Search Results (3)

Although the extrudate swelling, melt fracture, and extrusion deformation of polymer micro-catheters in traditional extrusion molding can be eliminated via the double-layer gas-assisted extrusion (DL-GAE) method, some failure problems are generated under unreasonable process conditions. To ascertain the reasons for failure in DL-GAE molding of polymer micro-catheters, the influences of the interaction between the melt and double assisted gas on the DL-GAE molding of polymer micro-catheters were experimentally and numerically studied. Meanwhile, a DL-GAE die and experimental system were designed and constructed. We analyzed the influence laws of the melt and assisted gas on the DL-GAE molding of polymer micro-catheters, as well as reasons for the molding’s failure. Our studies demonstrate that under the condition of stable DL-GAE, as the melt flow rate increases, the wall thickness and diameter of polypropylene (PP) micro-catheters increase. When the melt flow rate continuously increases, the stability of the assisted gas is destroyed, resulting in the failure of DL-GAE. In addition, under synchronized pressures of a double gas-assisted layer, the diameters of the micro-catheters increase, but their wall thickness decreases. Under an individual pressure increase of the outer gas-assisted layer, surface bump defects are generated. Under an individual pressure increase of the inner gas-assisted layer, the diameters of PP micro-catheters swell prominently until they break. Therefore, although DL-GAE can eliminate extrusion problems of polymer micro-catheters, it is suggested that reasonable process parameters for the melt and double assisted gas should be satisfied and matched. This work can provide significant technical support for the DL-GAE of polymer micro-catheters during manufacture. Full article

In the process of the double-layer gas-assisted extrusion of plastic micro-tubes, the external size and surface quality of the micro-tubes are greatly affected by the size of the assisting gas inlet slit inside the mold. Therefore, in this experiment, a two-phase flow model was established based on a compressible gas and a non-compressible melt. The Polyflow finite element solution software module was used to solve the velocity field, temperature field, pressure field, and section size of the melt under the condition of double-layer gas-assisted extrusion in a mold under different gas inlet slit widths. The results show that, with an increase in the width of the gas inlet slit, the melt outlet velocity increases, the surface temperature increases, wall thickness shrinkage increases, and interior diameter expansion increases. In the process of gas-assisted extrusion, the thickness of the air cushion is affected by adjusting the size of the gas inlet slit, and, hence, changes the shape and size of the plastic micro-tubes. Full article

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. Full article