Numerical Prediction On Bubble Growth And Surface Quality Of Microcellular Foaming Material

Compared with general polymer materials, Microcellular materials exhibit many excellent physical properties because of their small and dense bubble structures. The bubble growth is the key factor to obtain desirable bubble structure. Numerical simulation is one of the effective means for the research of bubble growth in microcellular foam process.Based on the cell model,the governing equations of bubble growth were established respectively according to momentum transfer, mass transfer and polymer rheological state,then discretized in time and space. The program of bubble growth was written using MATLAB. Based on the established numerical algorithm, the mechanism of bubble growth and its influencing factors were studied. By comparing the bubble growth rate curve with the pressure variation curve and gas concentration gradient curve, it is found that the main driving forms for bubble growth are momentum transfer and mass transfer,and at different stages of bubble growth, the ratios between the two kinds of driving forms are not fixed. In the early stage, momentum transfer is the main driving form,while in the middle and late stage, mass transfer begins to dominate the bubble growth process. Then, the effects of physical properties of polymer, process parameters and initial values on the bubble growth were studied. By taking different values of single parameter, the curves of bubble radius, bubble growth rate, pressure variation and gas concentration gradient were plotted. The former two kinds of curves were used to observe the dynamic process of bubble growth, while the latter two served as the reference in analyzing the bubble growth mechanism. The results showed that the surface tension and system temperature on bubble growth rate are less affected. The former has a slight effect on momentum transfer process, the latter on mass transfer process; The increase of diffusion coefficient and Henry's law constant promote the process of mass transfer significantly, so as to accelerate the bubble growth; The bubble growth rate decreased with the increase of initial pressure and bubble radius,but the latter has no influence on the equilibrium bubble size. The increase of the bubble nucleation density reduces the final bubble size because the gas amount for the growth of a single bubble has declined with it.By studying the formation mechanism of surface roughness of microcellular foaming products,a calculation model for surface roughness was established. Then conduct a prediction on surface quality of microcellular products by combining numerical algorithm of bubble growth with injection molding CAE software. Using the orthogonal test method, the effects of different injection parameters on the surface quality of products were discussed. The results showed that with the increase of the melt pressure and melt filling rate,surface quality of microcellular products improved significantly. On the contrary,the influences of melt temperature and mold temperature are not obvious.Since melt pressure is the most significant factor on the improvement of surface quality, the growth process of single bubble under the GCP(Gas Counter Pressure)microcellular foaming was simulated. It is found that at the stage of gas counter pressure,the bubble growth rate decreased, which leads to a lower surface roughness. In order to investigate the mechanism,pressure variation curve and gas concentration gradient curve of GCP microcellular foaming were plotted. The results showed that at the stage of gas counter pressure,There is basically no difference in mass transfer between GCP and Conventional microcellular foaming, but the momentum transfer process hardly exist in GCP microcellular foaming, resulting in a reducing driving force and a decreasing bubble growth rate.