Multiple Layer Extrusion

Coextrusion permits multiple-layer extrusion of film, sheet, pipes, tubing, profiles, wire coating, and extrusion coating. It is used mostly in packaging applications to obtain desired barrier properties. The process eliminates the need for a laminator for plastic-plastic surfaces, is less expensive, and provides property enhancement. 

There are basically three types of multiple layer extrusion techniques (1) melt streams flow separately (2) melt streams flow separately and then together (3) melt streams that flow together. Examples of type 1 are shown in Figure 7.47. In this process polymers A and B are extruded through separate flow channels and then joined together outside the die. 

The most often used die geometries in multiple layer extrusion are slits and tubular film dies. However, there are also cases in which profile dies are used. Some quantitative design work can be carried out in the case of flow through flat film and tubular dies. The factors that can be dealt with are flow rates and pressure drops required to provide a given thickness in a multilayer exttudate. The major problem in multilayer extrusion is the instability in the flow. This will be discussed in a qualitative fashion in Section 7.6.3. However, the design equations can at least be used to estimate whether the flow will be unstable. 

Viscoelastic flow effects in polymer coextrusion. In this example we will present work done by Dooley [7, 8] on the viscoelastic flow in multilayer polymer extrusion. Dooley performed extensive experimental work where he coextruded multilayer systems through various non-circular dies such as the teardrop channel presented in Fig. 9.39. For the specific example shown, 165 layers were coextruded through a feedblock to form a single multiple-layer structure inside the channel. 

Multiple layer film extrusion Thermoforming Film insertion into mold... 

Increasing work in the field of HME and published literature reveals the innovative aspects of this technology. These include, but are not limited to, in situ salt formation, quick dispersing systems with foam like structures, coextrusion to prepare extrudates in the form of laminar structures with multiple layers, nanoparticles released from molecular dispersions manufactured by melt extrusion, and twin-screw melt granulation, which can provide continuous manufacturing of granules yielding consistent product quality attributes. 

Production of permeation-proof plastic fuel tanks can be realized using coextrusion with mbular die systems, followed up by hollow article extrusion blow molds. Using slit dies, multiple-layer, flat films or plates can be produced, which can then be deep-drawn. When the melt is united before the die, oxidizing gas can be blown into the airstream as an adhesion promoter, or adhesion promoter layers can be extruded into the material by means of coextrusion. Using a rotating mass distributor, sheathing with spiral markings can be made for switch wires, tubes and marbled profiles. 

The floor covering structure, can also be prepared by melt extrusion. In such a process, one or more polymer layers can be applied to a continuous web or substrate in a single extrusion operation. When co-extrusion is used to provide multiple layers in a single pass, a separate extruder is used to feed each melt to the sheet die block. 

In most coating operations a single layer is coated. When more than one layer must be applied one can make multiple passes, or use tandem coaters where the next layer is applied at another coating station immediately following the dryer section for the previous layer, or a multilayer coating station can be used. Slot, extrusion, slide, and curtain coaters are used to apply multiple layers simultaneously. Slide and curtain coaters can apply an unlimited number of layers simultaneously, whereas slot coaters are limited by the complexity of the die internals and extrusion coaters by the ability of the combining adapter, ahead of the extrusion die, to handle many layers. 

FIGURE 14.10 Production of multilayered sheet by extrusion (a) head-on view of two polymers entering an annular die like spokes on a wheel (b) spiral formation of multiple layers by a single spoke between counterrotating dies. 

The results fi om the multiple pass extrusion study as shown in Figure 2 represent a regrind scenario where the ratio of regrind is 100% and where no other components (such as matoial from barrier or adhesive layers) are being introduced into the recycle stream. 

Many commercial ceramic membranes nowadays come in the form of a monolith consisting of multiple, straight channels parallel to the axis of the cylindrical structure (Figure 3.6). The surfaces of the open channels are deposited with permselective membranes and possibly one or more intermediate support layers. The porous suppon of these multi-channel structures are produced by extrusion of ceramic pastes described above with a channel diameter of a few millimeters. Their lengths are somewhat limited by the size of the furnaces used to dry, calcine and sinter them and also by such practical considerations as the total compact weights to be supported during heat ueatment and the risk of distortion in the middle section. It should be noted that this type of honeycomb... 

Fig. 6.4-26 is a collection of diagrams of cross sections through ropes from single, double, and triple rotary bar roller presses. All rollers can be up to 1300 mm wide and accommodate a multitude of nozzles for strand forming (Fig. 6.4-27). Wide sheets can be multi-layered by extrusion through slots in multiple rotary bar roller presses or by laying several slabs on top of each other (Fig. 6.4-28, Fig. 6.4-31a). Fig. 6.4-29 shows single, double, and triple rotary bar roller presses with standard (cantilevered) drive.

The external diffusion process is based upon the fabrication of a Nb-Cu-Ta composite in a manner analogous to the processing used for NbTi. The surface of a wire strand is plated or dip-coated with Sn which is then diffused and reacted to form NbaSn. The cost of this process is lower than that of current bronze processes, since the use of a Cu matrix removes the need for repeated anneals during drawing, and multiple extrusions are not required. Because the amount of Sn is not restricted to 13 wt.% of the matrix, a high overall critical current density can be achieved without the need for thin reaction layers and very fine filaments (--2 pm in diameter). Thus, a single extrusion with several hundred filaments, compared with thousands for the bronze process, is all that is required to obtain a high critical current density in an external diffusion conductor. 

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). 

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