chain segments decreases, distance between molecules decreases, and force between molecules increases, which makes dislocation between chains more difficult, which is manifested as an increase in the overall viscosity.
However, under same pressure, different plastics have different degrees of viscosity increase. Polystyrene (PS) is the most sensitive to pressure, i.e. viscosity increases rapidly with increasing pressure. Compared with low-density polyethylene, high-density polyethylene has less effect on viscosity by pressure, and polypropylene is affected by pressure equivalent to moderate polyethylene.
Fact that increasing pressure causes the viscosity to increase suggests that simply increasing pressure to increase flow rate of plastic melt is not appropriate. Excessive pressure not only cannot significantly improve filling of fluid, but also due to increase in viscosity, filling performance may sometimes decline, which not only causes excessive power loss and excessive equipment wear, but also causes problems such as flashing and increasing internal stress of product.
In addition, if pressure is too high, there will be injection molding defects such as product deformation, resulting in excessive power consumption. But if pressure is too low, it will cause material shortage.
Combining 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 reducing temperature. For example, for many plastics, when pressure is increased to 100 MPa, change in viscosity is equivalent to reducing temperature by 30 to 50℃.
Sensitivity of several plastics to pressure is shown in Table 3.
Table 3 Effect of pressure on plastic melt viscosity

As shear rate increases, viscosity of plastic generally decreases. But at very low and very high shear rates, viscosity hardly changes with shear rate. Under premise of certain temperature and pressure, degree of viscosity reduction of different plastics is different.

 

In other words, although viscosity of most plastic melts decreases with increase of shear rate, sensitivity of different plastics to shear rate (shear stress) is not same. Shear rate sensitivity of viscosity of several commonly used plastics is shown in Table 4.
Table 4 Sensitivity of plastic melt viscosity to shear rate

Implication of this for use is that within a certain shear rate range, increasing shear rate will significantly reduce viscosity of plastic and improve its flow properties. Nevertheless, it is better to make process adjustments in the range where melt viscosity is less 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 larger 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 resin melt fluidity.

Low molecular weight can reduce force between macromolecular chains and play a "lubricating" role, thereby reducing melt viscosity and reducing viscous fluidization temperature. If plasticizers and solvents are added, resin can be easily filled and molded.
Table 5 Methods of commonly used plastics to improve flow properties

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