Extrusion blow molding is a manufacturing process used to produce hollow plastic parts, such as bottles, containers, and tanks, by forming molten plastic around a mold using air pressure. The process begins with a thermoplastic material—commonly polyethylene or polypropylene—that is heated and forced through an extruder. This extruder pushes the softened plastic through a die to form a hollow tube called a parison. The parison hangs vertically, and while it is still hot and pliable, it is enclosed within a split mold.
Once the mold closes around the parison, air is blown into it—typically through a needle or pin inserted into the top or bottom of the mold—causing the plastic to expand outward and conform to the shape of the mold cavity. The internal air pressure forces the material against the cooled mold surfaces, allowing it to harden and take on the final shape. After sufficient cooling, the mold opens, and the finished part is ejected. Any excess material, such as flash around the mold parting line or at the top and bottom of the part (from the ends of the parison), is trimmed away.
One of the key advantages of extrusion blow molding is its ability to produce seamless, complex hollow shapes with consistent wall thicknesses, often in a single cycle. It’s particularly well suited to high-volume production of items like milk jugs, shampoo bottles, and automotive fluid reservoirs.