A continuous extrusion process for the manufacture of low-density microcellular polymers is presented. Microcellular polymers are foamed plastics characterized by a cell density greater than lo9 cells/cm3 and a fully grown cell size on the order of 10 pm. F'revious research on continuous processing of microcellular polymers has focused on control of microcell nucleation in extrusion. This paper presents an effective means for control of cell growth to achieve a desired expansion ratio with CO, as a blowing agent in microcellular foam processing. Also, a strategy to prevent deterioration of the cell-population density via cell coalescence during expansion is presented. Promotion of a desired volume expansion ratio and prevention of cell coalescence in microcellular foam processing were experimentally verified. By tailoring the extrusion processing parameters, microcellular HIPS foams with a cell density of 1Olo cells/cm3 and a controlled expansion ratio in the range of 1.5 to 23 were obtained.

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The main purpose of this research is to bolster mechanical properties of the parts, produced by an extrusion-based 3D printer, or fused deposition modeling machine, via increasing the content of continuous fiber yarn to its practical limit. In-melt continuous glass fiber yarn embedding was applied as a reliable and consistent method for simultaneous impregnation to produce continuous fiber-reinforced thermoplastic composites in the fused deposition modeling process. It is well known that the main weakness in the fused deposition modeling 3D printed products is their low strength compared to the manufactured ones by conventional methods such as injection molding and machining processes. This characteristic can be related to both inherent weakness of thermoplastic materials and poor adhesion between the deposited rasters and the layers. Although various attempts have been performed to tackle this issue, it is widely believed that using continuous fibers is the most effective method to serve this purpose if a reliable and consistent method is implemented. Since the mechanical properties of continuous fiber-reinforced composites directly depend on the content of fiber volume, maximizing the fiber content as well as producing an integrated part was assumed as the main objective. In this work, an analysis of various patterns of raster deposition was conducted, followed by the experiments and verification. The effective parameters on the fiber yarn volume, such as fiber yarn diameter, fiber yarn laying pattern, extrusion width, layer height, and flow percentage, were investigated and their optimal values were reported. The attained experimental results showed that, for polylactic acid-glass fiber yarn reinforced composite, with the extrusion width of 0.3 mm, the layer heights of 0.22 mm, flow percentage of 0.43, and the rectangular laying pattern, approximately 50% fiber-volume content can be achieved which resulted in tensile yield strength and modulus of 478 MPa and 29.4 GPa, respectively. There was an excellent agreement between these experimental results and predicted theoretically values by rule of mixture.

In this study, an innovative method was devised and implemented to produce continuous glass fiber–reinforced thermoplastic composites via a fused deposition modeling three-dimensional printer to enhance the mechanical properties of the printed products. In the extrusion-based, or filament-based, additive manufacturing process, namely, fused deposition modeling, the parts are basically formed via deposition of the material in the molten state, and thus embedding continuous fiber, in a solid form, is highly challenging. Hence, a nozzle system was designed and manufactured to feed the continuous fiber into the molten polymer simultaneously, which is called, here, in-melt simultaneous impregnation. With the presence of continuous fibers in the nozzle outlet, the feed of filament was calculatedly adjusted in the G-codes depending on the fiber volume percentage, to produce sound flow, and consistent deposition. Composite products were produced with various geometrical shapes. Via analysis and close control of the filament feeding, as a critical requirement to produce a sound printed product, composites with various fiber volume percentages were printed. Also, the mechanical properties of the printed parts with 30% by volume of glass fiber were measured. The results of the tensile test indicated that the continuous fibers were appropriately and effectively embedded that could result in remarkable increases in tensile strength and modulus of the samples, higher than 700%. The resulted values of tensile modulus were consistent with the values calculated via the rule of mixture. In addition, scanning electron microscopic images of the fracture sections suggest a sound adhesion between fibers and the matrix.

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