Tom Bishop
EASA Technical Support Specialist 

Did you ever wonder if the preheating instruc­tions from solvent varnish and solventless resin (hereafter we’ll use the term “resin” when it ap­plies to both) manufacturers were really all that important? The short answer is, yes, they are. Here we’ll expand on some of the reasons that preheating is a key step in the winding process. 

One of the first benefits of preheating is that it drives out moisture that may have settled on sur­faces, or been absorbed by insulation material. A little known aspect of pre­heating is that it relieves the mechanical stress cracks, termed “crazing,” on the magnet wire insulation coat­ing that occur during coil winding and insertion. Epoxy B-stage materials can be set by preheating, provided the preheating time and tempera­ture meet the epoxy’s curing requirements. Random wind­ings typically don’t use many B-stage materials; however, many of the lacing products for endturns are thermoset­ting. Form coil windings often have B-stage surge ropes, and some felt packings used for endturn coil bracing are B-stage epoxy loaded. 

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