All rights reserved. 07/31/1999. Reprinting and disclosure of this publication only permitted with prior written approval by the editor.

After it had become clear that polyethylene is an ideal thermoplastic for the production of large-volume containers to be used for long-distance transports, the development for the required machines gained new momentum.

From the processes used, only the rotational process and the blow molding process deserve closer attention, since these two processes account for nearly 98 % of the worldwide production of plastic drums.

For the rotational process, PE powder is used as the raw material. By continuously heating an oscillating and rotating mold, the material is plasticized and crosslinked while the centrifugal forces press material to the mold wall. After freezing during a subsequent cooling phase the product can be taken out of the mold.

The drums with a weight of approx. 14 to 18 kg produced in this way are only used for special applications. As a consequence of the long production cycles they are not cost-efficient for large-scale use.

The blowing process (fig. 4) is today considered standard technology for the mass production of plastic drums. Generally high density polyethylene with a low melt flow index (MFI) is used.

These raw materials are melted, shorn, and homogenized in long extruders and then are pushed out of an accumulator head at high speeds to form a parison of approx. 13 kg.

This parison has a diameter of about 250 mm and a wall thickness of about 18 mm and is then introduced in a mold, mounted in the closing unit, as it is called. This unit guides and closes the mold at the correct time, and the enclosed parison is then inflated inside the mold at high pressures. Cooling systems inside the mold serve to cool down the plastic parison from its temperature of about 190 °C (374 °F). While freezing, the parison takes over the contour of the mold, and can be taken out of the mold as a drum as soon as the closing unit has opened the mold. Only two flashes at the top and the bottom of the drum have to be removed and the final drum is available free of any scrap.

Since blow molding machines for the production of large-volume containers must be equipped with suitable molds, the size of the closing unit must be adapted to the size of the mold.

Blow molds weigh as much as 3.5 tons, and the closing unit must be designed not only to accommodate the mold on the so-called mold mounting platens but also to move the mold with high precision and high closing forces. It is the machine component which will impart all the design details to the article produced.

State-of-the-art closing units have a minimum mold closing force of 600 kN and are normally operated with hydraulic controls.

A long stroke of the platens in the clamping frame which is indispensable due to the large diameter of the article, is ensured by parallel guides which despite the high weight of the blow mold halves are able to move both fast and slowly, a prerequisite imposed by the process.

Closing units may be designed with or without tie bars.

Tie-bar free designs (fig. 5 ) are units with platen carriers running on rails. After the platens have been moved together, they are held together with the closing devices and the closing force of the unit counters the forces resulting from the blowing process.

The closing unit not only has to counter the significant expansion forces acting on the two halves of the mold during the blowing process. It must also provide the closing pressures to offset the forces acting during the welding of the parison at the top and at the bottom.

Fig. 5 Schematic diagram of tie-bar free closing unit

Designs with tie bars (fig. 6) usually incorporate four tie bars that serve as a parallel guide of the platens and accommodate the closing forces of the closed mold during the inflation process. Such machines have proved to be rugged and safe. Combinations with four combined platens are known all over the world, yet the closing technology is still prone to synchronization problems as the usual hydraulic system for the control of the movements is rather difficult to master.

Fig.6 Schematic diagram of four-platen/two-tie bar closing unit

Designs with tie bars (fig. 7 ) usually incorporate four tie bars that serve as a parallel guide of the platens and accommodate the closing forces of the closed mold during the inflation process. Such machines have proved to be rugged and safe. Combinations with four combined platens are known all over the world, yet the closing technology is still prone to synchronization problems as the usual hydraulic system for the control of the movements is rather difficult to master.

Three-platen closing units with two tie bars (fig. 8) are a development in closing units since - as is readily seen - the diagonally arranged tie bars permit a lateral introduction and removal of the parison and the blown article.

This flexibility of a closing unit, which meets all requirements in terms of the dynamic processes of mold change and article removal, also permits the installation of the required high-performance equipment for either accumulator head operation or continuous extrusion without major technical effort, even if the type of extrusion is to be changed subsequently.

All rights reserved. 07/31/1999. Reprinting and disclosure of this publication only permitted with prior written approval by the editor.