Fiber Orientation Analysis of Overflow Water-Assisted Injection Molding with Short Glass Fiber Reinforced Polypropylene

The fiber orientation is playing an important performance indexed for glass fiber reinforced polypropylene for water-assisted injection molding. Based on the viscoelastic constitutive equation (White-Metzner) and the fiber orientation model (iARD-RPR), the effects of fiber mass content, water injection delay time, water injection pressure, and melt temperature, which are on the fiber orientation along the flow direction and shear rate distribution of the melt, were investigated. Studies found that the orientation degree of the fiber along the flow direction was reduced with the increase of the fiber mass content, the extension of the water injection delay time, and the improvement of the melt temperature and that the orientation degree of the fiber along the flow direction was raised with the increase of the water injection pressure flow in the laminar flow state, but it was reduced with the increase in the turbulent state. It can be further learned from the shear rate distribution that decreasing fiber mass content, reducing the water injection delay time, lower melt temperature, and increasing water injection pressure in laminar flow conditions will increase the shear rate in the channel layer and the shear rate gradient along the thickness direction of the melt, while the water injection pressure in the turbulent state is on the contrary.

1. Introduction
As a medium-assisted molding technology in injection molding, water-assisted injection molding (WAIM) is favored by insiders in the industry because of its advantages of saving raw materials, reducing the weight of parts, enhancing physical and mechanical properties, shortening the molding cycle, and so on [1, 2]. Gas-assisted injection molding (GAIM) has the same strength as WAIM. These two molding methods are collectively referred to as fluid-assisted injection molding, and the molding principles of them are the same. However, as to fluid media, water has higher thermal conductivity, specific heat capacity, and incompressibility than gas, which makes WAIM have shorter molding cycles, higher internal surface smoothness of molded parts, and more abundant size of molded part than GAIM [3, 4]. The WAIM can be classified into short-shot water-assisted injection molding (SWAIM) and overflow water-assisted injection molding (OWAIM) in the light of whether the polymer melt is filled in the main cavity before water injection.

Based on the two molding methods above, the researchers conducted macroscopic and microscopic properties research such as water penetration length, residual wall thickness, and crystal morphology according to process parameters, physical parameters, and mold structure parameters. In terms of macroscopic performance, Liu [5, 6] and Zhang [7] have studied the effects of relevant process parameters on the water penetration of water-assisted injection products. Huang et al. [8] studied the influence of process parameters on water penetration length and residual wall thickness of polypropylene (PP) elbow pipe products by the single factor method. Polynkin et al. [9] simulated the effect of water injection pressure on the residual wall thickness of water-assisted injection circular pipe products. Pudpong et al. [10] used Moldflow to simulate and analyze the influence of process parameters on the residual wall thickness of overflow water-assisted injection PP products. Liu [11] studied the effect of crystal morphology on water-assisted injection HDPE products based on different melt temperatures and different molecular weights.

Nowadays, on the basis of the broad prospects of PP and the excellent properties of glass fiber, such as good hygroscopicity, heat insulation, and shock resistance [12], the research on the injection molding of glass fiber reinforced polypropylene (GFPP) has attracted much attention. In terms of performance research, Tang Xiao et al. [13] pointed out that GFPP has excellent comprehensive properties such as stable size, good fatigue resistance, and small shrinkage; Scheme revealed that GFPP has good molding flowability and convenient processing [14]; Zhou et al. find that the length of the glass fiber has an obvious effect on the mechanical properties of the composite [15] and fiber orientation is essential for enhancing the mechanical properties of glass-fiber-reinforced polymer [16]. In terms of the molding mechanism, Liu [5] revealed that the fibers in the WAIM of short fiber reinforced PP are along the flow direction at the interface between the polymer and the mold wall as well as between the polymer and water; Huang et al. [17] studied the fiber orientation pattern of water-assisted injection fiber-reinforced polypropylene parts and explained its formation mechanism by using the unique shear rate and cooling rate fields of melt filling and high-pressure water penetration into the cavity; Wang [18] studied the effect of the formation mechanism of PP or acrylonitrile-styrene copolymer on the transverse crystal formation based on relevant process parameters and physical parameters. In terms of fiber orientation, Loekett [19] qualitatively predicts the law of fiber orientation due to the flow that in the case of convergent flow and shear flow, the fiber orientation tends to the flow direction, while in the case of divergent flow, it is perpendicular to the flow direction; Zhou [20] has made some progress in theoretical calculation and computer simulation of fiber orientation in short fiber filled plastics and obtained some schematic diagrams of fiber orientation distribution; Fisa et al. [21] used scanning electron microscopy to study the fusion line of glass fiber reinforced polypropylene injection mold and observed that in the plate model, the orientation of the fiber in the fusion line area was almost parallel to the flow direction. By improving the short-shot water-assisted injection molding process, Yu et al. [22] found that the water injection parameters affected the fiber orientation in the front half of the injection molding process of short-shot glass fiber-reinforced polypropylene.

The above research shows that the influence of flow behavior on fiber orientation is very complex, but at present, the prediction of fiber orientation is mostly from the qualitative point of view, and the research on fiber orientation control has not yet been proposed. On the premise of determining the material, the relationship between the injection molding process parameters and fiber orientation is comprehensively grasped, which is beneficial to optimize the injection molding process parameters and improve the fiber orientation in the fiber reinforced injection molded parts.

In this paper, the fiber orientation along the flow direction has been studied by changing the physical properties and process parameters, such as fiber content, melt temperature, water injection pressure, and water injection delay time.