the manufacture of a plastic material , particularly provided with fibers , is frequently carried out in an extruder . various problems have arisen in this connection . for example , problems arise in working long or endless reinforcement fibers , for example rovings , into the plastic . on the one hand , the fibers are so badly broken up that they have only very small lengths . on the other hand , it has proved difficult to impregnate the fibers sufficiently . according to the invention , in order to feed in fibers , the plastic is moved in batches past a pre - plastifying worm . this is achieved by a secondary worm located next to the pre - plastifying worm . the secondary worm enables the plastic to be provided in a controlled manner with fibers outside the pre - plastifying worm , and enables the secondary worm to be designed in such a way that it satisfies the conditions for a good combination and mixture of the plastic with the additives , and particularly a good impregnation of the fibers .