Blow molding stretched-operation

Originally, the commercial PC resins were linear polymers with high shear viscosity and low melt strength, thus difficult to process in operation involving extensional flows, viz. blow molding stretching, foaming. Several years ago branched PC (bPC) became available. The resin is usually blended with linear PC at the ratio that on the... 

This is a two-stage process in which the material is injection molded around a core rod to form a preform. Then the preform is stretched through the action of a stretch rod. Finally, it is inflated and cooled, which results in a lighter biaxially orientation product. This process is used to produce scrap-free, close-tolerance, completely finished products that require no secondary operations such as trimming, which is used in extrusion blow molding. 

There are three principal types of blow-molding operations extrusion blow molding (Fig. 9-4), injection blow molding (Fig. 9-5), and stretch blow molding. 

Orientation processes also take place in blow-molding operations. A particularly significant aspect is biaxial stretch blow molding, in which the polymer preform is stretched both in the hoop and axial directions. 

There are several differences between the major stretch blow molding machines in the industry. Cincinnati Milacron offered an indexing type machine. It had the advantages as being very rugged and versatile. It was also a relatively simple machine to operate and provided the user a machine that produced the lowest cost bottle per machine cost. TTie ovens could be moved to allow for equilibration time to be adjusted for each different style preform. 

The thermally conditioned preform is transferred to a blow mold and stretched and oriented axially by an internal stretch rod, either immediately prior to, or simultaneously with the blowing operation that provides radial stretch and orientation. 

Limitations. Because the operating pressures used in blow molding are relatively low, it is difficult to achieve Class A surfaces out of the mold, particularly with EBM. Because the process is, in effect, a one-wall process (like forming processes), control of wall thickness can be a problem, particularly in areas where extensive stretching of the parison is required. Injection blow molding addresses this limitation to a degree, but at the cost of overall part size because shear flow is required to form the parison in the first place. 

Nakamura, Y., Injection Stretch Blow Molding. In Blow molding handbook technology, performance, markets, economics the complete blow molding operation, Munich New York 1989 pp xxvii, 1010 p. 

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