i have a couple year history and experience with making carbon fiber parts in my garage. i understand and use vacuum bags and the correct resin for my use. one of the things i really want to get done is a carbon fiber gas tank. here i have an example picture of something im trying to make
http://www.thumpertalk.com/forum/garage.php?do=viewattachment&attachmentid=64289
its pretty clear that the carbon doesnt match in the middle so it was 2 separate pieces and the same goes for the gas cap also. my biggest questions are how can i join the 3 molds together? and also how can i put some nuts into the tank so i can bolt it to my bike. you can see mount holes with nuts in them in the picture. also obviously the most important thing is a resin that is okay under heat since the engine is right near it (i can wrap the tank in heat wrap where its around heat) and also can withstand gas. im thinking ill layer it with the 6oz basket weave carbon that i get, maybe add in some kevlar layers? and then finish it off with one twill weave layer on top so its got the pretty look. any help? 
1: there are inserts you can buy that are nuts, that get locked in place. I’m sure if you add a large threaded insert, or hell, an actual nut, it would work as well.
2: you will need a heat shield, composites don’t really protect from the heat.
3: You can buy a tank liner kit to protect it all from the gas. I think it’s a cleaning solution, and then a lining solution (like paint) that coats the inside with a gas-friendly polymer. I think most shops should carry it.
yeah i think the best thing would to do would be to insert the correct nuts into the correct spot and then let them cure with the carbon. could you sow me a picture of what you were talking about? the nuts that lock into place. i asume the lining that your talking about would be best to just pour into the tank and then shake the tank all around so it will evenly spread. any ideas about the molds? i just dont know how to pare up and seal 2 different cured molds together
Yeah, it’s a kit that you just coat the inside with. NOrmally used after tank repairs and self-made ones.
I can’t help you on the multi-part mold. That is not my expertise, but I know others will chime in on it soon.
You will want more than just one layer of carbon fiber, as kevlar is not rigid enough to make all the layers kevlar. Two layers of a light weight kevlar should be enough. But you also want several layers of cf in there. Also on your mounting points you may want to do an extra layer of kevlar… rubber mount the fuel tank if possible.
You could probably make the molds so they all bolt together with flanges. I have never seen a fuel tank mold personally but i know it’s done all the time.
i usually do brake caliper guards and engine guards at 7 layers of my 6oz basket weave carbon. i was planning on doing something like 6 layers carbon plus 2-3 kevlar and finish with a 6oz twill weave for the cooler carbon look than the basket weave
I am in the middle of making a CF tank for my SV650 so while not an expert I can tell you what I have learned.
As for attaching nuts, etc. I have had success with allowing the finished piece to cure and then scuff up and epoxy a nut into place afterwards. This seems to work well as you end up with a solid surface that you can put a clamp on to hold the nut in place while it cures.
As for a tank liner I am using something called POR15 U.S. standard fuel tank sealer. It gets painted on before bonding the halves of the tank, and then you slosh some around inside to coat the seams afterwards.
For the layup I am doing: CF/CF/Kev/CF/Kev/CF/CF for rigidity as well as abraision and impact resistance.
Hope this helps.
would you be able to provide pictures of your molds and or how i could join 2 molds together?
Hi, this is my first posting here , i am also making a petrol tank for my bike here are some pictures of the top side molds the underside is made separatly and there gonna be glued together (hope my english typing makes any sense … )
Spondtronix… how did you get the seams of your mold so close and nice fitting?
I am working on a plug that will have to have a multipiece mold.
how would i even go about making a plug of my tank?
Hi, I made as good as possible fitting splitterplate from wood(mdf)
and attached it to the plug with a 2k filler (first put some tape on the plug)
Then cut to the tape and filler while it’s stil soft along the edge so you get a nice straight angle,
The plug is made of foam (styrodur) made in to a shape , then I covered it with a few layers of thin glas (twill weave with epoxy resin) , and filler , sand it, spray with a 2k paint polish it (and be glad your not making a racing car :cheesy:)
I don’t now if this is the best way but it works for me till now
Its just time and effort making the flanges from whatever your chosen material is and time plastercining/claying the edges using a sharp cornered tool to not end up with a huge radius on every join line.
so if you mold the carbon in 3 seperate pieces, how do you join the 3 together to be very strong? say if half the tank is under your seat like mine is and some of the weight of you is sitting on it
I have used modeling clay many times and the tooling gel coat always “melts” the clay to the point where it changes the shape of the tooling gel coat. Gel coat gets warm enough the clay gets very soft and changes it’s shape. I always end up having to repair the mold before i even make a part with it.
I do like the body filler idea… trimming off the excess before it cures - to create a straight edge.
I need to get some woodworking tools and a sheet of mdf.
I’ve seen over the years many people making molds here and i can say by far the best molds i’ve seen people used mdf for the plug flanging. Not even my sheetmetal idea works as well as mdf. Only thing better i think is a machined plug, but not many of us have access to such equipment.
aids: you ask a forum member to do some engineering magic to see how thick and ply orientation to see how well it will hold the weight and how it should be laid up.
also are you sure you NEED a 3 part mold? Think of it this way… cover your original part in a cast of plaster… how will you cut the plaster in lines to remove it? You want your mold seems to be on part lines, edges where possible.
Also if your mold comes apart, you may choose to lay up just the first few layers of each section. Then trim those sections, place them back into the mold. Now finish your lay up schedule and vacuum bag it. Then unbolt your mold and release the part.
Clay needs to be sulfur free. You can also use sulfur free plasticine or wax fillets. Wax fillets require a little practice but can work really well in certain applications. If working with wood, you can mold the flange to the plug with body filler, release from the plug, sand it down flat then rebond it to plug with epoxy and you will have a perfect sharp edge. I think peoples biggest problem with flanging is they think it should be done quickly, in some cases it can be but I find taking the time on flanges is critical to a good mold and part.
my clay probably does have sulfer in it then. where can i buy sulfer free clay? Is “plastecine” a brand name or just generic?
the wax filet … can that be heated to make it easier to work or is no heat needed?
Spondontrx: What material is the mold actually made of? I can never get my molds to come out that nice so I guess I have a lot more research to do yet.
For the mold I used first two layers polyester gelcoat , wait until
almost dry ,than one layer of glas (250gr.) let it cure , the next day sand the spikes of , and the next layers of glas laminated with polyester, if you try to do this in one take the polyester reaction heat deforms the gelcoat surface (so I discovered :rolleyes:)
I only use wax as a release agent
But the most imported thing is a good shiney plug sprayed with a 2k
paint and a lot of time … (i’m also still learning)