Blown Film Dies

Most dies produced today are spiral channel dies, though some antique side entry and spider dies are still in use. In spiral channel dies, the polymer melt is introduced through several ports into spiral channels, as shown in Fig. 7.9. It then flows partially up the channel and between the channel and the die wall. This type of smearing flow reduces any existing temperature differences in the melt. As the flow moves up the die channel, the spiral disappears and the melt flows into a relaxation chamber designed to allow the melt to lose the memory of its previous 

Die channel design, like extruder design, is often a compromise because of the wide array of materials that must be run through the system. Channels designed for tie layers and barrier polymers are generally small because of the small amount of these materials that Is needed. Problems may be encountered if a polymer used In small flows Is switched into a large extruder or die channel, since the residence time may exceed the polymer s heat tolerance. 

The die land refers to the section of the die where the die gap is constant, just before the polymer exits the die. The selection of the length of the die land is determined by the amount of shear orientation that is required. Typical values are a land length to die gap ratio of between 5 1 and 30 1, with a 10 1 ratio the most 

Blown film dies, like those for cast film, are generally manufactured from steel, and are often plated with chrome or nickel to harden the surface and increase its durability, as well as making it easier to clean. In some cases, the die lips are coated with PTFE-based coatings or special alloys. For processing of highly corrosive plastics such as PVDC, special high-nickel alloys can be used. 

For this reason, many blown film screws today include a barrier flight. This second flight in the melting section of the screw helps to ensure that no unmelted material exits from the barrel. Because blown film products are very thin, unmelts can be quite problematic, both structurally (as stress concentrators) and aesthetically. 

Finally, mixing sections are being used extensively for blown film extruder screws. The need for high melt quality in blown film requires that the material leaving the extruder and entering the die be as homogeneous as possible with regard to both compositional and temperature variations. 

Several types of blown film dies are available, varying in cost, complexity, and purpose. Side fed, bottom fed, and spiral mandrel dies are used for producing monolayer films. 

Coextrusion dies are used for multilayer products. Both monolayer and multilayer films may be extruded using rotating (oscillating) dies. These are all described in the following paragraphs. 

In many operations, the product is made up of multiple layers of polymer combined into a single film. Each layer serves a specific purpose, such as reduced cost, low oxygen permeability, strength, printability, heat-seal ability, and so on. These products are processed using a coextrusion die that is fed by two to five (sometimes more) extruders. 

The most common die used in blown film extrusion is the spiral mandrel die. In this die, the polymer is divided into a number of spiraling channels with the depth of the channels reducing in the direction of flow. The popularity of the spiral mandrel die is due to its relatively low pressure requirement and its excellent melt distribution characteristics. Spiral mandrel dies can be used with a wide range of materials over a wide range of operating conditions. 

A simpler die is the conventional crosshead die see Fig. 9.24. This design is more susceptible to weld lines however, with the correct design good blown film can be 

The distribution characteristics of conventional crosshead dies may not be good enough for application in blown film extrusion, where wall thicknesses are generally quite small (the typical range is 0.005 mm to 0.25 mm). Spiral mandrel dies can achieve good flow distribution and largely eliminate weld lines. As a result, spiral mandrel dies are widely used in blown film extrusion. 

The incoming polymer melt stream is divided into separate feed ports. Each feed port feeds the polymer into a spiral groove machined into the mandrel. The cross-sectional area of the groove decreases with distance, while the gap between the mandrel and the die increases towards the die exit. This multiplicity of flow chan- 

Rauwendaal [41] used the following assumptions to analyze the flow in the spiral mandrel die  

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Fig. 6. Extrusion of blown film A, blown-film die B, die inlet C, air hole and valve D, plastic tube (bubble) E, air ring for cooling E, guide roUs G,...

Control flow Contains adjustable matching inserts, no restrictor bar Has restrictor bar or flow dividers in each polymer channel but with blown film dies control is by individual extruder speed or gearboxes... 

