Wednesday, January 30, 2008

Another lesson learned on infusion. Unfortunately, I don't know which one.

After applying today's layer of fairing compound I continued the preparations for resin infusion of the beam bulkheads. When doing resin infusion you never stop having surprises. My approach to this technique is to read Henny's starter kit instructions and then try to anticipate what happens with a certain setup using common sense and what I remember from school in addition to Henny's tips.

The principles are after all very basic: Place your fabric, core and other elements of wish in the desired configuration, wrap in plastic, suck out all the air, then let the atmospheric pressure fill up all the empty spaces with resin. The hard part is to anticipate how the resin flows under different circumstances and in different distribution media.

Until today, I have infused all flat panels using inlet in one end and outlet in the other end. Today's part, however, had a non-rectangular shape and thus not ideal for this approach. I use a core with pre-drilled holes and 0° - 90° grooves for resin distribution. I place the grooves facing down. This works fine with my regular setup as the grooves distributes the resin very effectively. I decided to use one central inlet and outlet in the perimeter on this part and I did not anticipate any problems with this configuration. The picture below shows the setup just prior to infusion start:

So, based on my previous experience on the same core, fabric, peel ply etc I expected this part to be totally impregnated in 10 -15 minutes. I was quite surprised to see the extremely slow migration of the resin. Here after nine minutes:

And after half an hour of infusion:

This is what I suspect is the explanation: When doing the regular inlet one side outlet one side infusions, the inlet tube is at the end of the core. The resin migrates directly to the end of the grooves and distributes further along the part.
With this setup, however, the resin had to pass through the release film, the peel ply, then two layers of carbon fabric, through the holes in the core and THEN being distributed. I suspect the way from the inlet to the grooves were the bottleneck slowing the process down.

Anyway, after one and a half hour of speculations, most of the part was impregnated, the bucket of resin was finally empty and I left the building:

Final results will be reported when available.


Fram said...

First of all, I really enjoy the way you are using resin infusion and that my RI kit was of some help with that. But, the devil is in the details, so please re read my directions and you will see that the spiral tube must be in connection with the RDM (or in your case the grooves) What you did was making the resin break at the wrong end. The opposite setup, resin along the perimeter and vacuum in the middle, (which incorporate an in-build resin break)should work however. But beware, your RDM method is different on top and bottom which has the risk of an uneven resin front on top and bottom. But I think you are using a violent perforated core to prevent that.


Tor Rabe said...

Thanks Henny!

It's not the first time things would have been less exciting if I read your instructions more carefully. I never forget these mistakes, however. The core is the preperforated and grooved Divinycell H60.

And you confirm my theories, so the lesson is learned and I now know which one :)