Extrusion International 1-2025

47 Extrusion International 1/2025 sion blow molding have still a conical ow channel. As well the die as also the mandrel are conical at their ends (Picture 1). That helps to alter the wall thickness of the parison homogeneously over the complete circumference. Con- ventional conical dies are also well suited to compen- sate the elongation of the parison due to its increasing weight. But conventional conical dies are of extreme disadvantage in case you want to alter the wall thick- ness differently over the circumference of the parison. That is why it was necessary to break fundmentally with the traditional well established design strategies for dies to realize the patented (US 9,676,134 B2) GWDS- technology (Picture 2). It is the only available solution which can alter the wall thickness of the parison differ- ently over its circumference while it is extracted that can be used universally. This because the GWDS-technology is not subject to limitations neither regarding the die di- ameters nor regarding of the individual geometry of the part. To enable to alter the wall thickness of the parison differently over its circumference the mandrel should be designed to be not conical but to be predominantly cylindrical at its end. Additionally the die should either be cylindrical at its end or converging with the smallest diameter at its end. Furthermore the diameter of the mandrel should be smaller than that of the die end. Advantage of GWDS designed dies In this case the mandrel can be pushed out of the die without touching it. When this is done the end of the mandrel is no longer part of the ow channel so that the parison simply glides over the end region of the mandrel which is protruded from the die. Now the man- drel region which produces out of the die can be pro- led in any shape that is necessary for a special region of the parison. As long as the protruded pro led mandrel end is outside of the die and thus not part of the ow channel the thickness of the parison is determined by the ow channel gap which exists at the end of the die. The wall thickness of the parison is not at all affected by the pro led end of the mandrel which is situated out - side of the die. It simply glides over it without that the wall thickness which is built by the gap at the die end is not in uenced or modi ed by the pro led end region of the mandrel. This because it is no longer part of the ow channel. But when the part of the parison is dis - charged that needs the variation in the thickness over the circumference of the parison the mandrel is simply pulled up so that the pro led end region becomes part of the ow channel. Then the thickness of the parison varies over the circumference. This according to the changing ow channel gap which then exists at the die end (see Picture 2). The mandrel for instance is designed to be oval at its end for a simple round bottle. This is necessary to equal- ize the wall thicknesses of the bottle at both ends of the weld line and perpendicular to the weld line. But that oval shape is not at all suited for the upper part of the bottle. Therefore the oval end geometry of the mandrel is continuously converted into a round geometry over a short length. Now when the part of the parison is dis- charged which is responsible for the weld line region of the blown article the oval end of the mandrel matches with the end of the die. The more the in uence of the weld line vanishes the more the mandrel is pushed out of the die until the round geometry matches with the end of the die. This round mandrel geometry is then responsible for the parison thickness which is necessary for the rest of the bottle (Picture 3) Thus the GWDS-technology can be easily retro tted to every existing blow molding head. It neither affords a deformable die nor special actuators. It can be used with every existing blow molding machine without that the machine has to be modi ed in any way. Only the die of the head has to be exchanged. Although it needs only a solid die and a solid mandrel it enables to alter the wall thickness differently over the circumference of the parison while it is discharged. Without exception ev- ery blow molded article that is nowadays produced can be further improved in its quality in a very effective way using the GWDS-technology. The ow channel gap at the exit of the die can be easily altered by time match- ing the varying draw rations which exist for different locations of the part to be produced. A GWDS-die and mandrel do not need to be maintained in regular time intervals and they can not be responsible for potential machine break downs. This because a solid die and a sol- id mandrel cannot fail during operation. In case a head is immediately equipped with a GWDS-die the fabrica- tion cost of the head is comparable with that of a head which is equipped with a conventional conical die. The economic value of the GWDS-technology The GWDS-technology is a solution which cuts the cost to establish a dynamic radial wall thickness program- ming nearly to zero in comparison with the state of the art expensive PWDS-system. It helps to further improve the quality of blowmolded articles. But in the same time it saves raw material, reduces the energy consumption, cuts manpower requirements in the production, in- creases the capacity of the machines and thus increases MEASUREMENT TECHNOLOGY Situation for the welding line region Situation for the body of the bottle Picture 3: Cross sectional drawings of a GWDS-die designed for the production of round bottles