Wednesday, February 22, 2012

Bagged Baked and Sanded

The first step after the joints are wrapped in prepreg is to cover each joint in release cloth.  In this case I use a very fine woven coated nylon that lets excess resin through during cure.

Once all the joints are covered they get a layer of bleeder/breather material to distribute the vacuum across the joints and absorb excess epoxy.  Once that's in place the frame is slipped in a vacuum bag.  I use Stretchlon 800 which is a 500% stretchable material that can withstand cure at 250F.  You can sort of see how I form a "Y" shaped bag to slip the rear stays into.  I actually managed to bag and pull a vacuum with no leaks which is a border line miracle!

Here is Chuck's frame hanging in the oven post cure.  You can see the epoxy saturated cloth at the joints.  I added a support bracket and bearing to the oven fan so it didn't try to self destruct this time.
After the oven the frame gets unwrapped which sounds a lot easier than it is.  It requires pliers, a knife and a lot of work.  I ended up with a pretty good looking joint with only a few creases and ribs where the bag halves meet.

Some quick work with a file results in this.  I just take off the little ridges and high spots.  You don't want to be removing carbon.

Post cure weight is about 26g less than before the oven.  That means quite a bit of epoxy was removed.  The prepreg I use is 45% resin to start.  The cure process brings that down significantly.
Here is the BB joint after it was filed then coated in a thin layer of epoxy, sanded and re-coated.  Once this is sanded the joint should be nice and smooth and ready for paint.


Crash said...

I've been flowing you progress and overall, I'm very impressed with your work. I thought I'd share some more composite tips help you make stuff stronger, lighter and speed production up. Looks like you've done your research so I won't bore you with Classic Laminate Theory equations...

Try using the Airtech peel-ply coated with release agent. It pulls off with bare hands, major time saver. Its also a much finer weave so there's almost complete resin/breather separation.

Those ridges are very bad when they contain fiber...and you've compensated with overbuilding but here's what's causing the problem and why it's bad in T2T construction. A simple 2-half bag, under vacuum, loads the outer surfaces more than the inter surface contours, particularly when using a heavy weight bag like Stretchlon 800. This causes a chain reaction of problems. First, you get fiber orientation shift as the resin viscosity drops with heat. With a external heat source, this happens from the outside-in, causing fibers pull toward the seam making those ridges.

The outermost layers is responsible for most of the joint stiffness. Crimp the fiber and the loads shift to the resin. Bad. This also means your F/R ratios aren't uniform, leaving fiber voids in those seams. This causes flex and weak spots. Even slight off-axis fibers and cause flex and matrix loading. Remember, fibers always pull straight under tension... Sanding the outer surface cuts the fibers, which loads the matrix further. High loads on the matrix causes surface micro cracks. This is why failures start from the outside-in.

Ideally you shouldn't need to overbuild, and shit would be lighter and cheaper too. Here's some things that will help, in order of difficultly...
You can eliminate most of those ridge seams with bag pleats and thinner bagging film. This will even out the compression loads. You can use a much larger bag and pull in pleats while it's under partial vacuum as long as there's no bag tension. Pleats occur naturally as resin flows out and joint volume drops, and thiner bag means smaller more uniform pleating. Depending on your layup schedule, you might try stage layups. The idea here is to reduce resin volume in smaller steps instead of all at once. It means several trips in and out of the oven and a lot more bagging work but less sanding the black stuff. The top layers won't shift, so you can add a filler before the final layer and you'll have good cosmetics as well. Finally, you can pre-form the pre-preg before applying it to the joint. Done right, you'd have control of fiber orientation and fiber volume across your production.

Here's some related pictures:

Seams are all resin. Different process, different tools, but it's ideal. Hell, not much sanding either.

Another. What a bad ass picture here.

Prepreg joint forms at LOOK. The prepreg joints are formed before applied to the tube.

More pre-cut prepeg forms.

Glen J said...


Thanks for the in depth comment full of information. I'm still working through the bagging process and I may try a thinner bag in the future. All of your images show molded frames. The first illustrates how they can bleed off excess resin through very thin gaps at the parting line of the mold, just like air vents in some plastic injection molds. As for the pre-forms, those are great but not applicable to tube-to-tube construction. What do you think of the heated external pressure forms used by Parlee on their T-T joints?