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

Abstract

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.

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

“…Secondly, as the water pressure increased, the radial squeezing force of water directly acting on the melt/fiber interface also increased, resulting in an increase in the viscous friction between the fibers and melt, which also increased the shear stress acting on fibers. Nevertheless, the unstable water penetration at the front of part led to a random fiber orientation near the water channel, 26 and increasing the water pressure had little influence on improving this (as shown in Figure 13), which was quite consistent with the previous investigations 24,26 . Therefore, increasing the water pressure could significantly facilitate the ordered fiber orientation at the rear position, but it had a limited influence on the fiber orientation at the front position.…”

“…According to the previous investigations, the water penetration process is similar to the gas penetration process in GAIM 26,39 . In the water penetration stage, once water/gas was injected, it would cool the melt near the water channel layer, where the viscosity of melt would be high.…”

“…As we know, with the increase of water injection delay time, the time for the mold wall to cool the melt also increased before water was injected, which caused the high‐viscosity melt near the mold wall to thicken toward the center of the mold cavity 11,26 . This, in turn, decreased the melt volume flowing with water (shown in Figure 19), resulting in a reduction in the region where the melt was subjected to strong shear action 49 ; secondly, the penetration resistance of pressurized water increased rapidly with the increase of water injection delay time, which in turn reduced the velocity difference of melt between the mold wall and the water channel.…”

“…As shown in Figure 6, with the increase of water injection time, the shear stress gradient significantly increased along the thickness direction. According to previous investigations, the fluctuating penetration behavior of high‐pressure water occurred near the water inlet, resulting in random fiber orientation near the water channel and nonuniformity of residual wall thickness distribution 24,26 . In this work, the proportion of residual wall thickness was used to reflect the water penetration behavior, the proportion of residual wall thickness refers to the ratio of residual wall thickness at the selected position to the radius of mold cavity.…”

“…To solve the aforementioned equations, boundary conditions in the SSWAIM process must be specified 26,32 . A schematic diagram of the boundary conditions is shown in Figure 1a, and others are listed below.…”