Blow-molding (Wikipedia) is an thermoplastic forming process in which a hot polymer pre-form is injected with gas to press it against the inside of a hollow mold. It’s how most plastic bottles are made. Designer George Fereday got annoyed that the primarily-industrial process was so inaccessible and decided to build his own DIY version, which extrudes pre-melted polymorph plastic through a custom die attached to a normal caulking gun to create the hollow pre-form. There’s more detail over at Core77.
Homemade blow-molding gun