Parison thickness control

Electronic parison programming is an effective way to control material usage and improve both quality and productivity. The most common method used is orifice modulation (Fig. 4-4). The die is fitted with a hy- 

In regard to parison control, a compromise is necessary between the desired net weight and the need to maintain a sufficient safety margin over a set of minimum specifications, which include minimum wall thickness, drop speed, drop strength, dimensional stability, and fluctuations in net weight. Most of these parameters can be directly affected by the molder s ability to control the parison wall thickness. The most common and practical way of doing this has been to adjust the gap between the die and mandrel (Table 4-2). 

Can be permanently shifted laterally to correct parison drop path 

As above, with greater speed, accuracy, and flexibility 

Fixed circumferential wall thickness change time-consuming complex 

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Some of the parison thickness control is still essential and may be one of the two forms designed either to ... 

Parison thickness control was achieved by adjusting the die annulus using the moving core technique (see PST 6). At the commencement of parison formation the core diameter was 145 mm and the die 150 mm diameter giving a 2.5 mm wide annulus. Actual diameter of the parison was approximately 190 mm. 

The above problems were overcome by adjusting the processing conditions, e.g. accumulator extrusion rate, melt temperature, and parison thickness control. 

Industrial shapes, tanks, gasoline tanks, vehicles and hospital ware such as urinals are blow molded. These shapes require complex blowing patterns because of the odd and nonsymmetrical shapes involved. The parison thickness control is difficult and usually the wall thickness on these parts is not as uniform as on the symmetrical bottle shapes. In designing parts for this process which have specific wall thickness requirements for stiffness and strength, it is advisable to use heavier walls because of the possibility that control in the blow mold-... 

Parison cooling significantly impacts the cycle time only when the final parison thickness is large. In thin blown articles the mold is opened when the pinched-off parts have solidified so that they can be easily stripped off thus they are the rate-controlling element in the cooling process. For fast blow molding of even very thin articles, the crystallization rate must be fast. For this reason, HDPF, which crystallizes rapidly, is ideally suited for blow molding, as are amorphous polymers that do not crystallize at all. 

In the injection blow molding process, the parison is formed by injection molding of the preshaped parison onto a steel rod, as shown in Fig. 14.18. The rod with the molded thread already completed is moved to the blowing station, where the article is inflated free of scrap. The parison thickness distribution is determined in the injection mold without the need of further control. Some axial orientation is introduced during injection, resulting in an article with partial biaxial orientation. 

Figure 6.5 Examples of parison wall thickness control by axial movement of the mandrel...

Die shaping and programmed parison techniques can be combined. In the most sophisticated wall thickness control systems, the use of deformable die rings permits both the mandrel motion and the die shape to be controlled as the parison is extruded. This permits extremely accurate control of the thickness profile of the parison and hence of the blow molded container. 

In order to obtain a uniform wall thickness for complex shaped containers, it is necessary to be able to control the thickness of the parison as it extrudes. Wall thickness control is achieved by varying the die gap as the parison is extruded. There are a variety of different arrangements for the blowing step of the process. These include the use of single dies with multiple molds (e.g., vertical rotary wheel), and multiple dies with multiple wheels. For some examples, refer to the illustrations in Figures 2 through 4. For enhanced barrier properties of multilayer structures, combinations of polymer materials are used, such as ... 

The weight and diameter of the parison is controlled by the geometry of the annular die opening and the resin being extruded. As the molten polymer is extruded through the die, it swells to a thickness and diameter that is greater than the dimensions of the die opening. The amount of swell is dictated by a complex relationship between the internal shape of the die and the viscoelastic properties of the polymer. 

With a simple parison, the large-diameter sections of the botde have a thin wall and the small-diameter sections have a thick wad. Certain modifications of the die can control the thickness of the parison wad along its length, which results in a bottle with improved wad thickness distribution and better strength. High density polyethylene (HDPE) is the most common blow mol ding resin used to produce containers ranging in size from 30 cm to 200 L. 

By extrusion parison control it is possible to minimize the wall thickness variation and the extent of stretching and stretch orientation. These are the province of the processor when the designer is not familiar with BM. Knowledge is required to provide information on what is possible and to select the specific BM process that has the capability to mold the product. The designer should be aware of the possible failure modes and compensate for them in the design. There is little else the designer can do but select the best material and process to make the product. 

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