Pet Bottle Preform blow molding is a process for making hollow plastic parts. There are three unique processes for creating blow molded parts, including:
Stretch blow molding
This blow molding process involves the production of hollow objects, such as bottles, having a biaxial molecular orientation. Biaxial orientation provides enhanced physical properties, clarity and gas barrier properties, all of which are important in products such as carbonated beverage bottles.
There are two different stretch blow molding techniques. In the one-step process, the preforms are injection molded, adjusted to the appropriate temperature, and blown into the container – all in a continuous process. This technique is most effective in special applications, such as jars, where high productivity is not required.
In a two-stage process, the preform is injection molded, stored for a short period of time (usually 1 to 4 days), and blown into a container using a reheat blow (RHB) machine. Due to the relatively high cost of molding and RHB equipment, this is the best technology for producing large capacity products such as carbonated beverage bottles.
Injection blow molding
The material is injection molded during the injection blow molding process. The hot material still on the core pin is then indexed to the blow station where it is blown into the bottle and allowed to cool. The bottle is then transferred to the next station and ejected. Injection blow molding has more precise details than extrusion blow molding in the neck and mouth (thread) areas. Since the orientation is seldom occurring, there is almost no improvement in physical properties during the injection blow molding process.
Extrusion blow molding
The extrusion blow molding process begins with a conservative extrusion of the parison or tube using a mold similar to that used to make plastic tubes. The parison is then extruded downward between the two halves of the open blow mold. When the parison reaches the proper length, the mold closes, grasping and keeping the end of the neck open, and pinching the bottom end to close. A rod-shaped needle is inserted into the neck end of the hot parison to simultaneously form a threaded opening and expand the parison in the cavity. After the bottle has cooled, the mold opens to eject the bottle. Excess plastic is trimmed from the neck and bottom pinch areas.
Blow molding and injection molding
Injection molding and blow molding are the two most popular methods of producing high quality, low cost plastic parts and products, but they differ. The most obvious difference between injection molded parts and blow molded parts is the injection molding to make solid parts, while blow molding to make hollow parts.
If you need something that requires a hard wall, injection molding may be what you want. Some common examples of parts made by injection molding are caps, combs, and computer and television housings. Conversely, if you are looking for a flexible, operable, liquid-filled product, such as a bottle or cooler, blow molding is what you need.
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