Which of the following is NOT a kind of open molding?

Vacuum infusion is not considered a type of open molding because it employs a different process focused on creating a vacuum that draws resin into a closed mold, rather than allowing for open exposure to air during the molding process. In contrast, methods like hand lay-up, spray-up, and chopper gun involve applying the composite material directly onto an open mold without the use of a vacuum chamber.

Open molding techniques generally use a direct application method, allowing for manual layering of materials and resin without any enclosed environment. This results in less control over resin distribution compared to closed molding processes like vacuum infusion, which rely on atmospheric pressure to ensure complete wet-out of the fibers. Thus, vacuum infusion stands out as a distinct method that requires a closed system, differentiating it from the other open molding techniques listed.