Coextrusion dies

Table 8-18 Comparison of feedblock and multimanifold coextrusion dies...

A number of techniques are available for coextrusion, some of them patented and available only under license. Basically, three types exist feedblock, multiple manifolds, and a combination of these two (Table 9-18). Productions of coextruded products are able to meet product requirements that range from flat to complex profiles. Figure 8-35 (a) shows a typical 3-layer coextrusion die and (b) examples of rather complex profiles that are routinely extruded. 

The design of a die for an appiication is a very complex process. The design must be such that regions with long residence times do not exist, and it must provide uniform flow at the exit. Moreover, coextrusion dies must be designed with iayer uniformity and without process instabilities known as the wave instability and the zig-zag instability [i2, i3]. These instabilities are beyond the scope of this book and thus are not discussed further. Like transfer lines, the velocities in a die shouid be at least i.5 cm/s for thermally stable resins (with antioxidant systems) and at least 3 cm/s for resins that are not thermally stable. 

Tubes and blown Aims can be produced as multilayer structures by employing multiple extruders and coextrusion manifolds and dies. Figure 12.44 is a schematic representative of a conventional and new spiral coextrusion die. The designs can be used for both blown-film and blown-molding parison dies. In the extrusion of tubes, such as rigid PVC or PE pipe, the extrudate passes over a water-cooled mandrel and enters a cold-water bath whose length depends on the tube thickness the tube leaves the bath well below its Tm (if it is crystalline) or Tg (if it is amorphous) and is sectioned to the desired lengths. 

Castillo, R., Coextrusion Die Design Doubles Number of Layers for Packaging Films and Blow Molding Parisons, SPE ANTEC, 2001. 

Michaeli, W. Coextrusion dies for thermoplastics. In Extrusion Dies for Plastics and Rubber, 2nd Ed. Hanser, 1992 234-238. 

Table 3-7. Comparison of Feedblock and Multi-manifold Coextrusion Dies.

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. 

Figure 3.6 A concentric mandrel coextrusion die Battenfeld Gloucester)...

As discussed in chapter 3, there are two main types of coextrusion dies concentric and stack. Though both of these types are used in industry and most die manufacturers have a preferred design, there are several factors to consider with regard to the use of CO extrusion dies. 

Figure 9.38 shows a multi-manifold blown film coextrusion die enabling extrusion of three different polymers. 

Coextrusion is practiced on a wide scale in blown film. There are many five-layer blown film coextrusion dies used in the industry five-layer films are now considered a commodity [41], Even seven-layer dies are not unusual. Some coextrusion dies use as many as 8 to 10 layers. Most of these multi-layer dies are used in high-barrier packaging for food. Conventional blown film coextrusion dies have a concentric arrangement of spiral mandrel manifolds. In some cases conical spiral mandrel sections are used, while in other cases the spiral mandrel section is machined into a flat horizontal surface. The latter arrangement is referred to as a pancake coextrusion die, because the different sections of the die are stacked like pancakes. A schematic of a pancake coextrusion system is shown in Fig. 9.39. 

Profiling can be used with feed block coextrusion systems and also with vane-type coextrusion dies as shown in Fig. 9.41. With 3-D flow simulation the rearrangement of the layers can be predicted. Particles defining a flat interface can be tracked upstream to the input plane to determine the appropriate initial layer geometry. Examples of this approach are discussed by Perdikoulias et al. [45]. 

Coextruded films are produced by a tubular-blown film process and a flat-die, chill-roll casting process. Capital and operating costs for blown-film vs cast-film coextrusion lines are strongly dependent on product mix and utilization. Equipment suppliers provide comparative economic evaluations for specific products. Practical cast-film equipment has been discussed previously (3). Coextrusion dies are unique. Extruders used before the die and take-away equipment used afterwards are standard equipment for single-layer film manufacture of blown or cast film (see Extrusion). 

Tubular-Blown Film Dies (Circular Dies). Tubular coextrusion dies were the earliest dies used to make multilayer plastic film. Successfiil design requires formation of uniform concentric layers in the annular die land formed by the mandrel and adjustable or nonadjustable outer die ring. Early designs included center-fed dies that had the mandrel supported by a spider (4). Feedports arranged a concentric melt stream that was pierced by the mandrel as it flowed to the die exit, forming anniJar layers. Limitations of this early design were discontinuity and nommiformity cansed by spider-induced weld lines in the layers. 

Tubular coextrusion dies are expensive, and care must be taken when disassembling and reassembling them to clean or change parts. Discussions of additional practical design, maintenance, and operating considerations have appeared (6-10). 

Pol5uner rheology information is critical for designing coextrusion dies and feed-blocks. The flow characteristics of the pol5mier must be considered when selecting materials for coextruded products (see Rheology). 

The best designed die or feedblock does not necessarily ensure a commercially acceptable product. Layered melt streams flowing through a coextrusion die can become unstable leading to layer nonuniformities and even intermixing of layers under certain conditions. The causes of these instabilities are related to non-Newtonian flow properties of pol5uners and viscoelastic interactions. 

Viscosities of non-Newtonian polymers are dependent on extrusion temperature and shear rate, both of which may vary within the coextrusion die. The shear rate dependence is further complicated in that it is determined by the position and thickness of a polymer layer in the melt stream. A polymer used as a thin surface layer in a coextruded product experiences higher shear rate than it would if it were positioned as a central core layer. There are several types of flow instabilities that have been observed in coextrusion. 

Coextrusion Dies. Another type of die used in the extrusion industry is the coextrusion die. This type of die is used to make a multilayered product in one step. There are two main coextrusion systems the feed block system and the multimanifold system. In the feed block system, the different plastic melt streams are combined in a feed block and then fed into a regular single manifold extrusion die (see Fig. 18). 

In the multimanifold system each plastic has its own entrance and manifold in the coextrusion die. The different melt streams are combined just before they exit the die, so that minimum interface distortion can occur. The advantage of the multimanifold system is that plastics with widely different flow properties can be combined. As a result, there is a wide choice of materials that can be combined through this extrusion technique. The disadvantage is that the design of the die is more complicated and therefore more expensive. 

Fig. 7. Multiple cavity coextrusion die with multiple melt-distribution manifolds.

Multilayer films are produced by both cast- and blown-film coextrusion. One of the key enabling technologies in the manufacture of these multilayer films has been preencapsulation of the VDC resin (205). Preencapsulation means that the VDC resin extrudate is completely encapsulated with a more thermally stable polymer, such as an ethylene-vinyl acetate copolymer, prior to entering the coextrusion die. Using this technique, the VDC resin is not directly exposed to the... 

Table 7-12. Comparison of Feedblock and Multimanifold Coextrusion Dies...

Figure 12.57 Multiple cavity coextrusion die showing three separate melt-distribution manifolds, which join to...

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