For gel processing, the components, UHMWPE, optionally filler, processing oil, and lubricant are mixed. The components are preferably mixed in a continuous fashion, e.g., in a twin-screw extruder or a Brabender extruder, or a screw extruder with a blown film die. After mixing, the mixture is shaped. Shaping will depend upon the... 

Fig. 12.41 Typical spider-type tube or blown film die.

Unlike the coat hanger flat film dies, no simple final film adjustment is possible by lip flexing. Consequently, the order of magnitude of thickness accuracy in tubular dies is 10% as compared to 5% in flat sheet and film dies. This larger margin of accuracy in blown film dies is compensated by die rotation, which permits the distribution of the thickness variation across the entire width of the product. 

Fig. 12.42 Schematic representation of tubular dies, (a) Side-fed manifold die. (b) Blown-film die.

It is worth noting that, although in principle tube and tubular blown film dies are similar, in practice they are quite different in function, size, and complexity. Blown film dies are much longer, have a very small die lip opening, and are subject to more stringent product uniformity criteria because there is no sizing equipment downstream. Furthermore, blown film products are almost exclusively LDPE, and occasionally HDPE and PP. On the other hand, HDPE and rigid and plasticized PVC are the common polymers for pipes and tubes. 

Photograph of a coextruded blown film die followed by blown film with external and internal cooling. [Courtesy of Windmoeller Hoelscher (Lincoln RI).]... 

Figure 17.11 Examples of single layer blown film dies include side fed type (top left), bottom fed with spiders type (top center) and others are spiral fed types...

When compared to other blown film dies they each have advantages... 

Coextrusion can be performed with flat, tubular, and different shaped dies. The simplest application is to nest mandrels and support them with spiders or supply the plastic through circular manifolds and/or multiple ports. Up to 8-layer spiral mandrel blown film dies have been built that require eight separate spiral flow passages with the attendant problem of structural rigidity, interlayer temperature control, gauge control, and cleaning. Many techniques are available for coextrusion, some of them patented and available under license (Chapter 5). 

Several unique products are made by extrusion and the dies needed to make these products are classified as 1) sheet dies 2) flat-film and blown-film dies 3) pipe and tubing dies 4) profile extrusion dies and 5) co-extrusion dies. Furthermore, each product type has unique hardware downstream of the die to shape and cool the extruded melt. To aid the reader, detailed illustrations of the various die designs and the complementary downstream cooling and shaping hardware are shown. 

Fig. 3 Schematic of spiral mandrel blown film die operation (1) ring-shaped melt distribution (2) die body (3) spiral flow mandrel (4) sizing ring (5) spreader (6) film bubble (7) frost line (8) solidified film (9) bubble collapsing rollers (10) nip rollers (11) external bubble cooling air (12) internal bubble cooling air inlet (13) internal bubble cooling pipe and (14) heated internal bubble air return.

Figure 7.4 Tubular film apparatus (a) extruder (b) rotating blown film die (c) haul-off unit (d) pretreatment unit (e) wind-up unit (f) edge trimmer (g) longitudinal cutter (h) transverse cutter...

Thin film is inflated from extruded tube consequently to the die exit. The blown film dies could be in general divided to spider and spiral types, as presented in Fig. 4.3. Since spiral dies provide more uniform melt distribution and eliminate weld or knit lines caused by the spider holding the mandrel, they almost replaced... 

Ffe. 4.3 Schematic of spider and spiral blown film dies [43]... 

Fig. 3-12b. Blown film dies, with the spiral groove method used as the best method to distribute melt flow evenly distribution can be improved by lengthening the spirals and/or increasing the number of distribution points.

The purpose of the die is to form the extrudate into a shape that is close to its final shape. There are many types of dies and a vast number of variations within each type. In this book, the different types of blown film dies will be covered in a later section. This section discusses three common characteristics that may occur when polymer flows through any type of die die swell, melt fracture, and die drool. 

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