Recyclable Plastic structural articles and method of manufacture

A method of manufacturing a plastic structural article, the method comprising:

• providing a bead reservoir containing expandable polymer beads maintained in a pressurized state above atmospheric pressure;
• blow-molding a plastic preform in a mold cavity using compressed gas to form the preform into an elongated member comprising an at least partially formed elongated tubular plastic shell wall defining a hollow interior cavity;
• extending a plurality of hollow pins to pierce and extend through the at least partially formed elongated tubular plastic shell wall, the plurality of hollow pins comprising a first group of the hollow pins and a second group of the hollow pins;
• introducing a pressurized gas into the hollow interior cavity through at least one of the hollow pins so as to cause the elongated tubular plastic shell to fully conform to the mold cavity;
• after said fully conforming of the elongated tubular plastic shell to the mold cavity, venting the hollow interior cavity to achieve a controlled reduction in cavity pressure to below that of the bead reservoir pressure;
• cutting at least one fill port through the elongated tubular plastic shell wall while maintaining the mold cavity pressurized above atmospheric pressure during said controlled reduction in cavity pressure;
• after the reduction to below the bead reservoir pressure, filling the hollow interior cavity with the expandable polymer beads maintained in the bead reservoir in the pressurized state by opening a bead fill valve coupling the mold cavity to the bead reservoir while further controlling the cavity pressure by regulating the flow of gas exiting the mold cavity so as to maintain the expandable polymer beads in a compressed state relative to atmospheric pressure while the cavity is being filled therewith;
• injecting a hot, at least partially vaporized heating medium into the filled interior cavity through the first group of the hollow pins while removing the heating medium from the interior cavity through the second group of the hollow pins for a period of time and then injecting the hot, at least partially vaporized heating medium into the interior cavity through the second group of the hollow pins while removing the heating medium from the interior cavity through the first group of the hollow pins for a period of time so as to partially melt and expand the expandable polymer beads to substantially fill the interior cavity of the shell, in turn causing the beads to bond to one another and to the shell wall;
• injecting air into the interior cavity through one of the two groups of the hollow pins while evacuating air and condensate formed by the heating medium from the interior cavity through the other of the two groups of the hollow pins for a period of time, and then injecting air into the interior cavity through the other of the two groups of the hollow pins while evacuating the air and condensate from the interior cavity through the one of the two groups of the hollow pins for a period of time so as to provide cooling to substantially limit further expansion of the expanded beads, in turn forming the plastic structural article;
• and
• releasing the plastic structural article from the mold cavity.