A production line for the manufacture of strips of vinyl siding comprises an extruder extruding a strip of hot plastic material, such as vinyl. Next, an embosser supplies a decorative pattern to the heated strip. A cooling apparatus receives the heated strip and immerses the strip in a water bath to lower the temperature to a selected temperature range. The material, now reduced in temperature, is then passed to a former having dies creating joining runs in the cooled, embossed siding strip. Finally the strip runs through a final cooling tank which chills the strip to harden the joining runs created by the former. The cooling apparatus consists of a water bath tank with entry and exit guides for the strip, which are mounted above the level of the water in the water bath tank. A third guide, such as a roller, is positioned between the entry and exit guides and is selectively extendable in depth into the water bath so as to selectively extend the strip deeper into the water bath and thereby vary the dwell time of the strip to regulate cooling.