Viscoelasticity of plastic melt
DATE：2022/5/7 8:38:30 / READ： / SOURCE：This station
Viscoelasticity of plastic melt
Previous experimental results have shown that when the plastic melt containing foaming agent passes through the die channel, the position of foaming point has a great impact on the foam quality. In this section, we will focus on various parameters affecting the position of foaming point, introduce a large number of experimental results and explain the relationship between various parameters. EVA film production line www.handern.com comA. M. kraynik observed the foaming behavior of plastic melt containing foaming agent in the flat and direct channel of flow path die, and based on this, put forward a mathematical model to reflect the relationship between various parameters affecting the position of foaming point. He divided the flow channel into three sections: inlet section, steady flow section and outlet section. Refer to Fig. 4-13 and Fig. 4-14. The pressure curve of the melt in the inlet section deviates from the straight line due to the influence of inlet effect. The polymer melt without foaming agent will also produce import effect in the inlet section, but it is smaller than that with foaming agent. The pressure line of the steady flow section is a straight line. The melt flow is stable and fully developed, the pressure gradient remains unchanged, and its viscosity can be calculated by the following formula. In the outlet section, due to the bubbles in the melt, the melt undergoes phase change and changes from single liquid phase to gas-liquid two-phase flow. With the continuous expansion of bubbles in the melt, the proportion of gas phase will increase continuously, the viscosity of the melt will also change continuously, the pressure curve will deviate from the straight line, and the pressure gradient is no longer a constant. Therefore, the inlet section and outlet section cannot be calculated by formulas (4-1), (4-2) and (4-3). In order to find out the relationship between various factors affecting the position of foaming point, the inlet pressure correction and outlet pressure correction; Shear stress of melt at the wall of runner; R. L -- corresponding to the radius and length of the runner. EVA film production line www.handern.com com
If the curved part of the pressure curve is replaced by a straight line, the following relationship exists for each pressure. For the convenience of calculation, two dimensionless parameters, 0 and a: the physical meaning of 0 and a, are introduced. 0 represents the ratio of the pressure drop before bubbles appear in the melt and the pressure drop to the outlet after bubbles appear in the melt when the melt passes through the channel; A represents the ratio of the average pressure gradient of the melt before bubbles and after bubbles in the channel. Q volume flow rate CD of the melt containing foaming agent at the runner port Han confirmed the correctness of formula (4-13) with the test results. EVA film production line www.handern.com com
Hansen, D. mbrgg, y. qyanngi and others have long pointed out that by increasing the extrusion rate, the position of the foaming point of the melt in the flow channel can be moved out of the die, which is conducive to obtaining high-quality foam with uniform and fine pores. Equation (4-13) can be used to analyze the relationship between the majority affecting L, which is of great practical reference value. However, P should be determined when using this formula; It is difficult to determine the value. Generally, it is measured by observing the foaming behavior in the transparent channel, determining the distribution of axial pressure along the channel and determining the pressure of foaming point. In addition to the difficulties in equipment, the accuracy of this method is also relatively low. Later, M.L. Fridman, q.y. sabsmi, N.E. Nikolaeva and G.R. barshtein proposed a relatively simple method. They used the relationship between the compressibility of the melt containing gas phase and the hydrostatic pressure to find out the location of the foaming point, as shown in Figure 4-16. When the pressure of the polymer melt containing gas is higher than the critical foaming pressure P:, the gas is dissolved in the melt, and the melt is in a single-phase state, and its compressibility is the same as that of the melt without gas. However, when the pressure is lower than P, the excess 190 ℃ gas in the melt is separated to form a gas phase, and the melt is a two-phase fluid, and the compressibility is significantly increased by 20 · HDPE + 0.6% AC, so the slope of the pressure curve is obviously P_ The experimental results change, and the corresponding pressure of this transformation point is p The calculation result (4-14) is the critical pressure P of bubble expansion, the > 10 ordinate is the ratio of the volume of gas phase contained in the melt to the volume of the melt, and the abscissa is the static pressure on the melt. The intersection of two pressure lines with different slopes is p, P. The critical pressure Po of bubbles in the melt can also be directly affected by the viscoelasticity of the polymer melt. Fig. 4-16 and Fig. 4-17 show the foaming of two plastic melts containing foaming agent when they pass through the same channel, It should be noted that the people or B in the two figures are not the same. The ordinate of the diagram is pressure and the abscissa is flow. It can be seen from figure 4-16 that the gradient of the inlet and outlet pressure line is greater than that of B, that is, the viscosity of a is greater than that of B. The outlet residual stresses of a and B are very close, indicating that their elasticity is close. In the die, B foams earlier than the first one, and the bubble appears earlier. Due to the action of shear force in the flow process, the bubble is easy to collapse and the bubble density increases. The melt viscosity and shear heat are large, which is unfavorable to improve the output. EVA film production line www.handern.com com
Therefore, these two melts are not suitable for extrusion foaming. Figure 4-17 shows the pressure variation diagram of the other two materials a and B. the pressure line inclination of B is larger than that of a, indicating that the viscosity of B is larger than that of a, but the residual pressure at the human outlet is larger than that of B, which also exceeds the critical pressure P of foaming, so that the foaming point moves to the outlet die, which is beneficial to improving the foaming quality. Therefore, when selecting the polymer for extrusion foaming, it is required that the viscosity of the polymer melt should be lower and the elasticity should be greater, which is conducive to pushing the foaming point out of the die, It is beneficial to improve the quality of extrusion foaming. The previous experimental results show that there are two different existing states of gas in the plastic melt, one is melting in the melt, the existence of gas phase is not observed, and the other is that the gas dissociates in the melt and forms bubbles. These two different situations make the melt have two different rheological properties. Therefore, when studying and analyzing the influence of various parameters on the rheological properties of plastic melt containing foaming agent, we must pay attention to the conditions of the melt.
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