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Fiberglass for composite builds

Mike Arnopol

Mike Arnopol

Inactive
Commercial User
Jan 4, 2005
3,423
2,813
71
Chicago
Disclosures
Owner of MAS Soundworks
#1
Going to try my hand at a foam core composite build. In looking at the fiberglass sites mentioned, they state that the 7781 is not a good choice for tight radii. There are so many fiberglass choices--I'm wondering if there is a better compromise in terms of strength to ease in application. Lots of radii----1/2" on the outside and maybe smaller with the fillets on the inside. I know that Zac was using the 7781 and Robby was using some type of 10 oz. Also, on the Thayercraft site he's pretty emphatic about treating (before cutting) even the overhung cut edges to prevent unraveling. Any thoughts? (going to make some smaller boxes for a 3012ho, 3015, and a couple of little Faital 8"s)
 
#2
Mike, I would suggest doing a search here and on greenboy's site - Art of Noise Audio has been doing foam/glass composite fEARfuls for a while now, and Robby has been "perfecting the technique" w/every build. You might be able to get him to divulge the secrets. Last I knew he was going w/pink styrofoam, epoxy glass, and Kevlar reinforcements.. Not sure what he's using for laminations, might be mentioned in the threads..
 
#4
Type 7781 fiberglass is a structural glass (not the strongest, such as s-glass, but still stronger than required) used in many applicaitons including aircraft construction. It really has no place in building a cabinet, although certainly can be...you would just be spending money on strength which is not required for this application. Any cheap 6-8 oz cloth, 3 ply layup will work.

Most important to any composite layup is the resin to cloth ratio. Make sure you follow the manufacturers recommendation and you will have a strong cabinet even with the cheapest of glass product.


Mike Arnopol said:
Going to try my hand at a foam core composite build. In looking at the fiberglass sites mentioned, they state that the 7781 is not a good choice for tight radii. There are so many fiberglass choices--I'm wondering if there is a better compromise in terms of strength to ease in application. Lots of radii----1/2" on the outside and maybe smaller with the fillets on the inside. I know that Zac was using the 7781 and Robby was using some type of 10 oz. Also, on the Thayercraft site he's pretty emphatic about treating (before cutting) even the overhung cut edges to prevent unraveling. Any thoughts? (going to make some smaller boxes for a 3012ho, 3015, and a couple of little Faital 8"s)
Click to expand...
 
#6
Thanks, guys.
Yeah, Robby, it seems to make a lot of sense (at least for the baffle and back) to do the first layup before assembly. It's funny, I've collected a garage full of the prettiest red and white cedar. For the past 8 years I've been longing (but haven't had the time) to build some cedar strip kayaks. Same theory--- the cedar is merely a sandwich substrate and the strength comes from the fiberglass/epoxy layup. And the way theses guys build them is that the bottom and top portions of the kayak are completely 'glassed first, and then the top and bottom are assembled together just using fiberglass tape on the inside and outside. And these kayaks take a beating. So it makes sense to lay up the panels first. What about this business that the guy at Thayercraft is saying about only cutting the glass into pieces after there is some sort of treatment to prevent unraveling? I'm assuming that you guys just trimmed to slightly oversize and then trimmed down after setup? Check out his site (it's where Zac got his glass) He makes it sound like the glass is going to lose it's integrity and unravel----I've never heard of that before.
 
#7
Mike Arnopol said:
Thanks, guys.
Yeah, Robby, it seems to make a lot of sense (at least for the baffle and back) to do the first layup before assembly. It's funny, I've collected a garage full of the prettiest red and white cedar. For the past 8 years I've been longing (but haven't had the time) to build some cedar strip kayaks. Same theory--- the cedar is merely a sandwich substrate and the strength comes from the fiberglass/epoxy layup. And the way theses guys build them is that the bottom and top portions of the kayak are completely 'glassed first, and then the top and bottom are assembled together just using fiberglass tape on the inside and outside. And these kayaks take a beating. So it makes sense to lay up the panels first. What about this business that the guy at Thayercraft is saying about only cutting the glass into pieces after there is some sort of treatment to prevent unraveling? I'm assuming that you guys just trimmed to slightly oversize and then trimmed down after setup? Check out his site (it's where Zac got his glass) He makes it sound like the glass is going to lose it's integrity and unravel----I've never heard of that before.
Click to expand...


I've talked with Steve Thayer a bit, he gave me a great deal on my last cloth order. When you cut 7781, it hardly unravels at all. Thicker roven woven will come apart pretty easily, but 7781 doesn't unravel enough to cause a problem. Just leave enough of an overhang and trim off the excess after it cures with a razor blade. 2-4" is enough to leave some wiggle room when glassing.

One trick to prevent it from unraveling is to put masking tape down on the cut line before you cut and then it doesn't fray. Just leave on the tape since you'll have an overhang. But like I said with 7781, there isn't an issue IME.


