Hi everyone, I am hoping someone could assist me in finding a solution to a problem I have that keeps re-occurring? Having made a tool out of carbon tooling pre-preg under standard conditions and vacuum bagged on a flat surface (the dimensions are roughly 1/2m by 1m), and with a tight flatness tolerance of under 2mm all over, I still keep getting a bowing effect that takes me out of tolerance even by less than .5. I have tried different bagging techniques, postcuring with consistent weight over under surface and adding core stiffeners to the back etc, but i cant seem to get the flatness below 2mm over the entire surface. Help, it is costing me a fortune lol.:mad:
What is your laminate schedule? If you do not use a symmetric laminate (mirror image either side of the neutral axis) the part will warp (bow) as the part cures/shrinks.
Does the sheet bend, like a sheet of thin plywood, or does it warp, like a potato chip?
Yes the lay-up was symmetric and three previous tools were fine. I must add that there are flanges that appear to become more dominant under vacuum than the component area where the bowing effect is occurring. One other method i tried was to reinforce the underside with lengths of carbon like a grill effect but this only undulated the surface. I would mention that it was fine when it first came off the pattern but started to bow when shrinking and becomes worse under vac. Basically, i am trying to find a different solution to what i have already tried regarding the reinforcing/straightening as there are two existing and costly tools that need to be used. Some new ideas would be greatly appreciated as there is only the need to bring the tool back by probably a millimetre to put it within the tolerance with a bit to spare.
I put the tools back on to the master when trying the different reinforcing solutions and had limited success but not enough flatness was achieved to be able to use yet.
How thick is the layup on the tool? Have you thought about creating some kind of sandwich structure to boost the thickness. I often do 1" thick tools to maintain stability.
I have not clearly understood the way you made the mould.
Anyway, if you follow CAREFULLY all the procedures described here
it’s impossible to go wrong.
Unless you use a metal master, then you have to use insulating blankets on the bag surface to avoid uneven heat distribution into the laminate.
thanx Roberto, very useful link.
At the end, pag 13, there is the scheme for an autoclave mold. And for a resin infusion mold? Usually, how is the thickness request?
Pag 12 there is the scheme for a carbon sheet…also molds are made with a similar scheme? I usually use polyester resin and mat…with fabric you make molds with a similar balanced scheme, right? And for a complex shape, or a semi-spherical shape, as you plasce fabric?
The manufacture of the carbon tool followed the links schemes pretty closely as it was laminated off a PU tooling block to a 1-8-1 standard lay-up that has worked in the past. I believe thermal expansion of the PU block is a factor and envelope and surface bagging techniques have been employed but still this post-cure bowing is taking place, I have made sure the lay-up is a mirror image both sides of the middle. Any tips on how to stop it from happening again would be great but at the very least I need to reinfordce two tool already manufactured. On one, carbon spurs were bonded across the width and interlocked down the length to try to flattern the surface and on another, a 40mm core sanwich panel was laminated onto the back and although it worked to a point, it wasnt enough so any new ideas are most welcome.
How thick is the finished tool? What are the specifics of the prepreg materials you are using, areal weight, resin, woven or non-crimp, etc.
As mentioned above, have you tried using a core material from the beginning to make the mold a sandwich panel rather than laminating core onto it after the fact?
A standard 1-8-1 carbon tooling lay-up was followed with a mirror image fibre orientation from the middle. It was cured on a PU tooling block pattern and under normal cure conditions. When the tool was demoulded it stared to bow over time so the flatness was negatively effected and out of tolerance, even the post-cure failed to settle the tool back. It is worth noting that the first three tools were good, the last two werent. Envelope and surface bagging was tried to promote flatness but this didnt help. I would love someone to determine why this has happened but also it would be great if a reinforcement solution to the two dodgy tools was forthcoming as I need to use them. I have already tried a 40mm core skin over the back and also some carbon spurs along the width and length but with limited success.
To try to straighten a carbon tool is almost impossible, because you have to deform it in the opposite way first and then apply so much reinforcements to withstand the spring back. All of that as to work in the autoclave environment and hoping it wouldn’t leave fabric marks on the mould surface.
Could you post a picture of the mould or the master just to have an idea about the reasons of the problems?
I met your kind of problem in the past a few times, and they were all related to human error: the mould has been taken off the master without the necessary care before the free standing post cure, and that lead to stress induced warping of the mould.
I’m sorry, but using polyester and mat is all another story.
I have very little experience with polyester, since I started working in the composites with epoxies from day one. (more than 30 years ago)
I don’t see any reason to use polyester. Too many problems with that. It’s not cheaper anymore too…
yes I agree, polyester has many problems. I started with this resin only for a reason of money…now I’m starting to use epoxy and fabric for the molds, and it’s another world.
The only positive of polyester, is that don’t require post cure, and for a home use, this is positive.
Now I’m trying molds with epox and fabric. Your link is very interesting, my question is…mold for resin infusion needs thickness as autoclave? Then about 5 mm? I think they can be thinnest, 3 mm?
They can be as thin as you want, if they are able to retain their shape under the tensions and loads of the vacuum bags.
You can help yourself including 20-30mm. 90° flanges on the outer side of the mould flanges.
It has to be airtight too, of course, unless it’s small enough to be enclosed INTO a bag.
Not sure i can produce photo as I do not own the tool and I dont know where I stand regarding sensitivity etc at the moment but i can tell you that tool started to bow as soon as vacuum pressure was released after cooling which is why i wondered whether the tooling block bowed under thermal expansion and the carbon mould cured to that shape so when the block has cooled and vac comes off, the carbon tool springs back to its cured state?It is originally 5.5mm thick with 1-8-1 lay-up. Could moisture in the porous PU tooling block have anything to do with it? it is also only 55mm at its thickest so probably suscepible to bowing under heat?
Getting a photo is difficult as I do not own the tools, If you remember I mentioned that i had previous successes under same conditions but i remembered that originally the PU tooling block was at least 100mm thick but because of machining etc, that thickness is now reduced to about 55mm. Could this make the pattern more susceptible to warping uder thermal expansion? or could moisture inside the pattern be a factor under curing conditions?. I was wondereing whether the carbon has cured in that condition which is why it springs back when cold and vacuum is released?
sorry, did not think previous thread was posted properly
Is the fabric that you are using a balance fabric (equal thread count in warp & fill, and the same yarn/tow is used in warp and fill)? If the fabric isn’t truly balanced then you have to be very carefully and attentive on the +/-45 layers.
The only way to warp a mould in this way is to cure it at too high temperature. To avoid distortion, you have to cure the laminate onto its master at the lowest temperature permitted by the technical sheets. Usually no more than 50°C.
Then, it has to be CAREFULLY taken from the master and post cured following the T ramp from the data sheets. If the part is big/heavy, must be adequately supported.
Initial low temp. curing tooling prepregs exist just for this reason…