Parison thickness

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. 

Fig. 14.17 Example of a parison thickness variation by parson programming. [Reprinted by permission from Modern Plastics Encyclopedia, Vol. 53, McGraw-Hill, New York, 1976-1977.]...

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 5.17 Predicted time sequence of the parison thickness during the inflation of a fuel tank with thickness contours(mm) (Tanifuji, S.-l. et al, Potym. Eng. ScL, 40, 1878,2000) John Wiley and Sons Inc. reprinted with permission.

In blow molding, the die swell ratio is the ratio of the outer parison diameter (or parison thickness) to the outer diameter of the die (or die gap). Die swell ratio is influenced by polymer type, head construction, land length, extrusion speed, and temperature. 

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-... 

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. 

The parison formation process can be considered as an annular extrudate swell and sag problems of viscoelastic fluids. The parison swell and sag have competing effects on the parison thickness parison swell results in a thicker parison whereas parison sag results in a longer and thinner parison. 

Figure 6 shows the comparison of simulated and experimentally measured parison thickness distributions from different die gap profile. It can be seen that the parison thickness measured on the cooled parison are smaller than those obtained by simulation. This is because... 

In order to keep the inserts a constant size, the overall dimensions will not be changed. The Laser Mike and pillow mold will be used to set the parison thickness at 0.3048mm. The thickness of the compression region of the flash pocket will be modified four times, keeping the length of the pocket constant. The optimum thickness is determined and three more inserts will be machined with the optimum depth as a constant dimension and vaiying the length of the first pocket. The HDPE parts will then... 

The extrusion blow molding machine that was used in this experiment was a Bekum BMO-4025. The mold that was used to make the test specimens was a 0.473 liter, boston round with an insertable pinch-off To find the parison thicknesses at the pinch off region, a compartmentalized (pillow) mold was also used. The pillow mold is a series of pinch offs that are spaced 20mm... 

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