Preforming Thermoplastic Ducts

A method for preforming a thermoplastic member to form a preform generally corresponding to a desired configuration of a thermoplastic duct defining a passage, the method comprising:

• providing a hollow tube defining a longitudinal cylindrical cavity having a cross-sectional size about equal to the cross-sectional size of the duct such that the cylindrical cavity corresponds to the desired configuration of the duct;
• providing a funnel extending longitudinally between first and second ends and extending longitudinally from an end of the tube such that the longitudinal cavity of the hollow tube extends from the second end of the funnel, the funnel tapering in the longitudinal direction toward the tube from the first end to the second end, the first end of the funnel having a cross-sectional size larger than a cross-sectional size of the duct and the second end of the funnel having a cross-sectional size about equal to the cross-sectional size of the duct, such that the second end of the funnel is smaller than the first end of the funnel and generally corresponds in size to the end of the tube so that the funnel tapers toward the tube;
• providing an inner member received by a passage defined by the thermoplastic member, the inner member defining an outer surface that corresponds to the desired configuration of the duct;
• a plurality of electrical resistive heaters disposed on the tube and the funnel, the heaters extending longitudinally along the tube and the funnel;
• inserting the thermoplastic member into the first end of the funnel;
• urging the thermoplastic member through the second end of the funnel and into the hollow tube to slide the sheet longitudinally in the funnel such that the tapering shape of the funnel causes the sheet to bend to the diameter of the cylindrical cavity defined by the tube and the desired configuration of the duct, thereby configuring the thermoplastic member to the desired configuration of the duct;
• during at least one of said inserting and urging steps, heating the thermoplastic member to a processing temperature between about 5° F. and 70° F. less than a glass transition temperature of the thermoplastic member;
• holding the sheet at the processing temperature for a processing hold interval and thereafter turning off the heaters such that the thermoplastic member cools in the tube before being removed therefrom;
• and
• joining longitudinal edges of the thermoplastic member to form the duct.