Factors affecting bubble expansion

日期:2022/5/6 8:50:24 / 阅读: / 来源:本站

Factors affecting bubble expansion
There are many factors affecting bubble expansion in liquid, which can be divided into two categories: one belongs to raw materials, including the variety, specification, performance and dosage of raw materials. For example, the type, degree, solubility and diffusion coefficient of foaming agent belong to this category; The other belongs to processing and forming conditions, including forming process, process parameters and structural parameters of forming equipment, such as forming temperature, pressure, shear rate, die geometry and size, etc. these parameters have a strong impact on the thermal dynamics of bubble expansion. Based on the calculation and experiments, the relationship between various parameters and bubble expansion is obtained, which is representative. It is introduced as follows. Casting film production line www.handern.com com
The effect of melt viscosity on bubble expansion was observed. The experiment was carried out under the condition that the melt was not subject to shear. The initial pressure in the mold cavity was 0.177mpa under the condition of mold filling, and the other bars reflected the effect of melt viscosity on bubble expansion process. The ordinate of the figure is the bubble radius and the growth rate of the bubble radius, and the abscissa is the time experienced by the melt in the mold cavity. In the later stage of bubble expansion, the main factor affecting bubble expansion gradually changes to the diffusion rate system. At this time, the influence of melt viscosity on bubble expansion rate becomes less obvious, and the bubble difference also decreases. The elasticity of the elastomer of the melt can be characterized by the relaxation time. A large value indicates that the elasticity of the melt is large, which is 0 for Newtonian fluid. The influence of melt elasticity on bubble expansion process is carried out from two opposite directions. On the one hand, increasing the melt elasticity can reduce the resistance to bubble expansion, which is conducive to accelerating bubble expansion, such as the calculation result of the third item on the right of equation (3-24); On the other hand, as the fourth calculation result on the right of formula (3-24), the large elasticity of the melt increases, and the ability to store energy in the melt increases, which makes the melt more flexible when entering the mold cavity. (0) slow attenuation, which will increase the resistance of bubble expansion process. Because the elasticity of polymer melt has the above two opposite effects on the bubble expansion process, when these two effects exist, the final performance should be the combination of the two effects. Figure 3-13 and figure 3-14 can illustrate this. The ordinates of the two figures are the same, which is the bubble radius and the growth rate of the bubble radius, and the abscissa is time. Other conditions of the expansion system are basically the same. There are four kinds of melts with different elasticity. As shown in the figure, when (0) = 0, the initial radius of bubbles in the melt and the growth rate of bubble radius increase with time λ The value increases obviously with the increase of time, and the difference decreases gradually with the increase of time. In the melt with large input value, the growth rate of bubble radius decreases the fastest. When t (0) = 6.4x10 ³ MPa, the situation is different. As shown in Figure 3-14, the influence of melt elasticity on the initial bubble radius and bubble radius growth rate is different from that in Figure 3-13. When the value is the largest, the initial bubble radius is the smallest, the bubble radius growth rate is the smallest, the x value decreases, and the bubble radius growth rate and initial radius increase, but the initial bubble radius and bubble radius growth rate in the melt of = 0.09s are larger than those in the melt of a = 0 Newtonian type. This is the comprehensive result of the third and fourth items in equation (3-24). www.handern. com
In_ In the melt with (0) = 0, the elasticity of the melt is conducive to the expansion of bubbles. Refer to figure 3-15 for the reason. The longitudinal coordinate of the figure is stress. The scales of the longitudinal coordinates on the left and right sides are different and the units are the same, but the quantity on the left is 3 orders of magnitude higher than that on the right. The abscissa is time, and the other conditions are the same as those in Fig. 3-13. It can be seen from Fig. 3-15 that the stress generated by bubble expansion in the melt of L = 5S is much smaller than that in the Newtonian melt of = 0. Therefore, a large melt is conducive to bubble expansion. www.handern. com
3. The diffusion coefficient of gas. Increasing the diffusion coefficient of gas D can accelerate the expansion speed of bubbles. Figure 3-16 shows the influence of gas diffusion coefficient on bubble growth. The ordinate is bubble radius and bubble radius growth speed, and the abscissa is time. Curves 1 and 2 represent two gases with different diffusion coefficients respectively. It can be seen from the figure that when time: > 5S, the bubble expansion speed is proportional to the square root of gas diffusion coefficient, Theoretically, this is the stage in which the expansion is controlled by the gas diffusion rate, and then look at the initial stage. For example, when = 0.01s, the ratio of the square root of the diffusion coefficient is 2.2 and the ratio of the expansion rate is 3, which shows that the bubble expansion in the initial stage is strongly affected by hydrodynamic forces. Generally speaking, the difference of curves 1 and 2 with different diffusion coefficients is relatively large in the pre foaming stage, but gradually decreases in the later stage
4. Gas liquid interfacial tension. The interfacial tension between melt and bubble acts as a resistance to bubble expansion. Figure 3-17 shows the influence of different interfacial tension on bubble expansion. The conditions are as shown in the figure. The ordinate is the growth rate of bubble radius and bubble radius, and the abscissa is time. In the God period of bubble expansion, the influence of melt with different interfacial tension on bubble expansion rate varies greatly, but the difference gradually decreases, Finally, it converges into a line, which can be explained by the second item on the right of (3-24). When the R value increases, the calculated value also decreases accordingly. For commonly used polymer melts and gases, the interfacial tension value a is about 0.3, and the influence of interfacial tension becomes smaller. www.handern. com
5. Foaming agent foaming agent is mainly consumed in three aspects: forming bubbles, diffusion loss and remaining in plastic. To improve the utilization efficiency of foaming agent, we should try to reduce the consumption of the latter two. Whether it is physical foaming agent or chemical foaming agent, it needs to emit gas to make the polymer melt or liquid foaming and expansion. The concentration of gas molecules in the melt is directly proportional to the pressure in the bubble. Therefore, increasing the concentration of foaming agent in the melt can improve the bubble expansion rate. The experimental results also confirm the above relationship. Figure 3-18 shows the effect of foaming agent concentration on bubble growth. The conditions are shown in the figure, t_ (0) = 3.2x10mpa, the cavity pressure is 17.7kpa (1.77kgf / cm), the ordinate is bubble radius and bubble radius growth rate, and the abscissa is time. It can be seen from the figure that the bubble expansion rate accelerates with the increase of foaming agent concentration, especially in the initial stage of expansion (t = 0.01s), the bubble expansion is affected by hydrodynamic effect, the foaming agent concentration is doubled, and the bubble expansion rate is increased by 3.9 times,

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