Making a gas tank - some questions for the wise

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#1

Hi all,

I have already made myself a carbon/kevlar gas tank which does the job but isnt perfect. I used standard epoxy resin and coated it with an internal sealant to protect the resin however my race fuel is eating through the protective barrier.

That said, I wanted to try my hand at making another, better model over the coming winter.

So, with that in mind I have a ton of questions and I apologize if a lot of this has been covered elsewhere. I have searched these forums and cannot find these specific answers.

So, I assume that I am to use a Vinyl Ester resin when making a gas tank? It is my understanding that the resin is far more resilient than standard epoxy? Is this correct?

I have a mold that I have painstakingly made and it does the job. I am looking to reuse this mold but I havent used Vinyl Ester before. Are there any particular things that I need to do to protect the mold/facilitate mold release? I have countless hours into this mold and do not want to risk any damage to it.

Finally, am I on the right track with just vacuum bagging vinyl ester? Should I be looking at infusion at this point? I want a high quality part but it must be able to be done in my own garage. I would really apprerciate some guidance here.

Thanks in advance!

#2

For the mold release question, wax will release all resins. If you want the best finish though semi perm releases are the best, although a bit expensive and cumbersome to use for one off parts.

Vacuum bagging is labor intensive but can produce high quality parts if done right. If you have practice with vacuum bagging I would suggest you stick with that for now. Infusion does have many advantages though but will require some practice.

#3

Is this for a car or bike?

#4

Bike, here are a few shots of my first attempt

There are a few imperfections in the finished product, mostly where I have to force the fabric into a tight corner and the bagging didnt pull it into place enough. I am all ears for suggestions on how to prevent this. I am guessing its the 3M Super 77 adhesive but I dont know the proper way to use it. Is it as simple as spraying adhesive into the mold, putting your fabric where you want it, and then wetting it out?

Back to mold releases, I use Partall release wax but should I consider using PVA? or Frekote? I have both but havent really messed with either.

Again, sorry if these are foolish questions. I really need to wrap my head around this process from start to finish.

#5

I just read something on this today, on a boat site, some guy wanting to make his own tanks.
http://www.boatdesign.net/forums/fiberglass-composite-boat-building/vinyl-ester-fuel-tank-good-idea-39141.html

#6

The only thing I am worried about, is the fuel. Race fuel can be nasty stuff. Make sure you choose a resin that can withstand the fuel.

Also keep in mind that the interior of the tank should be a resin rich layer, or even coated. Bagged and infused laminates are not resin rich, and that can bring problems as well.

#7

With th 3m i would suggest spraying the fabric with a light dust coat this would keep the weave straight as u lay it in a press it nicly into the corners. A fuel tank is something im doon over winter need to research more into it myself

#8

Thanks for this. I plan to recoat the entire inside of the part once it is done with the initial vacuum bagging. Finally I will also coat it with a more resilient rubberized protective coating made specifically for fuel cells. I am taking no chances here.

That is a great idea and I will experiment with it to see how it works.

#9

Usually for tank I am using Vinyl and not epoxy. Vinyl is extremely very good in Carbon fiber tanks. The infusion system makes Aerospace products. So if you can use Vinyl and infusion then you will make perfect product :slight_smile:

#10

I guess I need to do some research on home infusion. I dont know what equipment is needed or anything other than a very basic understanding of how it is done…and I mean basic :-p

I will take a look…and yes definitely using Vinyl Ester for this tank…

#11

Still working on this intermittantly so reviving this thread a little bit.

I have made the bottom half of my tank and am now going to make the top, and more challenging, half and I have a bunch of questions. Here is my mold:

and for comparison this is what the part looks like out of the mold so you have a firm understanding of the shape


As you can see the mold has to be split in two to facilitate release of the final part which makes things tricky. Anyway here are my questions:

  1. the seam between the two halves isnt perfect. What can I use to temporarily fill any imperfections that will allow me to make the part but release easily when it comes to demolding?

