We have been preparing to vacuum bag the pattern of the center section for a couple of days. Time runs once the resin is mixed and a good preparation is half the work.
The pattern is covered by thin abachi sheeting (1.2 mm, 1/20 inch). This creates a good surface, which is filled and sanded easily—an approach used often in rc-models. Abachi veneer has to be primed to prevent unnecessary resin consumption. As neither the foam core nor the primed veneer are well wettable, a coupling layer is recommendable. We used 86 g/m² glass fabric and an epoxy resin with a processing time of 110 minutes (HP-E110L). The HP-E110L provides enough time to prepare everything before vacuum bagging.
We proceeded as follows:
- Cutting and priming the abachi sheets:
- Making a vaccum bag that is large enough (not as trivial as it sounds!)
- Laminating the glass fabric, setting the pre-assembled abachi on top of the foam and vacuum bagging the „turkey“:
Right now, everything is curing at -0.3 bar (-43.5 psi) and a temperature of about 20 °C (68 °F). We will see the result tonight!
One word that fits perfectly to what happend today in one of my workshops is:
- an act, process, or instance of joining in close association
Defintion taken from Merriam-Webster’s Dictionary, December 19th, 2015.
The last couple of weeks, we have been cutting core by core until we had all six segments ready that compose the pattern of the center section. One issue was that the foam panels were too small and had to be glued together to obtain the needed length of 2.84 m (9 ft 4 in).
We tried silan modified polymer from the hardware store. It worked quite well, but this adhesive stays very elastic after curing. This is troublesome, when you have long pieces. Combining a highly elastic adhesive on a small contact surface with long lever-like pieces is a bad idea.
Thus, we tried the good old polyutherane foam spray, which worked better but not perfectly. Foam spray is suboptimal when you need small gaps between pieces. Honestly, this foam is too foamy and gross in handling for our kind of application. It was good enough to glue the panels together, but I really needed a better solution for joining the six sections!
I found finally the solution: one component, foam building, fluid polyutherane adhesive. I know this adhesive already for some years and used it a couple of times before. However, the European Union (EU) started to regulate its distribution a couple of years ago. Some important componet is believed to be carcinogenic. It was sold in every hardware store before regulation. Now, it is hard to get for private persons… I said „hard“ but not impossible! Either you buy it in Switzerland—a neighboring country outside the EU were it is not regulated—or you do like I did and buy it from Bacuplast Faserverbundtechnik GmbH (Germany). They sell it provided you read a somewhat lengthy technical information sheet and sign a disclaimer. Believe me, this U-600 is great and worth the extra trouble.
After having the the right adhesive, we started to join section by section until we had the left and right halves. Today, these two halves had their wedding. A moment I have been eagerly waiting for the last weeks. Questions had been plaguing my mind: Will the parts fit together? Did we make a mistake while hot-wire-cutting? How will the shape look like?
Not a single of these fears was truly justifiable. It was a perfect day for a perfect wedding:
Yesteday, after a somewhat longer preparation time, everything was ready for cutting the first foam core of the center section’s master model. I chose styrofoam for the core, because it can be cut with the hot wire and it is easily sanded further in shape afterwars. I had to exepriment a little bit with the glue to get the sheets together. Though UHU-Por is ideal for styrofoam, aligment cannot be corrected after joining. This is a potential problem when you have large pieces. I ended up in using a silan modified polymer.
Here’s a time-lapse video of the procedure: