High-Stalk Extrusion

The machine direction tear strength of the film is relatively low, so that a HMW-HDPE grocery bag made from a resin that increases the MD tear value would provide a significant advantage in the marketplace. 

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An unusual feature of fabricating HMW-HDPE into film is the length the molten polyethylene travels during the film-forming process before the bubble is inflated. The bubble shape is illustrated in Figure 6.17 and is referred to as high-stalk extrusion. This extrusion shape is used in order to produce film with more balanced orientation (more balanced properties in the MD and TD direction). After the melt exits the die, the melt is drawn down by about a factor of 20 in the machine direction before the bubble is inflated with a blow-up ratio of 4, i.e., the diameter of the bubble is four times the diameter of the die. Hence, the melt is drawn down by a factor of 80. Consequently, a final film thickness of 0.5 mils is obtained with a die... 

Figure 6.17 Illustration of high-stalk extrusion of HMW-HDPE polyethylene.

Because of its low melting point and high chemical stability, HDPE is easily processed by most conventional techniques (injection molding, blow molding, rotational molding, and extrusion). Blown HDPE film is manufactured orr high stalk film lines specialized techniques have also been described. 

Figure 3.9 Pocket bubble on left and high (long) stalk bubble on right (The Blown Film Extrusion Simulator)...

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