Extrusion International 1-2025-USA

54 Extrusion International 1/2025 BLOWMOLDING – FROM THE RESEARCH GWDS the Revolutionary Economic, Innovative and Safe Solution to Further Improve the Quality of Blow Molded Parts The wall thickness of blow molded articles is always the smallest at those locations where the stretching ratio is the greatest when blowing the parison into the nal geometry of the article. This is at least true for small articles which have to be produced using a die that is smaller than 50 mm in diameter. As an example for small bottles the wall thickness is always the smallest at the bottom region perpendicular to the welding line. As at least a certain wall thickness is demanded to meet the required useful properties the ow channal gap of the die has to be increased to reach the aspired wall thickness. As a consequence the wall thickness at both ends of the welding line where no stretching occurrs gets much bigger than it is intended and that it would be necessary. That affords more raw material, and it increases the energy consumption. Additionally it extends the cycle times, as the wall sections which are too big need to be cooled for a longer time. But this also limits the capacity of the blow molding machines and thus also the productivity of the company. For a long time it was not at all possible to alter the wall thickness over the circumference of the parison at a locally limited area for small blow molded articles while the parison is discharged. For greater die diameters the well established PWDS- solution can be used to alter the wall thickness over the circumference while the parison is extracted. But the PWDS-system is rather sophisticated and therefore ex- tremely expensive. Furthermore the PWDS-system has to be maintained regularly as it possesses different de- vices, which might fail and cause machine break downs. In both cases this can not be avoided that in those times the machine is not available to produce parts. That is why for many articles it is even questionable whether there exists a reasonable relation between the neces- sary investment and the bene t that is reached. So for decades there was a need for a solution, which can be used for all blow molded articles independently wheth- er they are small or big and independently what geome- try they might have. And of course for a long time there was also an economic solution that costs a minimum of a PWDS-system, that needs no regular maintenance and that cannot cause non-operating steps. Comparison between rules to design conventional dies for blow molding and those for GWDS-dies The general designing rules for dies for extrusion blow molding have been established in the last century and are still state of the art nowadays. They have not been put into question and thus kept unchanged. Most dies which are actually designed and offered for extru- Picture 1: Cross section of a conventionally designed conical die Picture 2: Cross sections of cylindrical GWDS-dies with different position of the mandrel in order to alter the wall thickness of the parison over its circumference in a locally limited way Die Flow channel Mandrel Pro le not active Same velocity V=V Pro le active The velocity keeps unchanged V=V