The ease with which modern plastics can be injection molded makes them very suitable for the production of many different products and, today, plastics are often used as substitutes for metal. Polypropylene (PP) is one of the most widely used thermoplastics globally since it is very useful, cost-effective and flexible for molding. However, the amount of harm to the environment caused by plastic waste has become phenomenal and the recycling of plastics has become a serious aspect of environmental protection. PP, as the most commonly used plastic material, was selected for use in this study. It has a melt flow index of 15 g/min and its recyclability, fluidity, and physical properties, as well as manufacturing conditions, were explored in relation to the number of times the material could be recycled (TR). A cavity pressure sensor was used to measure the viscosity index of the recycled plastic after multiple cycles of plasticizing and injection, part molding, scrap-recycling, and crushing. A paperclip-shaped test specimen was used to determine PP fluidity and crystallinity of specimens with different TRs. Tensile tests were used to detect differences in the tensile strength between specimens made from Raw-PP and recycled PP. The results showed that PP that had been recycled several times had a higher melt flow index, material fluidity, melting peak area, crystallinity, crystallization rate, and crystallization temperature. Repeated injection and recycling of the material had reduced the length of the molecular chains and broadened the molecular weight distribution. This improved the fluidity and increased crystallinity. The increase in fluidity made cavity filling easier, reducing the cavity pressure as well as the viscosity index. The results of this study showed that the recycling of the PP could improve the physical properties of the products to a degree and also went some way to further the benefits of a circular economy. The recycling of injection-molded PP material can be added to renewable energy technologies and used in environmental impact assessment.

In this study, fiber breaking behavior, fiber orientation, length variation, and changes in melt flow ability of long glass fiber reinforced polypropylene (L-FRP) composites under different mold cavity geometry, melt fill path, and plasticization parameters were investigated. The matrix material used was polypropylene and the reinforcement fibers were 25 mm long. An ultra-long-fiber composite injection molding machine (with a three-stage plunger and injection mechanism design) was used with different mold cavity geometry and plasticization parameters. Different screw speeds were used to explore the changes in fiber length and to provide a reference for setting fiber length and parameter combinations. Flow-length specimen molds with different specimen thickness, melt fill path, and gate design were used to observe the effect of plasticizing properties on the flow ability of the L-FRP composite materials. The experimental results showed that the use of an injection molding machine with a mechanism that reduced the amount of fiber breakage was advantageous. It was also found that an increase in screw speed increased fiber breakage, and 25 mm long fibers were shortened by an average of 50% (to 10 mm). Long fibers were more resistant to melt filling than short fibers. In addition, the thickness of the specimen and the gate design were also found to affect the filling process. The rounded angle gate and thick wall product decreased the flow resistance and assisted the flow ability and fiber distribution of the L-FRP injection molding.

Processing equipment and parameters will highly influence the properties of long-fiber-reinforced injection-molded thermoplastic composites, leading to different fiber lengths and orientations. Thus, maintaining fiber length during the injection molding process is always a big challenge for engineers. This study uses long-glass-fiber-reinforced polypropylene with 25 mm fiber length and a special-built novel injection molding machine with a three-barrel injection unit, including a plasticizing screw, an injection plunger, and a packing plunger, to fabricate injection molding parts while retaining long fiber length. This study also discusses the influence of process parameters, such as back pressure, screw speed, melt temperature, and different flow paths, on the properties of long-glass-fiber-reinforced composites. The experiment results show that a higher screw speed and back pressure will reduce the fiber length in the injection-molded parts. However, using appropriate parameter settings can maintain the fiber length to more than 10 mm. It was found that by increasing the back pressure, the cross direction of the fiber orientation can be increased by up to 15% and the air trap volume fraction can be decreased by up to 86%. Setting appropriate back pressure under a low screw speed will increase the tensile strength. Finally, it was found that the single-edge-gate path results in a higher tensile strength than that of the single-sprue-gate path due to the retainment of longer fiber length in the injection-molded part.

