Core Material Woes

[Scrapmaster87]

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

 

[Scrapmaster87]

This is the 3rd trial after 20 hr under vacuum and it's failure(s)

 

[Chad]

have you tried the birch with the thinner epoxy? I'm not an expert, but #2 with the birch may get you there.

I did a couple experiments with basswood and fiberglass but not near as thorough as your tests. I don't have the gear for vacuum bagging so these were hand layups.

Mine looked like this

1. flat spare tile about 1ft square
2. parchment paper
3. paper towel
4. peel ply
5. wetted fiberglass
6. basswood
7. wetted fiberglass
8. peel ply
9. paper towel
10. parchment paper
11. another spare tile with some weight on top ( maybe 10 lbs )

One thing i found was the basswood soaked up so much epoxy it seemed to create dry spots in the fabric. For the second test I put down a light coat of sanding sealer on the basswood to try and make it a little less porous. That seemed to make the result much more uniform with no obvious dry spots. I didn't test for strength though, this was more just testing my ability to get the layup right.

I know having sanding sealer between the epoxy and core material is a no-no but basswood is like a sponge and i put on a very thin coat.

 

[SDramstad]

I havent used coremat so cant help you there. Have you tried end grain balsa? I just started playing with it and have been pretty impressed.

 

[OKTurbo]

I’m no kind of expert on this, but I have done some big carbon fiber fins. Search for my Praetor thread. I did the carbon fiber seperately as a resin infused sheet. I cut the fin skins out of the cured sheet and epoxied them onto the core. Turned out very strong.

The keys to the infusion for me was using the membrane tube and the flow/check valve. No catch pot needed and a perfect infusion.

The other key for me was to glue the composite sheeting onto the fin core with thicker structural epoxy. Extra epoxy soaking into the core just adds weight.

hope that helps..

 

[OKTurbo]

Here’s a link to my infusion setup.

https://www.rocketryforum.com/threa...tor-will-the-fins-stay-on.149293/post-1879489

 

[TonyL]

I know from work that if you are just looking for strength, the correct method is to have enough plies for the strength you want, and forget about the core.

A caul plate is the normal way to make both sides smooth [like in a press, vacuum not always necessary].

An external scrim layer of fiberglass improves sanding so that is a good approach.

A bandsaw will generally tear the fibers. An abrasive wheel is the usual hand-cutting method.

Coremat I believe is intended for bulking up boat hulls, so it is not a great core for fins. The shear strength of the core material is the most critical characteristic. End grain balsa or PVC foam core material would be my recommendations for a thin core.

Core thickness generally only adds stiffness, not strength as the tension load is carried by the face sheets and shear loads in the core material.

hope this helps,

Tony

 

[n27sb]

Coremat I believe is intended for bulking up boat hulls, so it is not a great core for fins.

Totally agree. I have used it in bulking up molds. It helps prevent print through of your support structure. It is resin rich and heavy.

The shear strength of the core material is the most critical characteristic.

The best way to improve the shear strength of core materials is to squeegee a thin slurry of epoxy and microspheres on the core. Squeegee off the excess while pressing the slurry into the pores.
Apply your lay up on the core before it cures.