Repeat High Temperature Thermal Cycling

I've been thinking about the possibility of large boat building with prepreg. I think there could be substantial benefits to using prepreg as it's much easier to work with, minimal sanding, grinding, reduced fumes, increased strength, less waste, weight, etc. I think it could be possible to insulate a barn at least a decent amount and then use diesel vortex heaters to bring the barn up to temp. Could even thermoform the foam core?

Issue is I expect you'd need to do the boat in sections as some of the fiber would be required to hold the structure together as you add more.

Would repeatedly bringing epoxy up to curing temperatures damage it? Also worried about the boat sagging because of the heat.. Has anyone tried anything like this?

Could you use directed heat on smaller areas like is done for some aircraft parts?

 

I don't know if you have experience of prepreg and large structures but it might require the exposure of prepreg to ambient temperatures for longer than is wise.You could probably find a way to control the ramps and dwell times with nothing more complicated than an Arduino,if you are into that sort of thing.Lots of thermocouples on the face of the laminate will be essential to accurately understand what is happening with the resin.Putting them on anything else isn't a good idea as you need to know what the resin is undergoing.Additionally,the thermal mass of a large piece of tooling and it's attendant support structure might have adverse effects on the temperature gradients specified by the resin manufacturer.You can source low temperature prepregs that cure at 70 deg C rather than the more common 120 deg C but they are rather more expensive and the ambient temperature is closer to their cure temperature.If you resolve to experiment then I would suggest building up to large boats in stages to gain experience as the mistakes are less costly.

 

*Sanding could be more or less than other methods. It would not be less than infusion.
*It is not easier to work with because the job needs to be done in a short time once the prepreg is warmed to room temperatures. There is a much smaller window of opportunity to fix errors compared to other methods, like hand-laying or infusion.
*Minimal grinding depends on the organization of the job. Infusion is likely to have the least grinding. Grinding the surface to prep it to laminate bulkheads, etc. will be the same regardless of method.
*The fumes should be the same for the same volume of resin used. This would depend on the laminated schedule.
*The strength depends on the lamination schedule, the ratio of resin to fiber required and the workmanship.
*The prepreg would have more expensive waste. Instead of only the fabric wasted, it would be fabric and resin wasted.
*The weight depends on the laminate schedule.
The one advantage is that the fibers are already wetted with resin to a pre-determined ratio. The accuracy would depend on the quality of the product, just as with other systems.

 

@wet feet

No experience with prepreg and large structures. Hoping to learn. I'll definitely start on something small.

My understanding with the ramp cycles is that they are largely to reduce time spent in an autoclave while reducing the chance of the resin running.

If I'm not concerned with speed and can do a slow ramp it'd still work fine?

@gonzo

Thank you for the input. My limited understanding has me looking at some things differently, looking to learn where I am off.

For sanding/grinding, could you not apply more of the prepreg prior curing such as on vertical surfaces and even underneath surfaces, then rely on vacuum bagging to keep it together?


I had thought that you had multiple weeks at room temperature to laminate? Even after that I had expected you could continue to laminate but it might start to be more of a mechanical rather than chemical bond. Totally shooting from the hip here so I might be off.

For the fumes, I expect that the majority of the off gassing would be during the mixing and curing stages, during which in person contact is minimized with prepreg.

I think the excess fabric could possibly be refrozen and should result in less waste then mixing epoxy but I'll give you infusion as being less wastful.

For strength I have seen that hand layup < vacuum bagging < prepreg < infusion, but I've also seen some horror videos, especially of production boats where the epoxy does not fully penetrate and parts of the layup remain dry. That and the possibility of a leak mid infusion has me worried about the idea.

 

The resin in prepregs doesn't run,the vacuum bag prevents it happening,you did know that vacuum bags are always used with the material?The ramp is to optimise the balance between flow rate and cure as the little flow that does occur in order to fill the weave and allow the tiny air pockets out needs to happen before the curing process has gone too far.I wouldn't use autoclave and big boat in the same post myself as the two tend to be incompatible.

You might have an open time of a couple of days maybe three if you have a cool environment and then it is wise to get the latest layers cured.Even so the rolls need to be returned to the freezer overnight and brought out again a couple of hours before work starts.Fumes are somewhere between negligible and non-existent.

You would be amazed at how little brand new prepreg is used outside aerospace and high end motor racing.Many of the producers of carbon decorative trinkets are buying from outfits that take material that has just exceeded it's use by date for critical applications and sell it on.If you need to provide items with a traceable certificate of conformity,this stuff won't pass muster but it will be fine for decorative stuff or items with generous safety factors included.

Infusion can work well and although I have very little experience of the process,I have friends that are competent and they tell me there are two ways to get it right;one is to buy the software that determines the plumbing for the feed tubes and the other is to make lots of mistakes as you learn more about the process empirically.You can,of course,consult the professional composite engineers and they will advise you.They will very likely advise you not to use date expired prepreg and the cost of quality stuff plus their fees adds up really fast.

 

There are bigger boats buildt with prepreg, for example this one: https://www.aeoloscomposites.com/post/aeolos-p30-production-has-started

You don't do multiple heatings on the same part. Major parts are cured and assembled using structural adhesives or wet laminating. For example every one of this could be one piece done in its own mold: hull, deck, bulkheads, liner, furniture. It's not so different in principle from "normal" practices.

Just like wet systems prepregs are tailored to the individual application. Open time (freezer to oven) can vary from hours to days, curing temperature from 70°C to over 120°C, pressure can range from normal vacuum bag to needing a real autoclave.
There are also prepreg specific things like special surface layers formulated for cosmetics and glue films for core bonding. The molds have to be constructed with resins that can withstand temperatures above the curing ones so they remain stable while hot. There is usually a lot of debulking involved, you pull a full vacuum after placing a few layers in the mold in order to consolidate them.

"Insulating a barn" isn't the problem, that's easy enough, just some sheet steel and fiberglass batts. Designing the ventilation ducting and valving system to keep the air inside at a constant temperature so the entire part experiences the same temperature everywhere is something different. You don't want your boat coming out of the oven like a bad made pizza with burnt edges and a moist center.

 

That link brought back some memories! I remember in the early 80's using moulds made of high temperature polyester and glass for prepreg work.The thing to do before using them was to postcure the polyester and the stink as the free styrene was driven out of the matrix will live with me forever.We learned a bit about the challenge of the different thermal expansion of glass and carbon in an interesting way;some of the components were made using three cure cycles-the first for the outer skin,the second to bond the core to the outer skin and then the final inner layer.With one of them,we had extracted the mould from the oven just before tea break so that it could cool down before we did anything else with it.Just as we walked back into the workshop there was a loud bang as the shrinkage of the glass had just popped the partly complete component loose.I can't now remember what we did to get round the problem.

 

You apply all the prepreg for the part. The only grinding is on parts that have to be joined with tabbing or adhesives.

You lay all the prepreg and then cure it in one shot. It is crucial to keep the temperature even on all the parts. The thicker parts may need a cooling system.

It depends. A skilled crew will not have a dry laminate. In fact, it is easier to inspect any areas that are dry and correct it with a hand layup. Hand layup, can be vacuum bagged. Prepreg and infusion must be vacuum bagged. A resin rich laminate will be thicker, therefore stiffer. It will be cheaper for the stiffness. You need to qualify what strength you need. For example, flexural, tensile, compression, etc. Also, what parameters are you using; weight, thickness, price, etc.