Seeking Lightweight Materials for an Oversize Moulding

Pros suggest alternatives to wood for a 15-inch-wide cove moulding installation. October 28, 2014

Question (WOODWEB Member) :
We did a poplar prototype section of a 15" wide cove moulding last week. It is an interior paint grade project where the edges of the cove blend directly into sheet rock to create a curved transition from the flat ceiling to the recessed upper ceiling surface. This is for an oval tray style recessed ceiling so the 15" cove is curved. The weight of this giant poplar cove was 7 1/2 pounds per running foot and the oval tray ceiling is almost 100 feet around. This is a whole lot of weight hanging up there.

I'm wondering what alternative materials I might experiment with that would machine with moulder knives and be much lighter than poplar. Ultralight MDF is an alternative and might save a little weight. Another option might be sign making foam board but I have no idea about adhesives for building up layers or how it would machine with the moulder.

I've discussed ribs and a wire frame base for plaster as an alternative but the customer isn't too excited about that option. I've also sent it off to Flextrim for evaluation although I'm concerned about stability of the material at a 15" moulding width. At the least backing ribs would be needed for strength and significant movement could ruin the installation. Are there any other thoughts and suggestions?

Forum Responses
(Architectural Woodworking Forum)
From the original questioner:
I should add to the above that I'm not totally happy about the amount of movement we would see in poplar across the 15" wide face of this cove. That is why a nice light weight species like pine is out - just too much movement.

From contributor B:
Personally I would never do anything that big in wood. That should be done in plaster whether precast in sections or done in place. Long term I believe that the wood movement would be a problem and create a constant joint problem between ceiling and wall. A composite or foam is an alternative. No matter what it will be pricey.

From Contributor G:

Why wouldn't you just do a 1/4 barrel corner with plaster? Just a few forms and hang wire mesh on it. The plaster guy will take care of the rest. It'll be seamless.

From contributor M:
Look into Celtec by Interlam. It's a sheet material that is thermoformable. Not sure about its limits with compound curves, but plastics are a lot easier than wood in these regards.

From Contributor O:
Historically, this was always done relatively easy with plaster and lath and/or wire, on site in place. It doesn't matter that your customer prefers wood. Despite his wishes, there will be too much movement, and there are better ways.

From contributor L:
I have to agree with Contributor O. We use Celtec for some things and it would probably form to that compound curve in a male/female mold at 300F. Don't know about its long term stability.

From contributor C:
What about creating a series of molds, and casting each section from a pourable foam? A lot of movie props are done this way.

From contributor K:
You could try Fypon - they do custom.

From contributor R:
Rigid urethane sign board machines well. I'd suggest nothing lighter than 10 lbs/cubic foot for smooth painting. What about the mouldings used on exterior work under stucco. Isn't that polystyrene? Cut with CNC hot wire machines I think. That would require filler and lots of finishing time.

From contributor N:
A friend of mine's dad is a drywaller and he would do massive crown moulding out of polystyrene. He didn't buy a machine, just would bend wire to the profile and hook it up in loop form to a current - same idea as a soldering iron. Not sure exactly how he finished it but I know it was not very labor intensive. I think he just skimmed it with lightweight all-purpose mud then painted it. His wife knew how to do textures with paint so he could get all kinds of finishes.