The shear of the plastic melt is thinner
The plastic melt is a non-Newtonian fluid. A process that is closely related to injection molding is the shear thinning of the plastic melt. The viscosity of the fluid does not vary with the shear rate. This fluid is called a Newtonian fluid, such as water , The low molecular weight liquid or solution is a typical Newtonian fluid, and if the viscosity of the fluid depends on its shear rate, such a fluid is a non-Newtonian fluid, most of the plastic melt behaves as a non-Newtonian fluid. There are also a number of non-Newtonian fluids. The properties of the plastic in the molten state are shown in the coordinates of Fig. 4, showing a curve in which the shear stress rises rapidly and then slowly rises, and there is no yield stress, which is the plastic melt Shear thinning flow characteristics. That is, the increase in shear rate is faster than the increase in shear stress, as shown in Fig.
Corresponding to the shear thickening of the phenomenon. However, the common plastic melt is characterized by shear thinning, that is, with the increase in shear rate, the melt viscosity to be reduced, the viscosity of plastic melt in the mold cavity to help flow and fill.
Plastic injection molding process through the barrel to heat, and then through the injection molding machine nozzle, into the mold of the main channel, the flow channel and the mold gate, and finally into the cavity. The shear rate and viscosity relationship of the melt through the various parts are shown in Fig. 5, which shows that the plastic melt has a high viscosity in the cylinder and a small flow velocity. After reaching the gate, the shrinkage of the gate makes the Melt flow rate increases, increasing the shear rate, reducing the melt viscosity, is conducive to the melt filling. The wide MWD resin is larger than the narrow profile resin.
Several factors that affect viscosity
Viscosity is one of the most important basic concepts of plastic processability. It is a quantitative representation of the mobility. The factors that affect viscosity are melt temperature, pressure, shear rate and relative molecular mass.
(1) The effect of temperature It is known from the previous analysis that the viscosity of the plastic is a function of the shear rate, but the viscosity of the plastic is also affected by the temperature. Therefore, only the shear rate is constant, the study of the temperature on the viscosity of the practical significance. Generally speaking, the sensitivity of plastic melt viscosity is stronger than the shear effect. The results show that the viscosity of plastic melt decreases exponentially with the increase of temperature. This is because the temperature rise, will inevitably make the intermolecular, molecular chain between the movement to speed up, so that the plastic molecular chain between the winding down, the distance between the molecules increased, resulting in lower viscosity. Easy to shape, but the product shrinkage, but also cause decomposition, the temperature is too low, melt viscosity, flow difficulties, poor formability, and flexibility, will make the shape of the product stability is poor.
(2) But the different plastic viscosity for the degree of temperature is different. POM is the most sensitive to changes in temperature, followed by polyethylene, polypropylene, polystyrene, the most sensitive to the number of cellulose acetate, Table 1 lists some of the commonly used plastic for temperature sensitivity. Very sensitive to the plastic, temperature control is very important, otherwise the viscosity changes greatly, so that the operation is unstable, affecting product quality.
Table 1 some of the plastic viscosity affected by the degree of temperature
Plastic
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CA
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PS
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PP
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PE
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POM
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Sensitive to Temp
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Highest
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Higher
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High
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Less higher
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Least higher
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In practical use, for the temperature sensitive melt, you can consider in the molding process to improve the molding temperature of plastic to improve the flow properties of plastics, such as PMMA, PC, CA, PA. However, for poorly sensitive plastics, increasing the temperature is not significant for improving the flow properties, so generally do not use the way to improve the temperature to improve its flow characteristics. Such as POM and PE, PP and other non-polar plastic, even if the temperature rise is very large, the viscosity is reduced very small. Also, to improve the temperature must be subject to certain conditions, that is, molding temperature must be allowed in the plastic molding temperature range, otherwise, the plastic will be degraded. Forming equipment loss, working conditions deteriorate, more harm than good. The use of the size of the activation energy to express the viscosity of the material and the relationship between temperature, a quantitative significance. Table 2 shows the activation energy of some plastics at low shear rates.
Table 2 activation energy of some plastic kJ / mol
Plastic
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HDPE
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PP
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LDPE
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PS
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ABS
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PC
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Activation energy
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26.5～29.4
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37.8～40
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49.1
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105
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88.2～109.2
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109.2～126
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(2) the effect of pressure on the plastic melt between the molecules, the molecular chain between the tiny space, the so-called free volume. So the plastic can be compressed. Injection process, the plastic by the external pressure can reach up to dozens or even hundreds of MPa. Under this pressure, the distance between the macromolecules decreases, the range of motion of the segments decreases, the distance between molecules decreases, and the force between molecules increases, resulting in the difficulty of inter-chain misalignment, Big.
