There are three main processes of plastic blow molding: injection, extrusion and stretch, and these differ mostly in the pre-form stage. In plastic blow molding, plastic resin is softened and formed into a parison, or a hollow cylinder of molten plastic, 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 mold cavity.
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. Controlled and constant air flow is essential to the quality of the final outcome of the blow molded plastic, as varying wall thicknesses could 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.
Due to the durability of plastic, blow molded plastics are widely used – once a thermoplastic resin has been softened and set in a shape, it is strong and resistant to damage from the elements, usually having a fairly long lifespan. There is not a joining line on the final plastic product due to the nature of the plastic blow molding process and this improves the strength of the wall. The wide varieties of shapes and sizes that can be manufactured, combined with the broad choices of plastic materials to use allow blow molding manufacturers to create products according to their custom blow molding needs.
It is also possible to inject colors into the plastic before setting it to further customize the product. Blow molded plastics are an affordable option for many manufacturers due to the cost-effective and efficient nature of the blow-molding process. The fast turnaround times – as little as a few seconds per part – combined with minimal material wastage and high-volume output are further advantages to using blow molded plastics.