Coextrusion manifolds

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. 

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. 

Coextrusion produces multilayer laminates in a single process step. Two or more extruders feed different molten polymers into a multi-manifold die which layers them directly, or into a modular feedblock which layers them before feeding them into the die. This is used primarily in the packaging field, to sandwich an impermeable barrier layer between two commodity outer-film layers, and often includes adhesive tie layers to bond the barrier layer to the outer layers. 

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). 

Fig. 8 Coextmsion feed block manifold and sheet die (A) (1) sheet die with flow restriction (2) adapter plate (3) feed block asembly (4) core material layer inlet (5) upper material layer inlet (6) lower material layer inlet and coextrusion multimanifold sheet die (B) (1) lower melt channel (2) upper melt channel (3) lower choker bar (4) lower choker bar adjustment bolt (5) upper choker bar (6) upper choker bar adjustment bolt (7) flex lip.

Multilayered laminates with an ABS outer layer can be produced by coextrusion. In this process two or three different polymers may be combined into a multilayered film or sheet. Adhesion is enhanced by cooling the extruded laminate directly from the melt rather than in a separate operation after the components of the sheet have been formed and cooled separately. In one process flows from individual extruders are combined in a flow block and then conveyed to a single manifold die. All the polymer streams should have approximately the same viscosity so that laminar flow can be maintained. 

A number of techniques are available for coextrusion, some of them patented and available only under license. Basically three types exist feed-block, multiple manifold, and a combination of these two (Table 3-7). 

A third approach combines the feedblock and multi-manifold types, and provides further processing alternatives as the complexities of coextrusion Increase. This approach has been used successfully in barrier sheet coextrusions where requirements preclude other alternatives. The feedblock is placed on the manifold or manifolds, permitting the combination of materials of similar flow characteristics in the feedblock while feeding dissimilar materials directly into the die. 

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

It is also desirable to be able to vary and control the thickness of the plies. As there are two or more extruders feeding the separate inlet channels, the first step is to calibrate them in terms of output rate vs. screw speed. As previously discussed, gear pumps can be used. The next step is to establish the required width to thickness and takeoff speed, and convert this value into an output rate for each resin. From such output rates and calibration curves, screw speeds are established. When a feedblock is not designed properly, problems such as excessive pressure drops, shear rates, or residence time can affect the quality of the extrudate. The tear-drop shape manifold has proved beneficial for coextrusion (see Fig. 3-12a, part 4). 

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. 

Figure 9.40 shows a multi-manifold sheet die for two-layer coextrusion. 

The third coextrusion technique uses multi-manifold external-combining dies, which have completely separate manifolds for the different melt streams as well as distinct orifices through which the streams leave the die separately, joining just beyond the die exit. This technique is also referred to as multiple lip coextrusion. The layers are combined after exiting while still molten and just downstream of the die. For fiat film dies, pressure rolls are used to force the layers together. In blown... 

Keywords extrusion die, die design, biaxially-oriented PP (BOPP), heat sealing, manifold, coextrusion, extrusion parameter, process control, flow. 

A growing trend in extrusion blow molding is to coextrude parisons that contain up to seven layers of different materials. However, as of this time, coextrusion is limited to machines that only use one (1) parison as on a wheel and the continuous-type machine. Since the different materials combine in the head tooling (see Fig. 35), the use of manifolds for multiple cavity is not feasible, nor are accumulator machines (see Coextrusion). 

Two basic die types used in flat-die coextrusion systems are multimanifold dies and the feedblock/single-manifold die. A hybrid combines feedblocks with a multimanifold die. 

Fig. 4. Exploded view of modular feedblock and single-manifold die for three pol3oners forming a five-layer 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, the different plastic melt streams enter the die separately and each material has its own manifold. The different melt streams combine close to the die exit to make the multilayered product (see Coextrusion). 

Coextrusion. Coextrusion is a commonly used technique to combine two or more plastics passing through a single extrusion die. There are two major coextrusion techniques the feed block system and the multimanifold system. In the feed block system the different plastics are combined in the feed block module (see Fig. 18) and then enter into a regular extrusion die with a single inlet, manifold, and outlet. 

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.

The basic coextrusion process consists of the generation of two or more melt streams and their confluence while in the melt phase. The number of separate extrusion systems is determined by the number of polymer types. This is typically 2, but occasionally 3 and exceptionally up to 10. Each polymer type to be incorporated in the structure is separately melted, pressimized, and (optionally) filtered in parallel extrusion systems before flowing into the coextrusion hardware. The optimum method of bringing the separate melts together depends primarily on their respective flow behaviors. The melt layers must remain distinct but well bonded in the process from the point(s) of confluence through to sohdification. There are basically two hardware configurations in use for common polymers the multi-manifold die and the injector block. Combinations of the two are also possible for complex structures (8,9). 

The review in coextrusion also applies with coinjection providing similar advantages. Two or more injection molding barrels are basically joined together by a common manifold and nozzle through which melts flow before entering the mold cavity by a controlled device such... 

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

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