Need help with construction

General Composites Discussions Composites Talk
#1

First of all… I’ll get a real pic of the part later. second, thanks in advance…

now then… here is a side view of a part the we may be consigned to do. it’s 5’ X 5’ and about 8 inches thick at thickest and maybe 1 inch at the edges. it has to support 20,000 lbs total gross what ever. I don’t think there is a shear strength requirement nor an impact resistance requirement. it will be used to cover xfer boxes for local power grids. the thought is what if a semi parks on it, or a car wrecks and some how goes airborne and falls right on it kind of thing to give you about the same idea of what it has to withstand that i have as of now. I’m thinking about 15 layers of 1.5oz mat on top and bottom, with a honey comb or some type of foam as a core. we will be using PER (recomendations welcomed) resin, and to start out with hand layup until it becomes more advantageous to make a (i’m thinking) VIP process for it. I’d rather do it the best way, although the only requirements we have so far are cosmetic appearance and strength. i’m also thinking about resin waste from hand layup and structural weakness from too much resin.

the plan so far to get us started with the part is… mold inside and outside, with a flange 6-8 inches larger than the sides of the lid. placed every 6 inches, bolts with spacing sleeves to ensure proper thickness (and for compression of parts.) I’m not quite sure yet how to cut something that big, because we don’t have the tools for an object that big, but i’m thinking something along the lines of a bandsaw and to smooth out the edges a 12"X6" handheld band sander. something like that. and then some type of non slip covering is require for the outside of the part.

so questions are:

1 how do i figure stress in parts for composites.
2 who do i contact for a custom sized foam core.
3 what resin should i use.
3.5 can infustion be used for up to a 1/2 inch thick laminate OR
3.51 what would be the best way
4 how should i cut the parts when the come out of the mold.
5 and any recomendations for a nonslip coating that will stick to the resin chosen

kind of long, but all input is welcomed.

#2

kind of a separate thought.

I would like to do it in 3 parts. the top part, the lower part, and put it together around the foam core in another step. i would think that would allow for stronger laminates since there would be no fear of collapsing or crushing the core during infusion or bagging.

EDIT: the pic

#3

some pics

#4

i have no idea! But very interested to see what other have to say!

#5

Boatbuilding would be the closest structure of similar specs that I can think of. Transom thicknesses sometimes reach 4 - 6". The foam cores used there are primarily made of PVC based foams. Divinycell, Klegecell, & Renicell are all produced Diab Inc. Another structural foam is Corecell. Many of the foams are sold with channels or slits to aid in infusing thick laminates.

I’m sure GGROP can point to someone that does foam CNC machining & cutting. The core looks pretty simple, so if you had a bandsaw with a custom table or even a table saw, you could easily cut the cores yourself. The manufacturers would provide precut cores if you contracted for a number of units.

Entire boat hulls are infused with complex stringer and hull designs, so you should be able to infuse that product.

Non skid is another boat building porduct with numerous manufacturers. One option is to have the non-skid as part of the mold - similar to the diamond pattern used by boat builders. Look at fluorescent light fixture covers. Some come with excellent textures that can be used as non skid patterns in a mold surface.

Also, don’t overlook thin, marine grade plywood as a skin layer over the foam. It would provide an excellent impact resistant layer over the core and contribute a great deal to the strength of the overall structure. It’s would be completely encapsulated, so rot isn’t an issue. Infusing probably wouldn’t be as viable with wood as part of the core though.

Just throwing out some ideas I’ve come across in researching the boat building trades.

#6

so… something like, foam core (what type of adhesive) to the skin layer of wood… then a few layers of fiberglass over the top for weather resistance. the core would take up most of the space, the wood on the top and bottom of the foam would act as the strengthening “agent” and the fiberglass would be just a protection layer.

so it would look something like this.

so where can i find someone who knows structural information about composites and can tell me how thick i need to make each layer to put me into the durability range i need to be in?

#7

One word of caution with wood as a core material is that VER is preferred over PER because the VER has better waterproof properties. Overtime water may migrate through a PER based laminate. Happens with cheaply made boats all the time (and the reason no one wants a boat with any wood under the fiberglass).

Make some test pieces and break them to see where the failures happen. My guess would be the foam will fail (pulling away from the skin) before anything else.

Can’t help you the strength estimates. Diab has a pdf file with all the mechanical property formulas on their site for foam cored structures.

#8

off the top of anyone’s head, how much of 1.5oz mat will it take to make a .5 inch thick laminate… roughly