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.

Improved process moldability and part quality of short-glass–fiber-reinforced polypropylene via overflow short-shot water-assisted injection molding

Water-assisted injection molding (WAIM) is a promising molding process developed based on conventional injection molding (CIM). It has been a research hotspot in recent years and is still receiving extensive attention from many scholars and industries because of its significant potential advantages in practical applications. However, compared with CIM, since the additional water-related parameters are involved, the process moldability of thermoplastics is significantly reduced, especially for fiber-reinforced thermoplastics, which stunts the development of WAIM process. In this work, short-shot WAIM with an overflow cavity (OSSWAIM) was developed to address the problems and broaden the application scope of WAIMs. The results showed that compared with overflow WAIM (OWAIM) and short-shot WAIM (SSWAIM), OSSWAIM could significantly improve the process moldability and part quality of fiber-reinforced thermoplastics, especially for thermoplastic composites with a high fiber weight fraction. Besides, it was also found that water penetration had a slight influence on the fiber orientation near the water inlet, but had a significant influence on the fiber orientation near the end of mold cavity. Finally, three processing parameters affecting the water penetration, i.e., water pressure, melt temperature, and water injection delay time were investigated in terms of their influences on the fiber orientation within OSSWAIM.

Numerical Simulation during Short-Shot Water-Assisted Injection Molding Based on the Overflow Cavity for Short-Glass Fiber-Reinforced Polypropylene

Compared with water penetration condition of short-shot water-assisted injection molding with or without overflow cavity, it can be known from theory and common knowledge that short-shot water-assisted injection molding with overflow cavity has many advantages, such as it can save materials and energy. Then, the effects of melt short shot size, water injection delay time, melt temperature and water injection pressure on the penetration of water after penetration, and the orientation distribution of short fibers during water-assisted injection molding of the overflow cavity short-shot method were studied. It is found that the melt short shot size had the greatest influence on it, followed by water injection pressure, water injection delay time, and finally, melt temperature. With the increase of the melt short shot size, the thickness of the residual wall of the whole main cavity becomes thinner, the orientation of short fiber along the melt flow direction becomes higher, and the degree of fiber orientation changes becomes lower. In the front half of the main cavity, with the decrease of water injection pressure, the delay time of water injection, and the melt temperature, in the front part of the main cavity, the residual wall thickness becomes thinner, the fiber orientation along the melt flow direction becomes lower, and the fiber orientation changes degree becomes higher; in the latter half of the main cavity, the influence of the water penetration and the orientation distribution of short fibers along the melt flow direction are not significant.

Suppression of the hierarchical structure of water-assisted injection moulded iPP in the presence of a β-nucleating agent and lamellar branching of a β-crystal

Unexpected lamellar branching of the β-crystal structure is observed in water-assisted injection molded iPP parts with a high content of β-nucleating agent.