The diameter of a micro-tube is very small and its wall thickness is very thin. Thus, when applying double-layer gas-assisted extrusion technology to process a micro-tube, it is necessary to find the suitable inlet gas pressure and a method for forming a stable double gas layer. In this study, a double-layer gas-assisted extrusion experiment is conducted and combined with a numerical simulation made by POLYFLOW to analyze the effect of inlet gas pressure on micro-tube extrusion molding and the rheological properties of the melt under different inlet gas pressures. A method of forming a stable double gas layer is proposed, and its formation mechanism is analyzed. The research shows that when the inlet gas pressure is large, the viscosity on the inner and outer wall surfaces of the melt is very low due to the effects of shear thinning, viscous dissipation, and the compression effect of the melt, so the melt does not easily adhere to the wall surface of the die, and a double gas layer can be formed. When the inlet gas pressure slowly decreases, the effects of shear thinning and viscous dissipation are weakened, but the gas and the melt were constantly displacing each other and reaching a new balanced state and the gas and melt changed rapidly and steadily in the process without sudden changes, so the melt still does not easily adhere to the wall of the die. Thus, in this experiment, we adjusted the inlet gas pressure to 5000 Pa first to ensure that the melt do not adhere to the wall surface and then slowly increased the inlet gas pressure to 10,000 Pa to reduce the viscosity of the melt. Lastly, we slowly decreased the inlet gas pressure to 1000 Pa to form a stable double gas layer. Using this method will not only facilitate the formation of a stable double gas layer, but can also accurately control the diameter of the micro-tube.
The amount of supercritical CO2 dissolved
in polystyrene
(PS), dissolution rate, and solubility under static conditions at
170–190 °C and 7.5–9.5 MPa were calculated by utilizing
volume-changing-method experiments and numerical simulations. By comparison,
the instantaneous error can be guaranteed to be less than 15%. The
two results are in good agreement, and the reliability of the simulation
method is verified. Based on the obtained results, another parameter
was added to the tested model, and the dissolution rate of supercritical
CO2 in PS under different shear conditions was numerically
simulated. The effects of temperature, pressure, and shear rate on
dissolution were analyzed. The results show that when the temperature
and pressure are constant, the dissolution rate of supercritical CO2 in PS with shear increases significantly compared with that
without shear. The conditions that enable the maximum dissolution
rate are 190 °C, 9.5 MPa, and a shear rate of 240/π. With
the abovementioned pressure and shear rate conditions, the maximum
solubility can be obtained under the temperature of 170 °C.
In order to reveal
the dissolution process, the adsorption kinetics
and diffusion theory are combined and used to describe the adsorption-diffusion
mechanism. This can not only predict the solubility of supercritical
CO2 in polymer melts but also describe two important parameters
of supercritical CO2 in the dissolution process: dissolution
amount and dissolution rate, which can provide a good theoretical
basis for microcellular foaming. To verify the feasibility and accuracy
of the theoretical calculation method, an experimental device for
the volume-changing method under static condition was established.
The results showed that the theoretical calculation value was in good
agreement with the experimental value. In addition, the dissolution
amount and dissolution rate of supercritical CO2 in three
polystyrene melts with different molecular weights under different
temperature and pressure conditions were measured. The results showed
that the difference of polystyrene molecular weight can cause the
change of dissolution rate during the dissolution process, that is,
the larger the molecular weight, the slower the dissolution rate.
The solubility and diffusion coefficient of supercritical CO2 in polystyrene (PS) dynamic melt were studied by using a new constant pressure experimental device. By comparing the experimental results with those of other researchers, the validity of the experimental device and the reliability of the calculated results are verified. The solubility and diffusion coefficient of supercritical CO2 in polystyrene dynamic melts at different temperatures and pressures were obtained. The numerical calculation method, dissolution process, and experimental results are analyzed and compared with that of the static melt. Finally, the effects of stirring speed, pressure, and temperature fluctuation on the solubility and diffusion coefficient are also analyzed.
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