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Design requirements for blow molds

by:Yosion Machinery     2022-08-30
The extrusion blow mold is mainly composed of two half female molds, which are generally composed of a mold body, a mold neck, a mold cavity, a blank cutting sleeve, a blank cutting opening, and a guide column. Requirements for profile design Structural design of extrusion blow molding mold The position of the parting surface of the mold should be selected to make the mold symmetrical, reduce the inflation ratio, and facilitate the product demoulding. Therefore, the position of the parting surface is usually determined by the shape of the blow molding cavity. Most blow molds are designed as two half-molds that cooperate with the parting surface. For bottles and containers with irregular shapes, the determination of the position of the parting surface is particularly important. Improper position will cause the product to fail to be demolded. Or cause the bottle to be scratched. At this time, it is necessary to use a mold with an irregular parting surface, and sometimes even a multi-parting surface mold composed of three or more movable parts, which is conducive to product demoulding. For a container with a circular cross section, the parting surface is set through the diameter; for an elliptical container, the parting surface should pass through the long axis of the ellipse; the parting surface of a rectangular volume can pass through the center line or the diagonal line, the latter of which The blow-up ratio can be reduced, but the wall thickness at the corner opposite to the parting surface is smaller. For some products, multiple parting surfaces need to be set. The container handle should be located along the parting surface. The cross-section of the handle should be square, and the corners should be transitioned with arcs to optimize the wall thickness distribution. The handle holes are generally formed by inserts. The handle can also be molded separately by injection molding. (2) The design of the cavity requires the blow mold cavity to directly determine the shape, size and appearance of the product. The surface of the mold cavity for PE blow molding should be slightly rough. Otherwise, the mold cavity will be poorly vented, and air bubbles will be trapped, causing the product to appear.“orange peel”surface defects. It also results in a low and non-uniform cooling rate of the product, resulting in different shrinkage rates throughout the product. Due to the low temperature of the PE blow mold and the small inflation pressure of the parison, the blown parison will not be wedged into the trough of the rough cavity surface, but will be located at or across the wave crest, thus ensuring that the product has Smooth surface and provides tiny mesh channels for easy venting of the mold cavity. Sandblasting the cavity can create a rough surface. The sandblasting particle size should be appropriate. For HDPE blow molding molds, a coarser particle size can be used, and LDPE a finer particle size. Etching the cavity can also form a rough surface, and can also form patterns on the surface of the product. When blow molding highly transparent or high gloss containers (especially PET, PVC or PP), polish the cavity. For the blow molding of engineering plastics, the mold cavity generally cannot be sandblasted. In addition to etching patterns, it can also be polished or matted. The size of the mold cavity is mainly determined by the external dimensions of the product and the shrinkage rate of the product at the same time. The shrinkage rate generally refers to the difference between the size of the cavity at room temperature (22°C) and the size of the product after 24 hours of molding. For example, in blow molding of HDPE bottles, 80% to 90% of the shrinkage rate occurs within 24 hours after molding. (3) Mould incision The mouth part of the blow mold should be a sharp incision to facilitate cutting the parison. The minimum longitudinal length of the jaw for cutting the parison is 0.5 to 2.5 mm. If it is too small, it will reduce the thickness of the joint seam of the container, reduce its joint strength, and even easily cut the parison without blowing it up. If it is too large, the tailings cannot be cut off. , even the mold cannot be completely closed. The shape of the incision is generally triangular or trapezoidal. In order to prevent the wear of the incision, the inserts made of cemented carbide materials are often used to be embedded on the mold, and the angle that the end of the incision expands to the surface of the mold varies with the type of plastic, and LDPE is preferably 30.°~50°, HDPE take 12°~15. The opening and closing of the mold is usually controlled by compressed air, and the closing speed is best adjusted to meet the requirements of different materials. For example, when processing PE, the mold closing speed is too fast, and the incision is easy to cut through the parison, so that the parison cannot be well welded.
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