Robby
 
#8
Thanks, man.
I've been obsessively following yours, Zac's and Pete's threads.(here and on you site) I just had 2 new hips installed 2 weeks ago. (really!!!)(I'm an old fart---56) I'm walking a mile a day already and have taken enough time off from touring so that in a couple of weeks I'll probably be cool as far as starting the projects. I've been building cabs since I was 15 (my first one was a Voice of the Theater and my second a big honkin" 18" folded horn!). I have a very nice professional workshop ----tons of experience with Baltic Birch but a first time with fiberglass. I've got book upon book on strip kayak builds and the fiberglass work pretty much carries over.(me thinks) I was going to get my epoxy from U.S. Composites. One thing I know from my years of lurking on the kayak building sites is that there are just as effective and much cheaper alternatives to West Systems. I have used West Systems to repair my DB trunk with extensive fiberglass patching. (by the way, it's made with Spectra). The medium kit from U.S. (one gallon resin, 42.6 oz. hardener) is only $65. ---is medium cool or do you think I need fast? It makes sense that the tighter weave 7781 holds together better as far as cutting, and thanks for the tape suggestion.It seems like a worthwhile tradeoff in terms of a little bit harder on the radii, but more stable on the cutting. It appears that Steve still has some of the black glass that Zac used---I think $4/yard. 2 layers in and out or 1 inside and 2 outside?Seems like that with the 7781 that 3 layers might be overkill. Got a very good deal on the 3015 and the 3012HO. I've got a big Delta Unisaw and I've used it with a regular blade to cut the pink stuff before---easy, very clean cuts. Robby---are you also glassing the side panels first?
 
#10
Unravelling when cutting is only a minor annoyance and not worth stressing over. Use very sharp tools to keep it to a minimum. I use an inexpensive set of dressmakers scissors and they work nicely. And as you say, cut oversize then trim down later with a sharp blade when the resin is hard, or even better when it's semi-hard.
 
#11
Mike Arnopol said:
Thanks, man.
I've been obsessively following yours, Zac's and Pete's threads.(here and on you site) I just had 2 new hips installed 2 weeks ago. (really!!!)(I'm an old fart---56) I'm walking a mile a day already and have taken enough time off from touring so that in a couple of weeks I'll probably be cool as far as starting the projects. I've been building cabs since I was 15 (my first one was a Voice of the Theater and my second a big honkin" 18" folded horn!). I have a very nice professional workshop ----tons of experience with Baltic Birch but a first time with fiberglass. I've got book upon book on strip kayak builds and the fiberglass work pretty much carries over.(me thinks) I was going to get my epoxy from U.S. Composites. One thing I know from my years of lurking on the kayak building sites is that there are just as effective and much cheaper alternatives to West Systems. I have used West Systems to repair my DB trunk with extensive fiberglass patching. (by the way, it's made with Spectra). The medium kit from U.S. (one gallon resin, 42.6 oz. hardener) is only $65. ---is medium cool or do you think I need fast? It makes sense that the tighter weave 7781 holds together better as far as cutting, and thanks for the tape suggestion.It seems like a worthwhile tradeoff in terms of a little bit harder on the radii, but more stable on the cutting. It appears that Steve still has some of the black glass that Zac used---I think $4/yard. 2 layers in and out or 1 inside and 2 outside?Seems like that with the 7781 that 3 layers might be overkill. Got a very good deal on the 3015 and the 3012HO. I've got a big Delta Unisaw and I've used it with a regular blade to cut the pink stuff before---easy, very clean cuts. Robby---are you also glassing the side panels first?
Click to expand...


The layers you need will depend on how thick core you use. The thicker the core, the less glass you'll need. It is best to use the same amount of glass on both sides so that you don't get any warping and it is better for strength as well.

The type of epoxy will depend on your working temps. If you are around room temp or above, then the 3:1 thin epoxy works well. If colder, 4:1.

I am currently glassing all the panels separately and then joining them. Makes it easy to keep them square and makes layups much less complicated.

Robby
 
#12
makes a lot of sense. I was thinking of doing 2 layers inside 1 outside, joining them up, using tape in the inside joints and then doing a continous piece around the sides and then doing the back and baffle. If I read correctly, Zac did his baffle without glassing the inside cutouts? Just epoxy and micro filler? Just thinking out loud here. Robby, did you assemble the cab sides and back (already glassed) and then use a router with a round over bit for the corners? Or did you pre-assemble, round the corners, glass, and then assemble? By the way guys, I really appreciate you being patient and helping out. Very cool environment here.
 
#14
Mike Arnopol said:
makes a lot of sense. I was thinking of doing 2 layers inside 1 outside, joining them up, using tape in the inside joints and then doing a continous piece around the sides and then doing the back and baffle. If I read correctly, Zac did his baffle without glassing the inside cutouts? Just epoxy and micro filler? Just thinking out loud here. Robby, did you assemble the cab sides and back (already glassed) and then use a router with a round over bit for the corners? Or did you pre-assemble, round the corners, glass, and then assemble? By the way guys, I really appreciate you being patient and helping out. Very cool environment here.
Click to expand...

I've found it best to round over the edges before assembly. When I tried routing after assembly, the guide busing on the bit rests against the foam core and doesn't stay straight easily. In the case of a router table with a guide, it won't make much of a difference.

Robby
 

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