Fiberglass rigidity

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#1

I need some suggestions.

I’m learning to make composites. I have a quantity of AeroMarine 300/21 epoxy and 6 oz. fiberglass. So far my huge disappointment is that the laminated fiberglass comes out extremely flexy, somewhere between paper and cardboard. I have a project that requires making structural flat sheets 24"x5" and my fiberglass does not even come close to being suitable. By the looks of it, even getting 2x thicker material won’t do the trick, it would sag under its own weight.

I need some suggestions on how to easily improve the rigidity. My best idea so far is to make a foam sandwich with some kind of supporting frame inside and to fill the rest of the space with foam. Tried to make supporting frame elements by molding strips of fiberglass onto drinking straws, it’s hard :frowning: I suppose I could just get some Elmer’s foam board and stick fiberglass onto it, but I’m not sure how sturdy that construction would be.

#2

First off, you could move to carbon fiber to increase the stiffness. Second, you could create some kind of sandwich structure like carbon-foam-carbon or carbon-balsa-carbon. The best way to increase the stiffness is to increase the thickness.

What is the application?

#3

Household stuff, mostly learning what I can do with fiberglass and how to do it.

Carbon is on my to-do list, I have some ordered and it’s in the mail, but it’s 5-8x the price of fiberglass per square foot. Would carbon (T700 12k 300gsm) be rigid enough to make, say, a light-duty shelf out of a single layer without reinforcement?

What’s the normal procedure to get foam for the sandwiches? None of the places where I got my stuff carry foam boards. Mixing and pouring foam seems hard. (Particularly maintaining constant thickness.) Tried to make some boards with 3/16" Elmer’s foam (styrofoam), the result looks rigid but fragile, I think it’ll do for now…

And one more question: isn’t uncured epoxy supposed to be sticky? One of the reasons I’m having a hard time is that my fiberglass sheets don’t want to stick to each other. Or to anything, for that matter. I’ve seen wet paper stick better. I have to put joints under weight for a few hours to achieve any degree of physical bond. I’m wondering if that’s normal, if I’m doing something wrong, or I got a bad batch of epoxy.

#4

1: How many layers? Your second post said a single layer of carbon?
2: see above. NO. A single layer of anything will do NOTHING. For a household shelf, I would go with at least 6 layers of carbon, all depending on what it’s holding.
3: Mounting points will bend as well, even if the board itself is pretty strong. Don’t be shocked.
4: resin isn’t always “sticky”…it all dependson viscosity, and type. If you are doing simple flat sheets, you can put a waxed/released board on top (glass, metal, anything else that is non-porus and heavy)
6: as WyoWindWorks said, sandwiches work wonders. Take this as an example of a good “shelf”: 2 layers 10oz carbon- 1/4" pvc(et al) foam board - 2 layers 10oz carbon. This will be very strong for all your needs, and only 4 layers of carbon. Take the foam board OUT, and your strength will ~half, for only 4 layers carbon.

#5

OK, this is shaping to be considerably more expensive than I anticipated. Also, I need more mold surfaces.

Take this as an example of a good “shelf”: 2 layers 10oz carbon- 1/4" pvc(et al) foam board - 2 layers 10oz carbon.

In this example, would you layer everything at once, or laminate the carbon, wait for it to cure, and then stack things together?

Can you suggest a good source for the foam board, that is reasonably priced and deals with small quantities? Either a brick and mortar chain or a reputable online store. Tried to google, found one online store that sells 1/4" PVC foam board for about $5/sqft in small quantities, and a distributor near me that sells the same foam board for $2.4/sqft but is 4 by 8 foot sheets (I don’t need THAT much.)

#6

Any core—foam, wood(balsa wood is a widely used core) that separates the skins will stiffen a panel substantially at little gain in weight. Google composite sandwich construction for the numbers, you will be impressed.

I used plain old blue insulation board from Lowe’s/ Home Depot with 3 layer BID and epoxy on each side after sanding the foam to shape. The result was so stiff I kept a piece lying around just to show the skeptics. There is no discernible flex under moderately high hand pressure, as in trying to break the part.

Sandwich construction with cheap e-glass will make a much stiffer panel than single layer glass or maybe even single layer carbon fiber—assuming equal amounts of fabric/ resin. Do some homework, it will be worth your time.

#7

BTW, in a composite propeller project recently with about 7 pounds of glass, 12 ounces of epoxy and 12 ounces of aramid (Kevlar), the stiffness was low until wooden core dowels wre laminated in the middle AND postcuring occurred. Final stiffness of this 14 pound prop (50%) epoxy resin was easily 8-10 times that before the “core” and postcure.

