Flow visualisation of water assisted injection moulding process

It is a critical requirement to have an insight into the mechanism of flow-induced fiber orientation in short-shot water-assisted injection molding (SSWAIM) of fiberreinforced polymer for improving the structural rigidity and service life of molded parts. However, this mechanism is still unclear, which stunts the development of SSWAIM. In this work, the mechanism of flow-induced fiber orientation in SSWAIM parts of short-glass-fiber-reinforced polypropylene (SGF/PP) was clarified through numerical research and experimental verification. The results showed that the difference of fiber orientation distribution at different positions both in the radial direction and along the flow direction between SSWAIM parts and conventional injection molding (CIM) parts was mainly due to the strong flow field caused by the high-pressure water penetration in SSWAIM. Moreover, fiber orientation in the SSWAIM part depended not only on its position both in the radial direction and along the flow direction, but also on the processing parameters. At the front of SSWAIM part, fiber orientation changed greatly in the radial direction and presented an obvious "shell layer-core layer-water channel layer" hierarchical structure across the part thickness, whereas this phenomenon became more inconspicuous with increasing the distance of the selected position from the water injection inlet.Short-shot size is the principal parameter affecting the fiber orientation, and within the range of investigated processing parameters, smaller short-shot size, shorter water injection delay time, higher water pressure, and lower melt temperature could significantly facilitate fiber orientation across the part thickness.

Section: Resultssupporting

confidence: 87%

Numerical and experimental investigations on the mechanism of flow‐induced fiber orientation in short‐shot water‐assisted injection‐molded short‐glass‐fiber‐reinforced polypropylene

Zhang

Kuang

Liu

et al. 2022

“…However, as the melt was injected into the cavity, it quickly solidified once it touched the water inlet and tightly wrapped the water inlet, which caused the penetration behavior of water at position S1 to become unstable. With the decrease of melt temperature, the solidified layer of melt surrounding the water inlet became thicker, aggravating the unstable penetration behavior of high‐pressure water at this location, 55,56 which in turn induced the random alignment of more fibers. Thus, decreasing the melt temperature from 230 to 210°С led to a slight increase in the relative thickness of the ordered region at position S1.…”

Section: Resultsmentioning

confidence: 99%

Formationmechanism of high‐pressure water penetration induced fiber orientation in overflow water‐assisted injection molded short glass fiber‐reinforced polypropylene

Zhang

Liu

Jiang

et al. 2021

Recently, there has been growing interest in water‐assisted injection molding (WAIM) not only for its advantages over gas‐assisted molding (GAIM) and conventional injection molding (CIM), but also for its great potential advantages in industrial applications. To understand the formation mechanism of water penetration induced fiber orientation in overflow water‐assisted injection molding (OWAIM) parts of short glass fiber‐reinforced polypropylene (SGF/PP), in this work, the external fields and water penetration process within the mold cavity were investigated by experiments and numerical simulations. The results showed that the difference of fiber orientation distribution in thickness direction between WAIM moldings and CIM moldings was mainly ascribed to the great external fields generated by water penetration. Besides, fiber orientation depended on the position both across the part thickness and along the flow direction. Especially in the radial direction, fiber orientation varied considerably. The results also showed that the melt temperature is the principal parameter affecting the fiber orientation along the flow direction, and a higher melt temperature significantly facilitated more fibers to be oriented along the flow direction, which is quite different from the results as previously reported in short‐shot water‐assisted injection molding (SSWAIM). A higher water pressure, shorter water injection delay time, and higher melt temperature significantly induced more fibers to be orderly oriented in OWAIM moldings, which may improve their mechanical performances and broaden their application scope.

“…Water penetration remains stable, and the residual wall thickness is uniform. Wu and Liu12,13 investigated the flow visualization of cavity‐filling process in WAIM by experiments, which showed that the penetration of water was quite steady and uniform during the whole filling process, and the water bubble kept almost the same diameter. The water penetration behaviors of simulations are in accord with experimental results.…”

Section: Resultsmentioning

confidence: 99%

“…To make a comparison of the water penetration behavior and residual wall thickness of simulations and literatures,12–17 polypropylene (PP) is taken as a study resin in this article. Cross‐WLF viscosity model parameters of PP are shown in Table I, which are obtained form MOLDFLOW material library.…”

Section: Model and Methodsmentioning

confidence: 99%

Numerical simulation on residual wall thickness of tubes with dimensional transitions and curved sections in water‐assisted injection molding

Yang¹,

Zhou²

2012

Residual wall thickness is an important indicator which aims at measuring the quality of water-assisted injection molding (WAIM) parts. The changes of residual wall thickness around dimensional transitions and curved sections are particularly significant. Free interface of the water/melt two-phase was tracked by volume of fluid (VOF) method. Computational fluid dynamics (CFD) method was used to simulate the residual wall thickness, and the results corresponded with that of experiments. The results showed that the penetration of water at the long straight sections was steady, and the distribution of the residual wall thickness was uniform. However, there was melt accumulation phenomenon at the dimensional transitions, and the distribution of the residual wall thickness wasn't uniform. Adding fillet at the dimensional transitions could improve the uniformity of the residual wall thickness distribution, and effectively reduce water fingering. Additionally, at the curved sections, the residual wall thickness of the outer wall was always greater than that of the inner wall, and the fluctuations of the residual wall thickness difference were small.