Assuming cosmetics are of no importance, if one were to introduce say a 90 degree bend in a sample of carbon fiber wetted out and well on its way to cure, i.e., where the material behaves somewhat plastically and bends with a bit of resistance, would the sample’s material properties be much different once fully cured? Does moving the composite effect the epoxy’s molecular structure in a negative way, and if so, by roughly how much?
It makes no difference if you do it for just one layer. That is essentially what prepreg is. It is partially cured.
I partially cure the first layer of cf before laying it on almost every part. It solves a lot of the problems with air bubbles etc on parts with deep tight corners and curves when a vacuum is not an option.
The downside (which is a benefit for my purposes) is that it will lose some of it’s adhesive strength. If you try and bond two layers of partially cured cf, you might find that you can rip them apart with you hands when fully cured if you left it too late. For additional layers, you should lay them while they are still tacky and slightly wet at the latest.
If there is any difference in strength, it is too small for me to notice. Cf and epoxy continues to get more rigid for days after drying.
Unless you are making parts for a plane or something equall crucial, I wouldn’t bother worrying about minor differences in strength.
The trouble is timing. Pre-pregs are so slow to cure at room temperature that you don’t really have to worry about it. But if are using a room-temp curing epoxy, then you have a windows of opportunity. If you leave it too long and the resin has cured too much then you can actually damage the matrix by moving it, especially if you are trying to bend it 90 degrees.
That’s easy to solve by buying the right resin for the job. Room temp resins come in fast medium and slow cure. My slowest hardener takes over 24 hours to gel.
The speed can also be used to your advantage. I sometimes choose my fast hardener which gels in 10 minutes because I can predict exactly when it will get tacky. Even with fast hardener you still have 30 minutes to an hour before it has cured beyond the point where it is tacky and maliable. I never take that long to lay a job.
I often get better results making my own prepreg and laying it partially cured for the first layer than I do with $150 / yard bought in prepreg. Btw, this process can also be done using heat cure resin for you diy prepreg. It then behaves exactly like the bought-in stuff. The only potential difference with pre-made prepreg is a more precise estimation of the resin to cf ratio.
There is no magic to prepreg.
Sure, I was just pointing out the potential to damage the matrix in answer to the original question. You need to be aware of these things if you’re going to avoid them.
The thread isn’t really about commercial pre-preg vs home made pre-preg, and they aren’t the same thing at all. If it works for you then great, but you’re comparing an extremely precisely controlled machine operation to brushing resin onto some fabric on a table. There’s no magic to CNC machines either, that doesn’t mean it won’t churn out patterns better and faster than it could be done by hand.
That’s not to say I don’t think it can or should be done, as you said you’re getting good results with it. My point is that it’s not a controlled process, and there’s factors you have to be aware of and think about.
Thanks for the replies.
Good points. I 100% agree that prepreg offers precise control over the resin to fabric ratio. If I was building wings for a commercial plane I would only use high quality prepreg. I would only use the method that produced the strongest results.
In terms of answering the original question, my point of view is that there won’t be any noticeable differences in strength from moving a semi-cured piece of cf fabric or from not using prepreg for a large percentage of users. Any concern about not moving the material in time can be fixed with a slower hardener. Aside from the control over the resin ratio, I don’t think there is any difference between home made or factory made pre-preg in terms of them being effected by being moved mid cure.
As you said, it isn’t the method you would use where precision and accountability is required.
In the field I am in, people usually add a layer of Kevlar or fiberglass when additional strength is required anyway. There is a lot of smaller companies with minimal equipment so it is often necessary to use a few “tricks” to get a nice glossy presentation layer.