DIY Prepreg

A little more info on that would be interesting. Was it basically the cloth went through a vat of resin and then the excess got squeezed out?

 

yes, that's exactly how it works, in our case we were using 22oz triax in a 12" wide roll which came off the roll at the top, went down under a hold down bar which kept it in the resin bath, then straight up through the pinch rollers which were fully adjustable and were initially set up by trial and error by weighing a length of glass dry on a gram scale and then run through the machine and then weighed again wet to determine the glass/resin fraction. Once we had the desired ratio we could use a feeler gauge to get repeatable settings.

Steve

 

Thanks for the replies. One question, though. If resin infusion is so great, why is the company in the video I posted wasting their time with in-house wet prepreg? Especially in something as critical in the structure as aircraft? How would it be difficult to have a proper resin to fiber ratio when the resin is pre measured before wetting out?

 

Agree. I prefer pre preg to pre wet. With prepreg, you can always get a higher fiber content. It is almost to the point of being porous due to high void content when cooked with a low vacuum. High vacuum or autoclave curing is the only way to go.

With infusion, the resin fills up all voids (or so) and tends to level off creating excess. Nice surface but if you compare it to a prepreg finish, the surface of the prepreg is rough, almost fiber like. In fact to make a prepreg surface smooth and shiny, a veil adhesive film loaded with micro baloons is added before cooking to impart a shiny finish. It adds weight but much more efficient than spraying the surface with sanding primer to remove imperfection.

One of the process in making prepreg is the dry film technique. A precisely measured dry film resin/adhesive is laid on top of the fabric/fiber and melted under vacuum/pressure to fuse. This way the fiber to resin ratio is much more predictable.

 

A summary of the 4-10-2014 link is that prepreg allowed the use of a thicker, stronger resin than infusion. So in their application, they got ~17% more strength/less weight.

 

With the DIY "prepreg" as described in this thread it is simply wet lam done ahead of time, it is virtually impossible, if not actually impossible to achieve, say, a 65% fiber content by spreading resin with a squeegee whereas it is easy to achieve with infusion. What i really like about infusion is that the average boatbuilder or even skilled amature can achieve very high quality laminates without having to spend thousands of $$$ on setup. Genuine prepreg on the other hand is not a technology for the small shop or backyard builder. For this reason i see resin infusion as the most important advance in composites in my 40+ of boatbuilding.

Steve.

 

Not exactly. When the composite started its development, it was a backyard industry operating in warehouses or large garage. Walk in oven was constructed out of used gas home heaters and lumber framing insulated with glass wool and lined with metal sheets. Circulating fan is ordinary axial type and vac pumps are the carbon vane type. The only thing expensive is the programmable temperature contoller which controls the firing of the gas in proper sequence to achieve the proper heat profile. A large chest freezer was used to store the prepreg.

Labor is easy to train as there is not much skill involved in measuring and cutting cloth which has the rigidity of a cardboard. The only difficult part is when the weather is warm and you have to coax the tacky prepreg to lay down flat and go easy on the sharp corners. A scizzor, a utility knife, and a nylon push stick is practically the tools.

Well, the mould is a little expensive as it needs high temperature resins and tooling gel coat. Prepreg is expensive but off cuts are thrown back in the freezer to be later reused.

Admittedly, resin infusion is a cheaper method, a little heavier, but gives good surface finish. By volume, I have seen some charts that resin infusion has become popular and is equal in production with prepregs but aerospace and supercars will always go for for prepreg.

 

Right Rx, but even doing it as you describe it is much more expensive to setup than infusion. When I was building snowboards we did some experimenting with prepreg and while we were already using heated presses with microprocessor controllers so didn't need ovens i did have to build a long chest refrigerator which i just built out of 2" eps foam. There are a lot of guys building large boats in their backyards with resin infusion with no more extra equipment than required for vacuum bagging and the results are vastly better than even the most professional wet lam from only 10yrs ago, even if not quite aerospace quality.

Steve

 

Agree. What I was trying to emphasize is that it started as a backyard industry. Those guys, the early innovators who did not have access to autoclaves contributed much to the development of the industry. Infusion came later.

 

Yes, this is the case with most innovation, the thing is though, is that pre preg has never stuck for the small shop or backyard builders but with resin infusion just about anyone can afford the equipment and learn how to do quite large projects economically (as long as they can get past the epoxy for everything lobby)

Steve.

 

Auntie Frannies, WEST System may have some resins which would work. If naught else, look into their ProSet line. It's much more designed towards being post cured than their standard stuff, if nothing else. And to some degree, System Three Resins make resins along similar lines.
Though I'm not familiar with either of them making resins specifically designed for DIY prepreg apps. But likely it's worth a few calls, especially to WEST Systems, since you're in MI.

That said, SP Systems (Gurit) has a proverbial rainbow of types of resins & pre-impregnated cloths designed for shops of all sizes & levels of technical expertise. Including some with the resin already in the cloth, where the resin doesn't get soft & turn into a liquid form to glue the reinforcements together, until it's baked.
If memory serves, it's their SPRINT line.

 

Thanks for the info, UNCIVICLIZED.