filament construction formed from heat shrinkable material such as a thermoplastic by way of example only and not limitation exemplary fiber materials may include polyester polypropylene nylon and combinations thereof according to the potentially preferred practice the yarn is conveyed through the heat treating unit 24 at a rate of speed such that the yarn reaches a state of temperature equilibrium in its cross sectional dimension but not at all segments along its length in particular it is preferred that the temperature and yarn speed be set such that the water in the wet segments is at least partially evaporated but the temperature is not raised significantly past the boiling point of the water in this regard it is to be understood that some portion of the applied water may remain in the yarn following heating as will be appreciated during heating the water acts as a heat buffer to limit temperature elevation within the yarn at the wet zones conversely when the dry zones are subjected to the same treatment the temperature is elevated to substantially higher levels since no buffering force is in place it has been found that by bringing the wet segments to a point of nearly complete water evaporation that the dry segments may be heated sufficiently to provide a substantially uniform heat profile in the cross section significantly following heat treatment and any subsequent dyeing and finishing operation the filaments within the.