While making my first vacuum bagged part, I pulled about 12-15" of vacuum. The finished part really looks wierd. The fibers are ‘squished’ against the surface in some places and it doesn’t look so hot.
My question: how much vacuum should you pull when making a part?
Ideally you should pull at 27-30 inches of mercury. 12-15 should be fine though. A common misconception of vacuum bagging is using it to hold the fabric down where it normally wouldn’t stay down. Although the vacuum does a very good job at that, you should not rely on it as a cureall. Make sure your fabric is down flat on to the mold’s surface before putting it into the bag. Don’t expect the bag to do that for you. Remember the main point of vacuum bagging is to maximize strength to weight ratio by squeezing out excess resin.
Another thing that might help you fix that problem is to keep minimal movement around and inside the bag. Tape the excess fabric down to prevent it from running/floating out of place. Make sure the bag is being pulled down in a correct pattern where it won’t bunch up together and fold the fabric along with it.
Good luck and let me know how it turns out next time. It’ll probably take you another couple of tries before you will get something that will look right. Patience.
Evan
Evan - thanks for the tips!
I made my second try last night. Instead of using the polyester resin, I used epoxy resin (Aeropoxy). It was much easier to work with because of the longer pot life and more liquid characteristics. I tried laying up the fiber a little differently this time. Before, I was pouring resin into the mold and then laying up the fabric. I thought that would help me avoid surface voids. I was ending up with way too much resin, especially in the corners. That’s worked well for making molds, but not for making parts. Last nite, I went back the way of wetting the fiber on some poly plastic and flipping it over and putting it in the mold. Much less resin, but my part came out starved for resin (dry in spots).
I am currently experimenting with making household lightswitch covers because they are small, relatively flat, but have a little curvature to them. My biggest problem is just what you said - getting the fiber to lay down in the corners. I was hoping the vacuum would pull it to the corners, but I guess not. My best results have come from just putting a ‘caul plate’ on top of the fabric and using that to apply pressure. Is that the only way to get the fabric to lay down properly?
Here’s what I usually do.
Apply a layer of resin on the lightswitch plate and let it gets tacky, it can take anywhere from 20-45 minutes. While I wait for the resin to get tacky, I place the fabric on a piece of plastic wrap (food wrap will do) and wet the fabric out. Now both of them are going to start to get tacky at the same time because you’re using the same batch. Once it gets tacky enough, apply the fabric to the plate. This helps the fabric stick to the plate. I would also place the plate onto an object to raise it off the ground and use more bagging surface to allow the bag to wrap around the side and to the other side of the plate.
I also measure the weight of the fabric that I’m working with. Then I use that same measurement to mix my resin. 1 to 1 ratio is a good way to prevent you from wasting too much resin.
Good idea to switch from poly resin. It’s not a good resin at all for carbon fiber. It’s also not very structurally sound compare to epoxy. If cost is important to you then you should try vinyl esther resin. It’s a combination of both poly and epoxy although epoxy is still the way to go.
Evan