18.4 BLOW MOLDING
Blow molding is a modified extrusion- and injection-molding PTOCCS3. In extrusion blow molding, a tube (usually turned so that it is vertical) is first extruded, then damped into a mold with a cavity much larger than the tube diameter, and finally blown outward to fill the mold cavity (Fig 18.9a) Blowing is usually done with an air blast at a pressure of from 350 kPa to 700 kPa (50 psi - 100 psi). In some operations the extrusion is continuous and the molds move with the tubing.
The molds close around the tubing, close off both ends (thereby breaking the tube into sections), and then move away as air IS injected into the tubular piece. The pan is then cooled and ejected. Corrugated pipe and tubing are made by continuous blow molding in which the pipe or tubing is extruded horizontally and blown into moving molds.
In injection blow molding, a short tubular piece (parison) is first injection-molded (Fig. 18.9b). The dies then open and the parison is transferred to a blow-molding die. Hoc air IS injected into the parison, which expands to the walls of the mold cavity. Typical prod¬ucts made are plastic beverage bottles and hollow containers.
Multilayer blow molding involves the use of coextraded tubes or pansons and so permits the production of multilayer Structures. A typical example of multilayer structures is plastic packaging for food and beverage having such characteristics as odor and perme¬ation barrier, taste and aroma protection, scuff resistance, the capability of being printed, and the ability to be filled with hot fluids. Other applications are in the cosmetics and the phar¬maceutical industries.