Study on the Influence of Runner and Overflow Area Design on Flow-Fiber Coupling in a Multi-Cavity System.

Fiber-reinforced composites (FRPs) are characterized by their lightweight nature and superior mechanical characteristics, rendering them extensively utilized across various sectors such as aerospace and automotive industries. Nevertheless, the precise mechanisms governing the interaction between the fibers present in FRPs and the polymer melt during industrial processing, particularly the manipulation of the flow-fiber coupling effect, remain incompletely elucidated. Hence, this study introduces a geometrically symmetrical 1 × 4 multi-cavity mold system, where each cavity conforms to the ASTM D638 Type V standard specimen. The research utilizes theoretical simulation analysis and experimental validation to investigate the influence of runner and overflow design on the flow-fiber coupling effect. The findings indicate that the polymer melt, directed by a geometrically symmetrical runner, results in consistent fiber orientation within each mold cavity. Furthermore, in the context of simulation analysis, the inclusion of the flow-fiber coupling effect within the system results in elevated sprue pressure levels and an expanded core layer region in comparison to systems lacking this coupling effect. This observation aligns well with the existing literature on the subject. Moreover, analysis of fiber orientation in different flow field areas reveals that the addition of an overflow area alters the flow field, leading to a significant delay in the flow-fiber coupling effect. To demonstrate the impact of overflow area design on the flow-fiber effect, the integration of fiber orientation distribution analysis highlights a transformation in fiber arrangement from the flow direction to cross-flow and thickness directions near the end-of-fill region in the injected part. Additionally, examination of the geometric dimensions of the injected part reveals asymmetrical geometric shrinkage between upstream and downstream areas in the end-of-fill region, consistent with microscopic fiber orientation changes influenced by the delayed flow-fiber coupling effect guided by the overflow area. In brief, the introduction of the overflow area extends the duration in which the polymer melt exerts control in the flow direction, consequently prolonging the period in which the fiber orientation governs in the flow direction (A11). This leads to the impact of fiber orientation on the flow of the polymer melt, with the flow reciprocally affecting the fibers. Subsequently, the interaction between these two elements persists until a state of equilibrium is achieved, known as the flow-fiber coupling effect, which is delayed.

Processing Effects on the Through-Plane Electrical Conductivities and Tensile Strengths of Microcellular-Injection-Molded Polypropylene Composites with Carbon Fibers.

Polymers reinforced with conducting fibers to achieve electrical conductivity have attracted remarkable attention in several engineering applications, and injection molding provides a cost-effective way for mass production. However, the electrical performance usually varies with the molding conditions. Moreover, high added content of conducting fibers usually results in molding difficulties. In this study, we propose using microcellular (MuCell) injection molding for polypropylene (PP)/carbon fiber (CF, 20, and 30 wt%) composites and hope that the MuCell injection molding process can improve both electrical and mechanical performance as compared with conventional injection molded (CIM) parts under the same CF content. Both molding techniques were also employed with and without gas counter pressure (GCP), and the overall fiber orientation, through-plane electrical conductivity (TPEC), and tensile strength (TS) of the composites were characterized. Based on the various processing technologies, the results can be described in four aspects: (1) Compared with CIM, microcellular foaming significantly influenced the fiber orientation, and the TPECs of the samples with 20 and 30 wt% CF were 18-78 and 5-8 times higher than those of the corresponding samples molded by CIM, respectively; (2) when GCP was employed in the CIM process, the TPEC of the samples with 20 and 30 wt% CF increased by 3 and 2 times, respectively. Similar results were obtained in the case of microcellular injection molding-the TPEC of the 20 and 30 wt% composites increased by 7-74 and 18-32 times, respectively; (3) although microcellular injection molding alone (i.e., without GCP) showed the greatest influence on the randomness of the fiber orientation and the TPEC, the TS of the samples was the lowest due to the uncontrollable foaming cell size and cell size uniformity; (4) in contrast, when GCP was employed in the microcellular foaming process, high TS was obtained, and the TPEC was significantly enhanced. The high foaming quality owing to the GCP implementation improved the randomness of fiber orientation, as well as the electrical and mechanical properties of the composites. Generally speaking, microcellular injection combined with gas counter pressure does provide a promising way to achieve high electrical and mechanical performance for carbon-fiber-added polypropylene composites.

Effect of Gas Counter Pressure on the Surface Roughness, Morphology, and Tensile Strength between Microcellular and Conventional Injection-Molded PP Parts.

