Using the original part as a mold?

General Composites Discussions Composites Talk
#1

Hi all-
I ran into this board while looking into making some parts for my motorcycle. Thanks for all the good info here.

So here’s my newbie question. I’ve seen a bunch of stuff about making a mold of a piece, and using it to form the cf part. Is it possible though, to just use the original part as the mold? Could you just take the original part, wax it, or put on a release film and then vacuum bag a single layer of fabric on top. When this cures, pull the single layer off the original part and laminate more layers onto the back to build up thickness.

One downside I see to this would be that your finished product would be slightly thicker than the original part. If you could live with this though you could end up saving several steps.

Another possible problem I could see is that the epoxy would have to be cured pretty well before you separate the single layer from the original. Would there then be adhesion issues between the first layer and the next? Also, would a single layer be too flimsy to maintain its shape during the second vacuum bagging?

Anyone care to shoot down this idea? I figure someone must have thought of this, and the fact that I haven’t seen anyone doing it must mean that there are reasons why, but other than what I listed, I can’t think of any.

Thanks!
Dave

#2

Dave,

You’re not that far off. What you’re doing is using the OEM part as the male mold instead of making a female mold. Many many many people do it this way and here are the pro’s and con’s:

Pro’s:

  1. No need to make female mold which takes up time and resource

Con’s:

  1. As you have figured out already which is the fact that your piece is going to be thicker than the original.
  2. Surface finish is not going to be smooth. In order to achieve this you’ll have to build up the top layer with clears and sand it down smooth and clear coat it one last time.

You don’t have to do one layer at a time. If your original piece is tough and strong, you can do as many as 3-5 layers at a time. Too many layers at once might cause excessive heat which will warp your male mold. If you have to do it more than once, just continue to add on layers instead of removing it from your male mold.

Good luck,
Evan

#3

Evan’s spot on with his pro’s and con’s. A lot of people do use the original piece as a male mold, but for one off parts there are plenty of cheap mold making mediums out there.

You can make a simple plaster mold of your piece that’ll work well for 1-5 pieces if you’re careful. There are plaster upgrades like Hydrocal and other gypsum cements that’ll last longer than that since they’re stronger. The only real issue is with weight, especially for larger pieces, and it’s a messy process. For me though, I personally prefer a firm hi-temp resistant RTV silicone rubber. A little more expensive than fiberglass and resin, but infinitely easier to use. Just mix rubber with catalyst, mix, and poor into a box with your part layed up and let cure for 24-36 hours. You don’t have to worry about heat like you do fiberglass and resin, and there’s no involved steps in making the mold, other than establishing your base for you plug in clay.

#4

sorry for bring an old post back from the dead, but this is what i was planning on doing. i realize that the new part will be wider than the oem part. to reduce the thickness, could you first lay 1 layer, and cure it, then make it stronger by putting more layers on the underside of the “1st” layer so that it will only be “marginally” bigger than oem. then if this is the case, you would have to place your “final” layer first, if you’re using different types of cf. for example im using silver hybrid 2x2 twill as my top layer and plain weave cf with polyester “cross-stitching” for strength. so i would have to use the silver layer first, then after it cures, then i would put 1-2 layers of the other cf on the underside? i hope this makes sense… please post up if you have any suggestions.

#5

The problem with doing that is that initial layer will be very flexible. It will be very difficult to lay more layers on without distorting it.

In the end you will probably spend more time by having to do finish work the part vs. making a mold and having the part pretty much ready to go out of the mold. There’s a reason why everyone builds molds even for one part.

#6

where can i find this stuff? online, locally, maybe a brand name? all the foams and things i have found locally are the suck.

#7

It would greatly help us out if new members would list where they are located , but Evan can get you anything you need (I sure have given him plenty of sources), as long as you have the money…SHOW EVAN the MONEY!!!