Cores, injection molds cooling

Most PET botties are produced by injection blow mol ding (71) the resin over a steel-core rod. The neck of the bottie 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 bottie. The finished bottie is ejected and the rod is moved again to the injection-molding station. This process is favored for single cylindrical botties, but cannot be used for more complex shapes such as botties with handles. 

The thermal properties of fillers differ significantly from those of thermoplastics. This has a beneficial effect on productivity and processing. Decreased heat capacity and increased heat conductivity reduce cooling time [16]. Changing thermal properties of the composites result in a modification of the skin-core morphology of crystalline polymers and thus in the properties of injection molded parts as well. Large differences in the thermal properties of the components, on the other hand, lead to the development of thermal stresses, which also influence the performance of the composite under external load. 

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. 

An alternative approach is to cool the parison completely after the injection molding step, and remove it from the core rod. The parison can then be stored or shipped elsewhere before blow molding. In this case, it is necessary to reheat the parison to the desired temperature before the blow-molding step. This approach is seldom used for ordinary injection blow molding, but is not uncommon for stretch blow molding, which is discussed in Section 12.4. 

Two-Shot Injection Molding. Two-shot or oveimolding refers to a process whereby either different colors or different materials are molded into one part. In this process, the first material or color is injected, then the mold is rotated, and the second shot is injected as depicted in Fig. 1.27. An alternative method is to use a retractable core. In this case, the first material is injected, cooled to solidify, and then the core is retracted to allow injection of the second material as shown in Fig. 1.28. Bonding is accomplished through either strictly mechanical means or by adhesion between the two components through diffusion of the chains. This can result in parts with two materials combined with-... 

In injection blow molding, the rtielt is injected into a parison cavity around a core rod (Figure 1.4). The test-tube shaped parison, while still hot, is transferred on the core rod to the bottle blow mold cavity where the bottle is blown and cooled. Injection blow molding is generally used for bottles less than 0.5 liter in size. This type of blow molding allows for a scrap free product and for design of intricate shapes such as tamperproof closures. It is impractical for containers with handles. 

In injection blow molding, the parison is injected into a preform cavity and around a core pin in the exact quantity required to form a container. The preform mold is kept at a precisely controlled temperature, which is just a little cooler than the melt temperature. After injection, the mold opens, and the core pin and the still warm preform are rotated 120°. A blow mold then closes over the preform, and air is injected through the core pin. After the container is blown, it is rapidly cooled by contact with the walls of the blow mold, which are kept at around 102-122°C by cold air or fluid circulating through the mold passageway. The mold then opens, a second 120° rotation occurs, and the part is stripped from the core pin. Then a third 120° rotation of the transfer head returns the core pin to the preform injection mold, and the cycle is repeated. 

This is a two-stage process in which the material is injection molded around a core rod to form a preform. Then the preform is stretched through the action of a stretch rod. Finally, it is inflated and cooled, which results in a lighter biaxially orientation product. This process is used to produce scrap-free, close-tolerance, completely finished products that require no secondary operations such as trimming, which is used in extrusion blow molding. 

Analysis of extruded and injection molded iPP shows that a skin-core structure is formed due to high shear and thermal gradients upon cooling. These frozen-in tensile stresses in the skin can act as the stress component in the environmental stress cracking process [4]. In injection moldings, molecular weight has a secondary effect. This effect is that residual orientation in the direction of flow increases with higher... 

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