How many layers of glass, you may ask, I don’t know! Most of that glass weight was in tow, not Bid, with a few layers of bid wrapping the tow. My spool of e-glass tow lost 7 pounds of weight, that’s how I know how much glass is there.

#8

I used plain old blue insulation board from Lowe’s/ Home Depot

R-Tech foam insulation board? Saw that one, but it was starting at 1/2" thickness. It also did not look very sturdy. But it was super cheap.

Went to the local distributor and got a 32 sq.ft. sheet of 1/8" PVC foam. $37+tax. Cut it in half and glued it together, with a layer of fiberglass between sheets. Looks pretty good so far, waiting for it to cure.

#9

Weird. My epoxy does not want to stick to my PVC foam :open_mouth: It pretends to stick, but the bond is very weak. I just took apart two of my fully-cured sandwich boards with a flat-head screwdriver. I get clean sheets of PVC foam without a trace of epoxy, and neat-looking, flat and transparent sheets of laminated fiberglass.

PVC had a plastic film on one side and I made sure to take it off. It failed to adhere on both sides.

Do I need to sand the foam with something like 80-grit sandpaper before using it?

P.S. Trying to put them back together, wiped all surfaces with isopropyl alcohol, made sure to thoroughly wet the foam (as opposed to fiberglass). Waiting to cure … On the positive side, sandwiches were quite rigid up until the point where they fell apart.

P.P.S. Cleaning and wetting the foam was ineffective, looks like it held a bit better this time, but there was still almost no adhesion to the foam. Rebuilding the sandwiches again, this time after sanding the surfaces.

#10

It’s not very sturdy but apart from being cheap, the costs-benefits ratio is quite good. I would recommend them for small projects.

Marius
Cheap fiberglass panels

#11

Epoxy not sticking to the pvc foam?Something is wrong there.What make of foam?Perhaps contaminated with something since it released so cleanly?

#12

Where to start? Where to start?

Okay, let’s start with the relationship between thickness and stiffness. It is not a linear relationship. Stiffness increases exponentially as thickness increases. I’ve attached a document that puts some numbers to it (courtesy of my friends at Saertex). Generally speaking, a panel that is built with a couple of layers of glass separated by a foam core will be stiffer than a couple of layers of carbon stuck together. Simply put, your laminate is going to be a whole lot stiffer by changing thickness rather than going for a higher modulus material.

On the subject of how to build and what to use, I’m not against experimenting with different materials but you should have a rudimentary understanding of the materials or you are going to waste money (but I guess education is never cheap :D). For instance, foam cores need to exhibit certain physical characteristics to fulfill the needs of the project. Is it cheap you are going for? Ultimate stiffness? Durability? There are many factors that go into the decision.

One of the myths of epoxy is that it will stick to everything. Are you sure your board is really PVC? Maybe it’s some other plastic. There are literally hundreds of formulations that can be made into board stock. There are also countless surface treatments that can be applied to that board that may promote adhesion or it could just as easily fight adhesion. There is no guaranty that epoxy will stick to something you picked up at the hardware store.

The bottom line is you can build stiffness in a variety of ways. You can increase modulus. You can increase thickness with a core. You can add geometry to the part. You can use a combination of all these things. Of course, the best way is often subjective so it’s not necessarily easy to say what is the “best” way. In order to figure out what works for you requires some education. The best and usually cheapest way to learn about materials is to lean on your suppliers. By suppliers I’m not referring to the lumber guy at the big orange box store. I’m talking about guys who actually know the materials and can offer cogent advice - composites salesmen at composite stores. One thing I’d suggest, figure out specific questions. Vague questions get vague answers.

#13

AVT in Indianapolis sell partial sheets of urethane foam.It’s called Last-a-Foam.

#14

Great post, Roger. Nail on the head.

OP, I’m 99% sure I know the PVC sheet you have picked up. The stuff I know is white, but it may come in various colours. It has a clear plastic film on one side which you mentioned you peeled off, but then it also has a somewhat hard and smooth outer layer on both sides? Whereas if you look at the cross-section of the sheet, you will see the middle of the foam is a bit more porous and spongy?

If that’s the case, then I can see you would have delamination issues. I’ve used the stuff before for building patterns, I certainly wouldn’t use it as a ‘structural’ core. The resin won’t bond easily to the smooth outer surfaces without a mechanical key. Once scuffed up, it tends to bond ok, but the sheet itself doesn’t have great mechanical properties. Might be ok to use as a shelf considering that doesn’t really require the laminate to do any real work, but I’d use wood myself.