Coaxial Twin Screw Extruders

There are two versions of CTS extruders commercially available. In one version, molten material from the outer screw is transferred to the inner screw where it is pumped towards the die. In this case, the inner screw is used for forward pumping see Fig. 10.59. 

If the channel depth of the outer screw is reduced to zero, all the polymer melt is pumped toward the die hy the inner screw. This inner melt removal (IMR) screw was Invented by Kovacs [20] of Midland-Ross Corporation. It Is essentially a very complicated version of a barrier screw and does not offer any obvious advantages over conventional barrier screws. 

Another version of a CIS extruder is the solids draining screw (SDS) originally developed by Klein and Tadmor [21] of Scientific Process Research, Inc. In this design, unmolten polymer drains into the inner screw see Fig. 10.60. 

Material is transported upstream in the inner screw and plasticated. At the end of the inner screw, the polymer, which is now molten, is pumped back into the channel of the main screw. A modified version of the SDS screw for use in a molding machine was patented on September 22, 1981 [22]. Another modification of the SDS screw involves using a barrier type main screw to improve the solids draining process. This barrier SDS screw was patented on June 14, 1983 [23]. The SDS screw is claimed to give higher output and lower energy consumption. However, from a functional analysis it is difficult to see why recirculation of a fraction of the polymer flow would increase output or reduce power consumption. 

The mechanical design of CTS extruders is considerably more complex than conventional single screw extruders. Since material has to leak through holes in the main screw, there is a chance of plugging and of stagnant areas. Also, maintenance and operating procedures of CTS extruders will be considerably more complex than single screw extruders. 

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