Hood mold questions

General Composites Discussions Composites Talk
#1

We are in the process of creating a CF/fiberglass hood and I have a couple of issues I could really use some help on. You can see our process at http://www.home.earthlink.net/~ferretantics/cfhood.html .

This is the first time I’ve done a fiberglass lay-up for an automotive application where surface finish is so important. My experience is primarily composite nest for tooling. The original plan was to do a hand lay up with one or two sheets of CF and then backed with fiberglass, laminated with epoxy resin. But now I may vacuum bag it. I’m still doing research but I’m sure I’ll have more questions on that later.

Now that the plug is ready I need to know what material to use and the best way to create a good edge with positive draft around the parameter of the modified factory hood?

Do I need to place stiffing ribs around the parameter and maybe one in the center to maintain stability and strength in the mold? I may be pulling a few test parts off the mold. In all the post I have seen and read where they are doing vac bagging the molds look pretty flat without support but it’s hard to tell for sure. Supports have a tendency to pull the surfaces where they are bonded so that maybe the reason they are not used.

I would really appreciate some input.

Thanks for the help!

Jerry

#2

Can anybody help?

#3

Im a little confused as to what you mean. Like non drying putty around the edges?, or, what type of glass? etc…

#4

Thank you for taking time to respond, I really appreciate it.

I have no idea what to use. Putty sounds like a good idea, I never thought about that, is that what people normally usually use? Can clay or sheet wax be used? What kind of putty would you recommend? Do I need to worry about it separating from the hood in the middle of creating the mold.

What about bracing, is it required?

Thanks again,

Jerry

#5

With out actually being there its kind of hard to say whats the best way.

When making a mold I use putty A LOT. Mostly on the edges of the part as to create a radius/fillet at the base of the mold. Kleen Klay is probably what your looking for.

As far as covering putty with glass/resin…Ive never had a problem with it in any way at all. Although I wouldnt use it to hold anything in place.

I would brace the back side of your mold just as a saftey thing. It would suck to not do it, and then have your mold warp, crack, or whatever. Ive crushed a plug before, its a mistake you only make once.

#6

THANK YOU!!

That certainly puts me on the right track. I think I’ll use the clay around the edge, trim it with positive draft and then use some Formica strips under the clay to create a small flat shelf.

Do you think I can get away with bracing just around the parameter without anything in the center?

Jerry

#7

Im sure you could get away with it. If you made the center of your mold relatively think then I doubt it will break.

#8

For stiffening ribs you could do what i did on one of my molds. I used pipe insulation cut it in half with a stanley blade, it already has a slit down its length anyway so i followed it through to the other side. This gave me semicircular sections 1 metre long so i cut these to the required length and dabbed a little superglue here and there and placed them on my mold. at this point i must say the thickness of my mold was to its finished state. I then just glassed over these and once set the foam insulatiion had shrunk away a little and not stuck to the glassfibre thereby creating profiled glassfibre ribs.You can buy paper rope made for this purpose which will do exactly the same thing.

If your mold is the correct thickness to start with, at a guess i`d say about 10 to 12mm by adding ribs like this you should have no flex or distortion. You must leave the mold on the plug untill fully cured and not be in a rush to pull it.

By the looks of your work so far it would appear that you know what you are doing so i hope what i have said above does not sound like teaching your granny to suck eggs.

I have never made hood mold but i`m sure there are people on here that have so hopefully they will chirp in with some advice.

Baz

#9

I kind of dont understand what youre really looking for in answers but from what I do understand.

to make the flange, you can use some aluminum sheeting found at lowes (cant remember what its used for. Or your formica idea should work.

I use non drying clay to make the fillet from the part to the flange.

For ribbing…Ive heard of lots of people getting burned by glassing in right against the mold. Seem the shrinkage likes to make your bracing print through to the mold surface. Ive remedied this by using popsicle sticks or paint sticks stacked strategicaly and then using carboard as the bracing.

You can glue in the sticks to elevate the carboard a tiny bit and disperse the tension. Then put down your cardboard and glass over it. You should be able to do that cheaply and effectively.

Nice work! I wish I had access to that stuff you guys have. Large scanners and huge bed CNC mills…sheesh the things I could do.

Im about to hand carve a skull out of my foam. I wish I could get away with the cnc stuff

#10

Thanks guys for the help.

Lol, thank you! I’ve done a lot of customs and tinker around in tons of things but this hood project is taking me into territory I’m not very comfortable with and I’m kind of a perfectionist. I don’t mind the trial and error thing but it would be great if the next step is a good one. So I certainly appreciate the help.

The rib and aluminum ideas are great. I’m fairly certain the mold will pull in the areas where the ribs attach especially if I glass them in while the mold is wet but I’ll place them on the perimeter. Then add a rigid brace (board) down the center attach it to the perimeter bracing and after the mold cures I’ll attach it in a couple spots on the mold. I Hope that works.

Thanks again! I have some really great friends. I sent you a PM I might be able to help you with your skull project.

Thanks a lot guy I do really appreciate your help and suggestions. I’m going to have more questions if and when I decide to try vac bagging.

Jerry