Scrapmaster87
Well-Known Member
For my 4.6" OD big bertha upscale I'm trying to make the (carbon fiber) fins with a Coremat core to reduce weight and add stiffness. I'm building it with a 5-motor cluster (38 + 4x 29's), so the rear end is already plenty heavy, hence a desire to cut weight by cutting my teeth on more advanced techniques. I have 3mm 3-ply birch on hand, but I was unimpressed with it the last time I used it. I essentially was able to rip off the fiberglass lamination by hand, leaving the core maybe 90% intact. I have since switched to a different (and thinner) epoxy since then.
I've done 3 trial lamination so far, all failures to to achieve what I want: 1.25" x 6" coupon able with withstand most of my weight when trying bend it of the edge of my workbench. I want these fins to not break when they decide to be shovels on a hard landing.
1st trial: Vacuum pump failure
Skin: 1x 1.43oz plain weave e-glass (each side)
Lamination: 2x 6k 4-HS carbon fiber (each side)
Core: 4mm Coremat
Mold: Pane of glass protected by parchment paper, peel-ply both sides, craft felt as breather on top. Hefty kitchen trash bag serving as vacuum bag.
Result: Vacuum pump failed (returned under warranty), everything dried out when the vacuum released and the Coremat expanded. Coupons are very weak
2nd trial: Non-bagged near success
Skin: 1x 1.43oz plain weave e-glass (each side)
Lamination: 2x 6k 4-HS carbon fiber (each side)
Core: 4mm Coremat
Mold: 2 Panes of glass, each protected by parchment paper. Weighted with 5-lbs.
Result: Everything came out saturated. Top skin failed in tension per my test, almost strong enough for me.
3rd trial
Skin: 1x 1.43oz plain weave e-glass (each side)
Lamination: 3x 6k 4-HS carbon fiber (each side)
Core: 4mm Coremat
Mold: Pane of glass protected by parchment paper, peel-ply on top only, breather cloth on top. Nylon bagging film for bag.
Result: When I cut the coupon to shape on my bandsaw, I was getting frayed fibers of carbon fiber, almost like you would see when cutting a kevlar sandwich. I can see the occasional void under the fiberglass skin. The top surface came out very lumpy, which I'm assuming this is due to the variable density of the Coremat. The strength of the last coupon was absolutely not here. Even though the vacumm held overnight (valved off, didn't requre pump), the core had dried out. Failure was in shear of the core material, interesting to say the least.
With the variable density of the Coremat, it seems like I'm locked into needing a top mold (so a pane of glass) so the the fins come out smooth. The biggest problem here is keeping the Coremat saturated under vacuum! I need some help finding which direction to take this so I don't end up with a hundred failed coupons before getting it right.
Things I can do now:
1) Pre-saturate and cure the Coremat under weight. Layup the carbon and fiberglass as a 2nd operation and cure under vacuum
2) Do the whole layup without vacuum like the 2nd trial (just with the extra carbon layers to hit the strength I want)
3) Use 2mm Coremat instead of 4mm
4) Repeat 3rd trial by place a layer of parchment paper on top of the breather clot, followed by a pane of glass (or other suitable flat surface).
5) Abandon the Coremat and switch to the craft birch I have. Will still need to do test coupons to prefect getting the epoxy to soap in properly.
6) Insert something I didn't think of
Things I could do, but need to buy materials and/or equipment to do:
1) Go to vacuum infusion - keeping resin off the top mold will be be interesting
2) Us end grain balsa or a foam. I stayed away from foams origionaly because I was concerned about a shear failure like with my 3rd coupon.
3) Skip the core and just use more carbon or an aramid fiber
I've done 3 trial lamination so far, all failures to to achieve what I want: 1.25" x 6" coupon able with withstand most of my weight when trying bend it of the edge of my workbench. I want these fins to not break when they decide to be shovels on a hard landing.
1st trial: Vacuum pump failure
Skin: 1x 1.43oz plain weave e-glass (each side)
Lamination: 2x 6k 4-HS carbon fiber (each side)
Core: 4mm Coremat
Mold: Pane of glass protected by parchment paper, peel-ply both sides, craft felt as breather on top. Hefty kitchen trash bag serving as vacuum bag.
Result: Vacuum pump failed (returned under warranty), everything dried out when the vacuum released and the Coremat expanded. Coupons are very weak
2nd trial: Non-bagged near success
Skin: 1x 1.43oz plain weave e-glass (each side)
Lamination: 2x 6k 4-HS carbon fiber (each side)
Core: 4mm Coremat
Mold: 2 Panes of glass, each protected by parchment paper. Weighted with 5-lbs.
Result: Everything came out saturated. Top skin failed in tension per my test, almost strong enough for me.
3rd trial
Skin: 1x 1.43oz plain weave e-glass (each side)
Lamination: 3x 6k 4-HS carbon fiber (each side)
Core: 4mm Coremat
Mold: Pane of glass protected by parchment paper, peel-ply on top only, breather cloth on top. Nylon bagging film for bag.
Result: When I cut the coupon to shape on my bandsaw, I was getting frayed fibers of carbon fiber, almost like you would see when cutting a kevlar sandwich. I can see the occasional void under the fiberglass skin. The top surface came out very lumpy, which I'm assuming this is due to the variable density of the Coremat. The strength of the last coupon was absolutely not here. Even though the vacumm held overnight (valved off, didn't requre pump), the core had dried out. Failure was in shear of the core material, interesting to say the least.
With the variable density of the Coremat, it seems like I'm locked into needing a top mold (so a pane of glass) so the the fins come out smooth. The biggest problem here is keeping the Coremat saturated under vacuum! I need some help finding which direction to take this so I don't end up with a hundred failed coupons before getting it right.
Things I can do now:
1) Pre-saturate and cure the Coremat under weight. Layup the carbon and fiberglass as a 2nd operation and cure under vacuum
2) Do the whole layup without vacuum like the 2nd trial (just with the extra carbon layers to hit the strength I want)
3) Use 2mm Coremat instead of 4mm
4) Repeat 3rd trial by place a layer of parchment paper on top of the breather clot, followed by a pane of glass (or other suitable flat surface).
5) Abandon the Coremat and switch to the craft birch I have. Will still need to do test coupons to prefect getting the epoxy to soap in properly.
6) Insert something I didn't think of
Things I could do, but need to buy materials and/or equipment to do:
1) Go to vacuum infusion - keeping resin off the top mold will be be interesting
2) Us end grain balsa or a foam. I stayed away from foams origionaly because I was concerned about a shear failure like with my 3rd coupon.
3) Skip the core and just use more carbon or an aramid fiber