The extrusion blow molding process is based on glass blowing to produce glass bottles, but uses PET, or biaxial oriented polyethylene terephthalate, as well as high density polyethylene, both of which are considered the best thermoplastics for extrusion blow molding. The plastic bottles are lightweight, usually clear or tinted, and strong enough to withstand the pressure of carbonation in certain beverages.
Extrusion blow molding bottles are made to hold beverages like juice and pop, liquid soap, detergent and motor oil for consumer and industrial applications within the food and beverage, automotive, cleaning products, medical, cosmetics and drinking water industries. They are extruded into two mirror molds, and therefore have a visible seam dividing the bottle in half. Compared to other blow molding process, extrusion blow molding generates little waste, all of which is recyclable.
Extrusion blow molding begins with the formation of a parisan, which is a plastic tubular preform. Plastic pellets are loaded into a hopper and fed down into an extruder screw with a heating unit. As the pellets pass through the screw, they become molten and are pumped toward the die head. The liquid plastic flows horizontally and then downward, and emerges as a soft tube preform. The preform, which is called a parisan, is then extruded by compressed air as 2 molds simultaneously come together to form a cavity in the shape of the container.
The plastic balloons out and is stretched thin into the mold cavity, taking its shape. The mold cavity is then chilled by cold water running through the molds. The newly formed part will always have a seam where the two mirror molds came together, and a small point at the bottom where the plastic balloon started. Some parts may have excess plastic that needs trimming from the neck or parting lines.
The newly formed plastic parts are then put through post forming operations, including drilling, die cutting, milling, painting and adding decals or labels. Each container or part only takes a few seconds to form, and may be done in high volumes. They are formed in a continuous or intermittent method.