Blow molding is a process commonly used to create plastic objects. In general, the process consists of generating a tubular form, referred to as a parison. The parison is placed into a mold and is injected with a gas. This causes the parison to expand to the shape of the mold, while leaving a hollow interior. The process is frequently used in the production of plastic containers and bottles. There are three types of blow molding: extrusion, injection and stretch.
Extrusion Blow Molding
Extrusion blow molding is the most basic and simplest form of blow molding. In this process, molten plastic is extruded directly over a mold in a tubular shape (parison). In blow molding, the mold is typically water-cooled. The mold is then closed around the parison. Gas is injected through the top of the parison. The gas fills the parison in much the same way that a balloon is filled by blowing into it. When the molten plastic touches the cooled walls of the mold, it solidifies the plastic into its final form. The plastic container is then ejected from the mold, and any excess is trimmed. This process allows for a wide range of shapes and sizes, as well as neck sizes.
Injection Blow Molding
The injection blow molding process occurs in two stages. In the first stage, thermoplastic is melted in a barrel, generally through a combination of barrel heating and sheer force applied by a screw. The molten plastic is then injected into a preform mold that contains a mandrel or blow stem (a hollow component inside the die-head). In the second stage, the mandrel shapes the parison. The preform mold is typically heated to maintain the molten state of the thermoplastic material. The parison is then extruded over a mold, which closes around it. Gas is injected, and the parison expands to the shape of the mold. The plastic solidifies on contact with the cooled mold walls. The piece is ejected from the machine and is trimmed. Injection blow molding is preferable for smaller containers.
Stretch Blow Molding
Stretch blow molding can be achieved in two ways. The first, referred to as injection stretch blow molding, begins with creating a preform through injection. The preform is then heated to a specified temperature and placed in the mold. In the mold, the preform will be mechanically stretched and then inflated with gas.
The second method, called reheat and blow molding, involves the use of premade preforms. The preforms are generated off site and purchased by a factory, for example. The preforms are then placed into a machine to reheat the plastic and to inflate the preforms in the molds.