blow molding
Blow molding makes hollow plastic parts by inflating soft, hot plastic against the inside of a mold, exactly the way you blow a bubble of gum inside your mouth, except the mold gives the bubble a useful shape. It is how nearly every plastic bottle, jug, and hollow container is made. You start with a hot tube or a preform of plastic, trap it in a mold, and blow compressed air into it so it balloons outward and takes the mold's shape, then cools and is released.
There are two main flavours. In extrusion blow molding, a molten tube called a parison is extruded downward, the mold closes around it, and air inflates it into shape, used for milk jugs and shampoo bottles. In injection stretch blow molding, a test-tube-shaped preform is first injection molded, then reheated and simultaneously stretched with a rod and blown with air, which stretches the material in two directions at once. That biaxial stretching is the secret behind clear, strong, lightweight PET soda bottles.
The biaxial stretch is a lovely process-structure lesson: pulling the polymer molecules in two directions aligns and partly orients them, which raises strength and stiffness and improves clarity and gas-barrier properties, letting a thin bottle hold pressurized soda without ballooning. The honest limits are that blow molding makes hollow, thin-walled parts only, and wall thickness is hard to control precisely, so it tends to end up thin at the corners where the plastic stretched the most. It is cheap and fast for containers but not for solid or high-tolerance parts.
A PET soda bottle is injection stretch blow molded: an injection-molded preform is reheated, stretched lengthwise by a rod and blown outward by air at once, so the biaxial orientation strengthens the thin wall enough to hold carbonation pressure.
Biaxial stretch-blow orients the molecules two ways at once, turning a thin PET wall into a strong pressure vessel.
Blow molding makes only hollow, thin-walled parts, and wall thickness is hard to control, tending to thin out at corners where the plastic stretched most.