Extrusion International 4-2025

37 Extrusion International 4/2025 T he past has shown that even engineers have prob- lems to use innovative solutions. This especially in case that only fewother companies have already proved that the solution is really of great value. A good exam- ple for that is the innovative GWDS (Gross Wanddick- ensteuerung) technology which is patented since the year 2011. It is an advantageous solution to optimize the wall thickness distribution over the circumference of the parison during the extrusion blow molding pro- cess. The new technology stands in competition to the PWDS (Partielle Wanddickensteuerung) which has been developed and has also been patented about 50 years ago. This technology still keeps a world-wide monopoly position. As the patent protection has lapsed long ago several companies have already copied the solution and offer it in the market. What is the difference between the two technical solutions? The PWDS is a very complex solution where the die is dynamically deformed while the parison is discharged. In many cases also the core is deformed statically. In the early days the wall of the die was shifted by hydrau- lic pistons. In the meantime often servo drives are used to shift the wall. Whereas the wall of the core is posi- tioned by adjusting screws which are placed around the circumference of the core. It is necessary to implement special software to the machine controller additional to the software which is part of the basic machine. This is necessary in order to shift the die dynamically. Special steel alloys are necessary that allow as well for the dy- namic as also for the static deformation of the wall of the die and the core. Important components have to be maintained in regular time intervals. Furthermore the danger exists that a part of the complex PWDS system fails in spite of a regular maintenance. This inevitably causes a machine stop for a certain time. In contrary the GWDS solution needs only a solid die and a solid core. Consequently it is no longer necessary to deform a steel with high forces. Naturally it is also not necessary to implement additional software to the com- puter of the machine. What is of great advantage for the companies which produce blow molded articles is the fact that a solid die and a solid core need not to be maintained and that they can nor fail during the production and cause a machine break down. How do the two different solutions work? The PWDS system alters dynamically the size of the flow channel gap between the die and the core at defined positions locally limited while the parison is extracted. In order to achieve that two or maximum four actuators are mounted to the die in an angle of 180 or of 90 degrees. The wall of the die can con- sequently be pushed or pulled at four defined po - sitions. The size of the flow channel is continuously modified right and left of the point where the ac - tuators are positioned. The wall follows its natural bending behavior. Additionally the wall of the core can be pushed to the outside with the help of the adjusting screws. By adjusting the wall of the core the wall thick- ness of the parison is influenced over its entire length. The GWDS solution is based on a know how and an ex- perience which has been gathered when running experi- ments toproducepipes. During special experiments itwas found out that it is possible to push the core out of the die without that that has critical influences on the quality of the pipe that is produced. But in the first moment this knowledge does not helpwhen extracting a parison. This because it is state of the art in extrusion blowmolding to design as well the end of the die as also that of the core conical. Most experts still postulate: “That’s how it was always done and that has proven to be successful!” The core of a conical die cannot be pushed out of the die as the core would hit the die or the flow channel gap of the die would get much too big. But what is said against using a cylindrical die apart from the fact that dies for extrusion blow molding al- ways have been conical. The core of the die can be pushed out of the die in case the die is designed cylin- drical and in case the core is smaller in diameter than the die. Then the area of the core which is positioned out- side of the die can be profiled in any necessary way ac - cording to the needs of the article which has to be pro- duced. As long as the profiled area is outside of the die it is not part of the flow channel. So the wall thickness of the parison is not at all affected by the profiled core area. The parison simply glides over the profil end of the core without being modified in its wall thickness as it is formed by the end gap of the flow channel. In case it is necessary to alter the wall thickness of the parison the pin is simply pulled up and the profile becomes part of the flow channel. As a consequence the wall thickness of the parison alters according to the gap of the flow channel which alters over the circumference. Picture 2: Comparison of the results which have been obtained using a conventional conical die (on the bottom) and those that have been reached using a cylindrical GWDS die (on the top) G=710 g tb=50 s G=869 g tb=65s