preform

A preform is a semi-finished, shaped piece that serves as the starting point for producing a final object. The term is used most often in plastics and glass manufacturing, though it can refer to staged forms in other processes. Preforms are designed to be heated and re-formed into the finished product.

In plastics, the most common example is the PET preform used for beverage bottles. These preforms are produced by injection molding with a hollow body and a finished neck, which defines the bottle opening. The preform is later heated and blown in a bottle-blowing machine to form the final bottle in a process called stretch-blow molding. This approach enables high-volume production and efficient transport, because preforms are compact to ship and can be stored until needed. Materials besides PET—such as other polyesters or polyolefins—can also be used for preforms, depending on the intended application and barrier requirements. The neck finish, wall thickness, and finish details are precisely machined in the preform to ensure proper sealing and strength in the final container.

In glass manufacturing, the analogous starting piece is often called a parison or gob rather than a preform. A molten glass mass is shaped into a forming tool and then blown or pressed to produce bottles or other hollow shapes. The concept is similar, but terminology and equipment differ from plastics.

In other industries, preforms may refer to partial shapes used in metal forming, composites, or powder metallurgy. These preforms provide a near-net-shape starting point that reduces processing time and waste in subsequent forming or sintering steps.