Blow Mold Design | How Blow Molding Works

Blow Mold Design

Blow mold designs are comprised of two separate steps. These are creating the mold cavity and the mold core. The interior and exterior are created separately but in relation to each other. The mold core forms the interior shapes. The mold cavity creates the surface shape and textures. The heated plastic injected between these two layers forms the solid vessel.




Cavity – Cavity Blow Ratio = W˃D


Cavity – Core Blow Ratio =W˃2D


The molds are filled with melted plastic parison sticks. It begins to solidify as soon as it hits the mold. The blow molding occurs at this time. It stretches the plastic across the mold walls using pressurized air. Blow mold designs have to avoid 90⁰ angles to ensure structural integrity. When designing the product you must keep in mind that the product will shrink about 1.65% during the cooling process. Double walls in blow molding design allow for engineers to create structure within the plastic product.