  2. as you can see it is a fairly complex shape with lots of compound curves. Ideally I would like to make this out of a single piece of cloth for aesthetic reasons. Do you think it would be possible?

  3. In my first attempt I had issues where the fabric would pull away in the tightest corners leaving a void. Is this a “Super 77” fix or is there some other way?

3a) If super 77: I have used this before however it leaves a bit of a surface flaw. Did I spray a little too much or is there something that needs to be done other than spray it on the fabric and stick it into the mold?

  1. The top and bottom halves need to be attached together. This must be a very strong seam, so much so that I am considering using a mechanical fix although this means putting holes in a tank which is generally not a good idea. Is there a bonding material that has the strength to survive all the vibration and stress that will come onto this joint so I can avoid drilling holes in the tank? It would also need to be chemically resistant to the race fuel that will be in the tank. Thoughts?

Top and bottom awaiting bonding

Well, thats enough for now =) thanks in advance for putting up with my incessant questions!

#12

For the gluing part: Plexus MA 300 or Plexus MA 420. Check the datasheet for chemical resistance, or ask your reseller.

Super-77: Will be hard to use invisible. In general gluing fabric to the mould will leave traces. Perhaps stick the fiber in on a half-cured gelcoat. (epoxy or VE)

#13

Yep, using the super 77 you end up with pits on the surface when you solvent wash the part.
To avoid this I spray a clear duratec into the mold to start with. Even though the glue is present between the carbon and clear it is not visible.

#14

+1 - We always spray Duratec as well, the high gloss clear, makes for a nice finish out of the mould and helps seal the part for clear coating (less pin-holes & fish-eyes) by using this product first. For the tank, you should do this…

#15

What is the shelf life of this Duratec stuff? I see it is only available in gallons and as a home creator I really dont see my using that much any time soon. Also how is it used?

#16

I can check the TDS on it to see, we go through about a drum every 3 months so it doesn’t hang around here to know if time causes issues with it.

It’s a simple product to use, easier than spraying clear, we prep the mould and put 3 to 5 coats of wax in it, then spray Dura-tec Hi-Gloss clear into the mould and let it sit overnight. Generally, between the 18th hour from spraying the mould we begin the lay-up. We never let the Dura-tec sit more than 24 hours in the mould as it can begin to have adhesion problems to the part. I have no idea why, potentially it may be too hard at this point for the resin to bind.

Once laid-up, we infuse the part, on a 10 hour vac timer, if we are in a rush we will post cure the part in our oven with a ramp-soak up and down, max temp it will see is around 110 degree’s F. At this point, when cured, we pull the part same as any part with or without Dura-tec.

The difference is a gloss part right from the mould with a pretty sealed surface, it is a gloss finish but I would not suggest one good enough to polish and deliver a part based on. We still clear the part afterwards, but it makes for a quicker process for us with less time wet-sanding and re-clearing.

Hope that helps, we also use it most times as a “gel-coat” on the plug surface before we make a mould, we’ll wax the plug and spray it with Dura-tec and then begin our mould process, idea is the Dura-tec releases from the plug as the mould surface ahead of the surface coat. It is very durable…

To your point on supply, the stuff isn’t cheap and to my knowledge the smallest quantity available is 1 gallon. Call Dura-tec and ask for some samples to try, the item is 904-040, Crystal Clear Hi-Gloss, catalyst is MEKP @ 2%, there is also a thinner you can use to thin it down, you can go as high as 20% but we have had issues with it that high, we try not to go over 15% when thinning it is required.

Hope that helps, if you need more detail PM me - tank looks good! Keep it up

#17

I keep mine at -18 deg c and just pull it out of the freezer 3 hours or so before use. Make sure it’s up to room temp before you open it due to condesation issues.

#18

It’s available here in quarts.
http://www.expresscomposites.com/duratec1.html

Nice work on the tank!

#19

Thanks gang, really appreciate the info thus far. Any ideas on filling the gap between the two top mold halves?

Edit bump! Any thoughts on this?

#20

Bump! Any thoughts on what I can use to fill the gaps between the two mold halves?