Extrusion International 4-2025

38 Extrusion International 4/2025 Comparison of the processing possibilities of the two solutions The PWDS system is limited in its use. It can only be used in a middle die diameter range. The smaller the di- ameter of the die gets the stiffer it becomes. That is why there actually exists a border diameter of 50 mm under which a die is too stiff that it can be deformed in a nec- essary way. This is the reason why even today all blow molded articles have more or less wall thickness differ- ences in their bottom region which are not intended. This applies to the great amount of articles which are used in packaging applications and which are produced using die diameters which are smaller than 50 mm. Dies having a diameter that is bigger than 800 mm get that expensive that it does not make sense to use a PWDS system due to economic reasons. In contrary to the PWDS system there exists no die diameter limit to use a GWDS solution. That is why the wall thickness distribution of all small blow molded ar- ticles which for instance are used in packaging can be further improved in their wall thickness distributions. The wall of small bottle is actually thick at the ends of the welding line and thinner perpendicular to the weld- ing line. No PWDS system can be used to reduce such wall thickness differences in the bottom region of the articles. Many experts still postulate it is caused by the blow molding process and it is unenviable! When using a GWDS die and a profiled GWDS core the wall thickness distribution of every article can be improved that is produced by extrusion blow mold- ing. Consequently also the wall thickness distribution of small bottles can be easily optimized so that the wall thickness in the bottom region gets aligned. In order to achieve that only the end of the core has to be designed oval and has to transition further up continuously into an ideal round geometry for the body of the bottle. This reduces the wall thickness where no stretching occurs. However the wall thickness in both regions perpendicu- lar to the weld line is increased. The wall of these re- gions is more and more stretched until the cooled wall of the mold is reached. An even wall thickness can also be reached in the bottom region of a blow molded arti- cle when using a GWDS die in case the relation between the main and the secondary axis is dimensioned in the right way. In the amount the influence of the weld line diminishes the core is more and more pulled up during the extraction of the parison. This then results also in a good thickness distribution in the transition region of blow molded articles. But restrictions exist even regarding the possibility to optimize the wall thickness for articles which are pro- duced using a die which has a diameter that is suited for a PWDS system. This especially because in many cases the die cannot be deformed in a way which is optimal suited for the article that has to be produced. The actuators of the PWDS system must be positioned in an angle of 180 or of 90 degrees. This does not match with the geometry of most technical articles which have to be blow molded as only few articles have a symmetric geometry. Further- more also the possibility to match the flow channel ge - ometry with the requirements of the individual article is also limited. The reason is that the wall of the die can only be deformed in a steady way. Sharp differences of the flow channel gap which are necessary for many technical products can therefore not be realized. As an example a 6 liter fuel tank is shown in Picture 1. The customer required a wall thickness of 1 mm at the end of the thread which is marked by the red line. In order to reach that the flow channel gap has to be in - creased dramatically at an extremely limited area. Picture 2 shows on the bottom the wall thickness distribution, the weight (G) and the cycle time (t) of the tank that has been produced with a conventional conical die system. On top the result is shown that has been achieved with a predominantly cylindrical GWDS die solution. Picture 3 shows the cylindrical GWDS die and the profiled pin which has been heavily profiled at a very small region. It is absolutely impossible to realize such a heavy locally limited change of the flow channel gap when using an expensive PWDS system. As the tank was produced us- ing an accumulator head the GWDS core has still a short conical end region. This is to close the flow channel gap while the accumulator of the head is filled with melt. As already mentioned the wall of a PWDS system can only be deformed steadily. This is the reason why it is absolutely impossible to profile the flow channel gap of a PWDS system in a way that the wall thickness distribu- tion of the tank can be significantly improved. Comparison of the economics of the both solutions By far the biggest difference between the two solu- tions exists in regard of the cost of purchase, the cost of production and the operational cost. Picture 4 dem- onstrates that impressively. On the left side is shown a complex 3DX system of S. B. Enterprise Srl. It is one of the many PWDS copies which in the meantime are of- fered on the market. The system uses maximum four servo drives to push or to pull the deformable wall of the die in order to alter the die gap at those locations. The very reverse is the GWDS solution which is shown on the left side and which needs only a solid die and a BLOWMOLDING – FROM THE RESEARCH Picture 3: GWDS die which is cylindrical at its end and a profiled GWDS core that is severely profiled in an a very limited area