Nanjing GIANT Machinery Co.,Ltd.

Knowing the respective bulk densities of different polymeric materials to be plasticated together makes it possible to provide separate plastication conditions in each suitable for a different material. This is done by constructing a Twin Screw Extruder with two or more different plastication sections. The plastication section of an Single Screw Extruder typically has a frusto conical shape with the circumference of the cone increasing in the downstream direction. As polymeric material passes through a plastication section of an extruder the space between the root of the screw and the interior wall of gear pumpthe barrel (the "channel depth") decreases, and the polymeric material is compressed. The compression ratio of a plastication section of a screw is determined by a ratio of the channel depth at the top of the frustum to the channel depth at the base of the cone (base of the frustum).

This is shown in gravimetric feeder as the ratio of H ₀ /H ₁ . The compression rate of a plastication section of a is determined by a relationship derived from the right triangle formed by a line intersecting the circumference of the top of the frustum and parallel to the longitudinal axis of, the intersection of such line with the base of the cone (base of the frustum) and the root of the screw. This relationship is the compression ratio divided by the quantity L/D where L is the distance from the top to the base of the cone frustum and D is the diameter of the annulus of the barrel. These elements are also shown in FIG. 1.

It has been found that when two or more plastication sections are constructed in a screw for plastication of a mixture of materials having different bulk densities, a sufficient compression ratio for each such section may be in the range of about 1.2 to about 5. The compression ratio for a section in which a material having a bulk density of 0.6 g/cm ³ or more is to be plasticated may preferably be in the range of about 1.2 to about 3.3, and the compression ratio for a section in which a material having a bulk density of less than 0.6 g/cm ³ is to be plasticated may preferably be in the range of about 3.3 to about 5. A sufficient compression rate for each such plastication section may be in the range of about 0.01 to about 15, preferably about 0.05 to about 13.7. The compression rate is selected such that, for a given temperature and pressure within the extruder.a greater length is allowed for the plastication section of the screw for those materials which are relatively more difficult to plasticate because of, for example, a higher bulk density and/or a higher melting or softening temperature. The plastication sections sheet production extrudermay be constructed in the screw in any order such that one section is followed downstream by Single Screw Extruder, the downstream section having a higher or lower compression ratio and/or a higher or lower compression rate than the upstream section. One or more such plastication sections may, but need not be, immediately adjacent.