Hi all, I started playing with composites about a year ago & have learnt a bit since then. All of these parts have been for micro light aircraft. The latest pieces I'm trying to produce are 10mm x 45mm x 405mm bars. My first attempt was just a simple layup sandwedged between 2 molds. The results were that it weighs the same as an aluminium equivalent. So the only way to improve on this would be to do a vacuum infusion.
I made a vacuum pump that pulls about 26Hg max, Is this enough?
How can I insure that the carbon tow is fully saturated at the bottom of the mold?

 The mold is "C" channel & I don't plan on using the second half on the mold.

not sure why but I think I ever read that unidirectional cf can not be infused...

Unidirectional sheet especially is very hard to infuse as the fibres are so tightly packed there are no gaps to allow the resin to flow.  A normal carbon woven fabric has lots of gaps to facilitate infusion.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Wow that's  a bummer to hear that.  Could you guys make any suggestions to what might be a better way of achieving the same result. I planned on doing a number of parts like this but if there is no weight saving then it would be easier to use aluminium. 

Did you already vacuumbagged with perforatedfilm and absorbing layer?

Matthieu Libeert
Founder MAT2 Composites X Sports
website: www.mat2composites.com

The first time I did a wet layup & sandwedged it between the two molds using clamps, no vacuum. I was trying to get a nice finish on all four sides. Some resin was removed but still had a very high resin to fiber content and was full of pin holes. I put as much CF in the mold as I could fit in being wet. It was about 13mm thick when cured. I had to tighten the clamps in stages or the hole lot would squeeze out the end like a tube of tooth paste.

To get a thinner dimension (10mm) under vacuum I have about twice as much CF as the first attempt.
Do you think it would help if I put the resin flow mat on both sides (top & bottom) and maybe sandwedge some  resin in the middle?

Your right about that Warren

I learned this the hard way! 

The resin during infusion stopped dead when it hit the unidirectional layer. It it had the same effect as if you put a sheet of plastic in your stack.

Fred 

First of all ... a good pump, that can deliver HARD vacuum is essential !!!

I have had very good results with unidirectional, using a wet lay up and a combination of mechanical pressure and vacuum.

I have made I-bars and U-profiles this way .. much stronger and lighter than same size alu profiles..

In fact I used square alu and stainless tubing as moulds for the profiles.

for the I-bars I milled grooves in two pieces of stainless steel, did a wet layup of 200 g standard twill against the mould, and several layers of unidirectional in between.. The finished parts do not have the most perfect surface finish, since i did not spend much time on polishing the steel...

For the U-profile and bars i used an aluminium square tube, and cut one side off, the one with the seam .. (in the mill ...easiest way ..) The sqare tube has a small radius in the cornes, which is a very good thing..  I used a stainless tube as inside mould, I had to mill that smaller in width to fit, and give me the desired thickness of laminate.

After initial layup i simply bagged them and clamped them very hard using a number of C-clamps.. I also found that one must clamp in the middle and work out towards the ends, the slight flex in the tubes i used makes this work rather well

The longest bar/profile was 400 mm

If you have trouble wetting the unidir in infusion, maybe you should try to wet out the unidir first and the set up a subsequent infusion?

Maybe my initial success is due to using a combination of unidir and ordinary twill--- hmmm...

"Sapere Aude"... Dare to KNOW!

The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind...

103% of all people do not understand statistics...

Do not adjust our mind, theres a fault in reality :-)

Thanks for the advice everyone.
Dravis, I'm trying to use tow or robing? Maybe I would have better success if I used Unidirectional mat instead. It's hard to get the tow sitting perfectly flat on top.

 I thought about trying injection molding, that way I could push the resin through with much more force. Also I will add a few more layers of twill weave spread throughout the thickness.
Any thoughts before I try? I'd hate to wast that CF, it's expensive.

As I wrote, I've never tried to infuse unidirectional CF (the stuff I have used is 10 cm wide "bands" uf Unidirectional CF), they wet out very well by hand and roller, and I have had no problems getting a good fibre to resin ratio.

I did not use pure CF tow in any of the bars or profiles I made. It seems a bit too messy and difficult to control ...?

I also did not make parts that were more than 6 mm in thickness at the thickest --- If i remember correctly, they were 10 layers of Unidir + a layer of 200 g twill on both sides.

I had a lot of pinholes in the first tries, but i then switched to pulling the vacuum first and then clamping from the centre.

"Sapere Aude"... Dare to KNOW!

The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind...

103% of all people do not understand statistics...

Do not adjust our mind, theres a fault in reality :-)