Extrusion Blow Molding

About the extrusion blow molding technology

How does it work?

In extrusion blow molding, molten plastic is extruded into a hollow tube (a preform) called a parison. The process begins by loading polymer pellets into the extruder where they are heated to melting temperature and mixed by the screw to form a uniform viscous mass. This melt is extruded through an annular die, forming a parison — a tubular preform with a wall thickness corresponding to the final product.

The preform is then enclosed in a cooled metal mold. It is important that the parison has the right temperature — hot enough to deform but not so hot that it sags under its own weight. The mold consists of two halves that close around the parison, pinching the top and bottom to create a sealed cavity.

Next comes the actual blowing. Compressed air (typically 5–10 bar) is introduced through a special nozzle into the preform, inflating it to the mold size. Under air pressure, the plastic stretches and presses against the mold walls, precisely reproducing every detail of the mold surface — reliefs, threads, inscriptions, etc.

Upon contact of the inflated preform with the cold mold, the plastic cools and solidifies quickly. The mold is made of a metal with high thermal conductivity (aluminum or steel) and often has a cooling system to speed up the process. Once the product has sufficient rigidity, the mold opens and the part is ejected. Excess parison material (pinched top and bottom) is removed in subsequent operations.

Production process specifics

Setting up the production cycle is lengthy — more than 1 day. This is due to the need for precise tuning of many parameters: melt temperature, extrusion speed, parison thickness, cooling time, blow pressure, and others. Each product and material requires individual adjustment. The technologist must achieve an optimal balance so that the product has uniform wall thickness, no defects (voids, warping, unevenness), and meets all customer requirements.

However, once tuned, the production time of a single item is about 1 minute, providing high productivity. Therefore, extrusion blow molding is used for medium- and large-scale production. The technology is not suitable for one-off items due to the lengthy setup, but it is ideal for batches ranging from thousands to millions of products. Automation allows operation with minimal operator involvement after setup.