Feed block coextrusion

There are two approaches to minimizing the interface distortion. One is to reduce the length over which the different melt streams flow together. This is done in multimanifold dies where the different melt streams are combined just before the exit of the die. Another approach is to modify the initial configuration of the layers in such a way that the final layer configuration is the one desired. This approach is called profiling, and this is a method frequently used in feed block coextrusion systems to achieve uniform layer distribution at the exit of the die. This principle is illustrated in Fig. 9.44. 

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]. 

The advantage of the feed block system is that it is simple, inexpensive, and allows many layers to be combined. The main drawback is that the flow properties of the different plastics have to be quite close to avoid interface distortion. This limits the choice of materials that can be combined through feed block coextrusion. 

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.

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). 

Feed block In coextrusion, a massive metal block in which the streams of the several extruders are brought together to form the layers of a single stream just before it enters the die. 

There are basically three different techniques for coextrusion. The first employs feed block dies where the various melt streams are combined in a relatively small cross-section before entering the die. The advantage of this system is simplicity and low cost. Existing dies can be used with little or no modification. Disadvantages are that the flow properties of the different polymers have to be quite close to avoid interface distortion. There is no individual thickness control of the various layers, only an overall thickness control. Figure 9.37 shows a schematic of a feed block sheet die. 

Another configuration used for coextrusion is a single die for all layers with a feed-block that combines and distributes the various layers so that they all maintain a uniform thickness across the exit of the feedblock. The design of the die is made to produce a product with the desired width and thickness while maintaining thick-... 

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). 

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). 

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. 

Pig. 6. Coextrusion system (single cavity die) using a feed block for three polymer streams. 

Coextrusion is promoted by the laminar flow of the melt in the feed block and die, which prevents the mixing of the various layers. The laminar flow is due to the low Resmolds niunbers (low inertia between flow planes) that result from the high melt viscosities (23,24). However, the generation of interfaces between the flowing materials requires that melt viscosities and melt elasticity between the layers he sufficiently matched to prevent the formation of flow insta-hihties (29-32). Therefore, coextrusion requires superior equipment and process control throughout. 

Multilayer structures can be formed by simultaneous coextrusion of different layers through a feed block die. The individual layers can be selected for their specific attributes, such as strength, barrier properties, etc, or for incorporation of recycled material. When the basic layers are incompatible, tie layers can also be included to form a cohesive composite structure. 

Multilayer spiral dies are becoming more common but are limited to rheological compatible polymers in the layers much as with the feed block method in flat dies. This is due to the long flow distances required for layer uniformity in the multilayer spiral dies, which makes them more prone to generate interfacial instabilities. Coextrusions with 5-13 layers are relatively common and products with as many as 1000 layers are known. 

For some applications, more than one polymer layer is desirable. For example, a food tray might need a strong, abrasion-resistant surface layer such as polypropylene combined with an inner layer such as poly(vinylidene chloride) to contribute low oxygen permeability. House siding of poly(vinyl chloride) can benefit from a surface layer of an ABS-type polymer with superior weathering properties. In order to produce laminated, multilayer film by coextrusion, several extruders can be coupled. Each will contribute a separate stream of polymer melt to be combined without mixing in a feed block from which a layered melt issues. Much better adhesion between film layers often can be achieved in this way than could be attained by coating one melt onto a previously extruded and cooled second film. 

Feed Zone Metering Zone Adapter and Coextrusion Block Die Body Die Lips... 

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