Better distribution of wall thickness of plastic preforms
The geometry of blow molded containers is becoming more and more complex. This is mostly the result of purely technical demand. However, it is often the designer who decides the shape of the plastic part, and it has nothing to do with the technical factors. For plastic parts manufacturers, it is not important why the plastic parts that he requested to manufacture use such a geometry. He only needs to use the industry's technical tools to produce plastic parts with the best wall thickness distribution.
        In order to create the desired wall thickness difference in the blisters, the nozzle and the mandrel may have a contour on the periphery, and the axial wall thickness may be changed by using a replaceable tapered mandrel. However, this often leads to undesired coupling between the axial and radial wall thickness controls, so that sacrifice must always be made in the optimal wall thickness distribution in both directions. If the plastic part to be produced has a very complex shape, the best wall thickness distribution can only be achieved by the dynamic change of the axial and radial billet thickness. In addition to programmable wall thickness control systems (PWDS) that have been used for many years, plastic part manufacturers now have radial wall thickness control in the form of curved ring technology. It can more accurately and better adjust the wall thickness of the material.
      Starting state
    In a circular blank shape, blow moldings to be produced often have strongly varying stretching conditions on the outer surface. The greater the local difference in stretch rate for a particular plastic part, the more interesting it becomes to change the wall thickness distribution of the blisters surrounding its environment, thereby obtaining the desired wall thickness at any point of the blow molding.
       The programmable wall thickness control system (PWDS) was developed several years ago and is specifically used to correct this problem. However, it can only be used properly if the diameter is 60mm or more.
 However, the process technology of such systems is limited to the differences in the shape of the blanks that can be achieved and the thickness of the blanks that can be obtained. It is also an extremely expensive and extremely complicated solution.
        skills requirement
        The purpose of the development of curved loop technology is to provide a simpler solution that can be used to blow all common die diameters and to control the wall thickness distribution of the blank more sensitively. In addition to this basic requirement, it includes
        ◆ must maintain a wide range of freedom when designing the flow channel geometry;
        ◆ The mold should of course withstand the pressures common to the process;
        ◆ The integration of new technology into the mold should not require a new parting line;
        ◆ If possible, the system should not create new areas where leakage may occur;
        ◆ When adjusting its shape, there should be no dead zone in the flow channel;
        ◆ The entire system should be preserved so it can be used for all possible molds;
        ◆ The positioning system must be easy to activate;
        ◆ Must have high positioning speed.
         Technical solutions
       A new production technology was developed to produce blow-mold inserts (curved groove), which is partially designed as a multilayer wall. These curved grooves can easily be converted into traditional molds or integrated into the mold.
        They are thick at one end and have a traditional flange collar that rests against this in the adjusted outer ring. The lower end of the flange forms a sealing surface, which is required in any case in the central parting line. So in this mold area, there is no major correction required for traditional molds.
        On the other hand, the upper end of the curved groove forming the die hole is designed as a multi-layer wall, which allows the flow path gap to be changed locally. This is particularly advantageous because the change in the flow path resistance directly at the end of the die is more effective than the change produced in the die. The wall of the flow path in this area is then assembled from a large number of very thin, nested walls, which can withstand the internal pressure of the melt and can be flexibly deformed.
        The deflection curve of the "leaf spring" shaped curved groove is very short. So it provides very sensitive purely linear elastic deformations at almost all points around. Taking a curved ring die with a diameter of only 43 mm as an example, FIG. 1 shows to what extent such a multi-walled loop can be deformed without plastic deformation. Because the flow resistance in the incision fluid changes with the three-dimensional dimension of the flow channel gap, a large local wall thickness change occurs in the blisters in this manner. In addition, when a curved loop is applied, there is no abrupt change in the flow path leading to a dead point because the curved groove geometry in the deformed area will always change gradually.
       The 32 mounting screws allow the material bubble to achieve a great deal of wall thickness distribution control, which is more suitable for final production than the four mounting positions available in the programmable wall thickness control system (PWDS) currently in use. Moreover, there are additional limitations with the traditional approach, where the two installation locations must be strictly one-on-one.
