Extruder cooling

The storage facilities at the end of the profile production line depend on the type of product (see Fig. 4.19). If it is rigid then the cooled extrudate may be cut to size on a guillotine for stacking. If the extrudate is flexible then it can be stored on drums. 

To separate any ethylene from the LDPE in the autoclave effluent, the pressure is let down in successive vessels and the ethylene flashes off (vaporizes). It is recycled to the compressors. The LDPE in a molten (hot liquid) state is cooled, extruded, pelletized, dried, and bagged. 

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...

This chapter will present the three most common uses for melt-fed extruders (1) pumping of resin from a compounding operation to a pelletizer, (2) large-diameter extruders used for finishing sections in resin production plants, and (3) secondary cooling extruders for foam sheet lines. 

Figure 15.17 Typical axial pressure and temperature profiles for a secondary cooling extruder running a PS-blowing agent mixture...

A typical pressure and temperature profile for a secondary cooling extruder is shown in Fig. 15.17 for a PS resin. 

Table 15.3 Typical Viscous Seal Channel Dimensions for a 152.4 mm Diameter Secondary Cooling Extruder...

For determination of the steady state shear viscosity the Instron capillary viscometer model 3211 was used at 190 C. Six capillaries were used, three each of diameter d = 747 and 1273 ym. The length to diameter ratio In each series varied from L/d > 0.6 to 60. The standard Bagley and Bablnowltsch corrections as well as that for the pressure effects (45) were applied. The extrudate swell was determined on air-cooled extrudates, 5 cm in length. 

U.S. Pat No. 6,578,368 [51] discloses a method for cooling extruded and molded cellulosic-polymer composites, among other plastics and plastic compositions the said method uses cryogenic fluid with a temperature below about 250° F, such as liquid oxygen, liquid nitrogen, liquid neon, liquid hydrogen, and liquid helium. 

Usually elaborate control systems cannot correct for problems such as those caused by a (1) worn screw and barrel, (2) inadequate drive torque, or (3) poor screw design. For example, such systems will not yield good temperature control unless all features essential to good control are well maintained. Burnt-out heating elements cannot be tolerated. Another common deficiency for liquid-cooled extruders is fouling or restrictions in the plumbing system or inoperative valves. Other factors of these types also exist. 

The term stretching is used to denote separate operations carried out after reheating of the already cooled extrudate or calendered sheet. It is mostly performed at lower temperatures than drawing and therefore requires higher stresses. Both drawing and stretching can be used to obtain monoaxial or biaxial orientations. 

Cross-flow quency n. In cooling extruded polymer filaments, refers to cooling air directed from one side cross the path of the filaments. There may be some type of suction on the opposite side to remove the heated air. 

Outflow quench n. Air for cooling extruded polymer that is directed radically outward from a central dispersion device around which the filaments descend. 

Contracool, Air-cooled extruder, Battenfeld Gloucester Engineering Co. Inc. 

In many cases, the temperature at the inside and outside barrel surfaces will not be known. In those situations we have to find another method to determine the heat fiux through the barrel. This issue was studied Radovich [277] who compared the cooling capability of air- and water-cooled extruder barrels. 

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. 

C.D Han, Analysis of the Performance of a Cooling Extruder in Thermoplastic Foam Extrusion, Conference Proceedings, 45 SPE ANTEC (1987)... 

Fig. 6. Foam extrusion line A, resin feed B, extruder for melting and gas incorporation C, gas supply D, gas metering and compression E, optical windows F, cooling extruder and G, die. P represents a pressure gauge. Courtesy of Pol5rmer Processing Institute.

MATHEMATICAL MODEL AND NUMERICAL ANALYSIS OF POLYMER MELT FLOW AND HEAT TRANSFER IN A COOLING EXTRUDER... 

This paper presents a mathematical model and numerical analysis of momentum transport and heat transfer of polymer melt flow in a standard cooling extruder. The finite element method is used to solve the three-dimensional Navier-Stokes equations based on a moving barrel formulation a semi-Lagrangian approach based on an operator-splitting technique is used to solve the heat transfer advection-diffusion equation. A periodic boundary condition is applied to model fully developed flow. The effects of polymer properties on melt flow behavior, and the additional effects of considering heat transfer, are presented. 

A polymer melt in a cooling extruder is assumed to be incompressible, and its flow is assumed to be steady-state and compliant with conservation laws for mass, momentum and energy. These laws are expressed by the following three partial differential equations the eontinuity equation. 

Since a standard screw geometry within a cooling extruder has a periodic flow chaimel, a periodic inflow/outflow boundary condition is applied to simulate fully developed flow. Validation tests showed that a single pitch of the screw with poiodic boundary conditions is sufficient to model polymer melt flow behavior in the whole cooling screw geometry. 

C. Rauwendaal, Screw Design for Cooling Extruders, SPE, ANTEC 2004. 

---