Multimanifold coextrusion

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

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

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.

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. 

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. 

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. 

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