Pin Single Screw Extruder
A pin single screw extruder for conveying polymeric material, especially elastomeric material, comprising a stationary twin screw extruder housing and an extruder screw rotatably mounted in the housing. A purality of radially outwardly extending pins are disposed on the extruder screw, at least a portion of which is embodied as a cylinder. The extruder housing, adjacent to the cylinder portion of the extruder screw, is provided with essentially helical conveyor paths and with openings for the pins.
Such extruders have been successfully used for many years. For example, DE-OS 22 35 784 shows a pin sheet production extruder from 1972 that is already adjustable with respect to the radial penetration depth of the pins. The pins extend to the extruder screw and are secured to the housing, whereby screw ribs of the extruder screw are provided with respective openings that enable the pertaining pin to pass through at this axial height.
Although it is often assumed that the greater throughput of such a pin extruder in comparison to an extruder without pins is based on the fact that the rotation flow is obstructed in the screw channel, more recent tests show that the throughput gain results from the increasing relative speed between the fixed pins on the one hand and the rotating screw flanks on the other hand.
A similar solution, which in principle is also suitable for single extruders, is known from DE-OS 26 50 248. With this arrangement, rows of pins extend in the manner of annular grooves that, however, are not continuous but rather are formed by a plurality of inwardly extending projections. Here also there results practically no conveying effect, but a great intermixing, as is desirable for plasticizing and homogenization of the extruded material.
A drawback of the known twin screw extruder is that a plurality of pins are required, so that the flow channel is greatly narrowed by the pin cross-sections. This produces a very great pressure loss, so that the advantage of the pin extruder with regard to the gain in throughput is again compensated for by the high pressure drop in the pin zone and the thus accompanying loss of throughput.
A number of other forms of pin extruders have also been proposed. Most of the solutions proposed since the beginning of the 70's and partially also realized have pins that extend radially inwardly, in other words proceeding from the extruder housing. Such pins are also partially combined with transfer-mix extruders in order to further improve the thorough mixing of the cold extruded material. However, the problem continues to be the throughput, which remains somewhat single screw extruder.