What is a blow molding machine

The Blow molding, also Extrusion blowing called, is a method for the production of hollow bodies from thermoplastics.

Manufacturing process

First, continuously or discontinuously, a tube made of hot, malleable plastic is ejected (extruded) vertically downwards from a flanged tool as a preform. The material thickness in the hose is regulated according to the shape of the finished workpiece.

A mandrel then moves into the hose from above or below. The two-part tool, which is still open at this point, then closes and envelops the hose together with the mandrel. Compressed air is then pressed through the mandrel into the hose, with which it is inflated and pressed against the contour of the blow molding tool and cooled. The plastic adapts to the given shape and becomes solid.

At the neck area, on the floor and at the seam between the two tool halves, plastic slugs are created, some of which are sheared off when opening, and some of them require reworking. This waste is reused (recycling).

The thickness of the plastic in the finished product can be controlled by varying the material thickness in the hose. In the case of symmetrical structures such as barrels, this can be calculated in advance; in the case of more complex forms, the material thickness is varied in a so-called zero series until the result is correct.

A characteristic of blow molding compared to injection molding is that hollow parts are produced and undercuts can also be produced to a limited extent without a slide (see mold construction). The wall thickness can only be varied indirectly compared to injection molding, and the tolerances that can be achieved are therefore greater.

The German company Kautex Maschinenbau GmbH from Bonn is the world market leader in the manufacture of production systems for multilayer plastic fuel tanks. More than 190 blow molding machines for the production of fuel tanks for vehicles have been built and delivered in the last 30 years.

BEKUM Maschinenfabriken GmbH from Berlin is the market leader in the manufacture of blow molding machines with around 600 employees in 5 production locations in Germany and abroad.

Types of plastic used

Polyolefins (especially polyethylene (PE) and polypropylene (PP)) are the most widely processed plastics. But Plexiglas (PMMA), polycarbonate (PC), polyamide (PA) and some mixtures are also processed. For technical articles also thermoplastic polyurethane or thermoplastic elastomers based on urethane (TPU).


The products created in this way are preferably encountered in everyday life as packaging items such as bottles, canisters or barrels. The volume varies between 0.05 and 10,000 liters. But also technical parts such as ventilation ducts, bellows (e.g. car steering and axle boots), case half-shells, roof racks or fuel tanks can be blown. Larger children's toys (e.g. slide vehicles) are also often made using this technology.

Injection blow molding is usually used for pressure-loaded containers (e.g. carbonated beverages).


  • Michael Thielen, Klaus Hartwig, Peter Gust: Blow molding of plastic hollow bodies. Processes, machines, tools. Hanser, Munich 2006, ISBN 3-446-22671-0

Category: Plastics Processing