During the blow molding process, a parison is clamped into a mold cavity, and compressed air is then forced into the tube with the help of a blow pin. The plastic expands, or “balloons” due to the air pressure and fills the shape of the cavity. The cavity is then cooled, usually through the process of conduction using liquid, and the molten plastic hardens and sets in the shape of the mold. The flow of air pressure into the parison needs to be carefully controlled to ensure consistent expansion of the plastic and uniform thickness of the product walls.
Varying wall thicknesses could compromise the strength and durability of the final product. Blowmolders use blow molded plastics to manufacture many different products including toys, blow molded cases and drums, plastic beverage bottles and containers, sports equipment and a broad range of products used in the consumer industry.
The purpose of plastic blow molding is to form a uniform finished product that is smooth and airtight and does not need to be joined together in any way. Blow molders can be continuous process machines, such as rotary wheel systems, producing a high-volume of products in a high-paced manner, or the machines can be intermittent, producing a certain number of products at a time rather than a continuous stream with single or multiple cavity capabilities. The processes of plastic blow molding can be extrusion, injection or stretch blow molding depending on elements of the pre-form process.
Injection blow molders are used in the manufacturing of smaller, precision-focused items while extrusion blow molders produce large quantities of simple, wide-necked products and handleware. Stretch blow molding is most commonly used in the production of plastic bottles, especially those for use in the beverage industry. Blow molders have the capacity to produce a broad range of product sizes from single-serve bottles to larger containers up to 30 liters in volume, with possible diameters of 24 inches.
Apart from high-volume production, minimal material wastage and the versatility of the process, other advantages to blow molders are the low costs of mold versus product output return and the durability of the products produced.