Last weekend, we vacuum resin infused the first mold of the cabin. It’s the first and most crucial one of three pieces, because it will be used to make the die for the clear canopy. We had to take two shots, as the first vacuum infusion failed. More on the reasons below.
Everything began with the dry lay-up of the sandwich. We made some simple tempates using Compoflex® SB 150. This peel ply is ideal for making templates because its drapable, but it does not deform as easily as woven fabrics, and it’s a consumable.
After having made the sandwich’s dry lay-up, we used the Compoflex® templates to make the peel ply layer. There is a Compoflex® variant specifically desgined for vacuum resin infusion (RF 150), but we have only the one for vacuum bagging (without flow mesh).
We used this type of Compoflex® as a substitute for usual peel ply, as the forces needed to peel it are much lower (prevents premature sepration of the mold). Though it has no flow mesh, we didn’t expect to get into trouble, beacuse the Soric® XF core should act as a flow medium. We were proved to be wrong, as we painfully recognized later…
As always, we used the MTI® vacuum line by DD|Compound. It makes a resin trap unnecessary, and we’ve used it in all our infusions. Highly recommendable.
Having vacuum bagged everything, we started to mix the resin and degas it. We used HP-E120RI from HP-Textiles. It’s an epoxy resin with 200 minutes pot life. About a month ago we made an infusion test together with the Soric® XF core. It worked pretty well, but we saw that it takes quite some time to degas. Degassing of 1.7 kg (3.7 lb) of HP-E120RI took us about 30 to 35 minutes, and we needed roughly 6.8 kg (15 lb) for the mold! To make the story short, it took ages to get the resin ready:
After opening the feed line, it became clear that we’ll get into real trouble. The flow speed was incredibly slow. If everything works well, it’s possible to infuse larger amounts of resin before it starts to heat up. We’ve done this, for example, in the mold of the center section.
Epoxy resin reacts with the hardener and creates heat. It’s an exothermic reaction. The larger the amount of resin is, the more it heats up because less heat is dissipated to the environment. Heat speeds up the reaction ending up in more heat being produced (pot life is halfed every 10 °C or 18 °F more). If you have bad luck, it can heat up so fast, that it hardens within minutes, although it takes usually one day to room temperature. This is exactly what happend in our first infusion: We needed very long to degas the resin, infusion speed was too low, the resin started to heat up quickly, and it „boiled“ at some point. Lucky us that it was cold and snowy outside and the resin did not catch fire.
We had to stop the infusion, although only 1/3 of the mold was ready. Usually this is a killer for a laminate, because infusion is usually used to create nice looking laminates. Here, we are only interested in properly wetting the sandwich, and we do not care about the looks. This is why we decided to let the aborted infusion cure and to infuse on top of it in a second shot.
Before we made a new infusion we had to understand why the first one failed. The main problem was probably the low permeability of the sandwich. If the feed line has direct contact to the Soric® core, low permeability shouldn’t be a problem. Here we had no direct contact to the core: The resin had to flow through a layer of Compoflex® SB 150, two layers of 105 g/m² glass and two layers 380 g/m² carbon. This was proably too much friction for the somewhat viscous HP-E120RI. The solution was, thus, to use a flow mesh the next time. We also decided to take the slightly less reactive and less viscous HP-E300RI with 300 minutes pot life:
The second photo shows clearly how much the infusion fronts outside and inside the core differ. We’ll probably not rely solely on Soric® in our next infusions and will use always a flow meash. We documented both vacuum infusions in a short video:
Here are some pictures of the mold after the laminate cured and the peel ply was removed:
We are making now the next parting surface. This surface will separate the aft part into a left and right mold.