The present invention provides in some embodiments, a method for insert molding a coupling between catheter segments with different diameters and/or a catheter having different diameters. Generally, the method can include the steps of providing first and second tubes having larger and smaller diameters, respectively and positioning the first and second tubes onto a core pin assembly having an outer surface at one section which forms a contoured inner profile for a joint section to join the first and second tubes. Additionally, the method can include injection molding the joint section to join the first and second tubes, to form the catheter tube with the contoured inner profile at the joint section.
