Parison programmed

A different problem occurs when the bottle has vertical rather than horizontal asymmetries, such as bottles with narrow waists or wide shoulders. In such cases, a one-time change in the die dimensions cannot produce even wall thickness. The solution is to provide the ability to change the die dimensions as the parison is being produced, so that a thicker profile is produced for the part of the parison that forms the wider part of the bottle, and a thinner profile where the bottle is thinner. This technique is called parison programming. 

A programmed parison is produced by moving the die mandrel up and down inside the die bushing to increase or decrease the gap between the mandrel and bushing, as the parison is extruded (Fig. 12.7). This process is also known as dancing mandrel blow molding. An alternative is to move the die bushing instead 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. 

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Figure 3.58 Moving mandrel used to generate a programmed parison.

Extrusion processes also can use programmed parisons and two sets of molds. The first mold is used to blow and heat-condition the preform—... 

Figure 13 Programmed parison wall thickness to yield even wall thickness in the final product.

FIGURE 14.32 A programmed parison designed to fit a particular mold configuration. (Data from Polyethylene Blow Molding, U.S.I. Chemicals, New York, 1970.)... 

Great care is taken that we design blow molds to avoid thin or weak regions that could result in premature failure. Molds are designed to avoid excessive draw into corners that would result in locally thin areas. For this reason, blow molded products invariably have rounded corners. Another potential source of weakness is the pinch-off line. To compensate for this fact, it is common to program the parison to produce a thickened base. 

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.]...

Fig. 14.28 Parison shape and thickness profile of the parison at different times with programmed mandrel and a region of partially confined extrudate. [Reprinted by permission from the Plamedia Research Corporation, Tokyo, Japan.]...

Why might you want to use parison programming die shaping Can both be used together Why would you want to do so ... 

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