in an opposite helical direction to that of the threads 33 of the screw 32 in fig2 more specifically the supply portion 21a of the cylinder 21 is composed of an enlarged diameter portion 23 and a converging portion 24 merged from the portion 23 into the compression portion of the cylinder 21 in this cylinder 21 is rotatably inserted a screw 22 which is composed of an enlarged diameter portion 25 and a tapered portion 26 positioned to radially face the cylinder converging portion 24 thus the plastic material fed into the clearance between the enlarged diameter portions 23 and 25 will wet the inner walls of the two and will then be subjected to compression in the clearance between the converging portion 24 and the tapered portion 26 under these conditions the plastic material is cenveyed to the nozzle where it attains its highest pressure level in another embodiment of fig3 on the other hand the supply portion 31a of the cylinder 31 is likewise composed of an enlarged diameter portion 35 and a converging portion 36 in this cylinder 31 on the contrary is rotatably inserted a screw 32 which has the same length and an uniform diameter the enlarged diameter portion 35 is formed at its inner periphery with a plurality of threads 34 which are arranged in an opposite helical direction to that of the threads 33 of the screw 32 the plastic material engages the helical grooves formed in the threads 34 of the enlarged diameter portion 35 and then is highly compressed by the screw 32 at the converging portion 36 the plastic material thus conveyed attains its highest pressure level at the nozzle tip according to another major feature.