Weight of wetted out core mat - various types

[UNCIVILIZED]

Hey guys, I could use a bit of help on this one. Specifically, I'm looking to build a few small composite parts, overall size say 1sqft & under. One's which need to be highly impact resistant, as well as light weight. Call it under 1lb/sqft or so. And the idea of going with cores is an intriguing one, for the stiffness to weight ratio, & to some degree, impact resistance. Plus I've done a decent amount of building of larger things, up to & including hull parts, using cores.

My question is, how much do some of the core mats weigh when wetted out with resin. Ones in the 2-6mm thickness range. And how do parts made with them fair with regards to impact vs. solid laminates, & also foam cored laminates? Plus, what specific cores would you suggest, & why?

For example, I could build something out of solid 4-5mm carbon & kevlar in the 1lb/sqft range. But I'm wondering if there's a way to go to get things either a bit lighter, or more damage resistant, if I use one of the core mats, & switch to thinner skins. Like 1.5-2mm skins on either side of such cores.

And any comments on using say 3mm corecell foam or similar in such an application?

Thanks

[TeddyDiver]

Weight saving with corematt is about 30% of same thickness solid GRP, nothing substantial in small parts I'd say unless there's a lot of them.
Depending of the shapes of the parts in question I'd use endgrain and strips of wood (spruce, ash or anyhting goes) and corecell mixed together if the geometry dictates so. As flat surface = strip, 2d = corecell, 3d = endgrain

BR Teddy

[Benz]

There is a supplier of small quantities of composite materials called Composite Envisions. On their website they have listed the amounts of resin that a square foot of each of their coremats soaks up. My favorite sort is Soric--it somehow has a honeycomb structure even in the thinnest (1/32") version.
The nice thing about mats as opposed to foam is that they're more flexible and sturdy before going in the project. Mat comes on a roll, 3/16" foam on a sheet that must be babysat and is very fragile.
Because mat soaks up resin and foam does not, I'll bet there's a better bond with mat--no chance of core sheer in an impact.

[Zach]

My two cents on small parts made in polyester, is that you can go all mat, say three layers... a layer of core mat, and a layer of mat over the top. A foot square you are looking at 2lbs finished if you gel coat it first. You can go thinner, but if you drop a winch handle on the part are you going to dent the core? Core mat doesn't dent, but it does delaminate if you try to use it as a core in the traditional sense. It makes cabin tops stiffer under foot in localized areas, makes hatches less springy... but that is about all it is good for.

The less extra glass material you put in the corners of a hatch or other molded part, the better if it is a female fitted part like a hatch cover. You also don't necessarily have to fully wet the core mat before laying the rest of the mat over the top. Just about the time it starts to ooze through the bleed holes is sufficient to hold the skins together as a stiffener. You want the top skin to be strong enough to take the load, and the core mat and inner skin just to be there to stiffen it up enough that you don't feel like it is going to collapse under your weight, and stress crack like mad.

Don't neglect that you can warp a shape into the surface too, as dead flat is flexible but a rib adds stiffness. Basically any time you add a 3/4 tall bump out, with a radius stepping up to the taller height the stiffer things get... If you can run it on a diagonal all the better as your panel doesn't use the stiffness as a hinge near as easily. This is especially important if you are going to add a hinge around a hatch. If you can mold in a detail that changes the shape close to the hinge, or a recess for the hinge, you can stiffen it up with the shape rather than thickness or core... Drain channels too.

With core-mat you'll get unequal loading on the skins, you won't have a balanced laminate, and you won't have as heavy a part as fully wetting it out... but it will be stiffer than the same part with the same number of layers of mat.

You can make a stiffer all mat part if you kick off fast, as the barcol hardness goes up... Try the first part at 2%, and see if you can get it to go off fast enough that the last layer is on and it kicks and changes color across the part. Sometimes you need a large weight/shaped panel to hold the piece in as it shrinks, though if you have it weighted it won't warp the part if the weight stays on longer than it takes for the temp to come to room temperature. If you try to do the same thing with directional fabric things go potato chip shaped, mat shrinks all directions about the same unless you have a resin ball.

If you are doing something without a mounting flange, I normally make the mold taller than it needs to be and rotate the part through a table saw to zip down to size, which means designing a flat surface big enough to clear the height of the fence. Making a one off mold just a bit deeper than what is required makes it easier to wet out perfectly at the top edge.

[UNCIVILIZED]

Thanks for the tips guys, very helpful. Especially the one on adding webs, or ridges (ribs) to the part, even if but small ones. Kind of like mini ring frames.

I'll look up that website too. As well as talking to the manufacturers of the stuff. Didn't realize that it was so fragile. Particulary the bit about delamination. And I also need to check it's compatibility with epoxy.

It sounds like Corecell might actually be tougher, though I'll check. But overall, in the end, a solid laminate might be better, especially given the lower densities of carbon & kevlar.

I suppose nature still has something of an edge when it comes to some cores. Such as using a thin veneer, or bending plywood. And were it a flat surface of any thickness, they'd certainly be high up on the list. The stength of Birch plywood is pretty amazing for any given thickness, as compared to some other species.

This "little" project is starting to take on a life of it's own, as it has me contemplating making lots of little, trick, accessory parts out of exotic materials. Makes me miss living near companies that commonly had 2nds of some of the more unique, & normally pricey, materials out there.

[Benz]

^^ The problem with wood is the danger of water intrusion, which you don't even have to be careful about with divinicell and other cores. Some of the mats are polyester-specific, others will work with epoxy as well. I believe Soric works with both. But if you're trying to bridge a large area with a flat panel, you can't do better than a real core--the thicker the stiffer--and there's dozens of core options depending on budget and engineering constraints. If we knew what you were building we could better give unwanted advice.