Hi Jim and Brian;
I don't understand your disclaimer "We are not talking about some sort of FRP structure here, but a headbox whose main material of construction is a resin specifically designed for this purpose."
FRP thermosetting plastics, as shown by GM in the Corvette, Boeing and others making stealth aircraft, etc, readily provide the dimensional stability and chemical inertness required for a headbox. An FRP laminate could have a Halar (ECTFE) or other high performance corrosion liner for complete non-stick properties, resisting microbiological growth and other causes of boilouts and other biocidal additions.
Industrial thermoplastics like PET, polysulfone, polycarbonate and even laminated glass (like car windshields) readily resist modest temperatures for paper-making without any loss of stiffness or strength.
Headbox construction for various products could consist of bolting together molded segments with carefully designed fluid dynamics properties for a variety of widths with a metal slice lip bolted in place if it it decided - though I doubt it should be - that only a metal slice lip is acceptable. Plastic shapes can be structurally supported with an external skeletal frame to ensure long term dimensional stability.
I proposed a crosslinked HDPE headbox approach chamber to P&G about 7 years ago precisely because it provides all the critical functions and properties they expect from a stainless steel unit with the engineering plastic fastened to a metal (SS) exoskeleton. The idea was too avante garde even though the non-metal equipment was 1/6 as heavy as a SS unit and was immune to crevice corrosion in seams and corners.
By the way, ALL white water piping around a paper machine can be plastic - CPVC being an obvious choice since it is rated to 210F and can be overwrapped with FRP for super stiffness. Solid Halar pipe or dual-laminate Halar/FRP wrap pipe would be permanently clean since nothing sticks to the surface.
However, before you/we can persuade anyone to construct a modern headbox from better, light-weight, non-metal materials, we might first consider our inability to persuade the industry (other than IPPEL in Brazil) to make rotating pressure vessels (dryer cans) from welded steel instead of material - gray cast iron - everyone KNOWS to be unacceptably dangerous and devoid of any redeeming properties. Continuation of this idiotic and unsafe practice supports your frequent observations on the dearth of quality management and intelligent leadership in the industry. I've been sorely tempted after each dryer explosion to request OSHA or at least one insurance company to ban new gray iron dryers, but instead have pledged to work around them as if they are potentially fatal bombs.
I hope this commentary contributes a tiny shred of encouragement to your LGMI mission, especially your persistent efforts to educate and inspire an industry that is too stupid to appreciate your insights or to realize how perilous its existence is.
Have a safe, healthy and productive summer.