Bladder molding pressure, resin viscosity and mold sealing...

[Bennisk]

thread327-166041

Hi all

I'm working on making a carbon fiber mountain bike fork, to a large extent by bladder molding, and I seem to be loosing to much resin out in the flash. My mold is a CNC'd aluminum sandwich mold and I don't have a bulletproof 100.0% fit between my two sandwich parts. The two solutions I have been considering (and haven't mastered yet, and am therefore asking you guys) are:

1. I have been contemplating with possible methods of sealing the mold better. I have a 10mm wide parting surface along the entire mold cavity. I would really appreciate if someone could give me hints on how this can be sealed nicely.

2. I'm planning to see if I can remedy my problem by solely playing around with pressure as the viscosity of the resin changes. How should I design my pressure profile?
My hunch is that I should start out with full pressure (some 4-8 bars) when the mold is still fairly cold (and the resin viscous), drop the pressure down (to let's say 1 bar) when the resin is getting really liquid and then when it's reaching a light rubbery state go back to full pressure.

Are these assumptions right? And am I potentially loosing some quality in the layup by having low pressure during its most mold-able phase, or are there any other implications to this method?

(Huge) thanks in advance!

 

[berkshire]

If you have a CNC'd mold, can you go back and put in a groove for an "O" ring seal.
Also are you using this joint mismatch to vent air as the bladder inflates, or do you have some other means ?
It is quite obvious that with that kind of leakage you cannot pull a vacuum to remove air first.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[Bennisk]

Thanks for your reply,

Yes, you are correct, I'm using the slight mismatch for the air to vent out as I inflate the bladder. As I believe is customary in the bike industry (from what I've seen). But my gap seems to be slightly to big. I was counting on a mismatch big enough to let the air out, yet tight enough to stop most of the resin from going out.

I guess using an O-ring seal might cause me further trouble as it will block the air as well, and using a vacuum liner is really not an option (I would think) due to relative complexity and associated production cost.

So I was thinking of some kind of a filler to fill up the biggest leakages. Potentially something I would apply on the parting surface of one mold half, somehow prepare the other half's parting surface with some release coating, bolt the mold together while the filler cures to the first mentioned half and creates a tight fit to the other half.

Have you ever heard of something like this being done? (Or do you have some better ideas?).

Regards,
B

 

[berkshire]

You could add a vent port to your setup if you used an 'o'ring seal. Then you could vacuum the air out of your mold prior to inflating your bladder. You could then use your method of inflating to a high pressure with the resin cold, and dropping the pressure as the resin liquifies without fear of trapping air bubbles in the layup.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[sjamesp]

Don't bother fooling around with anything too fancy in the way of changing the pressure, my experience with this has been that matching the cure profile is pretty tricky, and slight variations between batches make repeatability difficult. Quite often when you drop back the pressure, the increased loft of the fiber pulls air back into the part. Start with the simplest solution first. Cut some strips of a fine weave peel ply. Spray them very lightly with a bit of spray tack. put then around the flange surface of the part, so they are sandwiched between the mold halves, but not in contact with the part. This will still allow air to escape, but will inhibit the flow of resin. Depending on your gap width, you need a few plies, or you may need a long thin roll. most peel plies will build up about .003" thickness. If that fails, I have had limited success with making a thin glass caul as a filler, similar to what you describe. Take a couple of plies of a very lightweight glass, wet them out, and sandwich them between your mold halves. You will want both sides to release, as any filler will only last a few uses. Use a razor trim the laminate back from the mold interior. Handle the finished laminate carefully. If you wax it well, it will last a few uses. It depends on how thick it needs to be. Let us know how it goes if you try it.

 

[Bennisk]

Thanks for your great replies guys.

Sjamesp, I took you advice and took the simplest solution first
I've managed to solve the resin loss problem by increasing the cure time at a relatively low temperature to get the cure going somewhat, before I then increase the temperature to finish the cure. This raises my minimal viscosity during the cure and hence, the resin loss has decreased significantly.

But, now there is a new problem! (isn't there always one??
For every fork I make it becomes more and more difficult to get them out of the mold, and by now they are breaking when I try to get them out! And they leave resin clouds here and there on the mold.
This is the case despite that the surface smoothness of the mold has improved dramatically since the first cure cycles, because I have polished and waxed (>= 3 coats of Meguiars High Temperature Mold Release Was no.87) the mold thoroughly before each cure.

All that has changed between these first layups (when the fork released fine) and the most recent ones is that I now have a much smoother surface, and I have increased the bladder pressure from ~40psi to ~70psi.

So my questions probably are:
1. Does the increased bladder pressure have such a big impact on the release?
2. Is it possible that the smoother surface gives worse release (I'm imagining that the wax might be unable to adhere to the smooth surface and that might be the problem, however this explanation seems a bit illogical).
3. Should I be using a different release solution than this Meguiars wax no.87? Or should I maybe wax the mold while it is hot so all the micropores in the aluminum are more open when I wax?

With great thanks,
regards

 

[sjamesp]

Megular's is a crystalline wax and needs stripped every now and then. Wax from previous applications seed the crystallization of following coats, resulting in a gradually "coarser" texture to the wax, at a microscopic level. In polished aluminum tooling with little or no draft angle I have often had to strip molds every ten or so parts. Try a water based mold cleaner. Clean it until it (mostly) passed a waterbreak test. You can try a liquid wax, but all the waxes will do this to some degree. For aluminum I use Chemlease or Zyvax. They tend to last longer and work better at higher temperatures, though applying them is a bit more involved.
The other possibility is that unreacted epoxide groups are attacking the release agent due to the prolonged low temp B stage. Epoxies cure on sort of a two stage reaction. The first stage involves the the epoxy units forming a lot of OH groups. The resin actually becomes much more reactive shortly after mixing but before polymerizing. If you are holding the resin in this stage for too long, that may be the problem.
Increased bladder pressure may be offsetting shrinkage that helps to release the part, but that isn't too likely, or it may be that the majority of the resin shrinkage is now occurring at the hottest point in the cure cycle, and the part is actually slightly larger as a result. Its difficult to guess without actually knowing the cure profile of the resin. If its one of these, then the way the part sticks will tell you. Actual adhesion to the mold tends to be worse in corners and in hard to polish places, often leaving tiny bits of laminate stuck to the mold when it is bad enough. If it is a matter of the part being physically stuck in the mold because of the differing CTE's, or bladder pressure, you won't get bits sticking and you will likely see some scuffing somewhere on the part or the tool.

Hope this helps.

 

[Compositepro]

If your mold is becoming more polished, you may actually be removing too much of the wax in the polishing process. I don't have a lot of experience with wax releases but you do have to let the solvent in the wax dry before buffing. The buffing is supposed to smooth the wax layer, not remove it. Follow the manufacturers instructions exactly until you understand what is going on. Are you using a power buffer?

 

[Bennisk]

Thank you all for your much needed help.
I tried different curing heat and temperature profiles without any success, before giving up on the Meguiars wax. I replaced it with Frekote NC700 and all my problems are history
It seems like the highly polished mold just wasn't rough enough (!) for the wax to adhere properly!