Numerical and experimental studies for gas assisted extrusion forming of molten polypropylene

Abstract

In this study, the experiments of gas-assisted extrusion (GAE) for molten polypropylene were carried out under different gas pressures, the different extrudate deformations and sharkskin defects of melt were observed. To ascertain the effects of gas on melt extrusion, non-isothermal numerical simulation of GAE based on gas/melt two-phase fluid model was proposed and studied. In the simulations, the melt extruded profile, physical field distributions (velocities, pressure drop, and first normal stress difference) were obtained. Numerical results showed that the deformation degree of melt increased with increasing gas pressure, which was in good agreement with experimental results. It was demonstrated that the influence of gas pressure on the melt extrusion could be well reflected by GAE simulation based on gas/melt two-phase fluid model rather than simplified-GAE (SGAE) based on full-slip wall boundary condition used in the past time. Experimental and numerical results demonstrate that the gas pressure induced first normal stress difference is the main reason of triggering flow behavior changes, extrudate deformations, and sharkskin defects of melt. Therefore, the reasonable controlling of gas pressure is a key in practice of GAE, and the gas layer and its influence should be considered in GAE numerical simulation.

Study on the Extrusion Molding Process of Polylactic Acid Micro Tubes for Biodegradable Vascular Stents

“…The relationship between shear rate and volume flow rate is shown in Figure 8. It can be seen that the volume flow rate increases with the increase of shear rate, and the increasing trend gradually slows down, which verifies the correctness of the proposed viewpoint, a the viewpoint is consistent with the extrusion molding of PP micro tubes [20]. shown in Table 2.…”

“…The relationship between shear rate and volume flow rate is shown in Figure 8. It can be seen that the volume flow rate increases with the increase of shear rate, and the increasing trend gradually slows down, which verifies the correctness of the proposed viewpoint, a the viewpoint is consistent with the extrusion molding of PP micro tubes [20]. It is assumed that the gap between the die and the mandrel is equal at the extrusion die forming section, and the melt flow is balanced so each point on any concentric circle of the cavity cross section has the same velocity field and stress field.…”

Numerical and Experimental Studies on the Improvement of Gas Chamber Structure during Gas-Assisted Extrusion

“…Liang [ 24 ] introduced nitrogen into the HDPE melt round bar and die through the slit to form a stable gas cushion, which effectively reduced the die pressure drop and extrusion expansion rate of the melt and improved the quality and yield. Ren [ 25 , 26 , 27 ] found that the gas-assisted technology can overcome extrusion expansion, improve the surface quality of the micro-tube, and reduce the pressure, shear stress, and first normal stress difference of the melt inside the die through experimental research and numerical analysis. Liu [ 28 ] analyzed the effect of the inner gas cushion layer pressure on the melt pressure difference and the size of the micro-tube by numerical simulation and determined the control steps to stabilize the gas cushion layer during gas-assisted extrusion through experimental studies.…”

Numerical and experimental studies on the effect of thickness difference between up and down gas layers on sheet polymer gas‐assisted extrusion forming

“…Huang [13] and Liu et al [14] studied the formation and stability of gas layer film during GAE by means of experiments and simulations, and it was proved that the use of gas-assisted method can effectively eliminate some bad phenomena, such as extrusion expansion, pressure drop during extrusion, and the interface instability. For decades, many investigations in this filed, both experiments and simulations, [15][16][17][18][19][20][21][22][23][24][25][26][27] have shown the prospect and superiority of GAE technology, and many scholars [28][29][30][31][32][33][34][35][36][37] have demonstrated that GAE promotes the melt slippage phenomenon.…”