Several factors affecting viscosity

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

1, the influence of temperature

From the previous analysis has been known that the viscosity of plastics is a function of shear rate, but the viscosity of plastics is also affected by temperature. Therefore, only when the shear rate is constant, it is of practical significance to study the effect of temperature on viscosity. Generally speaking, the sensitivity of plastic melt viscosity is stronger than that of shear action. Studies have shown that with the increase of temperature, the viscosity of plastic melt decreases exponentially.

This is because the increase of temperature will inevitably accelerate the movement between molecules and molecular chains, thus reducing the entanglement between plastic molecular chains and increasing the distance between molecules, resulting in a decrease in viscosity. Easy to form, but the shrinkage of the product is large, it will cause decomposition, the temperature is too low, the melt viscosity is large, the flow is difficult, the formability is poor, and the elasticity is large, it will also make the shape stability of the product is poor.

But different plastic viscosity for the degree of temperature is different. Polyformaldehyde is the least sensitive to temperature changes, followed by polyethylene, polypropylene, polystyrene, the most sensitive to cellulose acetate, Table 1 lists the sensitivity of some commonly used plastics to temperature. For very sensitive plastics, temperature control is very important, otherwise the viscosity will change greatly, making the operation unstable and affecting the quality of the product.

in table 1 Some plastic viscosity is affected by temperature

In practice, for the melt with good temperature sensitivity, the molding temperature of the plastic can be considered in the molding process to improve the flow performance of the plastic, such as PMMA, PC, CA, PA. However, for plastics with poor sensitivity, increasing the temperature is not obvious for improving the flow performance, so the method of increasing the temperature is generally not used to improve its flow characteristics.

Such as POM and PE, PP and other non-polar plastics, even if the temperature rise is large, the viscosity is reduced very little. In addition, the increase in temperature must be limited by certain conditions, that is, the molding temperature must be within the permissible molding temperature range of the plastic, otherwise, the plastic will degrade. The loss of molding equipment is large, the working conditions deteriorate, and the gain is not worth the loss. The size of the activation energy is used to express the relationship between the viscosity and temperature of the material, which has quantitative significance. Table 2 shows the activation energy of some plastics at low shear rate.

table 2 some plastic, activation energy (unit: kJ/mol)

2, the influence of pressure

There is a small space between the molecules and the molecular chains inside the plastic melt, that is, the so-called free volume. Therefore, the plastic can be compressed. During the injection process, the external pressure on the plastic can reach tens or even hundreds of MPa. Under the action of this pressure, the distance between macromolecules decreases, the range of activity of the chain segment decreases, the intermolecular distance decreases, and the intermolecular force increases, resulting in the dislocation between the chains is more difficult, which is manifested as the overall viscosity increases.

However, different plastics under the same pressure, the degree of viscosity increase is not the same. Polystyrene (PS) is the most sensitive to pressure, that is, when the pressure is increased, the viscosity increases very quickly. High density polyethylene (HDPE) has less effect on viscosity than low density polyethylene, and polypropylene is affected by pressure equivalent to a moderate degree of polyethylene.

The fact that increasing pressure causes an increase in viscosity indicates that it is inappropriate to increase the flow rate of plastic melt by increasing pressure alone. Too high pressure not only can not significantly improve the filling of the fluid, and because of the increase in viscosity, the filling performance will sometimes be reduced, not only cause too much power loss and excessive equipment wear, but also cause overflow and increase the internal stress of the product and other ills.

In addition, the pressure is too high, there will be product deformation and other injection defects, resulting in excessive consumption of power. But the pressure is too low will cause a lack of material.

Combined with the influence of temperature on viscosity, it can be found that within the normal processing parameters of plastics, the effect of increasing pressure on the viscosity of plastic melt is similar to that of decreasing temperature. For example, for many plastics, when the pressure is increased to 100 MPa, the change in viscosity is equivalent to the effect of reducing the temperature by 30 ~ 50¡æ.

The sensitivity of several plastics to pressure is shown in Table 3.

table 3 Effect of pressure on viscosity of plastic melt

3, effect of shear rate

With the increase of shear rate, the viscosity of plastic generally decreases. However, in the case of very low shear rate and very high shear rate, the viscosity almost does not change with the shear rate. Under the premise of a certain temperature and pressure, the viscosity reduction degree of different plastics is different.

In other words, although the viscosity of most plastic melts decreases with the increase of the shear rate, different plastics are not equally sensitive to the shear rate (shear stress). The viscosity sensitivity of several commonly used plastics to shear rate is shown in Table 4.

table 4 Sensitivity of plastic melt viscosity to shear rate

The implication of this for use is that in a certain range of shear rates, increasing the shear rate will significantly reduce the viscosity of plastics and improve their flow performance. However, it is better to choose the process adjustment in the range where the melt viscosity is not too sensitive to the shear rate, otherwise the fluctuation of the shear rate will cause instability in the processing and defects in the quality of plastic products.

4, the effect of plastic structure

For plastics, at a given temperature, with the increase of the relative average molecular mass, the viscosity of plastics increases. The greater the relative molecular mass, the stronger the intermolecular force, so the viscosity is also high.

The smaller the relative molecular mass of the plastic, the less dependent the viscosity is on the shear rate; The greater the molecular weight is, the greater the viscosity depends on the shear rate. The resins with wide molecular weight distribution and bimodal molecular weight distribution have low melt viscosity and good processability. Because the low molecular weight chain part is conducive to improving the fluidity of the resin melt.

5, the influence of low molecular weight additives

Low molecular weight can reduce the force between large molecular chains, play a ""lubrication"" effect, thus reducing the viscosity of the melt, while reducing the viscosification temperature. Such as adding plasticizer and solvent, so that the resin is easy to fill molding.

table 5 commonly used plastic to improve the performance of flow

all in all, the polymer melt viscosity directly affects the size of the injection molding process easily. If the molding temperature of a plastic is controlled below its decomposition temperature, when the shear rate is 103 seconds-1, the melt viscosity is 50 ~ 500 pa-seconds, and the injection molding is easier. However, if the viscosity is too large, it requires a higher injection pressure, the size of the product is limited, and the product is also prone to defects; If the viscosity is too small, the overflow phenomenon is serious, and the quality of the product is not easy to ensure, in this case, the nozzle is required to have a self-locking device.