Blow molding parison formation

Key Words Blow molding, Parison formation, Finite element simulation. 

Blow-molding processes consists of five main operations plastication of the resin, formation of the parison, inflation of the parison, solidification of the part, and removal of the part from the tooling. The best process economics will occur with a part optimized for weight and a minimum cycle time. In order to have a minimum cycle time, the cooling operation must be the rate-limiting step. For the case study... 

Fig. 14.26 Diameter (SD, thickness (S7), and weight (Sw) swell of a parison extrudate from a commercial blow-molding machine equipped with CCD camera equipment and parison pinch-off mold based on the design of Shepak and Beyer (69). (a) A chain extended multibranched polyamide-6 resin (b) the former with 12% glass fiber of 10 pm diameter 60 1 L/D and (c) polyolefin modified polyamide-6 with some carbon black. [Reprinted by permission from A. H. Wagner and D. Kalyon, Parison Formation and Inflation Behavior pf Polyamide-6 During Extrusion Blow Molding, Polym. Eng. Sci., 36, 1897-1906 (1996).]...

A. H. Wagner and D. Kalyon, Parison Formation and Inflation Behavior of Polyamide-6 during Extrusion Blow Molding, AIChE J., 36, 1897-1906 (1996). 

S. Tanue, T. Kajiwara, K. Funatsu, K. Terada, andM. Yamabe, Numerical Simulation of Blow Molding - Prediction of Parison Diameter and Thickness Distribution in the Parison Formation Process, Polym. Eng. Sci., 36, 2008-2017 (1996). 

FIGURE 14-59 Extrusion blow molding formation of parisons. 

Two basic methods are used in this process to deliver material to the processing units. These are extrusion and injection. In the next step, the preformed material is expanded to form parison. There are many commercial variations on this basic technique some of which include continuous-extrusion-blow-molding, coextrusion-and-sequential-blow-molding, and injection-stretch-blow-molding. Both extrusion and injection molding are the subjects of later discussions below, we will concentrate here on the parison formation, its processing, and the related effects. 

The blow molding process therefore involves essentially two properly synchronized operations parison formation from the plastic material and blowing the parison into the shape of the desired part. There are two techniques for plasticizing the resin for parison formation. These are extrusion blow molding (which is the most common method and which is characterized by scrap production) and injection blow molding. The latter process is versatile and scrap free and is beginning to be more understood and accepted by processors. 

The first step in extrusion blow molding is the formation of a molten tube called a parison. The parison is engaged between two mold halves and, after closing the mold, is inflated by using compressed air, ensuring the polymer adheres to the mold shape. The material solidifies in contact with the cold... 

Extrusion blow molding is a continuous process capable of high production rates. This process (Figure 8.1) involves three main stages parison formation, parison inflation, and part solidification. It has a number of... 

Engineering thermoplastic blowmolding developments are gaining, such as the three-dimensional blow-molded automotive coolant ducting, and higher-MW, higher-melt-strength BPS for parison formation [16]. 

Figure 6.21 Extrusion blow molding machine and parison formation. Reprinted from [28] with permission from Carl Hanser Verlag Munich.

Annular extrudate swell is important for parison formation in blow molding. Finite element calculations and comparison to experiment, together with calculations and measurements of blown bottle thickness, are in... 

Because blow molding is performed on a cylindrical parison, the process is not well suited to the production of technical articles with eomplex forms that deviate substantially from the parison axis. Such forms can be produced by conventional blow molding, but only by using a large parison that, in its flattened form, blankets the complex mold eavity. The penalty for this is the formation of an exeessive amount of pinch-off scrap. 

Another key parameter in extrusion blow molding is extrudate swell a.k.a die swell. Parison formation depends enormously on die swell since it dictates its final dimensions. However, since it cannot be predicted from molecular structure and is very dependent on processing conditions and die geometry (see equation 2)... 

In recent times, the introduction of more sophisticated rheological methods and especially the ability to determine independently the shear and extensional viscosity of polymer melts, the design and correlation between molecular structure and rheology has been more accessible to industrial practitioners. Processes such as extrusion blow molding involve those types of deformations such as shear and extension, which can be determined using capillary rheometry and extensional viscosity measurements, which can be related to the formation and blowing of the parison. 

The viscosity-shear rate curves of the samples are shown in Figure 1. The branched PVDF samples have a similar viscosity profile to those of the reference samples across the entire shear rate range. The change in the viscosity is mostly controlled by the molecular weight of the samples. The melt viscosity of the resin plays an important role in the first step of the extrusion blow molding process where the resin is processed under moderate shear rates well below the onset of melt instabilities. This is extremely important for the formation of a smooth parison and ultimately in a quality product. 

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