Blow molding is the process of inflating a hot, hollow, thermoplastic
preform or parison inside a closed mold so its shape conforms to that of
the mold cavity. A wide variety of hollow parts, including plastic
bottles, can be produced from many different plastics using this process.
There are three basic blow molding processes:
Stretch Blow Molding
This process involves the production of hollow objects, such as bottles, having biaxial molecular orientation. Biaxial orientation provides enhanced physical properties, clarity, and gas barrier properties, which are all important in products such as bottles for carbonated beverages.
There are two distinct stretch blow molding techniques. In the one-stage process, preforms are injection molded, conditioned to the proper temperature, and blown into containers—all in one continuous process. This technique is most effective in specialty applications, such as wide-mouthed jars, where very high production rates are not a requirement.
In the two-stage process, preforms are injection molded, stored for a short period of time (typically 1 to 4 days), and blown into containers using a reheat-blow (RHB) machine. Because of the relatively high cost of molding and RHB equipment, this is the best technique for producing high volume items such as carbonated beverage bottles.
Injection Blow Molding
In the injection blow molding process, the material is injection molded. The hot material, still on the core pin, is then indexed to the blow molding station where it is blown into a bottle and allowed to cool. For processing PET it is critical that this core pin be cooled. The bottle is then indexed to the next station and ejected. Injection blow molding allows more precise detail in the neck and finish (threaded) area than extrusion blow molding. Little, if any, improvement in physical properties is realized in the injection blow molding process since very little orientation occurs. Further, the injection blow molding process is normally limited to the production of relatively small bottles, i.e., 180 mL (6 fluid oz) or less. Eastapak polymer 9921 has been successfully used in the injection stretch blow molded process.
Extrusion Blow Molding
The extrusion blow molding process begins with the conventional extrusion of a parison or tube, using a die similar to that used for making plastic pipe.
The parison is commonly extruded downward between the two halves of an open blow mold. When the parison reaches the proper length, the mold closes, catching and holding the neck end open and pinching the bottom end closed. A rod-like blow pin is inserted into the neck end of the hot parison to simultaneously form the threaded opening and to inflate the parison inside the mold cavity. After the bottle cools, the mold opens to eject the bottle. The excess plastic is trimmed from the neck and bottom pinch-off areas.
Extrusion blow molding: