TANDEM EXTRUSION

Figure 13.1 Schematic of a tandem extrusion process for foams. The secondary extruder or cooling extruder will be the rate-controlling step for a properly designed process...

A comparison is made between a 3.5 melt flow conventional polypropylene homopolymer (Profax 6523) and a 7.0 melt flow, high-melt-strength, foamable PP homopolymer in extensional flow. The importance of extensional or elongational viscosity in the foam process is demonstrated and the way in which the rheological differences permit the production of low-density foam on tandem extrusion equipment is shown. 6 refs. 

Extruded polystyrene foam sheet is primarily produced in a single screw tandem extrusion line. Primary application of foam sheet is as a packaging material in items such as disposable dishes and food containers, trays for meal, poultry and produce products, and egg canons. 

THV Fluoroplastic can be processed by virtually any method used generally for thermoplastics, including extrusion, coextrusion, tandem extrusion, blown film extrusion, blow molding, injection molding, vacuum forming, and as skived film and solvent casting (only THV-220). 

Polyethylene foam sheet is used mainly in package cushioning, and also in roof insulation and extruded profile is also used as construction sealants and pipe insulation. It is made mainly by tandem extrusion. The first extruder melts the polyethylene and blends in liquid volatile organic foaming agent. The second extruder cools it to the optimum foaming temperature, and pumps it through a die into room-temperature air, where it expands into foamed sheet. 

Owens Corning manufactures a similar extruded polystyrene foam under the trade name FoamulaR. A single-screw tandem extrusion line is used to produce foam boardstock in a vacuum chamber connected to a barometric leg that acts as a vacuum seal [78]. Owens Corning also licenses their process to a number of international parties. 

More recently Pactiv, formerly Tenneco Packaging, has manufactured foamed polystyrene sheet and boardstock. A tandem extrusion line is used to produce polystyrene foam insulation products. 

Polystyrene foam sheet can be made by a variety of extrusion processes (single screw, twin screw) but is most commonly made using a tandem extrusion process as shown in Figure 11.6. This process uses two extruders in series, which effectively optimize the melting, mixing, and cooling unit operations. The diameters of the extruders currently used in this process are usually 115 mm for the primary extruder and 150 mm for the secondary extruder, with an output capability of 400 kg/h. 

Figure 11.6 A tandem extrusion process for the manufacture of polystyrene foam sheet. (1) Primary extruder (2) blowing agent addition system (3) screen changer (4) secondary extruder (5) annular die (6) cooling mandrel (7) S-wrap (8) winders...

Table 11.4 Polystyrene foam sheet tandem extrusion conditions using n-butane...

In the first operation, the paper and the foil are extrusion laminated using an acid copolymer as the adhesive. Next, the sealant layer is applied as an extrusion coating. The operation can be accomplished in two passes, or in a single pass in a tandem extrusion line as shown in Fig. 8.2. In such a line, both extrusion heads are usually capable of coextrusion, as well. To understand the process of extrusion coating and laminating, it is necessary to start with the process in its simplest form, which, by the way, is how it is still practiced today by some converters. 

Finally, increasing the channel depth will reduce the shear rate and viscous heating as discussed earlier this will result in lower melt temperatures. In fact, the channel depth is one of the most critical screw design parameters to control melt temperature. Deep-flighted screws are used when the viscous dissipation and melt temperatures have to be minimized. That is why screws used to extrude rubbers generally have deep channels. The same is true for cooling extruders in tandem extrusion lines for foamed polymers. 

TABLE 17.8 Morphological Characteristics of Linear PLA (PLA 2002D) and Branched (Proprietary Formnlation, Nature Works ) Foamed on a Tandem Extrusion System Using 9% Carhon Dioxide as the Physical Blowing Agent [78]... 

FIGURE 3.1 Experimental setup of the tandem extrusion foaming system. (From Lee P. C.,... 

A tandem extrusion line was used and its set-up is shown in Eig. 5. It consisted of a melting extruder with a mixing screw (screw diameter = 100mm, screw... 

A new low-loss cable was developed by improving the foaming degree of PE insulation. The foaming extrusion was performed in two extrusion lines. From the preliminary experiments in single extrusion line, optimizations of resin compositions and processing conditions on the foaming au value of the low-loss cable was produced in tandem extrusion line. The newly developed low-loss coaxial cable has 6 8% lower attenuation values compared to the conventional cable. 

To validate the model, visualization experiments were conducted on a tandem extrusion system using a highspeed CCD camera. Polystyrene (PS 685D) was supplied by Dow Chemical. The (fie diameter/length, processing temperature, gas content, and melt flow rate were 1.22tnm/14.30tmn, 133°C, 7wt% and 15.7g/min,... 

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