Secondary extruder

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...

Secondary Extruders for Tandem Foam Sheet Lines... 

The secondary extruder is typically larger in diameter than the primary machine, and it rotates at a lower screw speed. For example, a midsized tandem line would be constructed using a 114.3 mm diameter primary extruder and a 152.4 mm secondary extruder. The primary extruder would operate at a normal screw speed (50 to 70 rpm) and could generate the pressure required to run the downstream equipment. The secondary extruder would operate at a lower screw speed (10 to 15 rpm) so that the entry and exit pressure for the extruder are about the same. 

Cooling water flow to the barrel jacket coolers for the secondary extruder removes the excess energy in the resin dissipated by the primary extruder and also the energy dissipated by the screw in the secondary machine. For a properly designed line, the performance of the secondary extruder determines the overall rate of the process. 

Fig. 11 Foamed sheet extrusion line. 1 Primary extruder, 2 blowing agent metering and flow control, 3 screen pack/breaker plate, 4 secondary extruder, 5 annular die, 6 cooling mandrel, 7 guide rolls, 8 winders. [38]...

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. 

An annular die is used on the end of the secondary extruder. Die diameters range from 76 to 254 mm and die gaps range from 0.25 to 0.76 mm. Nucleation of the cells occurs near the exit of the die because of the pressure drop in the die lips. Controlled foaming continues outside the die lips as the material is stretched over a forming mandrel (Figure 11.7). The forming mandrel diameter... 

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...

From secondary extruder (I50mm single screw at 13 rpm). 

The cost of manufacturing thermoformed, polystyrene foam sheet parts is less dependent on raw material cost than other extrusion processes. This is largely due to the combined effects of additional energy costs required to operate two extruders, heat removal requirements in the secondary extruder, cost of pelletizing (densifying) regrind and the relatively low output of the process for the equipment scale and cost. Typical cost factors for the manufacture of thermoformed polystyrene foam sheet products include raw materials 35%, labor 27%, sales and administration 16%, depreciation 8%, utilities 7% and other 7%. 

In downstream, the pellets are sent to primary extruder. Mixing is done at 20 rpm. The temperatures from feed zone to die head are 180°C, 240°C, 240 C and 260°C. These mixed samples are then sent to a secondary extruder with a screw rotation speed of 90 rpm and the pressure adjustable die temperature is 220°C. The supercritical fluid (CO gas) is compressed at 21 MPa pressure using a positive displacement syringe pump and sent to the secondary extruder. The melt and supercritical fluid are mixed with the help of a distributive-type screw. This mixture is pumped to the pressure adjustable die, where the sample is casted and the supercritical fluid escapes out. A similar fabrication route has been used elsewhere [43,54-55]. 

Verta-Turret, Primary or secondary extruder, Entwistle... 

One of the most demanding situations for controlling melt temperature is in foam extrusion. In the secondary extruder of a tandem foam extrusion line, the cooling screw is usually made with very deep channels to minimize viscous heating. Also, multiple (usually six) thin flights with a large helix angle are used to increase the heat transfer capability of the screw. 

There have been few basic studies of flow in Buss Kokneters. Only in the 1990s do we have pubUcations of Elemans and Meijer [89] and Lyu and White [90 to 95] with which to seek to understand and model the flow mechanisms and the flow fields in the various machine elements. Certain things are clear, such as the fluid mechanics indicates the axially oscillating screw inducing an oscillatory output. The machine operates under starved conditions with alternating fully filled and starved sections. The secondary extruder into which compounds exit from the Kokneter is starved and the length of fill oscillates but the output is uniform. This would feed a palletizing die. 

---