Dimensions of the injection-molded semi-crystalline materials (polymeric products) decrease with the time that elapses from their formation. The post-molding shrinkage is an effect of secondary crystallization; the increase in the degree of polymer crystallinity leads to an increase in stiffness and decrease in impact strength of the polymer material. The aim of this study was to assess the changes in the values of post-molding shrinkage of polypropylene produced by injection molding at two different temperatures of the mold (20 °C and 80 °C), and conditioned for 504 h at 23 °C. Subsequently, the samples were annealed for 24 h at 140 °C in order to conduct their accelerated aging. The results of shrinkage tests were related to the changes of mechanical properties that accompany the secondary crystallization. The degree of crystallinity of the conditioned samples was determined by means of density measurements and differential scanning calorimetry. It was found that the changes in the length of the moldings that took place after removal from the injection mold were accompanied by an increase of 20% in the modulus of elasticity, regardless of the conditions under which the samples were made. The differences in the shrinkage and mechanical properties of the samples resulting from mold temperature, as determined by tensile test, were removed by annealing. However, the samples made at two different injection mold temperature values still significantly differed in impact strength, the values of which were clearly higher for the annealed samples compared to the results determined for the samples immediately after the injection molding.
The dynamic growth of the use of polymer construction parts manufactured individually and in a small series makes it necessary to improve additive methods in the areas of materials, equipment and processes. By observing selected phenomena occurring during the processing of polymer materials in other production technologies (e.g., extrusion and injection molding), it is possible to obtain solutions that positively affect the final performance properties of the products obtained in additive manufacturing technologies using thermoplastic filament. The aim of this research was to determine the effect of the diameter of the print head nozzle on the spatial structure (path width) and selected mechanical properties of samples produced by the FFF method with PLA material. The obtained results were compared to the samples with a solid structure produced using injection molding technology. In the experiment, the RepRap device for additive manufacturing was used, with the use of nozzles with diameters of 0.2 mm, 0.4 mm, 0.8 mm and 1.2 mm. The test objects were produced with a layer height of 0.2 mm, full filling (100%) and with constant remaining printing parameters. The conducted research allowed us to conclude that the use of layer heights lower than the standard ones gives favorable results for selected mechanical properties. The use of an extruder nozzle diameter of 0.8 mm allows one to obtain a macrostructure with a high degree of interconnection of layers and paths and favorable mechanical properties. The test results can be used in the construction of functional elements that are produced by fused deposition modeling (FDM) and fused filament fabrication (FFF) methods in prototype, unit and small-lot production.
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