Ok finally I have make a rough mold for a bicycle saddle and I have tried to make one carbon fiber saddle but I have many problems, see the pictures.
Where is my error?
Good job. It seems you are doing most of it right, as the mold is pretty nice.
Are you vacuum bagging? It kind of looks like you are because of the peel ply texture on the back of the part.
The dry areas on the mold side are from the fabric not being in contact with the mold. Make sure to work it into the corners better, and the bag doesn’t bridge. You need to have slack in the bag so it can conform.
The shiny area on the back of the part, towards the front, is from the peel ply and bag bridging.
Is your pump pulling good vacuum?
I have build the mold quickly for practice and testing materials, now I’m to making new perfert mold.
yes I used it a vacuum bagging with fridge pump at -0,45 atm.
I used peelply on back because after I must to attach a metal rail for attach the saddle to the bike seatpost and I have read that peelply it’s necessary for good join with glue.
this is a picture of the mold and part during a drying
The vacuum bag is too tight! It must have slack to conform to the shape of the mold. That should solve 90% of your problems.
I agree with TET.
1). your vacuum bag needs pleats ( think of a hanging drape curtain). Watch the video on YouTube.com how the guy vacuum bags a Honda Civic car hood/bonnet. Where the bag pleats on the sticky tape you will have to just stick tape into those pleated areas so there is no air leak.
2). Are you gel coating the mold prior to laying the cloth into the mold? Clear UV (Marine) gel coat is nice because it can offer a glue-like surface to stick your carbon fiber cloth onto. This will help eliminate that other 10% of bridging problem once you fix the 90% vacuum bag problem.
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Peel ply doesn’t need to be one solid piece over the mold. Cut the peel ply into smaller squares or rectangles and that will eliminate peel ply bridging.
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Are you doing a wet lay up ( not vacuum infusion) ? If so get your carbon more wet in the areas where gravity will want to work against you. But try not to let excess resin pool up in the middle of your mold. Easier said than done I know. I would wet the carbon fiber out on a table thouroughly, then place it into the mold. But if you are using gel coat in the mold do not wet out the first layer of carbon fiber outside of the mold. First lay the dry carbon into the mold, push it all down to the mold surface. Then fully wet out the carbon fiber, proceed wetting the rest of the carbon out on a table then placing it into the mold.
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If doing a wet lay up /vacuum method be sure to use at least one layer of breather cloth. Breather is about 3mm thick and has the ability to transfer vacuum pressure evenly over the entire mold surface. It also will soak up excess resin that could otherwise make for a brittle part.
lastly this is just my 2 cents… make that seat nice and thick with just carbon fiber, no fiberglass if possible. About 2 to 3mm thick laminate.
Your mold does look very good 
Getting professional results with vacuum bagged carbon fiber cosmetic parts and structural requires thinking outside of the box… I know as I have racked my brain thinking of methods to produce quality parts.
Just like this guy, i’ve been there…
http://www.youtube.com/watch?v=V9aX0WcDh7g
I’m always on the lookout for new ideas that are cheap. I’m still looking for a silicone rubber/plastic that is liquid and can be sprayed or rolled onto a mold for making a bag. Problem is many of those liquid products are expensive to just go and buy for experimenting with.
Try pleating your bag in at least 4 places and that will help you tremendously. Also the clear gel coat trick. let the gel coat cure just to the point where it is tacky but won’t come off on your finger when you touch it. Also get a mil thickness of about 15 to 20 mils (.015"-.020")
“Pleats” can be seen in this video …
http://www.youtube.com/watch?v=iylkt9aDB6Y
Ok it’s true I need to cut more vacuum bag.
This is my first carbon fiber part that I have make so I don’t need a god finish I want only to try technics and testing materials but when I will make a “real”
carbon fiber saddle yes I think will use transparent gel coat.
Ok well but peel ply it’s necessary alway? or only if I need to attach others parts?
Yes wet lay up not vacuum infusion. I have brushing the mold with resin and then I laying first layer, the others 3 layers were resin out the mold on a piece
of glass and then laying into the mold over the first layer.
The vacuum infusion it’s too early for me, I was born westerday in composites, I must first learn to walk and then to fly.
Next test all your tips with fiberglass and then I try again with carbon fiber.
Ok well now I have only white gel coat but I can buy clear gelcoat but I must spraing or brashing?
I have another question for release agents.
I used 4 times gloss wax and 1 layer of polivinol for release agents but I have read that polivinol leave matt surface, I can use only gloss wax for smoth surface?
I recommend using ONLY wax. PVA is no good in my opinion.
Peel ply or some other release film is needed on the back of the part so breather material doesn’t stick. It also gives a consistent texture, and allows for easy bonding of other parts to it.
For small parts like that, you are ok with brushing the gelcoat on.
Try getting some latex liquid from a craft’s store and use gauze bandage (or that netting I gave you…) as the reinforcement…
Thank you Werksberg, I will have a look at the local craft stores for the liquid latex
Netting?
Do you suppose the homemade latex bag would be reuseable… or just a one shot deal?
Do you mean absorbent filter? I have used this
1)Peelply: http://tinyurl.com/58a2n3
2)Micro holes film release: http://tinyurl.com/5q6vxf
3)Absorbent filter: http://tinyurl.com/5jf3pg
this is the last layes the breather cloth?
I can’t read Italian, but the pictures look as they should be for your description. The micro holed release film is for the resin to be sucked into the breather (absorbent filter), but all the top layers (breather, bag, etc) will come off the part. I have forgotten release film in the past, and had parts with unneeded layers of breather stuck on it!
Peelply is for secondary bonding, like for the seat rails. Otherwise, is is only for making a part have a surface finish that looks neat. Sometimes you will get wrinkles, and shiney unevenness on the part. Peelply sometimes can hide that with a dull finish.
How many coats of wax it’s recommended? I need to wax and after 1 hours unwax? but I don’t understand if I don’t need to unwax the last coat of wax.
Today I see on my mold many little squares marked by carbon fiber cloth, this is a mold release problem?
You apply the wax, let it sit for 5 minutes, and buff it off with clean cloth. Use about 5 coats for a new mold, wait about 10 minutes to apply the next coat. Do not leave any heavy amount for the last coat, it should be applied normally. Wait at least 30 minutes before using the mold.
It’s just like waxing a car really. Don’t let it sit too long, and it shouldn’t be visible when finished.
The little squares are likely resin residue that transferred onto the mold surface from the high points in the weave. It should buff off. A gelcoat surface on the part will prevent that.
I still have a 5 gallon pail of latex here…
I have never got around to trying to make a reuseable latex bag but I have latex molds for plaster of paris molds that are more than 15 years around here still.
Try a small test panel set up…
I need to use only transparent epoxy gelcoat with epoxy resin?
Tell me if I understood correctly, I apply gelcoat with brush and then I put dry first carbon fiber layer and wet this layer with gelcoat then I put immediately others carbon fiber layers wet out of the mold with epoxy resin, It’s right?
But why I don’t wait the gelcoat is dry and then apply all carbon fiber layers with epoxy?
I have heard of people using polyester gel coat, let it cure then use epoxy for the lay up. Not all epoxies are compatible with gel coat though.
I was just thinking maybe there is a clear(ish)epoxy that can be used as a surface coat for the part.
Tabletop epoxy is quite clear and gives a good finish.