But different plastics under the same pressure, the degree of increase in viscosity is not the same. Polystyrene (PS) is the most sensitive to pressure, ie, when the pressure is increased, the viscosity increases rapidly. High-density polyethylene and low-density polyethylene compared to the pressure on the viscosity of the smaller, the impact of polypropylene pressure is equivalent to a moderate degree of polyethylene.
The fact that the increase in pressure causes an increase in viscosity suggests that it is not appropriate to simply increase the flow of plastic melt by increasing the pressure. Excessive pressure not only can not significantly improve the filling of the fluid, but also because of the increase in viscosity, filling performance may sometimes have the possibility of decline, not only caused too much power loss and excessive equipment wear, but also cause the flash and increase Product stress and other defects. In addition, the pressure is too high, there will be defects such as product defects, resulting in excessive power consumption. But the pressure is too low will cause lack of material.
The effect of the combination temperature on the viscosity can be found to increase the effect of pressure on the melt viscosity of the plastic and the effect of the temperature on the viscosity of the plastic in the normal processing parameters of the plastic. For example, for many plastics, when the pressure increases to 100 MPa, the change in viscosity is equivalent to reducing the temperature of 30 ~ 50 ℃ role.
The sensitivity of several plastics to pressure is shown in Table 3.
Table 3 Effect of pressure on plastic melt viscosity
Number
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Name
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Melt temperature℃
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Pressure change range/MPa
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Viscosity increase
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1
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PS
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196
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0～126.6
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134
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2
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PS
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180
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14～175.9
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100
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3
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PE
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149
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0～126.6
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14
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4
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HDPE
|
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14～175.8
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4.1
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5
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LDPE
|
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14～175.8
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5.6
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6
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MDPE
|
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14～175.8
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6.8
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7
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PP
|
|
14～175.8
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7.3
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(3) the impact of shear rate With the increase in shear rate, the viscosity of plastic is generally reduced. However, in cases where the shear rate is very low and very high, the viscosity hardly changes with the shear rate.
In the temperature and pressure under certain conditions, different plastic viscosity reduction degree is not the same. In other words, although the viscosity of most plastic melts decreases with increasing shear rates, the sensitivity of different plastics to shear rate (shear stress) is not the same.
The sensitivity of the viscosity of several commonly used plastics to shear rates is shown in Table 4.
Number
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Plastic
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Sensitive
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1
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ABS（Most sensitive）
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Sequentially reduced shear sensitivity
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2
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PC
|
|
3
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PMMA
|
|
4
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PVC
|
|
5
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PA
|
|
6
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PP
|
|
7
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PS
|
|
8
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LDPE（Least sensitive）
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|
The implication of this is that in a certain shear rate range, increasing the shear rate will significantly reduce the viscosity of the plastic and improve its flow properties. Nevertheless, it is preferable to adjust the process in the range where the melt viscosity is less sensitive to the shear rate. Otherwise, the fluctuation of the shear rate will cause the processing to be unstable and the defects in the quality of the plastic product.
(4) the impact of plastic structure For plastics, at a given temperature, with the relative average molecular mass increases, the viscosity of plastic increases. The greater the molecular weight, the stronger the intermolecular force, and the higher the viscosity.
The smaller the molecular weight of the plastic, the smaller the dependence of the viscosity on the shear rate; the greater the molecular weight, the greater the degree of viscosity dependence on the shear rate. The resin having a broad molecular weight distribution and the bimodal molecular weight distribution resin have low melt viscosity and excellent workability. Because the low molecular weight chain is beneficial to improve the melt flow of the resin.
(5) the impact of low molecular weight additives Low molecular weight can reduce the interaction between the macromolecules, from the "lubrication" effect and thus reduce the melt viscosity, while reducing the viscosity of the fluidization temperature. Such as adding plasticizers and solvents, so that easy to mold filling resin.
Table 5 Commonly used plastics to improve the flow properties of the method
Plastic
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Method
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Plastic
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Method
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PE
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Increase the screw speed
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PS
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Choose non-crystalline grades
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PP
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Increase the screw speed
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ABS
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Increase Temperature
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PA
|
Increase Temperature
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PVC
|
Increase Temperature
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POM
|
Increase the screw speed
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PMMA
|
Increase Temperature
|
PC
|
Increase temperature
|
|
|
In short, the polymer melt viscosity directly affect the size of the injection molding process difficult. If the control of a plastic molding temperature below its decomposition temperature, the shear rate of 103 seconds -1, the melt viscosity of 50 to 500 Pa-second, injection molding easier. But 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, overflow phenomenon is serious, product quality is not easy to guarantee, in this case Require the nozzle to have a self-locking device.
Kent Yuan (Mr.)
Hangzhou Zhongwang Technology Co., LTD. |
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