So the phrase “pre preg” is one that is tossed around a lot. So much so, it almost has this mystical allure that parts made with it are more magical than others. So much so that I sometimes hear people tout it as “always being the best option to make parts.” But is it always the best option? Why does it seem to be talked about so much, while infusions and vacuum bagging seem to be pish posh at times. The world of composites is so complex, and so many shapes and parts can be made, no one process is always the best. I will try and dive a little bit into some of the pros and cons of pre pregs. For the sake of the article I won’t even reference wet lay parts. If you wanna see the result of we lay parts, read my “Not All Composites are Created Equal” Part 1and Part 2.
First. So what is pre preg? Pre preg is carbon fiber fabric that is “Pre impregnated” with epoxy. It is let to cure to whats called a B stage cure, and is then frozen to essentially stop the exothermic reaction and halting the curing of the epoxy. Because of this, one of the negatives to pre preg is it does have a shelf life and can go bad if not used in time. It needs to be kept frozen as well, so a storage freezer is needed. The addition of epoxy and extra process time by the manufacturer make it much more expensive per yard, but you won’t have the added cost of adding your own epoxy. It also, generally, has to be cured at a much higher temperature than regular epoxy. Note that this could be an advantage in some instances, and there are a multitude of epoxy mixes with pre pregs. Because its starting at a B stage cure, when parts are in the oven, the epoxy doesn’t really “flow” through the part like infusion, or even vacuum bagging, and generally an autoclave is needed. Think of an autoclave as a giant pressure cooker. Its easy for an autoclave large enough for car body parts to be over six figures. And then there is the running cost, too. The pressure of the autoclave almost makes the epoxy flow through the laminate when it warms up and softens. Yes, there are out of autoclave (OOA) prepregs available, but they still need a much higher cure temp. So the material is expensive. The process is expensive. The equipment to process it is expensive. So why use it?
There are several reasons why they are used. One is that they can lend themselves to making certain parts where the infusion process and vacuum bagging just doesn’t work well. Think of a small intricate shaped part. It wouldn’t make much sense to use infusion as there isn’t much room for your inlet and outlet line. Vacuum bagging may be difficult to get the carbon down and into all the corners without bridging. But pre preg on the other hand, with an out life of days, sometimes weeks, the tech can lay it up and take their good old time to make sure its laid down properly into the mold. Since the pre impregnated epoxy is at a tacky stage, it can be stuck into place and not move or shift aiding the tech in getting it into tight corners. Another situation may be on very thick laminates that take a while to lay up. Infusion could would work well here as well, and pre preg having epoxy on every layer, can be layered and layered without fear of any dry spots. A vacuum bagging setup runs the risk of the epoxy “kicking” before the laminate is finished or being way over saturated that the bleeder can not absorb all the excess epoxy. That brings up another reason. Since the epoxy is already in the carbon, this negates any need for a tech to have to mix epoxy, which eliminates the chance of an improper mix ruining parts. Pre preg can also be cut on a CNC table. This eliminates the need for a tech to cut out every single piece of fabric. It also lends itself to being easier to produce a more consistent product. Once the cutting table cuts in, a tech lays it in place, bags it, and pops it in the oven. Sound pretty easy right?
One of the reasons I feel a lot of the larger companies use pre pregs is more of a business decisions, rather than a part quality decision. At the end of the manufacturing line, the part produced is still carbon fiber and epoxy, under vacuum and pressure to yield a light, strong part. But the business side of it is where the magic can happen. As stated above, a CNC table can cut out all the shapes, eliminating the need for a tech to do that and reducing waste. A tech does not have to mix epoxy in precise amounts and lay it on, or wait for the infusion to finish. An autoclave will press the laminate together and squeeze out any air and voids that the tech may have missed. Essentially pre pregs allow for a lower skilled and lower man hour work force since machines can do a lot of the work. They take out the human element. Which means as a business they can more accurately estimate part cost along with manufacture times. But with the expense of the machines and equipment its a tough pill to swallow. Are the parts necessarily any better? They can be, but not always. If you can achieve the same resin to fabric ratio as with vacuum infusion as you can with pre preg, how could it be? Now, I will say that pre pregs benefit from a larger range of epoxy variables to exhibit certain characteristics, but thats another story for another time. I’ve seen my fair share of parts made with pre preg by other companies. One that comes to mind is the front splitter on Porsche GT3 Cup cars. They get banged up and broken and I’ve fixed several of them. Its neat to see how they are made. They are light. They are strong. But not really any different than some of the splitters we produce. Pre preg has its advantages and disadvantages for given parts and situations. But at the end of the day, its just another process to produce parts.
I created this little cheat sheet for some of the things to consider when making composite parts. Keep in mind this is very general rules of thumb here as I see it for my own shop and volume of materials and parts produced will change it for other people and shops.