Abstract: This relates to the increasing of the intrinsic viscosity of resins, particularly PET, by effecting a solid state polymerization of the resin. More particularly, the solid stating is effected in conjunction with an injection or extrusion unit wherein after having the IV of the resin increased, the heated resin may be fed dry into an injection unit thereby utilizing the heat of the heated resin to reduce heating cost. The solid state polymerization is effected by heating the resin in the presence of either a vacuum or an inert gas such as nitrogen.