Typical single screw extruder

From a first aspect, the present invention provides an extruder machine comprising: an extrusion die; a feed passage disposed behind the die and guiding a raw material thereinto; and further comprising characteristically at least one pair of blades which each has a helical edge formed at a predetermined lead angle and protruding into the feed passage from an inner periphery thereof, wherein the blades are driven to rotate so as to force the raw material towards the die.

From a second aspect, the present invention provides an extruder comprising: an extrusion die; a feed passage disposed behind the die and guiding a raw material thereinto; and further comprising characteristically a stationary core disposed in an intermediate region of the feed passage so as to define at least one annular passageways around the core and in fluid communication with the feed passage; and at least one pair of blades which each have a helical edge formed at a predetermined lead angle and disposed in the annular passageway, wherein the blades are driven to rotate so as to force the raw material towards the die.

In a single screw extruder, the screw typically has an external diameter corresponding to outer edges of the helical threads or flights. The flights may be under sized in relation to the barrel in the feed or hopper area so as to reduce feedthroat friction. Typically, a single keyway in the shaft area of the screw matches a keyway in a sleeve portion of a drive shaft so that a key positioned in the matching keyways is utilized to transmit rotational movement from the drive shaft to the screw. The key is a separate piece of rectangular metal which fits into the matching keyways. A disadvantage attendant with the use of keys is that they may be lost or misplaced. Also, worn keys can contribute to screw wobble.

As the viscosity of the thermoplastic material increases, it has been observed, that in single screws of the above described type, radial screw movement or wobble may develop. Clearance in the feed area between the outside diameter of the screw and the inside of the barrel permits movement of screw. As the screw wears, metal may be removed between the exterior of the screw and the inside of the barrel. This creates additional clearance which may cause even more wobble or radial movement.