BLOWN-FILM DIES AND TOOLING

The basic die used in making blown film is a tubing die that produces a thin wall tube. After the thin-wall tube is extruded, it is inflated by air pressure to form a large-diameter bubble of much lesser wall thickness, hence the name “blown film”. The tooling used is the tubing die, the venturi support with its cooling ring, and the cooling tower with its nip rollers. The latter is to hold the bubble shut (but not sealed）to contain the air pressure while pulling the bubble away from the die opening at several times the speed of extrusion. The tubing is expanded from 5 to 15 (or more) times the original diameter with a resulting wall thickness which usually ranges between 1/2 mil (0.0005 in.) to 10 mils (0.010 in.) but not in the same bubble! Any one film must be uniformly blown, or else disaster lurks in the bubble or in the next product using the film. Because of the very large expansion factor (thinning of the tube wall), the design of the tubing die becomes critical in several aspects, as described hereafter. Any variation in output around the extruded tube will be amplified by the blowing process, and will produce an out-of-specification product. Even though the material may be usable with wall variation(s), it is almost certain that the variable wall thickness will cause problems in the winding and handling of the film.

The primary requisite for a die for blown film is that it must make a tube with a uniform wall. A tube with a uniform wall will expand to make a uniform film. The die construction or design may have any one of several variations based upon the inlet feed point as well as the manner in which the melt is spread around the bushing. Tube sizes range from approximately 1 in. to 100 in. or more. Tube-wall thickness ranges from approximately 0.015 in. to approximately 0.075 in. Exactly which diameter to choose, and exactly which wall thickness to choose depend on the blow-up ratio and on the final film thickness to be produced. The designer should be well aware of the engineering process used to determine the tube diameter, the wall thickness, and the blow-up ratio. We suggest that working with an experienced engineer is probably the best way to develop this awareness. As of this edition, most of the specific knowledge is jealously guarded as ^proprietary information.” Thus this text confines itself to detailing the principles of design.

Figure 1 illustrates one of the simpler variations. The melt is fed from the bottom (note: vertical extrusion), and the inner mandrel is supported by a spider” element. The spider legs are a “necessary evil” because they support and center the spreader, but they also act as an unwanted restriction to flow. Their restriction can produce thin spots in the film unless counter-acting effects are introduced. To minimize the restricting effects, the flow Path through the die is equipped with several reservoir manifolds in the flow these manifolds eliminate the pressure drop caused by the spider legs.

They also eliminate the thin spot(s) in the extruded tube. This design technique is adequate when materials with a relatively high melt index are being processed. For plastics with a low melt index, another design will be more suitable，as illustrated in the following examples.