Pigment twin-screw extruder

In the premix process, all additives are premixed in a mixer and then fed to the twin screw extruder at the primary intake stage via a volumetric feeder. A portion of the polymer is generally mixed with the pigments in powder form to achieve a better pre-distribution of the pigment (see Fig. 4.31). 

To a continuing process the pigment dispersions are added directly to the plasticising equipment by volumetric or gravimetric measuring units. Further blending can then be done on a single- or twin-screw extruder. 

The pigments required for a specific color tone are intensively mixed with powdered plastic using dispersants (waxes), then dispersed in a twin-screw extruder (high energy consumption) and thereby embedded in the plastic. Laboratory mixtures and production often show different results. 

Table 5.2 Specific energy consumption values for pigment concentrates produced on co-rotating intermeshing twin-screw extruders (adapted from [42]) ...

Most plastics e.g. polyolefins and polystyrenes and their derivatives such as ABS (acrylonitrile-butadiene-styrene) and SAN (styrene-acrylonitrile) are supplied by the manufacturers in ready-to-use form with most of the above-mentioned stabilizers or simply need to be additionally stabilized with other additives, e.g. antistatic agents and HALS stabilizers, as required. On the other hand, in the case of other materials (e.g. PVC) it is the end user who adds the additives, pigments or preparations. This is normally done on fluid or high-speed mixers, although in the past gravity mixers or tumble mixers were also used. The mixture is then homogenized on mixing rolls, kneaders, planetary extruders or twin-screw kneaders and further processed. 

The Concentrate is a high level of pigment mixed with pulverized polyethylene. This preblend is then extruded on a twin-screw. The concentrate is letdown at end use levels and compounded on a single screw extruder. The compound is then injection molded to make displays. 

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