In order to improve the outstanding limitations of the traditional “secondary feeding” rotomolding process, Polivil Rotomachinery has developed a “one-time feeding” rotomolding process for the production of polyethylene structural foam sheets. An interest in research in this area is the ability to fill a material between the walls of the board that improves the properties of the sheet. These properties are mainly 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 their interest is to consider the recycling of materials.

The traditional "secondary feeding" process begins by putting a certain amount of material necessary into the outer layer of the mold-forming plastic part ("skin shell"), and once the material is melted in the furnace, the mold is taken out of the furnace and added. A blowing agent (such as a powdered additive premixed in polyethylene) is immediately placed in the furnace to complete the entire molding cycle (second melt, cooling, etc.). This process has safety problems because the manual operation of the high temperature mold lengthens the molding cycle, the length of the extension is proportional to the size of the mold, and the operation must be interrupted before the skin shell and the material suitable for dropping down are solidified (mold) The bigger the phenomenon, the more serious it is).

Unlike the "secondary feeding" process, the "one-time feeding" method puts two different polyethylene raw materials into a mold, one is powdered polyethylene, and the other is small spherical polyethylene mixed with a foaming agent. The powdered polyethylene has a lower melting point than the blowing agent and a different particle size for forming the outer skin of the plastic part. (Powdered polyethylene has a particle size of 500 to 1000 microns, and a small spherical (mixed foaming agent) polyethylene particle size of 3 to 4 mm). The powdered polyethylene is first adhered to the mold and the small spherical polyethylene continues to circulate and is continuously heated, and then the skin shell of the plastic part is formed and then subjected to a foaming reaction.

The material passes through the first stage of melting at 210 ° C, and the powdery material begins to adhere to the mold. Only the surface shell forming the plastic part is melted and propelled to the second stage at a temperature of 265 ° C. Ball wall. Since the internal pressure is generated by the decomposition of the blowing agent, this process continues until the internal pressure is balanced with the atmospheric pressure of the exhaust port. The small spherical polyethylene particles are sequentially foamed and enlarged, and finally the entire cavity is filled, and the polyethylene is solidified by cooling. The ideal process result is that the two materials are completely separated during the molding stage, and the foaming material is controlled by adjusting the processing temperature, melting and cooling time, rotation speed, and vent position. Furthermore, it must be avoided that small spherical polyethylene particles are adhered to the skin layer of the skin, and under normal conditions, the pellets are uniformly softened by heat and foamed due to the pressure generated by the decomposition of the foaming agent. Therefore, if the skin layer of the plastic part is not melted, unless the ball is completely wrapped, the small spherical polyethylene particles may sink (almost inevitable), heating and bringing the shell layer into contact with the small spherical particles to reach the softening point but has not yet occurred The expansion, the blowing agent only decomposes at the contact between the shell and the ball (here, the post-yield point), leaving visible marks on the surface of the plastic part.

The foaming process is a continuous process. When the temperature of the small spherical polyethylene is higher than the temperature at which the foaming agent starts to decompose in the mold, the pressure in the mold is not high, and there is room for the material in the mold to expand. At this time, foaming starts to occur, and the material expansion will occur. The air inside the mold is discharged from the exhaust port. Problems associated with the foaming process may be cracking, collapse, and increase of bubbles in the foaming agent. Excessively thin foamed plastic parts are detrimental to the mold closing flange and cause difficulty in demolding.

The foaming agent is generated near the exhaust port and the bubble wall that is growing up can be crushed and torn, reducing the mechanical robustness of the processed product, thereby forming a region with large bubbles (empty bubbles are not popular) . The long-term residence of the foam at high temperatures increases the duration of the expansion process, creating the risk of cell growth. The thrust applied to the cell walls by the blowing agent causes the cells to collapse, resulting in the aggregation of the cells, causing the cells to become larger and larger, the size of the cells to grow and the time of the second melting phase. It is proportional to the thickness of the foamed sheet (the heating control of the foaming agent decomposition reaction at the core of the sheet at a high temperature is more difficult).

In order to avoid all of these situations, it is necessary to increase the cooling rate as much as possible. Practice has proved that this is not enough, and the duration of the second melting stage is also reduced. This expansion process can be considered as a macroscopic growth movement of the polyethylene pellet particles, and the pellet grows from the epidermis shell to the lower pressure region (exhaust port region) if the cell growth is uneven and/or abnormal. This will result in an undesired large airbag area, inconsistent distribution of polyethylene pellets, increased blockage of cells, and/or obstruction or deflation of the venting holes. The inconsistent distribution of polyethylene pellets means that the number of small spheres allocated in some areas is insufficient and therefore cannot fill the mold cavity (even if it is completely foamed), so the rotation must be adjusted. If there are obstacles in the mold that hinder the growth of the foam, the foamed material cannot fill the mold cavity, especially at high temperatures and when the second melting stage is too short. What is more, when the cells may be evenly distributed but the vent hole size is small, the air is clogged and sealed, and to a certain extent, the air pressure will be equal to the internal pressure of the bubble and hinder the further growth of the cells. Therefore, the vents must be of the correct size or of sufficient quantity to allow the gas to drain smoothly. When the expansion material blocks the exhaust port, the pressure in the mold will increase. In some cases, the material will overflow through the closed mold flange. As a result, in addition to soiling the flange (causing production must be interrupted), the same is also caused. Damage to the mold.

The completed experiments show that the temperature of the first melting stage must be as low as possible to protect the integrity of the porous beads from softening. The minimum temperature used in the LogicaPRM30008C machine is 215 °C. Unless there is special aesthetic requirements, the thickness of the skin layer of the foamed sheet is generally 2 to 3 mm. To avoid the emergence of the small ball, the thickness of the skin layer can be increased to 3 to 4 mm, and has a complex shape of a plurality of reinforcing ribs. The thicker the foamed sheet, the longer the second stage melting time, because the dense skin shell heat transfer is poor. A higher rotational speed must be used to reduce the time each perforated bead is in contact with the skin shell. As for the structural foam board produced by the "one-time feeding method" in the design stage, the sheet should be such that the pores remain inside the panel to ensure that air can flow from a hole to an adjacent hole, and the air can be easily discharged. On the other hand, a certain number of vents must be designed on the mold to ensure smooth venting when the material is foamed. In contrast, during the molding process, the melting temperature for forming the skin shell must be set only to consider the process of adding only the solid material to the mold to set the temperature (no foam beads). It is not preferable to reduce the molding time by setting the temperature to 200 to 230 °C. The melting time of the first stage must be determined based on the desired skin thickness of the skin. When the material that has been loaded into the mold for the production of the skin shell during the melting stage is combined with the foamed pellets, the duration in the mold will increase by 10% because the heat is absorbed by the foam pellets. In this case, it is necessary to calculate the theoretical amount of the material mixed with the blowing agent, and it is necessary to consider the cavity size and the theoretical foaming density without the skin shell.