Creating a mold from an existing part

[Dynafoiler]

I'm wanting to create a mold from an existing fiberglass part. What steps should I take in constructing the mold? Be as detailed as you like.

Here is a little info on my desired outcome(s). I have a rare personal hydrofoil that i'm wanting to make some carbonfiber pieces to replace the fiberglass ones. This is just for my own personal gradification. Check out dynafoil.com to view the details on this craft.

Thanks,
Josh

[mongo75]

clean the part well, apply several layers of release wax, and then apply fiberglass, a few light plys at a time so it doesn't heat up and warp while drying. Keep building it up until sufficient thickness has been made, and then carefully peel it off the part.

[Luckless]

Personally I would be more inclined to using molding gels over fiberglass and resins. It does require you build a box around the part you want to mold, but flexible soft materials have less risk of accidentally damaging the original. Check out art or dental suppliers. (I know the stuff used for dentistry is usually pricey, but you can get some deals if you order in bulk beyond what a dentist office would normally use, plus the stuff I'm familiar with them using gives very clean and precise lines and holds up fairly well to storage and transport. Should also work well for multiple runs with the same mold.)

Take your time and design a supporting box for the mold if you're not doing it in a self supporting agent. That is, if you're doing it in something that doesn't cure hard. Soft wood is usually cheap and easy to work with, try and get roughly half an inch to an inch if you can around what you're casting.

A fairly cheap option is straight up plaster, but you need to be careful when you're making your mold. Plaster doesn't flex much, so you can't have any over hangs in what you cast, but you can take your time and do multiple castings. When mixed right (That is, slowly adding measured powder to measured cool water while mixing) you get a fairly strong and reusable mold (backed by a wooden box).

What you use really depends on just how complex the shape is.

[JRL]

You would get a much better, more detailed answer if you actually had an image of what part you were tyring to copy.

Otherwise the generic answer would be to add flanges. Clean the edges up with plasticine. Wax the part a minumum of 3 times. Add tooling gelcoat. Layup glass one to two layers per day untill desired thickness is reached. Let it cure for a week. Demold.

[JRL]

Originally Posted by Luckless

A fairly cheap option is straight up plaster

I've had good luck using a mix of plaster, epoxy, 1/2 chopped fiber. Comes out supper strong and zero warping or shrinking. I use 1000 cps epoxy with black die and white aluminum powder for the surface coat. Not so sure the powder is needed.

[jim lee]

Another approach on this thread .. Newbie- Need help on mold building

-jim lee

[Luckless]

Originally Posted by JRL

I've had good luck using a mix of plaster, epoxy, 1/2 chopped fiber. Comes out supper strong and zero warping or shrinking. I use 1000 cps epoxy with black die and white aluminum powder for the surface coat. Not so sure the powder is needed.

Any chance of getting a little more detail on just how you do this? I've never tried mixing plaster and epoxies, but it sounds interesting.

I've had fairly decent luck using properly mixed "Sheet Rock 90". You do have to handle them a little gingerly, as they're easy to break, but good mold forms are key. I did use a bag of sandable sheet rock once when the hardware store screwed up and had only that in stock for a week. In short, don't. Actually, just never use the 'sandable' plasters, just learn to use the tools properly so you don't leave a ridge.

I've worked with faster setting plasters for dry walling, but the few times I have it came out with a far coarser finish, and I wouldn't use it for molds.

[JRL]

I started out by buying regular art plaster. Followed the mixing instructions, only instead of using water, I used epoxy. This will leave you with a relatively soupy mix. So to add strength, and make it less soupy, I added 1/2" chopped fibers untill the material started to turn into a clay like cosistancy. Forgot to mention that I also used Q-cells. You really have to expirament and take notes on what ratios to add (as far as Q cells and chopped fiber goes). Unfortunately I didnt, but I knew what they were when I was doing it.

Ive only used this method twice. Both molds turned out perfect. The benefits of it is that you use so little epoxy, that I layed both molds up to about 3/8" at a time w/o any exotherm. Both molds have been sitting on my garage floor for a year and a half with no supports. Neither has lost their shape, shunk, or warped.

Heres a vid of one of the molds....Kind of....

http://www.youtube.com/watch?v=dhFHPLbt014

And heres where the finished parts ended up: