The geometry of blow molded containers is becoming more and more complex. Most of this is the result of purely technical needs. However, it is often the designer who determines the shape of the plastic part, regardless of technical factors. For plastic parts manufacturers, it is not important why the plastic parts that he is required to manufacture use such a geometry. He only needs to use the technical tools available in the industry to produce plastic parts with the best wall thickness distribution.
In order to create the desired difference in wall thickness in the blister, the nozzle and mandrel may have a contour at the periphery, which may be altered by the use of a replaceable tapered mandrel. However, this often leads to undesired coupling between axial and radial wall thickness control, so there is always a sacrifice in terms of optimal wall thickness distribution in both directions. If the plastic part to be produced has a very complex shape, the optimum wall thickness distribution can only be obtained by dynamic changes in the axial and radial blank thicknesses. In addition to the programmable wall thickness control system (PWDS) that has been utilized for many years, plastics manufacturers now have radial wall thickness control in the form of meandering technology. It can adjust the wall thickness of the blister more accurately and better.
Initial state
In the shape of a circular blank, the blow molded part to be produced often has strongly varying stretching conditions on the outer surface. The greater the local difference in the stretch rate of a particular plastic part, the more interesting it is to change the wall thickness distribution of the blister surrounding its environment, thereby achieving the desired wall thickness at any point of the blow molding.
The Programmable Wall Thickness Control System (PWDS) was developed a few years ago to specifically correct this problem. However, it can only be used properly if the mold diameter is 60mm or more.
However, the process technology of such systems is limited to the achievable shape and the difference in wall thickness of the blank that can be obtained. It is also an extremely expensive solution with extremely expensive technology.
skills requirement
The goal of the development of the curved ring technology is to provide a simpler solution that can be used to blow all common mold diameters, allowing for a more sensitive control of the wall thickness distribution of the blank. In addition to this basic requirement, it also includes
A wide degree of freedom in the design of the runner geometry must be maintained;
The mold should of course be able to withstand the pressures common in the process;
The integration of new technologies into the mold should not require a new parting line;
If possible, the system should not create new areas where leaks may occur;
When adjusting its shape, dead zones should not appear in the flow path;
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 a high positioning speed.
Technical solution
A new set of production techniques has been developed to produce inserts for blow molding dies (curved ring grooves) which are partially designed as multi-walled walls. These loop grooves can be easily retrofitted into conventional molds or integrated into molds.
They have thick walls at one end and have traditional flange collars that are attached to the adjusted outer ring against this. The lower end of the flange forms a sealing surface which is required in any of the central parting lines. 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-layered wall, which allows the flow path gap to be locally changed. This is particularly advantageous because the change in flow path resistance directly at the die end is more effective than the change produced in the mold. The flow path wall in this area is then assembled from a large number of extremely thin, nested walls that can withstand the internal pressure of the melt while being flexible.
Thus, the deflection curve of the "leaf spring" shaped loop groove is short. So it provides a very sensitive, purely linear elastic deformation at almost all points around it. Taking a curved ring die having a diameter of only 43 mm as an example, Fig. 1 shows the extent to which such a multi-walled bent ring can be deformed without plastic deformation. Since the flow resistance in the slit fluid varies with the three-dimensional size of the runner slit, this mode produces a large local wall thickness variation in the bubble. In addition, when the loop is applied, there is no sudden change in the flow path leading to a dead point because the geometry of the loop groove in the deformed zone will always change stepwise.
The 32 mounting screws allow the bubble to achieve a lot of wall thickness control, which is more suitable for final production than the four mounting positions available in the Programmable Wall Thickness Control System (PWDS). Moreover, there are additional restrictions in the traditional way, and the two installation locations must be strictly one-to-one.
It is naturally not appropriate to perform dynamic adjustments on the 32 installation locations of the production equipment. The dies shown in Figures 1 and 2 are both experimental tools. In the design of the mold, they can determine the best runner profile for new products quickly and inexpensively. With pure static adjustment, the proper shape of the flow path around the mold can be determined for each position along the length of the bubble. So the shape can be optimized after each injection to achieve the desired effect.
Initial test results of the production line
However, such a mold was originally designed for pure static adjustment and can now be easily converted into a dynamically controlled production mold as shown in FIG. In the local area, the mounting screws are removed and replaced by the starter motor, which completes the linear forward movement. This shaft is reliably coupled to an adjustment lip against which the ring can be locally deformed over a relatively large circumferential area. The adjustment lip distributes 14 small screws (Fig. 3) over the entire width, and the shape of the lip can be easily adapted to the needs of the product.
Because of the low weight, this system must be used for lifting, so the positioning movement can be done quickly. In order to demonstrate what can be done with the meandering technique, in one test, the runner was started at a point by activating the motor for 0.3 seconds during the bubble extrusion process. A location is chosen where the containers made from conventional molds always have undesirable thickness points. Figure 4 shows the test results performed on a stock head mold. A purely visual comparison of the resulting thin zone and the wall thickness of the relative position can explain the potential for significant wall thickness variations with the meander ring die.
The short transition zone on the edge of the resulting thin zone cannot be obtained by any other system in such a way.
At the German Institute of Plastics Processing (IKV) in Aachen, Germany, a two-year research project conducted high-intensity research and testing of the curved ring die. A die with a diameter of only 35 mm and 16 starter motors was designed (Fig. 5), and an algorithm for calculating the optimum mounting screw position to obtain a perfectly shaped ring groove was developed. Finally, the test bottle was designed to have three distinct zones for research purposes. The shape in the length direction of the bottle is changed from a rectangular bottom area to a circular shape through the elliptical 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 synchronization problem of data transmission, which avoids simultaneous activation of 16 channels.
It must be pointed out here that these studies show what is possible in principle. In production applications, the symmetrical bottles envisioned will also meet the traditional four adjustment positions. In such cases, the time problem of data processing will of course be overcome.
The meandering technique also has advantages in low cost optimization for mandrel profiles. The post-processing of the contour is not only time consuming, but it also always consumes valuable machine capabilities. The linear and flexible adjustable curved ring mandrel (Fig. 8) simplifies this step significantly because the contour can be optimized at the beginning of the mold. Similar adjustments can be made to the mandrel profile from one injection to another. So there is no risk of removing too much material at a particular point. With the looped mandrel, each change that does not result in an ideal positive result can be easily reversed in the next step by loosening the corresponding screw.
Future prospects
The possibility of changing the gap of the runner at any position on the circumference of the die opens up a whole new world for extrusion blow molding. Not only does it make it possible to make more complex parts, but the meandering technology extends the application of dynamic wall thickness control to all required die shapes, so there will be no longer any limitations on radial wall thickness control applications in the future. The head diameter is too small.
The curved ring technology provides an opportunity to significantly reduce production costs. This is accomplished not only by improving the wall thickness distribution of the blow molded part to reduce material consumption, but also by shortening the cycle time, which automatically occurs when unnecessary thick spots in the blow molded part are eliminated. It can be assumed that the difference between the required wall thickness distributions in the plastic part is actually obtained to be smaller. Moreover, the die used is simpler than the Programmable Wall Thickness Control (PWDS) die used to date and is therefore less susceptible to interference.
For curved rings, cost should not prove to be a disadvantage, because pure mechanical parts of the meandering die can be manufactured cheaper than conventional programmable wall thickness control systems (PWDS), with the necessary electrical conditioning systems and mature oils. The pressure regulating system also has cost advantages and technical advantages.
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