Blow molding cooling

Figure 6.1 6 Examples of water flood cooling blow molding molds...

B. H. Petersom Feasibility of Ceramic Castings for Conformal Cooling Blow Molds, Chemical and Materials Department, Arizona State University Tempe, Arizona,http //www.nisoe.edu/ieu/research papers/Ben... 

Blow Molding. Blow mol ding is a multistep fabrication process for manufacturing hoUow symmetrical objects. The granules are melted and a parison is obtained by extmsion or by injection mol ding. The parison is then enclosed by the mold, and pressure or vacuum is appHed to force the material to assume the contour of the mold. After sufficient cooling, the object is ejected. 

Production molds are usually made from steel for pressure molding that requires heating or cooling channels, strength to resist the forming forces, and/or wear resistance to withstand the wear due to plastic melts, particularly that which has glass and other abrasive fillers. However most blow molds are cast or machined from aluminum, beryllium copper, zinc, or Kirksite due to their fast heat transfer characteristics. But where they require extra performances steel is used. 

The nature of these processes requires the supply of clean compressed air to blow the hot melt located within the blow mold. Other gases can be used, such as carbon dioxide, to speed up cooling of the blown melt in the mold. The gas usually requires at least a... 

The crystallinity in PET soft drink bottles is about 25%. Because a more crystalline state is normal for PET, the amorphous content is increased intentionally by copolymerization and rapid cooling for the molten PET from the melt to a temperature below the glass transition temperature. Companies which perform high-speed blow molding of PET prefer PET resins made with small amounts of glycol and diacid comonomers. 

Blow-molding processes consists of five main operations plastication of the resin, formation of the parison, inflation of the parison, solidification of the part, and removal of the part from the tooling. The best process economics will occur with a part optimized for weight and a minimum cycle time. In order to have a minimum cycle time, the cooling operation must be the rate-limiting step. For the case study... 

FIGURE 18.11 Steps employed in simple extrusion blow molding of a test tube. From left to right mold closed softened material introduced air or other gas injected forcing the softened thermoplastic against the walls of the mold and, after suitable cooling, the mold is opened giving the molded plastic test tube. 

In continuous extrusion blow molding, the preform is continuously produced at the same rate as the article is molded, cooled, and released. To avoid interference with the preform formation, the mold-clamping step must be rapid to capture the preform and move it to the blow mold station. There are various modifications of this that allow essentially continuous operation. 

Parison cooling significantly impacts the cycle time only when the final parison thickness is large. In thin blown articles the mold is opened when the pinched-off parts have solidified so that they can be easily stripped off thus they are the rate-controlling element in the cooling process. For fast blow molding of even very thin articles, the crystallization rate must be fast. For this reason, HDPF, which crystallizes rapidly, is ideally suited for blow molding, as are amorphous polymers that do not crystallize at all. 

Most PET botdes are produced by injection blow molding (71) the resin over a steel-core rod. The neck of the botde is formed with the proper shape to receive closures and resin is provided around the temperature-conditioned rod for the blowing step. The rod with the resin is indexed to the mold, and the resin is blown away from the rod against the mold walls, where it cools to form the transparent botde. The finished botde is ejected and the rod is moved again to the injection-molding station. This process is favored for single cylindrical botdes, but cannot be used for more complex shapes such as botdes with handles. 

Extrusion blow molding. In extrusion blow molding, a parison or tubular profile is extruded and inflated into a cavity with a specified geometry. The blown article is held inside the cavity until it is sufficiently cool. Figure 3.56 [25] presents a schematic of the... 

As expected, the largest portion of the cycle time is the cooling of the blow molded container in the mold cavity. Most machines work with multiple molds in order to increase production. Rotary molds are often used in conjunction with vertical or horizontal rotating tables (Fig. 3.59 [14]). 

Heat Transfer in Blow Molding Estimate the cooling time of a 15 cm long, 4 cm in O.D., and 0.3 cm thick HDPE parison at 200°C, which is inflated onto a 10-cm-diameter and 15-cm-long cylindrical bottle mold at 15°C by 5°C cold air. Solve the heat-transfer problem involved. Use the p, k, and Cp data given in Appendix A. Assume that the inner surface of the bottle is at 15°C. 

Parison cooling is an integral part of the process that has been treated by a number of researchers (103-105). The principles are based on contact solidification without deformation, as discussed in Chapter 5. A special complication is the frictional heat generation in injection blow molding between the rod and the parison. 

To speed the cooling portion of the blow molding cycle, chilled air can help. For more... 

First station usually has multiple preform injection molds where preforms are formed over core pins. The preforms have hemispherical closed ends (resembles a laboratory test tube). The other ends have an open bore, formed by the core pin. External details, such as the thread and neck flange for a screw-top container, are directly produced by injection molding. While the preform is still hot, the injection split mold is opened and the preforms, still on the core pins, are rotated to the blowing station two. Here the preforms are enclosed within the blow mold, and introducing blowing air through the core pins followed with cooling produces the BM. Blow molds opened and the finished products, still on the core pins, are rotated to an ejection station where they are stripped off mechanically and/or air. 

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