Gents - I’ve obtained an experimentel block of 3# MDI foam and am curious how important it is to have the correct amount of flex within a blank. I’ve taken a 2" x 2" rail bone from said blank and started to bend it to find a point of snap. It almost completely wrapped around itself. Now IMO this is to much, correct? I did the same with a bunch of Walker and Bennett rail bones and they all had much less flex AND all seemed very consistant as to thier snap points. If the foam is to flexi won’t this result in the glassing schedule having to absorb this excess flex? thus creating stress cracks?
Or - Is more flex good? Or should all blanks have a consistant “just the right amount” of flex?
Cheers!
Ray Barnett
The cured glass/resin or composit matrix will have less flex than the foam, so the amount of flex the foam has is, relative to the skin, pretty insignificant. Boards fail when the structural parts fail… the skin and the stringer (if there is one).
In general, the flex properties of the core material are much less important than the flex properties of the skin. Although there’s a lot more to it than this, in simple terms, the core is just something you shape to get the structural integrity of the skin to bear the bulk of the forces boards are subjected to.
Think about hollow boards, that have have no core, or solid wooden boards, that have no skin. Whichever provides the greatest strength, the skin or the core, determines the flex and structural integrity of the board.
NJsurfer - Thanks for the input and I get your point however, my subject is a 3# MDI formulated foam example that will be glassed lets say “Mainstream” as this is how the foam will be offered. Double 6oz deck, single 6oz bottom Etc. Now if the foam is flexier than a regular blank then the glassing schedule will have to absorb this excess flex possibly resulting in glass damage I would suspect?
My question is if someone burst onto the scene with a new foam it would be expected to have the same stifness qualities of current foam available yes? Or would a manufacturer benefit from being able to offer different degrees of flex quality?
It isnt much of an issue because boards dont flex as much as your test - your test doesnt say much about a finished board’s abilityt to resist snap. A finished flexible longboard might have a deflection of 1/2" in the center max under normal conditions. Cyclic fatigue (this includes deck deformations) if the core and skin is the real killer.
Now if you want some useful info from such testing, if you had bent the sample far enough before/without breaking:
a. how much force did it take to bend?
b. did the foam spring back to its original form when released?
c. what is the true density of the sample?
d. can density variation be controlled in mfg to extrapolate such results over many blanks?
IMO, there are too many variables to assert that a single sample test counts. A single sample test is a little better than zero sampling, three samples begins to tell you something, 30 samples is the correct statistical minimum for data gathering.
True my test was a bit elementary. It did tell me though that the new foam was significantly more flexi which raised my curiosity about possible glass damage.
Okay. Its been my experience that foam flexility, as well as some woods, is very dependent on density. Its possible that the #3 you were bending was less than that. Clark foam was very flexible but will snap way before bending over. PU foams are also noteriously tricky to work with (mfg) and its also possible that the sample you tested wasnt mixed or cured properly. The manufacturer should be able to tell you whether you tested a representative sample.
Having said that, keeping other variables constant, more flexible foams will yield more flexible boards. However, they will also heel dent much worse/faster if the foam isnt very resilient.
Btw, I love the term “rail bone”! Ive been wanting a good term for those offcuts.