Polymer processing blow molding

Rheological characterization on thermoplastic melts measures material flow properties and provides vital information about polymer processing. Blow molding materials are expected to be more viscous than would normally be employed for the injection molding process [55]. It is also possible to separate the polymer by identifying their MFl values. 

F. N. Cogswell, The Scientific Design of Fabrication Processes Blow Molding, Plast. Polym., October, (1971). 

The type of process used also indicates the addition of polymer modifiers. Blow-molding requires that the polymer flows easily, while retaining good cohesive strength when molten, to avoid tears and voids. Melt flow modifiers are typically used for this purpose and include fatty acid esters, oleoamides, and adipates. Compounds of this type often also serve to prevent blocking (adhesion of one side of a bag to the other). 

Polymer scientists and engineers have developed a wide variety of processes by which to convert polypropylene into useable articles. These processes include fiber-spinning, production of melt blown and spun bond non-woven fabrics, film production, blow molding, and injection molding. The following sections describe these processes and the resulting properties of the product as a function of the process. 

Blow extrusion, in which the initial lump of polymer is formed by an extrusion process, is the most common form of blow molding and is shown diagrammatically in Fig. 16.10. A short length of plastic tubing is extruded through a crossed die and the end is scaled by the closing of the mold. 

Figure 7.80 Schematic illustration of blow molding process. From Z. Tadmor and C. G. Gogos, Principles of Polymer Processing, Copyright 1979 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

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. 

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