by KCStudly » Thu Aug 24, 2017 7:08 am
They're similar but not the same.
Yes the cotton canvas is the tensile element of the composite, but it and the set glue remain more flexible than the FG/epoxy. In fact I was able to bend my single sided PMF sample over to a surprising bend radius with no apparent adverse affect to the foam; unlike an unsupported piece of foam, the 3/4 thick sample with PMF bent right around in a 'U'-shape and, more or less came right back. Whereas an unsupported piece of foam would have snapped. The single ply of 6oz FG also could take a surprising amount of bend, it did not shatter or crack as some would have you believe (even after curing for a long time and retesting multiple times).
The PMF sample had the most give to it under point load (trying to indent with thumb pressure). The FG sample with just one 6oz ply was better. With two plies of 6oz I was happy with the point load and impact resistance. It still has a very slight amount of give under a heavy finger press point load but does not tend to indent locally, and the sample was much stiffer. Sure, the PMF has shown in many cases to heal itself from dents over time, but the 2-ply FG doesn't readily dent in the first place... this is not to say that the PMF dents easily, it is impressive, it just has a softer feel and will take a dent more readily without springing back instantly.
For me, with moderate off road plans, it was a very difficult choice, but I wanted the extra "armor". Like GPW says, the benefits of PMF are actually quite remarkable. It is surprisingly close to FG in many ways and has some great benefits that you can't get with epoxy/glass. It is hard to argue with a durable, flexible, self healing, and easy to repair material. After a few simple tests I still had a very hard time deciding which way to go. Cost and time were not necessarily my biggest concerns, and I have learned to deal with the PPE issues surrounding the use of epoxy (religious use of proper respirator, proper cleaning and storage of said respirator, disposable gloves and Tyvek style arm sleeves... while applying, sanding and cleaning up afterwards, including servicing the shop vac). These things are not small issues of time, effort and cost, and should all add into the decision. A big part of the decision, for me, came down to perceived quality; when I showed the unfinished samples to my learned friends, very few believed that the PMF could be finished to the same level of quality, or would maintain the perceived value standard that I had been striving to achieve on the rest of my build (not perfection, but a certain level of material quality, attention to detail, fit and finish that I had been working on for a very long time). This later part is very subjective and they had not done the same degree of "research" (essentially, reading this forum and doing personal tests) as I, so I admit that it is very subjective input. There are some very well finished PMF foamies out there, I'm not saying it cannot be done. However, most of the people that I presented the samples to were hands on fabricators and craftsmen in their own right, so I still valued their input and advice very much.
This is why we always say, "test". It is far better to invest a relatively small amount of your time and money to determine what are the best materials and methods for your own build. We all have different resources, skills, budgets, goals (time, creative, and otherwise) and planned end uses.
As stated above^, the engineering process is always a juggling act between intended function, cost, durability, and ergonomics (...and other factors).
"You can have it GOOD/FAST/CHEAP; pick two." - author unknown.
KC
My Build:
The Poet Creek Express Hybrid Foamie
Poet Creek Or Bust
Engineering the TLAR way - "That Looks About Right"
TnTTT ORIGINAL 200A LANTERN CLUB = "The 200A Gang"Green Lantern Corpsmen