A4e. single screw extruders - Standard single screw extruders, as shown in Fig. 4I, have the capability of mixing plastic materials fed through them. Mixing is a desirable function during an extruding operation although it is not the prime function of the machine. Although these machines do not have the blending power of continuous mixing equipment or other dedicated mixing machines, they have proven useful for modest mixing tasks. Common operations are the mixing involved when fillers, antioxidants, stabilizers, color concentrates, or other ingredients are added to the resin compound to be extruded. These machines are also capable of blending reground scrap material. The back pressure of an extrusion die develops some back flow in the material in the barrel, further adding to the mixing. In all such machines, the added ingredient must be metered properly into the feed hopper.

A4f. compounder-extruder mixing - uses a machine similar to an single-screw extruder, but whose prime purpose is mixing rather than extruding. The barrel of the machine contains a series of stators outside the flutes of the extruder screw. The stators are opposite handed from the screw and are of alternatingly different heights. Torpedo and screw modifications are also made in the machine. These involve varied pitch and diameter of the screw and spiral grooves in the barrel. The screw is configured to provide an extensive mixing section and a high intensity variable-shear section. In the central area, the screw and barrel are shaped to permit the escape of trapped air and volatile materials. A vacuum pump draws them off.

A4g. twin screw extruder mixing – is performed on machines with two feed screws side-by-side in a double barrel. The screw flights usually overlap and intermesh. The screws usually rotate in the same direction, which means that there is a wiping and shearing action in the area of overlap. The material follows a figure-8 path as it moves through the machine and is thoroughly mixed. Fig. 4A4g shows the cross-section on one twin-screw mixer.

Fig. 4A4g The cross-section of a twin screw mixer showing how the material is mixed as it is transferred from one screw to the other.

A5. pelletizing and dicing of plastics - Commercial plastics intended for molding and extruding operations are most often supplied in pellet form. The advantage of pellets is that they can be easily handled, accurately weighed, and conveniently stored. Machines to create pellets from compounded material are of two basic types:

1) rolls that mix the material and convert it to a sheet that is slit and diced and, 2) strand pelletizers, or extruders with attachments at the outlet end to cut extruded rod-like shapes into small pieces. The final pelletizing may be carried out with the material either cold and solid or still hot as it emerges from a die.

A5a. mixing and dicing with two-roll mills - Two-roll mills can be used as a final mixing step to introduce and blend plasticizers and fine particles of solid additives. Typically, the horizontally-shafted rolls rotate in opposite directions and they pull and nip the material through the space between them, providing good shearing action. The exiting material is slit by strip cutters into ribbons that are fed to a cooling tank and then to a dicer. Fig. 4A5a illustrates the two-roll mixing action.

Fig. 4A5a Schematic view of two-roll mixing followed by slitting and dicing.

A5b. strand pelletizers - each consist of a screw or gear-pump extruder and die, and a rotating cutter that works against stationary blades. Each machine also has provision for cooling: by air, airvacuum, or water, either before or after cutting. The machines include a drying system if water is used, and a means to collect the pellets. Hot-face cutters cut the extruded strands into pellets while they are still soft. Cold-cutting systems cut the strands after they have cooled. With cold-cutting systems, the strands may be drawn and pulled through a water bath before cutting. With hot-face cutting systems the cutters act upon the strands before they are cooled by air, fluidized bed, water spray, and/or water stream. The method chosen depends on the properties of the plastic, particularly its melt strength and sensitivity to temperature and its ability to withstand a residence period at a high temperature.

A5c. underwater pelletizing - Material exiting from a mixer-extruder flows through heated multiple-opening extrusion dies into a water chamber where the strands are sheared into pellets by a rotating, multi-bladed cutter moving across the die face. Water circulated through the chamber cools the material and conveys the pellets through a discharge port, away from the cutting area, and to a dryer. Water provides a convenient handling medium. The dryer then removes the water from the plastic pellets by centrifugal force. Polyethylene, polypropylene, PET, polystyrene, ABS, SAN, and thermoplastic elastomers are pelletized with this method.