        It is naturally not suitable to perform dynamic adjustments at the 32 installation locations of production equipment. The dies shown in Figures 1 and 2 are all experimental tools. In the process of designing the mold, they can determine the best flow path profile for new products quickly and inexpensively. With purely static adjustment, the proper shape of the flow path around the mold can be determined for each location along the length of the blisters. So the shape can be optimized after each injection to achieve the desired effect.
        Initial test results of the production line
        However, such molds were initially designed for purely static adjustment and can now be easily converted to dynamically controlled production molds as shown in FIG. In the local area, the mounting screws are removed and replaced by the starter motor, and its spindle completes the linear advance motion. This shaft is reliably connected to an adjusting die lip against which the curved groove can be locally deformed over a relatively large circumferential area. Adjusting the lip of the die distributes 14 small screws over the entire width (Fig. 3). The shape of their lip can be easily adapted to the product requirements.
        Because of its low weight, it must be lifted with this system, so the positioning movement can be completed quickly. In order to prove what can be done with the curved ring technique, in one test, the motor was started for 0.3 seconds by extrusion of the billet, and the runner gap was closed at one point. A location was chosen where containers made from traditional molds always have undesirable thick spots. Figure 4 shows the results of a test done on a head mold. A purely visual comparison of the wall thicknesses of the thin areas produced and the relative positions can explain what might be a significant wall thickness variation with the curved ring die.
        The short transitions on the edges of the resulting thin areas cannot be obtained in any way by any other system.
        At the German Institute of Plastics Processing (IKV) in Aachen, Germany, a two-year research project conducted high-strength research and testing of curved ring die heads. A die with a diameter of only 35 mm and 16 starter motors was designed (Figure 5). An algorithm for calculating the optimal mounting screw position to obtain the ideal curved groove was developed. Finally, the test bottle was designed to have three different zones for research purposes. The shape of the bottle in the lengthwise direction is changed from a rectangular bottom area to a round shape via an oval lower bottle area ( FIG. 6 ). In the static test, an excellent thickness distribution can be obtained for each bottle area (Fig. 7). In the dynamic test, there is a problem of synchronization of data transmission, which avoids the simultaneous activation of 16 flow channels.
        It must be pointed out here that these studies show what is possible in principle. In production applications, the symmetrical bottles envisaged will also satisfy the traditional four adjustment positions. In this case, the problem of data processing time will of course be overcome.
        The curved ring technology can also have advantages in the low-cost optimization of the mandrel profile. The post-processing of contours is not only time-consuming, but it also always consumes valuable machine power. The linear and elastically adjustable curved mandrel (Fig. 8) simplifies this step considerably, since contours can be optimally optimized at the beginning of the mould. A similar adjustment of the mandrel profile can be made from one injection to another injection. So there is no risk of removing too much material at a specific point. With curved mandrels, each change without ideal positive results can be easily reversed in the next step by unscrewing the corresponding screws.
        Future prospects
       The possibility of changing the runner gap anywhere on the circumference of the die opens up a whole new world of extrusion blow molding. Not only is it capable of producing more complex parts, but the curved ring technology also extends the application of dynamic wall thickness control to all required die shapes, so that in the future there will no longer be restrictions on radial wall thickness control applications because The head diameter is too small.
        Quchu technology provides an opportunity to significantly reduce production costs. This is accomplished not only by improving the wall thickness distribution of blow molded parts to reduce material consumption, but mainly by shortening the cycle time, which occurs automatically when unnecessary thick spots in blow molded parts are eliminated. It is possible to assume the difference between the desired wall thickness distribution in the plastic part and actually get smaller. Moreover, the die used is simpler than the programmable wall thickness control (PWDS) die used so far and is therefore less susceptible to interference.
        For curved loops, the cost should not prove to be a disadvantage because the pure mechanical parts of the curved loop die can be manufactured cheaper than traditional programmable wall thickness control systems (PWDS), necessary electrical regulating systems and mature oils. Pressure adjustment systems also have cost advantages and technical advantages.
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