Air in a composite, under the microscope

So, a while back, I was making a trial part for the tub to understand how much resin a 12 layer infusion would take with a 10mm core. I cocked that up, but since then, I’ve also done an excellent infusion for my floor pan, and I wanted to compare the differences in them both for you under the microscope, because I’m that amazing, informative guy.

Pants Infusion – With Lots of Air in

bubblesIn doing the infusion, I inadvertently admitted some air into the infusion, and it ran over the part. Also, I capped the infusion off once complete rather than letting the pump run for ages to try and pull the air out. Now I know better and know it should be under vacuum until it gels off.

So, what you can see here is a scale at 0.1mm per subdividing line. Ignore the 5mm bit. So, these bubbles are anything between 0.1 and 0.5mm wide. Wherever there’s a bubble, there’s a weakness. To the naked eye, they just look like a very fine dot.

Good Infusion, Where I Got It Right

So, this yellow bitch is going on the car. I had infused the resin at 28 degrees, with the mould also at about 30 degrees. I used a brewing mat under the resin to warm it.

no bubble

This time, you can see no air bubbles, and the weave is easy to see. It’s at the same magnification as the part above, but it’s at a different weave. This is 300gsm twill weave (rather than 2/2 twill) but has less threads per twill than above – more tightly woven if you like. I also let the pump run all night to ensure absolutely no spare resin remained in the part. I also have a new technique to ensure there isn’t any air in the original input pipe, when it’s submerged in the epoxy.

Now, I’ve Keyed it!


In laying up the part, I put some strategically placed 1″ strips of peel-ply in where the chassis rails will be when it’s bonded in. Epoxy doesn’t stick to it peel-ply. When I took the part out and tore off the peel ply, I ended up with a nice keyed surface for the adhesive to the chassis. You can see it here. The crappy red fibers are just bits of the peel-ply I can’t get off. It’s incredibly thin nylon but the red threads are only at the edges. I don’t think they will (at all) compromise the quality of the adhesion.


3D Printing ideas

So, even though I’ve got my Velleman 8400 printer and not assembled it, I’ve been thinking of what to do with it. Needless to say, with 3D printing that started in the community there’s not only a great set of community based CAD packages, but people like Autodesk are in on it as well with TinkerCAD and a quite interesting suite of free apps as well (including using your i-device as a 3D scanner).

I think my first project (I have some 3D experience with CAD anyway) will be to rework my composite fuel tank. I had to junk the original plan – it wasn’t possible to really make a part from the moulds – I had returns greater than 90 degrees on some flanges and that just doesn’t work.

What I’ll do this time is print a framework that bolts together, and bond on to that framework a set of plastic panels. The end result will be a completed tank that can then be moulded from. I’m also going to simplify the design with far fewer faces, but some of the curves (radius curves so the carbon can go in) will be much easier to make more accurately. I think I’ll make two halves that slot into each other much like the top and bottom of a Matryoshka doll.

I will make the panels out of (maybe) 3mm polypropelene sheet. It has great release properties, but then of course, is difficult to bond to the framework. Nothing that a good keying and epoxy won’t fix. I can use other plastics of course, and if I’m a little rough with the cut, then I will fill the gaps with wax. It’s only a tank and not a display item.