Necks blow-molded

Fig. 21. Extrusion-molded neck blow-molding process, (a) Body section open, neck section c ose, nec sec ion re rac ed, (b) neck section extended to mate with parison nozzle (plastic fills neck section) (c) neck section retracted with parison tube attached (d) body section...

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. 

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. 

A very wide variety of container shapes and sizes can be produced by extrusion blow molding. This process is particularly useful if a bottle with a handle or with an offset neck is desired, since these cannot be effectively produced by injection blow molding. While it can be used to produce very large containers, it is not often used for small bottles less than 200 ml (7 fl oz) in volume. Injection blow molding is usually more economical for these small sizes. 

One development for coinjection equipment was the inclusion of separate hot-runner temperature control systems for the different resins, for example, in order to allow an EVOH core layer to be processed at a temperature 70 C lower than the PET skin layers. Another development was the use of computer simulation to design tooling that manipulates the barrier resin distribution to fortify critical areas such as wall sections. Both wide mouth and narrow neck coinjection stretch blow molded bottles are available. 

Figure 2.34. Injection blow molding is relatively slow and is more restricted in choice of molding materials as compared to extrusion blow molding. The injection process, however, affords good control of neck and wall thicknesses of the molded object. With this process it is also easier to produce unsymmetrical molding.

As blow molds do not have to withstand high pressure, a wide selection of construction materials is available. The ultimate selection will depend on a balance of the following factors cost, thermal conductivity, and required service life. The more commonly employed materials for small parts are aluminum and aluminum alloys, steel, beryllium copper (Be/Cu), and cast zinc alloys (Kirksite, etc.). Aluminum molds are excellent heat conductors, are easy to machine, can be cast, and are reasonably durable, particularly when fitted with harder pinch blades and neck inserts (Table 4-9). 

Figure 1.4 Three Station Injection Blow Molding Machine [Miller, 1983]. The parison is injection molded on a core pin (instead of as a tube in free air, as with extrusion blow molding) at the preform mold station (1). The parison and neck finish of the container are formed there. The parison is then transferred on the core pin to the blow mold station (2) where air is introduced through the core pin to blow the parison into the shape of the blow mold. The blow container is then transferred to the stripper station (3) for removal.

False neck n. In blow molding of containers, a neck construction that is additional to the neck finish of the container and that is only intended to facilitate the blowmolding operation. Afterwards the false neck is removed from the container. 

Finish insert (neck insert). In blow molding bottles, a removable part of the mold that aids in forming a specific neck finish of the bottle. 

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