Blow thickness distribution

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. 

Polystyrene was the first synthetic polymer used for blow molding during World War II and polyethylene was the first material to be implemented in commercial applications. Until the late 1950s, the main application for blow molding was the manufacture of PE-LD articles such as squeeze bottles. Blow molding produces hollow articles that do not require a homogeneous thickness distribution. Today, PE-HD, PE-LD, PP, PET, and PVC are the most common materials used for blow molding. 

Injection blow molding. Injection blow molding, depicted in Fig. 3.60 [25], begins by injection molding the parison onto a core and into a mold with finished bottle threads. The formed parison has a thickness distribution that leads to reduced thickness variations throughout the container. 

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. 

M. R. Kamal, V. Tan, and D. Kalyon, Measurement and Calculation of Parison Dimensions and Bottle Thickness Distribution during Blow Molding, Polym. Eng. Sci., 21, 331-338 (1981). 

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). 

Contour plots of the wall thickness distribution for the injection blow molding simulations. 

Another compromise is made when blowing multiple bottle sizes from the same preform. This may not be optimum from the perspective of polymer utilization, but it may save lots of money on inventories and mold costs. As shown in Section 12.3.2, one can design the preform for optimum wall thickness distribution by using finite element analysis. Sometimes polymer is less expensive than molds. 

Blow molding is complicated by the complex stress field set up in the materials when the parison is inflated. This amounts to a biaxial stretching of the molten polymer and it is difficult to obtain material data under these conditions so that simulation may be performed. Despite this, much work on the inflation stage has been done, mostly with the aim of determining the final thickness distribution. Recently parison inflation has been simulated using three-dimensional finite elements and with remeshing of the parison as it inflates to minimize error from element distortion. ... 

Parts have relatively good wall thickness distribution compared to processes, such as blow-molding and thermoforming. External comers tend to thicken, which can be an advantage in applications where wear is critical. 

Fig. 5. The thickness distribution of blown tube in one-stage injection stretch blow molding. Here, injection stage is 12 sec total blow stage is 1 sec, including pre-stretch of 0.3 sec.

Fig. 8. When the ratio of (injection stage to blow stage) is changed from 12 to 6. The thickness distribution of final product is affected significantly.

Blow moulding finite element simulation softwares are designed to assist process developers and part designers in the industry to save time and reduce cost of production. These softwares provide several capabilities such as prediction of wall thickness distribution, stress and extension ratios of the final product based on the specified processing parameters before the actual production. 

The process of SBM for a PET bottle was examined by using blow moulding simulation softwares - B-SIM and BlowView. These simulations are used to determine the final wall thickness distribution over the entire length of the carbonated beverage bottle. Even though too many assumptions were taken into account, the results obtained from the softwares were quite reasonable when they are compared with actual measurement. 

The use of simulation software to predict the process of blow molding can save considerable time and money in the product development and is becoming more widespread. However, for parison formation simulation, the current finite element (FE) software is suitable only for the situation where the die gap is fixed. In this work, a new method was proposed to apply the FE simulation to the varying die gap parison formation. In order to evaluate the availability of the new method, the predicted parison thickness distributions were compared with the experimental results. It is demonstrated that the new method has certain accuracy and reliability in predicting the parison thickness from a varying die gap. 

Figure 11.6 illustrates the general configuration of a film blowing operation. Molten polymer from the extruder is pumped into an annular die, where it is distributed around a tubular melt channel before emerging vertically as a relatively thick-walled molten tube. The top of... 

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