Figured I may as well try a second layer since I've got a few more days until I get the lighter cloth and come up with some better resin.
So...cut two more pieces of cloth - this time making them so the weave goes at 45 degrees to the original layers.
Mixed up some epoxy - still using the gflex but this time weighed it out instead of going by volume, I mixed up 8g total and it was more than enough to wet up both sides.
Since the original layer had had 24 hours to cure at this point I took some 80 grid sandpaper to it and roughed up the surface until there was no shine left and the weave of the first layer was exposed.
Then same basic procedure. Put the cloth down, poured a bit of epoxy over it, and used a gift card to spread it out and squeegee as much as possible off. Flip it over and do the same thing.
Then put it between two sheets of plastic wrap, some nice flat boards and a cast iron grill on top for weight. This time after 3 hours it was less tacky even though it was a bit cooler out. I guess I was more accurate measuring by weight instead of volume, and I was careful to give it more time mixing as well. Even so I gave it overnight.
This morning it's looking really good. VERY stiff and no longer able to twist. Though it is getting noticeably heavier with this 6oz cloth. Very curious to see how many layers of 0.74oz cloth it takes to get as stiff as I'd like..though again I suspect less flexible epoxy will help as well.
But what I've got now is definitely stiffer than the PVC sheet or Lexan sheet...and probably stiffer than the plywood. I'd be very tempted to try doing a full sheet this way and cutting the frame to see how it does.
The one big concern I have is hardpoints. Specifically where my screws go through to connect the layers. While this is stiff and tough it's also a little squishy. I suspect that even with washers trying to bolt layers together won't work well because it will squish and deform. The PVC and PLY did this a little as well...but they were both considerably tougher to squish than this. I know on composite aircraft they usually embed a bit of plywood where they want a hard point, or sometimes just mix up flox and epoxy to create a hard spot. And I could do either. But...that would require more machining steps or careful planning to make sure I prep the right areas before cutting. I'm kind of thinking that instead of drilling 3mm holes like I have in the past I'll drill 5mm holes then fill them with flox/epoxy - then go back and hand drill the holes back out using one of the previous frame pieces as a template. That seems easier than trying to get the work back into the CNC accurately to do a second machining pass.
The other thing I want to try is the technique my friend showed me that he learned at the EAA class (which it sounds like I'll be attending in October or January) They used polyester dress liner fabric as "peel ply" along with cotton batting to absorb extra epoxy. The trick is that epoxy won't stick to polyester. So after you wet up the FG cloth you put a layer of polyester fabric over the top of it then a layer of cotton batting over that - then put it between boards or vac bag it. The excess epoxy will get squished through the weave of the polyester and absorbed into the cotton batting - the end result is not a smooth surface since you have little dimples from where the epoxy went through the weave. But that's a good thing because it leaves a surface suitable for laminating additional layers on.
So I'll probably pick up a roll of batting and a few yards of polyester (Supposedly you can get 100% poly dress liner for about $1 a yard) and give that a try. Should make for a lighter final product without excess epoxy.
Not sure what’s causing it.
