? about a clam shell mold

[Chris Herzog]

Guys,

I've made a few shoe-box molds but now I'm getting ready to make a clam-shell type mold and have some questions.

Once the plug is finished and I'm ready to start making the first side of the mold, first what should I use to make the lip with. Then, what would be a good way to attach it so that when I'm ready to start the seciond side of the clam shell, I can remove the lip material without messing up the plug.

Any sugestions????

Thanks guys,
C Herzog

[SamSam]

You can use anything that will take wax, pva, etc. and be sturdy enough to lay up laminations against. Metal, fiberglass, plastic work. Formica over plywood. I use 1/8" tempered masonite with a white coating on one side a lot for moldwork. It's available at Home Depot for about $13 a 4x8 sheet.

If the lip is pretty much out of sight, like the bottom of a boat, angle brackets and screws can be used to attach it, the holes easily plugged with clay or something for the second half. Blobs of clay or tape can be used to hold the lip if you don't want holes.

It doesn't have to be excessively sturdy. Before gelcoat, the corner where the lip meets the mold is filled with modeling clay or something and the corner itself made as sharp as possible. Any small "vacancy" on the first parts corner will create a corresponding knife edge flake like projection on the second parts corner which will chip off. You want to start the mold with gelcoat, let cure, and then 1 layer of 1 1/2 oz mat catalized for a slow set to minimize shrink. That is allowed to set like overnight, then another layer. Then maybe two layers. Etc. You want to build it up slow to minimize heat and movement. Once you get one layer on the lip, it will be pretty sturdy and the more you put on the sturdier it gets.

It doesn't need to be perfectly straight or exactly perpendicular to the mold, as the second half will mold to it. You should watch out for bubbles or pools of gelcoat or resin along the corner where the lip meets the mold, as those will chip off and cause cosmetic and lamination problems along the seam. The first layer or two of mat should be very carefully applied with respect to air bubbles etc., any joints in the mat should have ripped edge meeting ripped edge to make a seamless transition. The first and second layer of mat on the plug should first carefully butt up to the lip and then the mat on the lip butt up against the mat on the plug. This assures the corner has glass in it, not bubbles or resin pockets, and will not chip. Individual strands of roven pulled from woven roven can then be layed lengthways in the corner for the next lamination or two, until a radius is built up that the glass will lay into by itself.

After the first half is done, take off the temporary lip and then make some "register" dimples in the permanent lip so when the second half is layed up, bumps will be formed to automatically line up the mold halves. The first part of a 3/8" or so drill bit works well, just to the point where the end meets the side of the drill and so leaves a dimple and doesn't start a hole. The rest of the plug and the new lip are then waxed, and the rest of the mold laminated. Before splitting the halves, drill holes for bolts to clamp the mold together. Even if eventually the holes wear out, the dimples and bumps will keep the mold aligned.

It helps to put two layers of gelcoat along the mold seam during layup of parts so you have less chance of going through the gelcoat when dressing up the finished part along the seam.

[Chris Herzog]

Great, thanks for all the tips! You have helped me out a bunch.

If I have any other questions about this I'll post them.

C Herzog