Machining Custom Crown from Synthetic Trim Material

Advice and feedback about making custom crown molding out of plastic trim materials, to match old trim for a curved porch restoration job. March 20, 2007

Does anyone have experience or an opinion on machining "Royal Wood" trim boards to make crown molding for an exterior application? I would be using their "1 x 6" stock. I am restoring a Victorian era curved wrap-around porch. To match the old crown molding profile, I need have this custom made. I am considering using Royal Wood because it is clear, readily available and it comes in 16 foot lengths.

Their website says this material can be machined, and that if the interior is exposed due to machining, that it will still be weather proof even if unpainted. But I will be painting the finished crown molding. However, my friend who will actually make the crown molding has never used Royal Wood before. Any input would be greatly appreciated.

Forum Responses
(Architectural Woodworking Forum)
From contributor B:
What you are talking about doing here is making your own flexible crown moulding. You could run into some problems other then those that might occur during the machining process.

I believe when you purchase ready made flex crown it is pre-twisted to make the upper and outer edge run the top outer radius while the lower inner edge runs the facia radius (outside curved descriptions - just the opposite for inside curves).

If you run your own crown I don't think you will be able to make the bend without the crown wanting to ride in flat up against your facia. This is because the bottom edge on an outside curve crown is running at a typically 3" to 4" smaller radius then the upper outer edge.

I'd suggest you take a piece of the un-profiled "Royal Wood" and seeing if it can be held up at the same angle as the crown and made to run the curve. Our business is curved mouldings and we make a lot of wood curved crown. I haven't worked with much flex crown over the years but do have a pretty good idea of how it gets applied.

From the original questioner:
I need to provide a bit more clarification. I was not selecting the Royal Board because of its flexibility. I just want to make crown molding. I'll use it straight on the many straight sections of the roof edge. And on the 8' radius portion of the roof edge, I will attempt to bend this with a series of notches along the lower edge as someone had advised me to do to bend wooden crown molding in this type of application.

I should post a separate question on how to best bend the crown around the curved edge of the roof. But for now, I simply need to know if the Royal Wood will work acceptably as a machineable substitute for real wood. Will we hit any surprises or have any regrets? I am just trying to make sure before I ruin about $160 worth of material.

From contributor G:
How did you know he was going to try to bend it? I think you might have to get the 8' radius crown out of something wider than 6".

From contributor B:
I assumed the original questioner was trying to bend it because in the second paragraph he said: "I am restoring a Victorian era curved wrap-around porch". As such it didn't even occur to me that he might be talking using it for the straights.

From contributor W:
There should be no reason you can't mill the Royal (though I don't use it). I prefer regular PVC as it mills very nicely with conventional tooling and is readily available, I find that Azek and Versatex have the finest "grain" (meaning core structure) and leave a comparatively smooth finish that will require a little sanding to smooth out. Primed (heavily) and painted it should work great, be indistinguishable from wood and last forever.

As far as bending you'll just have to try a piece and see how badly it deforms. I doubt kerfing will help much in your application given the "lean out" of that size crown. You could try heat but it's a PITA. You could also try flexible polyurethane if you're so inclined.

From contributor J:
“I'd suggest your taking a piece of the un-profiled "Royal Wood" and seeing if it can be held up at the same angle as the crown and made to run the curve.” This is very good advice. I don't think you'll get that stuff to bend (as a crown) the direction you want even with that gentle a radius.

From contributor Y:
Am I correct in assuming that this "Royal Wood" is neither? That is - neither "royal" or "wood"? Contributor B hit the nexus of the problem/solution with his mention of the differing radii.

From Gary Katz, forum technical advisor:
To contributor W: You are right. Azek and Kleer have the smoothest core and machine the nicest.

Couldn't he also use a heat blanket to bend the crown? I'm pretty sure that would work very easily. If you get PVC too hot, it turns to cooked spaghetti, so you'd want to cook it just right.

From contributor W:
Sure, he could use heat to bend it and probably with good results (though don't forget the blocking required for attachment of the bent material), but buying the blankets is big dough, renting them is difficult if not impossible, and trying to cook your own has yielded consistently poor results for us (we've tried lots of heating methods, none of which worked very well except on short lengths). I'd say get the blankets or don't bother with PVC (unless the radius is large enough to go for a bend without heat, best of all solutions). Still though, for the amount of trouble involved in bending PVC you're better off just buying the stuff in flex.

From contributor G:
In my area we can buy Azek brand mouldings so I have no idea why you'd be making it. You'd be happy with the finish and yes they can be heated in a blanket (looks more like a sleeping bag) and it bends well. I bought my blanket from my supplier and he discounted it because I was buying so much crown and board (for a single job).

Just heat up the moulding and push it to the shape you want – it’s pretty easier. Why bother with everything else. Just get the job done with the right tools. I always go out and get the correct tools for the job. If I never use them again I just sell it off. Someone else is looking for the blankets and I bet you'd lose less on it than renting it. Most of the time I just include it in my bid and it's charged off to the job. By the way - my supplier has some Azek that was heated and then shaped into a pretzel. It can certainly make the 8' radius.

From the original questioner:
A) The crown molding is being custom made because I am matching the original 1902 crown molding.

B) My friend has made some of the crown molding already, using a Bellsaw with a single blade. He reports that the Royal Wood shapes well, and is easier to cut than pine. He did note that it sands oddly compared to real wood, but is quite satisfactory.

C) As to making the 14 feet of curved crown molding, I had planned to follow the method described in Fine Homebuilding. But where wood was used, I planned to use a piece of Royal Wood. I had no idea that plastic wood could be bent with warming blankets. I'll have to think about this some more.