The investigation of the influence of injection molding parameters (injection velocity, mold temperature and injection temperature, and additionally, as a result of these three parameters change, injection time, hold time, and cooling time) and blowing agent percentage on selected properties of HDPE molded parts such as weight, density, mechanical properties (tensile strength and elongation at maximum force), surface state (gloss and color), and structure was the aim of this work. The examination showed, that the mold temperature has the main influence on properties and surface state of molded parts from solid and foam HDPE. The weight, density, mechanical properties and gloss of molded parts increased with the increase in mold temperature. The mold temperature also influences significantly the number and size of pores in molded parts. The addition of blowing agent in a quantity of 2% is sufficient to obtain parts with favorable mechanical properties and good surface quality. POLYM. ENG. SCI., 53:780-791, 2013. FIG. 8. The change of the tensile strength of molded parts from HDPE: without blowing agent in function of the injection velocity v w and the mold temperature T f (T w ¼ 2308C) (a); the mold temperature T f and the injection temperature T w (v w ¼ 45 mm/s) (b), with 2% addition of blowing agent in function of the injection velocity v w and the mold temperature T f (T w ¼ 2308C) (c); the mold temperature T f and the injection temperature T w (v w ¼ 45 mm/s) (d), with 4% addition of blowing agent in function of the injection velocity v w and the mold temperature T f (T w ¼ 2308C) (e); the mold temperature T f and the injection temperature T w (v w ¼ 45 mm/s) (f).
The investigation of the influence of mould temperature and blowing agent percentage on structure, and selected properties (weight, mechanical properties, surface state - gloss and colour) of moulded parts from HDPE was the aim of this work. The structure of moulded parts in cross-section perpendicular and parallel to the direction of polymer flow, and quantitative assessment of porous structure using microscopic image analysis were carried out. It was shown that the content of blowing agent in polymer influences not only the amount of pores in the part core, but also the thickness of solid skin. Lower mould temperature favors the formation of a fine porous structure. The formation of the porous structure influenced slightly the weight and mechanical properties of moulded parts. With the increase in blowing agent content, the gloss of parts decreased and also caused the change in their colour. Parts with higher amount of blowing agent were brighter.
Weld lines, created in the areas of collision of two flow fronts of plastic in the injection mold cavity, are the reason of lower mechanical properties and a worse surface condition of molded parts. In the weld line area, the V‐shaped notch is formed and its shape and size depend on injection molding conditions and properties of processed polymer. Addition of the foaming agent to the polymer can be one of the way to improve conditions of melt streams welding due to the higher velocity of the colliding streams of unfilled polypropylene (PP) and PP filled with talc. The examinations of mechanical properties showed, however, lower tensile strength of porous parts compared to solid ones, but in the microscopic observation and measurements of the geometric structure of moldings, in the weld lines area, better surface conditions were achieved for samples made of the foamed polypropylene. The size of V‐notch, determined by the total height of the raw profile Pt, depends also on the length of polymer flow path from the gate to the weld line area. The values of Pt parameter increase with the length of the flow path, but this increase is smaller for foamed polypropylene. POLYM. ENG. SCI., 59:1710–1718 2019. © 2019 Society of Plastics Engineers
Niekonwencjonalne metody wtryskiwania *) NONCONVENTIONAL METHODS OF INJECTION MOLDING Summary-Nonconventional methods of polymer injection molding allow to produce the parts showing properties and surface quality difficult or even impossible to obtain by the conventional injection molding process. These methods can be classified using different criteria, such as incorporation of additional materials or components into the molded part, melt formulation, melt manipulation, additional mold movement and special part or geometry features. Selected methods of nonconventional injection molding were described in the article. One of them is the microlayer injection molding, which enables to produce the parts consisting of a lot of very thin layers from at least two different polymers. Process of in-mold decoration is a combination of conventional injection molding and part printing. During in-mold lamination a multilayer laminate is positioned in a mold-parting plane and covered on one side by the injected polymer. Injection molding process can also be applied for metal-ceramic powders, mixed with thermoplastic and wax. By this method parts of good mechanical properties, complicated shape, made of almost unworkable materials, usually without additional machining can be manufactured. The push-pull process is characterized by the multiple polymer flow in cavity, in two directions, which causes significant macromolecule orientation in a core of parts and improves their mechanical properties in the flow direction. Multiple polymer flow in two directions lowers the possibility of weld lines appearance. During the rheomolding process a polymer melt, before its entering the mold or inside the mold, undergoes low frequency vibration to increase its rheological, mechanical and optic properties. Vibration reduces or even eliminates weld lines creation. Nonconventional methods of injection molding are performed using machines and equipment of more complicated constructions, their technologies are more difficult, but these methods allow to produce the products of specific properties required.
The weld lines in injection molded parts arise as a result of the collision of two fronts of the flowing material, which fills the mold cavity and are sometimes unavoidable. They decrease the mechanical properties and surface state of the moldings, due to insufficient connection of the fronts of melted polymer streams. The paper discusses the reasons for the occurrence of weld lines in molded parts, which have been divided into ones related to the design of the injection molded part, properties of the injected material, injection mold construction and injection parameters. The design of the molded parts should ensure even, smooth flow of the material in the cavity, but in the case of parts with many injection points, holes, varied wall thickness, complicated and irregular structure the weld lines can not be avoided. The type of material processed significantly influences the strength of the weld line area. Too high viscosity of the material, fillers and large particles of the coloring agent highly oriented in the weld line area, are the factors lowering properties of the molded part. Considering the design of the injection mold, the most important aspects in forming proper parts with weld lines are correct venting of the mold, small differences in the material temperature in all cavity areas, correctly selected injection points, avoiding jet filling of the cavity. The quality of such parts can be improved by changing the injection conditions, mainly by increasing the mold and melt temperature, also increasing the holding pressure and the injection speed. The possibilities of preventing the weld lines creation or reducing the negative effects of their occurrence have been presented, e.g. by changing the design and location of gates in injection molds, changing processing conditions or using unconventional methods of injection molding, like cascade cavity filling, push-pull and multiple live-feed injection molding, vibration process. Further examinations in the range of cyclic heating/cooling of the injection mold are suggested as the most promising regarding the quality of parts in the area of weld line, their strength and surface state.
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