The bottom line about poly mouldings is that 98% of them look like refugees from a K-Mart closeout sale. Shiny, overtly, proudly plastic. The perfectly smooth texture is what kills most poly mouldings.
However, recently a curious crossover is occurring in the framing world. As the quality of affordable wood moulding goes down, so does the quality of polystyrene improve. On a personal note, two wonderful, affordable, genuine wood finish mouldings I have relied on for some time were recently "redesigned" to what amounts to a plastic tube with a printed texture wrapped around a piece of wood. Yes, still a wood moulding by some measure, but the beautiful wood grain has been replaced by a miserable looking printed texture on plastic. This is happening a lot at the low and medium level of mouldings. I won't talk about the ever increasing percentage of unusable twisted and warped pieces of moulding included in every wood box. The best poly's now look better than plastic wraps on wood, by a lot. And the consistency and yield of poly is impressive.
So enter polystyrene mouldings. Mostly ugly. But recently a few types have emerged with surface textures and reflectivity qualities that look awfully good, even next to very expensive wood mouldings. I'm thinking of things like Universal Framing Product's Montalcino moulding in its "Valucore" line. Definitely a cut above.
But should you use poly mouldings? Probably not extensively. Most galleries will still turn up their noses. I've been trying it out at art fairs and such where perceived value trumps materials snobbery, and have been well received. The fabrication costs for poly over wood are pretty dramatic, and pricing reflects this.
What's not to love about poly...
--Not as rigid as wood. A 4" x 72" piece will sag enough due to gravity that frames that size need to have door skins or extensive wire bracing on the back.
--The Devil was involved in the design of the nasty, gets-everywhere so-called sawdust that is generated when cutting this stuff.
--Chop saws literally melt the stuff if you try to make a normal cut with a normal blade. You have do this really scary karate-chop cut that would probably maim you if you tried it with real wood. It helps a whole lot if you have a special "poly blade". But then you should have a special $130+ blade for cutting any kind of moulding, forget the blades down at Lowe's.
--The tiniest drop of glue on the cosmetic surface will create a melt mark which pretty much trashes the whole frame. You need to use the absolute minimum amount of glue while not creating a weak joint. Takes practice.
--Did I say most of it looks like c**p?
--Most serious galleries will show you the door.
What's to love...
--Relatively cheap, even very cheap.
--For the bigger sizes you wind up with a frame that doesn't take two strong guys to lift, and which does not kill anybody should it fall off the wall.
--A small percentage of the current selections looks pretty sweet indeed.
--Lightweight compared to wood, if you have to pay for shipping you will notice the difference.
--When you get your technique figured out assembling poly frames goes pretty fast.
--The yield is much better than wood. Almost 100% of poly moulding is usable out of the box, versus maybe 80% to 85% for wood.
--Your budget customers will love you.
It is theoretically possible to make nice poly frames with only a better-quality miter chopsaw from Home Depot, a sharp blade, and a tube of Super Glue Gel. A definite added plus would be an underpinner, hum that's a whole other story. But even minus an underpinner you can still rigidify the miter joints on the backs with mending plates and screws. But you have to rigidify those joints if you want your frame to survive twisting and torquing, no question about it.
So, really on my own behalf I only use poly for big art fair pieces where price point is king. There is an interesting parallel here between aluminum versus wood mouldings when aluminum first came on the scene. The same script will likely play out with poly versus wood versus metal, just give it time accelerated by a financial meltdown.