Top Skin

Step 3 INSTALLATION OF THE TOP SKIN

I used a slightly different method to remove the extra foam from the trailing edge of the elevator in preparation for the top skin. Instead of cutting it away with a Dremel or similar tool, I simply marked the .3" cutout with a straight edge and pencil and then used a utility knife to slice through the foam but not the glass.

I then used the utility knife to cut the foam away from the glass. I was afraid that the vacuum bagging would have forced a lot of epoxy into the cells of the foam and I would not be able to make this cutout this way but it worked fine.

Turned out really nice. I had to sand the glass face a little bit but this seemed a lot easier and a lot less messy than using a Dremel.

Of course, after doing the bottom skins via vacuum bagging, I had to to the top skins the same way. I took a lot of pictures here, this is sort of a step by step intro to vacuum bagging so bear with me if I get a little wordy. I started by microing the top surface of the foam as you would for a normal layup. In this photo I've already applied the micro and am starting with the first layer of UNI.

Wet out and lay on the glass as you would normally. This gets 2 layers of UNI cut 30 degrees to the selvage edge, each layer with the opposite orientation for the major fiber runs.

You need to trim the edges pretty close. I guess for some layups that are flat on certain edges this is not a big deal, like the trailing edge here. Given that it is all going to get trimmed off anyway, I decided to scissor trim with my Dritz electric scissors. I trim close once the layup is completely wetted out and squeegeed fairly dry.

Then add the peel ply. I cut it fairly close to the right size but leave it a little long on one end to help the "envelope" "breath", that is, prevent the top and bottom of the bag from sticking together and not letting the air come out.

I have been using two layers of peel ply. This might be overkill, but, the layups have turned out great, and, the peel ply and paper towels come off without too much trouble. I usually use a brush to press the peel ply onto the wetted out layup, this helps the peel ply stay in place.

Given this is an elevator with part of the layup extending around the front of the torque tube, I went ahead and taped the inside layer of peel ply around the front and onto the bottom. As the two sides of the bag collapse with vacuum, they have a tendency to make a ridge near the the middle of what is being bagged. I didn't want this to result in a ridge or worse the top skin not overlapping the bottom.

Now it's time to lay out the bag. Roll it out to the length needed. Make sure the hole in the end for the connector is on the top.

Add a quick lock seal to the far end of the bag.

After the peel ply, I add two layers of paper towel. This doubles as a breather ply and also helps to absorb the excess resin that works its way through the layers of peel ply as the vacuum is applied.

Insert the part into the bag from the open end. I usually tape the paper towels to the peel ply so they don't slide off or slide crooked when I insert the part into the bag.

Install the connector on the near end of the bag. The connector and rubber washer go on the inside. When I put the paper towel layers on I roll out enough so they cover the part and extend to beyond where the connector installs. This ensures there are air channels for the air to evacuate through all the way from the part to the connector.

The ridged washer goes on the outside of the bag. I don't have to tighten this with a wrench, finger tight is usually enough.

Add another bag seal to the near end of the bag once the connector is installed. These connectors are simply two pieces of plastic, one circular and one shaped like a C. The white/smaller one snaps into the blue, C-shaped outer one with the bag in between. Nice and simple, quick, and keeps a good seal.

Here's a shot of the connector and seal installed and ready for vacuum.

Now you're ready to put on the tube from the vacuum generator and pull a vacuum.

My system has a reservoir that helps maintain vacuum pressure and prevent resin from getting into the pump. Given the amount of peel ply and paper towels I use, it would be hard for this to happen. But, when you see how tight the bag becomes, it's easy to imagine excess epoxy working its way all the way to the connector and then through the tube to the reservoir.

It only takes a few minutes to get the vacuum established with this pump. If the bag wrinkles funny as it collapses, turn the pump off and then open a seal on one end or the other to let air back in. Re-adjust the bag or layers of paper towel and try again.

About halfway evacuated. The pump is still removing extra air here and is not yet pulling a vacuum.

Once the extra air is removed, the pump starts to pull a vacuum. I have the vacuum switch set to about 17" of Hg, the pump will run until this setting is reached, then shut off. If the bag leaks, the pressure will gradually "rise" until around 15" then the pump will turn on again. When I used the nylon bag and quick lock seals that came with this unit, I usually don't have leaks and the vacuum is maintained until the part is cured. I have, occasionally, unplugged the pump and just let the reservoir do the trick. If you suspect any leaks, however, or the gauge is indicating a very small one by moving, albeit slowly, leave the pump on to maintain the vacuum until the part is cured.

Here's the gauge once the pump turns off. I monitor it for a few minutes as I clean up to see if I am going to have to re-seal the connector (the most common spot for a leak) or even leave the pump on or not.

At 17" of Hg, the bag pulls down very tightly to the part. You can see the very texture of the glass layer beneath the bag, it stretches just enough to do this without getting too wrinkly.

Note I have not installed the micro over the glass to glass trailing edge. I did not want it to get smushed out by the vacuum pressure, or, worse, not ensure a glass to glass bond beneath it. Instead I just left the glass to glass bond and then bagged. I'll add the micro later after this cures. Better a bad micro to glass joint than a bad glass to glass joint, in my opinion.

Because the vacuum pulls the bag so tight, there is a chance for the part to warp as it cures. To prevent this, I used the foam leftovers from hot wiring the cores, trimmed a bit to avoid interference, with wood squares and weights on them to hold the elevator flat against the jig table.

Vacuum is still holding well, after about 5 minutes (check the file name on the picture of the gauge above) I have only lost less than half an inch of Hg. This is typical, the pressure will rise, about an inch or half an inch of Hg, then stabilize and not drop off any further.

I also added some square wood pieces, weighted down with some 10# weights, to prevent the trailing edge from curling up as the bag contracted during vacuuming. The bag gets so tight it wraps the glass around the trailing edge and causes it to curl just a bit. This should prevent this from happening as the top skin cures.

Once the part is mostly cured, unplug I take off one of the quick lock seal and let the air back in. I then remove the vacuum connector and reach inside and start to peel the bagging material away from the layers of paper towel. Yes, the bag does stick but like peel ply comes off cleanly.

I usually have to work my way down the bag peeling the bag away from anything it is stuck to.

I also usually have to turn the bag over then repeat the unsticking process on the back side.

I roll up the bag tightly and store it in a safe place. The rolling seems to help work out any wrinkles that may have developed from the previous bagging job.

Since I taped peel ply to the back side of the elevator, I need to remove that before the peel ply would come off.

I remove one layer of peel ply and the paper towels first. I suppose you could do all of the layers at once, but, since the ends of the elevator are not glassed I didn't want to risk separating any glass from the foam at the edges due to over-aggressive removal of peel ply and paper towel layers.

Then the second layer of peel ply is removed.

The top skin turned out very nice. You can see in this photo how good of a job it does forcing the glass, even 2 layers of UNI, into tight nooks and crannies. I will need to fill this with micro per plans in a separate step. I actually think I could have used less vacuum in this layup, the front of the cores have some dents and dings along the very front edge where the tube is exposed. The glass was forced into each and every one of them. This should not be a problem, however, that part of the elevator can't be seen once assembled.

I did the other top skin with the vacuum system. Not shown here are the weights I added, with the foam cutoffs used as protection for the shape, to make sure the squeezing didn't make the whole assembly warp.

I had a ding in the foam that I fixed with micro. Unfortunately, the micro wasn't firm before the vacuum was applied and I ended up with a divot as it got squeezed. I'll have to remember to fill these and let them cure if I vacuum bag the part.

That's as far as I have gotten on the elevators.