outer screw 132 this embodiment occurs for example when the inner screw 130 is fed core material 106 from feed throat 174 while the outer screw 132 is fed skin material 142 from feed throat 108 in these types of embodiments to achieve the later and more abrupt transition the poppet type valve 184 is desirably employed both to reduce the minimum amount of metal in contact with both melt streams thereby allowing for a more rapid switchover during injection and to extrude the core material 106 toward the periphery of the accumulation space 146 further as shown in fig7 one embodiment of the present invention eliminates contamination of the outer surface of molded parts by traces of core material by use of an elongated tip 202 on the outer screw 132 at the beginning of an extrusion cycle when the screw assembly 205 is fully forward that is the hydraulic cylinders 118 have pulled in platen 114 the end of the elongated tip 202 is nearly in contact with the rear face 204 of the machine nozzle 206 consequently as skin material 142 is extruded out of the elongated tip 202 any traces of core material 106 remaining within the barrel 102 and nozzle 206 are forced back toward the rear of the accumulation space 146 subsequent extrusion of core material 106 from the outer screw 132 will produce the desired toroidal shape to the interface between the two materials 106 142 the elongated tip 202 can contain a check.