Cell morphologies, mechanical properties, and fiber orientation of glass fiber-reinforced polyamide composites: Influence of subcritical gas-laden pellet injection molding foaming technology

Abstract

Advanced materials and new lightweighting technologies are essential for boosting the fuel economy of modern automobiles while maintaining performance and safety. A novel approach called subcritical gas-laden pellet injection molding foaming technology (SIFT) was performed to produce foamed polyamide/glass fiber (PA/GF) composite. Gas-laden pellets loaded with nitrogen (N2) were produced by introducing sub-critical N2 into PA/GF composite during compounding using a twin-screw extruder equipped with a simple gas injection unit. Compared to the commercial microcellular injection molding (MIM) technologies, gas-laden pellets enable the production of foamed parts with a standard injection molding machine, which is more cost-effective and easier to implement. To the best of our knowledge, this is the first attempt that the SIFT technology is being used for the PA/GF composites for making foamed parts. The tensile strength, fiber orientation, cell morphology, and densities of foamed PA/GF parts were investigated, and the shelf life of N2-laden PA/GF pellets was examined. Results showed that the N2-laden pellets still possessed good foaming ability after one week of storage under ambient atmospheric conditions. One week is a noticeable improvement compared to those N2-laden neat polymer pellets without glass fibers. With this approach, the weight reduction of foamed PA/GF parts was able to reach 12.0 wt. %. Additionally, a nondestructive analysis of the fiber orientation using micro-computed tomography suggested that the MIM and SIFT samples exhibited a less degree of fiber orientation along the flow direction when compared to the solid samples and that the tensile strength of both technologies was very close at a similar weight reduction. Cell size increased and cell density decreased as the shelf life increased. These findings showed that this processing method could act as an alternative to current commercial foam injection molding technology for producing lightweight parts with greater design freedom.

Lightweight and High Impact Toughness PP/PET/POE Composite Foams Fabricated by In Situ Nanofibrillation and Microcellular Injection Molding

“…Therefore, it is difficult to form a homogeneous and stable cell structure [ 13 ], and further deteriorates mechanical performance. Third, the mechanical properties of foamed products, especially stiffness or hardness, are generally inferior to that of solid samples [ 14 , 15 ]. It is due to the fact that the internal cells, which normally act as stress concentration points, may decrease the mechanical properties corresponding to the weight reduction [ 12 ].…”

Effect of viscosity on foaming behavior and surface quality of foamed Polyamide 6

“…11,12 To enhance the quality and performance of PA6 micro-foam products, it is imperative to address the issue of low melt strength. Several potential solutions include incorporating nano-clay, 11,13 fiber blends, [14][15][16] chain expansion, [17][18][19][20][21] and crosslinking. 22 According to research, adding nano-clay to PA6 can increase its activation energy, improving both the crystallization rate and nucleation efficiency.…”

“…The incorporation of glass fibers into polymer foams enhances their mechanical properties by increasing viscosity and melt strength. 14 This addition also improves foaming ability, simultaneously reducing shrinkage and warpage often produced during injection molding. 15,16 Additionally, the use of multifunctional epoxy-based chain extension (CE) can prolong the branched chain of PA6, ultimately improving its melt strength and foaming performance.…”

“…To enhance the quality and performance of PA6 micro‐foam products, it is imperative to address the issue of low melt strength. Several potential solutions include incorporating nano‐clay, 11,13 fiber blends, 14–16 chain expansion, 17–21 and crosslinking 22…”

“…Excessive clay addition inhibits the crystallization behavior of PA6 during foaming, leading to the inhibition of homogeneous nucleation of PA6. The incorporation of glass fibers into polymer foams enhances their mechanical properties by increasing viscosity and melt strength 14 . This addition also improves foaming ability, simultaneously reducing shrinkage and warpage often produced during injection molding 15,16 .…”