The invention of the single screw extruder
FIELD OF THE INVENTION My present invention relates to an twin screw extruder for thermoplastic resins or the like, e.g. as used to plasticize the material and feed it to a mold cavity of an injection-molding machine with one or more driven feed Single Screw Extruder lodged in a generally cylindrical housing and defining therewith an upstream compression zone and a downstream expansion zone.
BACKGROUND OF THE INVENTION The changeover from compression to expansion in such a housing, upstream of its nozzle, may be accomplished in various ways, as by altering the pitch and/or the depth of the thread or by widening the housing internally to form a mastication space. In each instance the plastic material exiting from the compression zone into the expansion zone experiences a certain decompression with resulting evolution of vapors which may be allowed to escape through a degasification port; If, as has heretofore generally been the case, the compression zones encompasses a substantial length (e.g. four or more turns of a single thread) of the feed screw, or of each of several intermeshing feed screws, then the helicoidal web forming the screw thread is subjected to codirectional axial pressures on successive turns as a result of the progressively increasing pressure differential.
With the pressure varying monotonically along the helicoidal web, . Such radial thrusts, in turn, create undesirable friction between the feed sheet production extruder and the surrounding housing wall, with local overheating of the extrusion system and resulting impairment of the uniformity of the plastic mass.
OBJECT OF THE INVENTION My present invention aims at avoiding these drawbacks in an extruder comprising one or more feed twin screw extruder, especially those of the one-thread kind, serving to plasticize an advancing heat-softenable mass as discussed above.
SUMMARY OF THE INVENTION In accordance with the present invention, the thread of the feed Single Screw Extruderis divided by one or more peripheral gaps into a plurality of sections axially adjoining one another within a compression zone. The presence of the gaps intermittently reverses the gradient of the pressure differential and establishes a more uniform pressure distribution throughout the compression zone here considered.
BACKGROUND OF THE INVENTION The changeover from compression to expansion in such a housing, upstream of its nozzle, may be accomplished in various ways, as by altering the pitch and/or the depth of the thread or by widening the housing internally to form a mastication space. In each instance the plastic material exiting from the compression zone into the expansion zone experiences a certain decompression with resulting evolution of vapors which may be allowed to escape through a degasification port; If, as has heretofore generally been the case, the compression zones encompasses a substantial length (e.g. four or more turns of a single thread) of the feed screw, or of each of several intermeshing feed screws, then the helicoidal web forming the screw thread is subjected to codirectional axial pressures on successive turns as a result of the progressively increasing pressure differential.
With the pressure varying monotonically along the helicoidal web, . Such radial thrusts, in turn, create undesirable friction between the feed sheet production extruder and the surrounding housing wall, with local overheating of the extrusion system and resulting impairment of the uniformity of the plastic mass.
OBJECT OF THE INVENTION My present invention aims at avoiding these drawbacks in an extruder comprising one or more feed twin screw extruder, especially those of the one-thread kind, serving to plasticize an advancing heat-softenable mass as discussed above.
SUMMARY OF THE INVENTION In accordance with the present invention, the thread of the feed Single Screw Extruderis divided by one or more peripheral gaps into a plurality of sections axially adjoining one another within a compression zone. The presence of the gaps intermittently reverses the gradient of the pressure differential and establishes a more uniform pressure distribution throughout the compression zone here considered.