Advice on making a fiberglass mold

Thanks. I try it with a one piece mold. That makes things considerably simpler.

 

Yeahbut, it's your call on whether it will work or not as far as positive draft and such.

As it's a one off piece, I would not worry a whole lot about getting the plug perfectly fine looking, as I think that with 3 separate pieces of 3 different materials (the original bumper, the foam and the fins) it would be very much a nuisance. If you can get a 'rough' mold that will produce a 'rough' bumper that is homogeneous, in that it is all one material and stable (fiberglass), it will be easy to finish that to perfection.

So, my advice would be to make the plug as close as you reasonably can to what you want without getting all overparticular about it, make a fairly light but stable mold , then a part that is strong enough to function as a bumper, that you can then finish to faultlessness . If you wanted to make more bumpers, that finished bumper could be used as the plug to make a finely finished, sturdy production mold. The most important thing would be to prep it all so the mold will come off the plug without any release problems.

Have you thought about what's needed to attach it to the car?

 

That all makes sense. Once I have something close to what I need in fiberglass, I can simply do bodywork on it. The bumper attaches with screws under the license plate and with flanges around the border under the fender line and behind the wheel. My plan was to end the mold with a flange an inch or two short of the ends as shown in the photo. Then I can remove the plastic portion of the plug and finish the ends of the bumper off the car. I plan to but some indexing holes on the mold and plastic portion of the plug to ensure it all lines up off the car. Does this sound reasonable?

 

Is that so you can copy the flange on the old bumper, and then be able to mold in a flange on the new bumper that it will bolt right up to the upper car body?

 

Thats correct. I want to try to replicate the flanges as close as I can so that it attaches just like the factory bumper. I know fiberglass doesn't like sharp bends so it could be an issue. I think I can reinforce the inner corner of the flanges with a fillet made with epoxy and flox possibly. Its not under much stress - just helps the bumper maintain shape. The main mount is under the license plate.

 

When you make the mold you want to go slowly. A moderate, regulated room temperature, no hot mixes of resin, and reinforcements applied in light layers allowed to set up before the next is applied to keep exotherm heat to a minimum. That will help a lot to keep the mold stable with little to no warping.

 

Sounds good. I am planning on making the mold using epoxy with an initial 2 oz cloth to minimize weave, then about 6 layers of 8oz cloth with a slow hardener. I am hoping using an epoxy with slow hardener will minimize the chance of warping, give me more working time, and result in a stronger mold even if its a little more expensive. I was planing on fairing all the inside corners of the mold with epoxy/flox/cabosil mix. I was planning on reinforcing with with 1/2 inch conduit but having the mold flexible may help it release. I might create the mold without the reinforcement - pull it off, the put it back on and add reinforcment.

 

Mold Progress

I was able to pull a decent mold off the plug. The only problem was that the PVA was separating l little from the polypropylene portion of the plug. This resulted in a few ripples but its good enough for a one-off mold. I dont think I really needed PVA on the polypropylene since its pretty slick already. I decided to do away with the little vertical fins for design reasons so I/m going to bondo over those before making the final bumper.