Plastic blow molding is widely used because it is well-suited for the manufacturing of durable hollow parts and because it can be used to form standard and common plastic shapes, but also can be custom designed to create plastic products that are specific to a customer’s unique needs.
While injection molding and rotational molding are more precise methods of plastic molding, plastic blow molding offers the advantage of high-volume capacity, fast turnaround times and relatively low costs. The process usually takes a matter of seconds and therefore blow molding machines can produce high-volumes of durable plastic products in short periods of time with minimal finishing requirements.
In the basic process of plastic blow molding, plastic is melted and softened and formed into a parison which is then clamped into a mold. A blow pin is then used to force compressed air into the parison, and the molten plastic expands to fit the shape of the cavity. Controlled and constant air flow is essential to the quality of the final outcome as varying wall thicknesses compromise the strength and durability of the product.
Blow molders have to ensure the constant expansion of the plastic in the mold to achieve uniform wall thickness in the product. The cavity is then cooled, usually by conduction, and the hardened plastic product is removed from the mold, sometimes with the assistance of ejector pins. There are three main processes of plastic blow molding: injection, extrusion and stretch. Injection blow molding is a combination of standard injection molding methods and plastic blow molding and is typically used to create smaller products that require greater precision.
Extrusion molding allows for a wide variety of shapes and sizes, with the added benefit of handleware capabilities, and yet results in a thin seam where the two mirror parts of the cavity joined. Stretch molding is commonly used to produce vast quantities of plastic bottles.