Control elements for low-foaming hollow blow molding
DATE：2022/1/18 8:43:03 / READ： / SOURCE：This station
Control elements for low-foaming hollow blow molding
Commercialization of hollow blow foam is difficult because the cells are difficult to control during inflation. Many published research results are only at the laboratory level. According to the successful product production experience and research results of a few companies, the following control elements can be summarized.
①The chemical foaming agent can usually be uniformly premixed into the raw materials for hollow blow molding. In this way, it is easy to ensure the uniformity of the distribution of the blowing agent in the plastic, thereby also ensuring the requirement of uniform distribution of cells in the final product.
② The best position for injecting the inert gas into the plastic melt should be as close to the machine head as possible. Therefore, when injecting inert gas through the screw, the injection port should be at the head of the screw. In order to make the gas evenly distributed in the plastic melt, the inert gas injection port should be located at the position off-center of the screw head, so as to achieve a certain stirring effect by means of the rotation of the screw itself.
③ Since the inflation process is unavoidable for hollow blow molding, all hollow blow molding foam products have different degrees of cell deformation (longer, longer). However, if the degree of deformation of the foam can be controlled within a certain limit, the final quality of the hollow blow molded part can be guaranteed to be within an acceptable range. Therefore, the design of low-foaming hollow blow-molded parts is required to avoid excessive thin walls as much as possible. Generally, large bubbles are most likely to be generated at the thin wall of the part. Such large bubbles are stretched too much in the process of inflation and stretching, and the rupture field collapses, resulting in waste or insufficient strength of the product.
④When making billets by extrusion method, pay attention to controlling the extrusion speed. The general relationship is that the faster the extrusion speed, the larger the resulting cell, the larger the cell, and the greater the possibility of excessive cell deformation or cell rupture during inflation.
⑤ During hollow molding, the relationship between inflation pressure and foam density should also be taken into account (see Figure 105). As can be seen from the figure, when the inflation pressure is high, the bubbles are destroyed more, and the foaming ratio decreases (or the density of the foam increases). If the inflation pressure is too low, the reproducibility of the cavity shape is poor. Therefore, under the premise of satisfying the reproducibility of the cavity shape, it is advisable to take a low inflation pressure as much as possible.
The surface quality of low-foaming hollow blow-molded parts can be improved by rapidly cooling the mold and reducing the blowing pressure. Generally, under good mold cooling conditions, the inflation pressure can be between 03 and 1.05MPa depending on the material, size and shape of the mold cavity.
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