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                            Soft polyethylene foam rotomolding process breaks through its limitations

                            2021-03-05

                              In order to improve the outstanding limitations of the traditional soft polyethylene foam technology, Polivinil Rotomachinery has developed a new "one-time feeding" rotomolding process to produce polyethylene structural foam plastic panels. The interest of research in this field is to be able to fill a material between the two walls of the board that can improve the properties of the board. These properties mainly include chemical-physical properties (such as heat insulation, specific gravity, floatability, etc.) or Mechanical properties (such as vibration resistance, structural strength, etc.). In addition to the availability of polyethylene structural foam products, another reason for its interest is to consider the recycling of materials.
                              The traditional "secondary feeding" process begins by putting a certain amount of necessary material into the outer layer ("skin shell") of the molded plastic part, once the material is melted in the heating furnace, the mold is taken out of the furnace and added Foaming agent (such as a powdery additive premixed in polyethylene), and immediately re-inserted into the heating furnace to complete the entire molding cycle (second melting, cooling, etc.). This process has safety problems because the manual operation of high-temperature molds prolongs the molding cycle, and the length of the extended time is proportional to the size of the mold, and the operation process must be interrupted before the skin layer and the material suitable for dropping down solidify (mold The larger this phenomenon is, the more serious it is).
                              Different from the "secondary feeding" process, the "one-time feeding" method puts two different polyethylene raw materials into the mold, one is powdered polyethylene, and the other is small spherical polyethylene mixed with a foaming agent. The melting point of powdered polyethylene is lower than the decomposition temperature of the foaming agent, and the particle size is also different. It is used to form the outer surface of the plastic part. (The particle size of powdered polyethylene is 500~1000 microns, and the size of small spherical (mixed with foaming agent) polyethylene particles is 3~4 mm). The powdered polyethylene first adheres to the mold and the small spherical polyethylene continues to circulate and be heated, and then forms the skin of the plastic part and then undergoes a foaming reaction.
                            When the material passes through the first stage of melting at 210°C, the soft polyethylene foam begins to adhere to the mold. Only the surface shell layer of the plastic part is formed to advance to the second stage of melting at a temperature of 265°C. At this time, the polyethylene Soften to form a spherical wall. The internal pressure is generated due to the decomposition of the foaming agent, and this process continues until the internal pressure is balanced with the atmospheric pressure of the exhaust port. The small spherical polyethylene particles are foamed and expanded in sequence, and finally fill the entire cavity, and the polyethylene solidifies after cooling. The ideal process result is that the two materials are completely separated in the molding stage. The control of the foamed material is by adjusting the processing temperature, melting and cooling time, rotation speed and the position of the exhaust port. Furthermore, it must be avoided that small spherical polyethylene particles are bonded in the skin shell layer. Under normal conditions, due to the decomposition of the foaming agent and the pressure generated, the small spherical particles are uniformly heated and softened and foamed. Therefore, if the skin of the plastic part is not melted, unless the ball is completely wrapped, the small spherical polyethylene particles may sink (almost unavoidable), heating and making the shell and the small spherical particles reach the softening point, but it has not happened yet. Expansion, the foaming agent only decomposes at the contact between the shell and the ball (here is the post-yield point), leaving visible traces on the surface of the plastic part.
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