Microcellular injection-molded parts have surface defect problems. Gas counter pressure (GCP) is one of the methods to reduce surface defects. This study investigated the effect of GCP on the surface roughness, morphology, and tensile strength of foamed and conventional injection-molded polypropylene (PP) products. GCP is generated by filling up the mold cavity with nitrogen during the injection-molding (IM) process. It can delay foaming and affect flow characteristics of microcellular and conventional injection-molding, which cause changes in the tensile strength, flow length, cell morphology, and surface quality of molded parts. The mechanism was investigated through a series of experiments including tuning of GCP and pressure holding duration. Surface roughness of the molded parts decreased with the increase in GCP and pressure holding duration. Compared to microcellular IM, GCP-assisted foaming exhibited much better surface quality and controllable skin layer thickness.

Effects of Injection Molding Process Parameters on the Chemical Foaming Behavior of Polypropylene and Polystyrene.

In the present study, semi-crystalline polypropylene (PP) and amorphous polystyrene (PS) were adopted as matrix materials. After the exothermic foaming agent azodicarbonamide was added, injection molding was implemented to create samples. The mold flow analysis program Moldex3D was then applied to verify the short-shot results. Three process parameters were adopted, namely injection speed, melt temperature, and mold temperature; three levels were set for each factor in the one-factor-at-a-time experimental design. The macroscopic effects of the factors on the weight, specific weight, and expansion ratios of the samples were investigated to determine foaming efficiency, and their microscopic effects on cell density and diameter were examined using a scanning electron microscope. The process parameters for the exothermic foaming agent were optimized accordingly. Finally, the expansion ratios of the two matrix materials in the optimal process parameter settings were compared. After the experimental database was created, the foaming module of the chemical blowing agents was established by Moldex3D Company. The results indicated that semi-crystalline materials foamed less due to their crystallinity. PP exhibits the highest expansion ratio at low injection speed, a high melt temperature, and a low mold temperature, whereas PS exhibits the highest expansion ratio at high injection speed, a moderate melt temperature, and a low mold temperature.

Enhancement in insulation and mechanical properties of PMMA nanocomposite foams infused with multi-walled carbon nanotubes.

In this study, PMMA/CNTs composite materials with carboxyl-multi walled carbon nanotubes (c-MWNTs) or untreated MWNTs were prepared via in-situ bulk polymerization. The as-prepared PMMA/CNTs composite materials were then characterized by Fourier-Transformation infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM). The molecular weights of PMMA extracted from PMMA/CNTs composite materials and bulk PMMA were determined by gel permeation chromatography (GPC) with THF used as the eluant. The PMMA/CNTs composite materials were used to produce foams by a batch process in an autoclave using nitrogen as foaming agent. The cellular microstructure, insulation and compressive mechanical properties of PMMA/CNTs composite foams were also investigated in detail. Compared to neat PMMA foam, the presence of CNTs increases in cell density and reduces cell size. The insulation and compressive mechanical properties of PMMA/CNTs composite foams were found to improve substantially those of neat PMMA foam. In particular, 22.6% decrease in thermal conductivity, 19.7% decrease in dielectric constant and 160% increase in compressive modulus were observed with the addition of 0.3 wt% carboxyl-multi walled carbon nanotubes (c-MWNTs).

Effect of vinyl-modified silica and raw silica particles on the properties of as-prepared polymer-silica nanocomposite foams.

In this study, the first comparative studies for the effect of vinyl-modified silica (VMS) and raw silica (RS) particles on the physical properties of as-prepared polymer-silica nanocomposite (PSN) foams are presented. First of all, the VMS particles were synthesized by performing conventional acid-catalyzed sol-gel reactions of TEOS in the presence/absence of MSMA molecules. The as-prepared VMS particles were then characterized through Fourier Transform Infrared (FTIR), solid-state 13C-nuclear magnetic resonance (13C-NMR) and 29Si-NMR spectroscopy. Subsequently, a series of PSN materials have been prepared through bulk polymerization of MMA monomers in the presence of VMS and RS particles with BPO as initiator. The dispersion capability of silica particles in polymer matrix was further observed by transmission electron microscopy (TEM) studies. The PSN foams can be further obtained by performing batch-foaming process on as-prepared bulky PSN materials with N2 as foaming agent. The cell structure analysis of the PSN foams was subsequently examined by the scanning electron microscopy (SEM) images. Gel permeation chromatography (GPC) was used to determine the molecular weights of as-prepared samples. It should be noted that the incorporated VMS and RS particles served as heterogeneous nucleation agent in polymer matrix under foaming process to reduce the cell size and increase the cell density of the PSN foams. Furthermore, the VMS particles exhibited a better dispersion capability of silica particles in PMMA matrix as compared to that of RS particles, leading to the PSN foams with smaller cell size and higher cell density. Effect of material composition on the thermal transport properties, thermal stability and mechanical strength of PSN foams were investigated by transient plane source (TPS) technique, thermal gravimetric analysis (TGA) and dynamic mechanical analysis (DMA), respectively.