Multilayered extrusion die

Chicago, II., 29th Aug-2nd Sept.l993,p.403-6. 6A MULTILAYER EXTRUSION DIE GEOMETRY FOR THERMOFORMED POLYPROPYLENE FOAM SHEETS AND BIAXIALLY ORIENTED FOAM FILMS Raukola J Savolainen A Tampere,University of Technology (TAPPI)... 

Co-extrusion has gained importance for the production of blown and cast films. Co-extrusion allows the property profiles of various PE to be combined. The film properties can therefore be adapted to the particular demands with extremely economical material consumption. Tried and tested designs of multilayer extrusion dies permit the formation of extremely thin layers, even under difficult rheological conditions. 

Another important product made with extrusion dies is the creation of multilayered materials. Multilayered sheet and film materials have two applications ... 

BLOW-MOLDED CONTAINERS. Initial evaluation of SELAR OH Plus in extruded blow molding containers was disappointing. Barrier improvements of only 30 to 401 were measured compared to the expected 300%.. The resolution of this discrepancy provided some valuable insights into the role of platelet orientation in barrier improvement and the effects of die swell in multilayer extrusion blow molding. 

Special extrusion dies have been built to coextrude and orient LCP with other thermoplastics. Recently, three-layer structures have been coextruded with such a die, resulting in PET-tie layer-LCP structures with a total thickness of 25 to 50 p.m and 10 to 30% LCP thickness [32]. These films have excellent oxygen and water vapor barrier properties as expected from the properties of LCP. Also, because only a thin layer of LCP is used, this multilayer film has very good performance-cost figures. In the future, it will be possible to coextrude... 

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. 

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). 

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. 

There are several other cases which are commonly encountered. Eor thin annular dies Eqs. 7.54 through 7.59 can be adopted directly by making appropriate replacements for W and h. Eor multilayer extrusion through an annulus in which the gap is too thick to apply the thin slit approximation, the appropriate equations are derived in Problem 7B.8. For more than two layers the equations for flow through parallel plates can be generalized as follows (Schrenk and Alfrey, 1976). Referring to Figure 7.51 we can obtain expressions... 

The process of laminating single-layer web materials to give flexible multilayer film structures has been an established method for many years. Alternate production techniques include extrusion coating and co-extrusion, where plastics are melted and extruded in thin layers through an extrusion die. 

Melt Extrusion. By far the most important method for producing film and sheeting materials reties on one or another of the various melt extmsion techniques (5). The main variations of melt extmsion are the slot (or flat) die-cast film process, the blown films process, and the flat die sheeting-stack process. These may be combined with one or more steps such as coextmsion wherein multilayer film or sheet is formed, biaxial orientation, and in-line coating (6). 

Extrusion. Sheet, film, and profiled articles are made by extrusion (20). The resin is melted and forced through a die plate or head. Variations include multilayer and blown film applications. In multilayer coextrusions, different combinations of plastics are separately but concurrently extruded to form layered sheet or film. In the packaging industry, specialty resins such as high barrier ethylene—vinyl alcohol copolymers are combined with heat- and impact-resistant thermoplastics for food packages. The properties of each resin layer are additive, as opposed to the "averaging of property" in blends. Multilayers are also used for blow-molded containers, films, and sheet products (see also Film AND sheeting materials). 

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. 

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. 

Multilayer co-extrusion is another technique used in the preparation of starch/ synthetic sheets or films [164, 263-266], in which TPS is laminated with appropriate biodegradable polymers to improve the mechanical, water-resistance and gas-barrier properties of final products. These products have shown potential for applications such as food packaging and disposable product manufacture. Three-layer co-extrusion is most often practiced, in which a co-extrusion line consists of two single-screw extruders (one for the inner starch layer and the other for the outer polymer layers) a feedblock a coat-hanger-type sheet die and a three-roll calendering system [164]. Biodegradable polyesters such as PCL [164, 264], PLA [164, 263], and polyesteramide, PBSA and poly(hydroxybutyrate-co-valerate) [164] are often used for the outer layers. These new blends and composites are extending the utilization of starch-based materials into new value-added products. 

Flat Dies (Slit Dies). Flat dies, also called slit dies because the orifice is a wide rectangular opening, are used in chill-roll, cast-film coextrusion. These dies are used almost exclusively for multilayer coextrusion with sheet thickness >254 /um, as well as in coextrusion coating processes (2), where a multilayer web is extrusion-coated onto a substrate such as paperboard, aluminum foil, plastic foam, or textiles. 

In general, as extruded products have become more complex with more demanding polymers, multilayers, and overall thinner walls, control has become much more important. Each type of extrusion process has its own requirements, but there are several common control strategies. With extruders the single most important objective is to deliver a steady flow of material to the die. Temperature control and a consistent pressure are critical to avoid surging which results in a varying product and often reduced output. Therefore, most extruders will have ... 

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