Blow molding pinch-offs

Green, Laskowitz, McConnel Evaluation of Design and Processing Parameters for extrusion Blow Mold Pinch-Off Design... 

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. 

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. 14.26 Diameter (SD, thickness (S7), and weight (Sw) swell of a parison extrudate from a commercial blow-molding machine equipped with CCD camera equipment and parison pinch-off mold based on the design of Shepak and Beyer (69). (a) A chain extended multibranched polyamide-6 resin (b) the former with 12% glass fiber of 10 pm diameter 60 1 L/D and (c) polyolefin modified polyamide-6 with some carbon black. [Reprinted by permission from A. H. Wagner and D. Kalyon, Parison Formation and Inflation Behavior pf Polyamide-6 During Extrusion Blow Molding, Polym. Eng. Sci., 36, 1897-1906 (1996).]...

In the continuous extrusion design process, the parison is continuously extruded between the open mold halves from an accumulator head. When the required length of parison has been produced, the mold is closed, trapping the parison that is severed usually by a hot knife from the die. Figure 6.7 provides a simplified schematic of a continuous BM process. Land or pinch-off areas on the mold compress and seal the upper and lower ends of the parison to make an elastic airtight part. Compressed air is introduced through the blow pin into the interior of the sealed parison that expands to take up the shape of the mold cavities. The cooled mold chills the blown object that can then be ejected when the mold opens. 

Extrusion stretch blow molding (ESBM) is a one-stage or two-stage process using two mold/mandrel sets where one is for preblow and the other for final blow. An extruded parison is first pinched off and blown... 

Figure 11.10 Schematic of the blowing stage, (a) The molten, hollow tube — the parison or preform — is placed between the halves of the mold (b) the mold closes around the parison (c) the parison, still molten, is pinched off and inflated by an air blast that forces its wall against the inside contours of the cooled mold (d) when the piece has cooled enough to have become solid, the mold is opened and the finished piece is ejected. (From Kaufman, H.S. and Falcetta, J.J., Eds., Introdur ion to Polymer Science and Technologies, John Wiley Sons, New York, 1977. With permission.)...

Aluminum and beryllium copper are often used for blow molds for plastic bottles. Beryllium copper has better heat transfer and resistance to wear, but is more expensive. Aluminum is used most because it is significantly less expensive and provides good heat transfer, although it does wear more quickly. Because blow molds are not subject to pressures nearly as high as the pressures in injection molding, the molds can be less rugged. Beryllium copper or steel inserts are often used in the parts of the mold most subject to wear, such as the pinch-off in an extrusion blow mold. A typical blow mold can produce about 12 million containers before it must be discarded, provided some refurbishing is done periodically. 

Aluminum is used for single molds, molds for prototypes, and large numbers of identical molds, as found on wheel-type blowing equipment or equipment with multiple die arrangements. (Regarding prototyping, it is best to utilize a material that the production mold will use to duplicate heat transfer conditions.) Aluminum may tend to distort after prolonged use. Thin areas, as at pinch-off areas, can wear in aluminum. 

Incomplete blow Extrusion rate too high Blow-up air pressure Blow-up time too short Parison is cut at pinch-off Reduce screw speed Increase blow air pressure Reduce mold closing speed... 

Parison blow-out Blow-up too rapid Melt temperature too high Pinch-off too sharp Blow-up ratio too high Program blow-up start with low air pressure and increase Align molds Use larger parison... 

Pinch-off n. (1) In blow molding, a raised edge around the cavity in the mold that seals off the part and separates the excess material as the mold closes around the parison. (2) In making tubular film, the paired rollers (pinch rolls) at the top of the tower that flatten the tube and confine the air in the bubble. 

Pinch-off blades n. In blow molding, the mating parts of the mold at the bottom that come together first, to pinch off the parison at the bottom and serve the tail, then, to help form the bottom of the part as the parison is inflated. 

Pinch-off tail n. In blow molding, the bottom tip of the parison that is pinched off and severed as the mold closes. 

Relief angle In an injection or blow mold, the relief angle is the angle between the narrow pinch-off land and the cutaway portion adjacent to the pinch-off land. 

With today s CADAM and CNC equipment, the blow-mold industry is producing large blow molds via machining rather than cast almninum (Table 2). However, there are some industries that because of the blow-mold cost and short nm volumes rely solely on cast aluminmn blow molds. They also live with the problem of aluminum pinch-offs, rather than insert steel or beryllium-copper. 

Generally, in volume production, pinch-off inserts are made of hard steel with the other portions of the blow mold produced from a nonferrous metal. The... 

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