Scientific injection molding technologies involve the integration and collaboration of cyber-physical systems and smart manufacturing. In order to achieve adaptive process control and production optimization, injection molding systems with real-time sensing have gradually become the development- and application-trend of smart injection molding. At the same time, this technology is a highly non-linear process in which many factors affect the product quality during long-run fabrication processes. Therefore, in order to grasp changes in the characteristics of plastic materials and product quality monitoring, the injection process has become an important research topic. We installed sensors in the molding machine (injection barrel, nozzle, and mold-cavity) to collect the melting pressure and used different materials (semi-crystalline and amorphous polymer; the melting-fill-index (MFI) is unified to 14.5 ± 0.5 g/10 min) to explore the influences of melting pressure variation and its viscosity index on the quality characteristics of molded products. The experiment reveals that a combination of barrel, nozzle, and mold-cavity sensing on the melt-pressure trend-based injection process-control incorporated with viscosity index monitoring can confirm the weight and shrinkage variation of the injection product. At the same time, the pressure and viscosity index value measured and calculated during the melt-filling of two materials with similar MI resulted in significant variations in the amorphous polymer. This study showed the possibility of mastering and controlling the rheology (barrel position) and shrinkage properties of polymers and successful application in various product-quality monitoring platforms.

Long-fiber polymers offer the advantage of a lower production cost because specific tool designs are required for conventional injection molding equipment to produce long-fiber polymer parts. The use of long fibers allows relatively high fiber aspect ratios to be obtained, thereby enhancing composite stiffness, strength, creep endurance, and fatigue endurance. However, the multigate design of the injection-molded part can result in weldline formation during the molding process, which reduces the structural strength of the molded part. Therefore, in this study, the surface quality, fiber compatibility, and structural strength of long-glass-fiber-reinforced polypropylene (PP/LGF) injection-molded samples were compared in the use versus nonuse of a mold-cavity overflow-well area and the mold-face infrared heating method. The experimental results indicate that the mold-cavity overflow-well area more greatly improved the surface roughness of the PP/LGF molded samples. Moreover, the infrared heating of the mold-face decreased the weldline depth of the samples. Optical-microscopy images and mold-cavity pressure distributions indicated that the weldline tensile strength and the interface compatibility between fibers and melts at the weldline region during the molding stage were higher in the use than in the nonuse of the mold-cavity overflow-well and mold-face infrared heating method.

Plastic foam molding methods include thermoforming, extrusion and injection molding. Injection foam molding is a one-time molding method with high production efficiency and good product quality. It is suitable for foamed plastic products with complex shapes and strict size requirements. It is also the main method for producing structural bubbles. In this investigation, we developed a structural foam injection molding technology using the gas supply equipment connected to the unique plasticizing mechanism of the injection machine and studied its influence on the specimens’ melt rheology quality and foam structures. In the experiment, the forming material was polypropylene (PP), and the gas for mixing/forming foaming characteristics was nitrogen (N₂). Additionally, in order to observe the rheological properties of N₂/melt mixing, a melt flow specimen mold cavity was designed and the change in the melt viscosity index was observed using a melt pressure sensing element installed at the nozzle position. With the nitrogen supply equipment connected to a unique plasticizing mechanism, the mixing of gas and molten plastic can be achieved at the screw plasticizing stage, where the foaming effect is realized during the melt-filling process due to the thermodynamic instability of the gas. It was also found that an increase in N₂ fill content increased melt fluidity, and the trend of melt pressure and melt viscosity index showed that the higher the gas content, the lower the trend. The foaming characteristic depends on the gas thermodynamic instability and the pressure release, so it can be seen from the melt fill path that, the greater the pressure near the gate, the lower the foaming amount and the internal structure (SEM) after molding; the farther from the gate, the greater the relative increase in the foaming growth/amount. This phenomenon will be more obvious when the N₂ fill content is increased.