Advice on making a fiberglass mold

I am trying to use some of what I've learned working on boats to make a
modified bumper for my car. I was hoping to get some advice. I am getting ready to make a female mold of my rear bumper plug. This is a much modified version of the original bumper. I am covering the plaster with epoxy resin, then waxing with mold release, then PVA. I am fairing some of the angles with modeling clay. Does this look like it could be done in one piece or do you think it requires a 2, 3, or 4 part mold? The mold includes the black diffuser on the bottom. If it requires a mult-part mold, what is the best method for creating the lip between pieces. Thanks Dave



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A good place to ask questions about that would be: http://www.compositescentral.com/index.php?

 

Thanks - Ill go there and do some research

 

You don't have enough pictures to tell if it would be a multi-piece mold. Pictures that would show if there are any negative angles that would lock the piece in the mold and make it impossible to remove from the mold. You also have to inform about what is the actual 'bumper', is it just the white part or what.

 

Thanks for the reply. There aren't any negative angles but quite a few 90 deg angles. I am most concerned about the fins at the bottom and around the tailight areas. That part of the plug is made out of polypropylene. In fact all the black parts are polypropylene which is quite flexible. I'm thinking the safest thing to do is split it up into 4 or 5 parts to ensure release. Maybe split in the middle both vertically and horizontally. Since the bumper is screwed to the car I was going to cover the screws with clay, then repair and reinforce these areas later. I am wondering if creating a fillet with clay at the base of the fins would improve my odds of a clean mold release. I have made smaller plugs before but they have all been simple shapes. This seems like it could be a problem to remove. I am open to any suggestions to improve my chances of success. This is a long term project so the amount of time or effort is not really a consideration.

 

Is there to be a strengthening / mounting flange all around the outside edges?

 

Yes - there will be a flange around the perimeter. I also plan to mold in some conduit to strengthen the mold to help keep it from warping.

 

Here's some stuff on making the lip...
http://www.boatdesign.net/forums/materials/about-clam-shell-mold-19365.html

If there will be a flange around the perimeter of the part, you will need a multipiece mold. As to where the dividing lines/lips should be, it's too hard to tell by a photo the locations. A thorough understanding of draft and negative angles will show you where the lips should go. If the part gets locked in the mold or the mold gets locked on the plug, it's a major problem. 1 1/2 or 2 degrees are usually the minimum. If the fins are the same "thickness", or the sides are parallel, when the part is de-molded, the fin sides will drag along the mold until they are out, with no clearance to wiggle the part or mold to assist in demolding. They might scratch the mold. ANY stickiness or resistance of the part and mold to separate will make it very hard to remove without damage. Even with each fin being perfectly of the same thickness, the position of each fin in relation to the other fins has also got to be perfectly parallel or the same problem will develop.

If the fins had 2 degree of draft on each side, I don't think there would be any problem with de-molding. They might be a nuisance to actually lay up in the mold, being deep and narrow, but they should release easily. The flange around the perimeter of the part will also be a real pain to lay up. Unless the mold is rotatable, it will be upside down work that you can't see unless you use mirrors.

Do you plan to make many bumpers?

 

I am only planning to make one bumper. One thing that may help is that the fins are slightly flexible when warmed up since they are made of polypropylene plastic. I'm thinking I might be able to warm the mold up a bit with a heat gun before removal. I was also thinking if I split the mold vertically on centerline using the center fin as the split, any negative angles between the fins would not matter. The fins are 7/16" wide. I'm not sure I can get the mold accurate enough to split a fin like this and have it come out the correct width. What do you thing about a four part mold split in half vertically and horizontally. I could also split it vertically slightly off center to get two fins on one part, and one fin on the other. Would it help mold removal to fillet the base of the fins with clay? I appreciate your advice on this project.

 

The mold will be as accurate as the plug. That goes for size and finish, so any dull areas, scratches, bumps etc will be in the mold and will be in the part made from it. Also, any negative draft will be a problem and that includes those small cavities in the license plate area and the key hole depression, etc.

I would wonder about the fins not being very accurate in size, being made of polypropylene plastic, I can imagine the edge being 7/16" wide, and the base being 7/16" also, but the flat area on the sides being slightly dished in and subsequently forming a negative draft. Splitting the mold on the center fin would still leave the space between it and the outside fins as a possible negative draft area. It doesn't take much at all to be a large problem.

Since the "plug" is nothing that can be used after the mold is made (I am assuming) it could be destroyed to get the mold off of it, if needed, and the mold could made to be destroyed to get it off the finished part.

The whole "plug" is somewhat flimsy, the white foam is probably pretty fragile, the fins look to be something screwed in place somehow. It would be difficult to get that all faired perfectly. Since only one part is to be made, the plug and mold wouldn't necessarily have to be perfect. If you could get a molded bumper that would be sturdy and pretty close to what you want, it would be easy to do body work on that to bring it to "perfection", using standard bodywork materials and methods.

Instead of a strengthening flange like this all around the perimeter of the part, if you just added more reinforcement(s) on the edges, you wouldn't need a flange on the mold. If that wasn't there, and if the fins and all the other little places had no negative draft, it looks like you could just make a one piece mold and the piece would come out pretty easily.

Or, the flange part on the mold could just be taped or hot glued onto the mold and be easily removed, that would eliminate that impediment to getting the piece out of the mold, which in turn would most likely eliminate the need for a multipiece mold.

The fins might have to be faired a little to create draft, but that wouldn't be hard to do.

Are you going to make the mold with the bumper on the car or with it removed?

 

I hope that all was understandable, if somewhat rambling.

I would remove the bumper to make the mold. It would be much easier to work on and there would be no chance of getting stuff all over the car. Before you removed the bumper, it should be braced on the inside to keep its shape, so all the edges line up with the edges of the car, and so all the attachment points will match those on the car.

 

Unfortunately I cant remove the bumper since it was formed in place. Luckily, the fins are wider at the bottom and taper slightly to the tips so there shouldn't be any negative draft. I can clay over some of the details in the license plate area and keyhole. Worst case, I could cut the fins off and then make some and attach them with epoxy. Maybe out of balsa covered with light cloth? Lots of good info to think about. Is there any negative aspects to making a multi piece mold? I am hoping to save the plug until I have a reasonable bumper in the event things go terribly wrong at some point and I need to start over. The plug is made out of expandable foam, then drywall putty, then epoxy with micro to hold it all together and provide a reasonable finish.

 

Making a multi piece mold adds more work and will make it more difficult to brace the mold so it keeps it's shape, but it's not a very hard thing to do. You'll probably have to make the mold heavier than if it was one piece.

How come the bumper can't be removed to make the mold?

 

Unfortunately all the foam is attached to the structure of the car rather than the remaining bumper parts. The bumper has to be flexed several inches to remove it - There was no way to make a mold that was removable - at least that I could figure out.

 

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It looks like you could do a one piece mold, which would be the best way as it cuts down the complexity and increases the chances of success. Putting seams/splits in the mold introduces anomalies which have to be compensated for with more materials and figuring out how to spread the stresses so the mold is stable and doesn't warp or twist.