Screw kneader for plastic material having a controlling mixing section

A screw kneader for a plastic material comprising a housing having parallel bores overlapping one another, a pair of longitudinal screws each rotatably mounted coaxially in a respective one of said bores, said screws including respective, successive, first, second and third sections, each of said first, second and third sections being respectively formed with threads, the threads of the first, second and third sections of the pair of screws respectively interengaging one another for stripping plastic material from the screws and from walls of the bores, the threads of said first and third sections having a thread direction to form conveyor sections for advancing the plastic material from an inlet region of the housing at said first section to an outlet region of the housing at said third section, whereat the plastic material can be discharged at an outlet of the housing, the threads of said second section of the screws having a thread direction opposite the thread direction of the threads in said first and third sections to form a mixing section in which the advance of said plastic material is opposed and mixing of said plastic material is produced in said second section, said threads of said second section being provided with openings to permit passage of up to 20% of said plastic material in a direction of advance, and transfer elements connecting opposite ends of the second section of said screws respectively with said first and third sections of said screws, said transfer elements of the pair of screws mutually engaging one another to strip plastic material therefrom and from the walls of said bores, each said transfer element having one end adjacent to the threads of a respective end of said mixing section and a second end adjacent to the threads of a respective conveyor section, said transfer element including first spaced helical threads extending from said one end partly along the length of said transfer element, said first spaced helical threads being abutted against and being aligned with the threads of said mixing section and having spaces therebetween communicating with said openings in the threads in said mixing section, said transfer element further including a second helical thread extending from said second end of the transfer element and merging with said first helical threads of said transfer element, said second helical thread confronting and merging with the thread of said respective conveyor section.