Heatpipe/Thermosyphon Augmented Mandrels to Improve Cure Quality and to Reduce Cure Time in the Thermoset Pipe and Tube Filament Winding Process
Authors: Ouellette Joseph 
Published: January 2011

Mandrels used in conventional filament winding processes for the production of (GFRP) fiberglass pipe are generally not actively heated. Mandrels, after being overwrapped by continuous bands of filaments impregnated with uncured resin, are then passively and indirectly heated as the resin/fiber matrix covering them is cured. Curing occurs by placing the mandrel and uncured laminate assembly in a convection oven or by radiating the mandrel/uncured laminate assembly with infrared heat energy for a period of time at elevated temperature to affect a cure of the composite laminate. Typically mandrels are rotated during cure to assure homogenous resin consistency within the matrix. When processed in this fashion, mandrels are typically the least heated part of the assembly; the heat energy being applied only to the outside surface of the uncured resin/fiberglass matrix. The energy thus applied must then be conducted first through the wall of the composite laminate. The laminate tends to be thermally insulative as it cures. Therefore the cure of the resin matrix occurs without any significant thermal input from the mandrel. Significant time and energy are required to bring the temperature of the mandrel and the inner surface of the laminate to a temperature which assures an optimum cure. A new application of a mature aerospace derived heat transfer technology now provides uniform, controllable and discrete energy to the mandrel. Mandrels incorporating this technology exhibit near isothermal temperatures, random point to point, on the mandrel surface. These temperatures can be set and controlled from below ambient to 220°C. Heatpipe technology provides these mandrels with essentially super thermal conductive characteristics due to the latent heat phase change heat transfer methodology used within them. Mandrels incorporating heatpipe technology absorb energy based on any localized energy input and transfer that absorbed energy throughout the mandrel in an isothermal manner. This super thermally conductive property provides additional uniform heat to the mandrel surface covered by the uncured resin matrix. When the necessary thermal energy input is provided, the mandrel now transfers that energy as heat uniformly throughout the mandrel surface. The mandrel, now being actively heated, lends that thermal energy to the cure sequence by heating the uncured resin/fiberglass matrix in contact with the mandrel’s surface. This extra energy provided to I.D. surface of the laminate results in a shorter duration cure due to an increase in the surface area actively being heated. Heatpipe thermally enhanced (HPTE); mandrels not only have characteristics described above but also permit the use of increased thermal energy throughputs which provide thermal energy transfer rates, unachievable with existing processes. This increased heat transfer rate can result in a further reduction of the cure cycle. When coupled with an induction power supply and induction coil, these HPTE mandrels can be heated directly while rotating. The induction power absorbed by these HPTE mandrels is of a magnitude that permits resin matrices to be cured entirely from the mandrel side or “inside out” without the need for a convection or infrared oven.