the secondary flow 52 passes through passage 53 it is combined with additive flow 58 additive flow 58 can contain a color blowing agent and or thermoplastic a check valve 59 is utilized to prevent back flow of secondary flow 52 into supply line 67 for additive flow 58 secondary flow 52 and additive flow 58 flow together without substantial mixing until they move into mixing section 60 mixing section 60 contains several mixing element 61b that have been previously described in u s pat no 3 051 541 3 051 453 3 195 865 3 206 170 3 239 197 3 286 992 3 328 003 3 358 749 3 382 534 3 394 924 3 404 869 3 406 947 and 3 506 204 as secondary flow 52 and additive flow 58 pass through mixing elements 61b a homogenous admixture 69 is formed which moves through passage 62 and finally back into passage 63 with primary flow 70 leaving mixing elements 61a as secondary flow 52 is being mixed with additive flow 58 primary flow 51 passes through slide plate aperture 57a and fixed plate aperture 57b where it may be combined with additive flow 64 additive flow 64 may contain a different color blowing agent and or different thermoplastic material or may be of different density or other property a check valve 65 is utilized to prevent back flow of the primary flow 51 into the additive flow 64.