Gas-Projectile-Assisted Injection Molding (G-PAIM) is a new injection molding process derived from the Gas-Assisted Injection Molding (GAIM) process by introducing a projectile to it. In this study, the short-shot method and the overflow method of both the G-PAIM and GAIM processes were experimentally compared and investigated in terms of the wall thickness of the pipes and its uniformity. The results showed that the wall thickness of the G-PAIM molded pipe was thinner and more uniform than that of the GAIM molded pipe, and the wall thickness of the pipe molded by the Gas-Projectile-Assisted Injection Molding Overflow (G-PAIM-O) process was the most uniform. For the G-PAIM-O process, the influence of processing parameters, including melt temperature, gas injection delay time, gas injection pressure, melt injection pressure and mold temperature, on the wall thickness and uniformity of the G-PAIM-O pipes were studied via the single-factor experimental method. It was found that the effects of gas injection delay time and gas injection pressure on the wall thickness of the G-PAIM-O pipes were relatively significant. The wall thickness of the pipes increased with the increase in gas injection delay time and decreased with the increase in gas injection pressure. The melt temperature, melt injection pressure and mold temperature had little effect on the wall thickness of the G-PAIM-O pipes. In general, the wall thickness uniformity of the G-PAIM-O pipes was slightly affected by these processing parameters.

Water-projectile-assisted injection molding (W-PAIM) is a novel molding process for plastic pipes with complicated shape. It utilizes high-pressure water as a power to push a solid projectile to penetrate through the melt to form a hollow space. In order to investigate the penetration behavior of the projectile during the water injection stage of W-PAIM process, numerical simulation of the water injection stage of a W-PAIM pipe with straight and curved segments was carried out. A turbulent flow for the driving water was considered in the motion equation, and the dynamic mesh technology was used to deal with the moving solid projectile. The simulation results, including RWT and the flow fields, were compared with those of water-assisted injection molding (WAIM) pipe with the same outer dimensions. It was found that the residual wall thickness (RWT) of the W-PAIM pipe is much thinner than that of the WAIM pipe. The projectile has a crucial influence on the RWT. The pressure fields of W-PAIM and WAIM are very similar in both straight and curved segments. The velocity field and strain rate field near the penetration front in W-PAIM are quite different from those in WAIM due to the drag flow caused by the projectile penetration.

The short fiber orientation (SFO) distribution in the water-assisted injection molding (WAIM) is more complicated than that in traditional injection molding due to the new process parameters. In this work, an improved fiber orientation tensor method was used to simulate the SFO in WAIM. The result was compared with the scanning electron micrograph, which was consistent with the experiments. The effect of six process parameters, including filling time, melt temperature, mold temperature, delay time, water pressure, and water temperature, on the SFO along the melt flow direction were studied through orthogonal experimental design, range analysis, and variance analysis. An artificial neural network was used to establish the nonlinear agent model between the process parameters and A₁₁ representing the fiber orientation in melt flow direction. Results show that water pressure, melt temperature, and water temperature have significant effects on SFO. The three-dimensional (3D) response surfaces and contour plots show that the values of A₁₁ decrease with the increase in water pressure and melt temperature and increase as the water temperature rises.

A preliminary and groundbreaking study on the fiber orientation pattern in gas-powered projectile-assisted injection molding (G-PAIM) part of short-glass-fiber-reinforced polypropylene was reported, the results showed that the fiber orientation pattern across the thickness of G-PAIM part was remarkably different from that in the parts molded via gas-assisted injection molding (GAIM) and conventional injection molding (CIM) as reported earlier, and the difference of fiber orientation pattern between G-PAIM, GAIM, and CIM parts was mainly due to the unique flow fields generated by projectile penetration. Additionally, fiber orientation states within the G-PAIM part depended not only on their positions both across the part thickness and along the flow direction, but also on the processing parameters. Particularly along the thickness direction, fiber orientation states changed significantly. The results also showed that gas injection delay time was the main factor affecting the fiber orientation states within the G-PAIM part, and a shorter gas injection delay time could remarkably induce more ordered fiber orientation along the melt direction. Within the range of the investigated processing parameter values, a larger ordered region, where most fibers tended to be orderly oriented along the melt flow direction, could be obtained under a larger gas pressure (8 MPa), higher melt temperature (250°C), and shorter gas injection delay time (1 s). This work provides a comprehensive guidance for the fabrication practice of G-PAIM parts of short fiber-reinforced thermoplastics.