Plexiglas Leading Edge Lenses: How?

I've been contemplating making a pair of custom leading edge lenses for my RV8's wings that are slightly wider than the standard Duckworks lenses. I don't see any discussion here on how to bend Plexi or similar polycarbonate sheet using heat, but know it can be done.

Can anybody offer some advice in doing a job like this? If I actually do make a set of lenses, I'd prefer to go about 3/16 on thickness and machine a .032" step prior to bending so the lens actually sits flush with the outer skin surface. Is this a workable idea?

I've never tried this type of application but I've softened and bent plastics using a rig I connect to a heat gun. I use standard copper pipe and fittings - one or two splitters and then 12" lengths of pipe or whatever length you need. I drilled small holes along the length of the pipes. The end result is a hot air distribution manifold and distribution. I've bent up to 1/4" thick material.

I've also had "some" luck simply using the oven.

A little experimentation will clue you in the the settings.

same thing

I was thinking of doing the same thing when I get to that stage in my build.
My plan is to make a mold, then vacuum form the lens.

LENS

LEXAN CAN BE BENT COLD..

Bill,
Acrylic will form at a bit lower temperature than Polycarbonate.

Make a male leading edge mold out of nose ribs and sheet.- or wood and foil.
Clean the surface.
Attach your stock to the nose. Clamp, whatever.

Go give your wife some cash and send her shopping.
Set your oven on 275-300 for Acrylic or 300 to 350 for Polycarbonate.
Place mold and stock in oven and watch it sag onto the wing shape in a few moments. Once it starts moving you can assist it by hand in the edge scrap area. Don't touch the lens area.
For piece that thick, if it does not move in 5 minutes, raise oven temp in 25 degree steps and repeat. The colder you can form it, the better the clarity should be.

It is thermoplastic so reheating is no issue

If you have sticking problem, you can coat the mold surface with moleskin (self adhesive) or just drape some felt over the mold and then form over the felt.

I would use Acrylic because it has a harder surface than Polycabonate and will resist scratching better. Also, if memory serves, Polycarbonate is hygroscopic and needs to be dried before forming or it will bubble.

I would not recommend machining step in Acrylic due to its brittle nature. If you try to machine and then form, it can be done but the "tooling" you need just gets more complex with the oven method. You may want to try the hot air heating method mentioned by HumptyBump for that.

I made flush fitting lenses for my 6. My Duckworks lenses did not seem to fit very well, and I noted that when I used the prescribed method of pulling them into the LE with the tape, it was the aluminum that distorted rather than the plastic lens! And there were still some gaps, and that step......

I used 3/16" acrylic heat-formed over a male mold made of plaster. To get the correct passive fit for the plastic, this mold needs to be 3/16" smaller than the outer surface of the LE. To achieve this, I first made a plaster female mold of the LE adjacent to the cutout. I then glued (using CA) 3/16" soft balsa into the female mold as a spacer, covered it with tape, and poured up the male mold. Let the plaster dry out for a few days before molding the
acrylic.

I cut the step in the acrylic with a Dremel using a square-ended cutter about 3/16" diameter. This was not as difficult as I had thought it was going to be. Basically, put the lens in the LE, mark with a fine marker and cut to the line. Put it back in the LE and mark it again, etc., until it pops in. The whole process is a lot of work but the result was perfect flush lenses and no distorted airfoil!

Stewart.

I don't have time to search for the thread right now, but I made my own about 3 years ago. There are instructions on how to do it, but it's something along the lines of heating up the plastic on a cookie sheet (can't recall the temp or for how long, but it's not much), then laying it over the wing's leading edge to cool and form into the new shape.

Worked for me. I made 2 like that. The plexi was from Home Depot.

Interesting discussion because I am getting ready to do the same thing. I was planning on heating some acrylic as Sonny mentions but I also want a "step" and was considering milling the relief before forming but I think it would be far easier to get some thick tape layed up on the LE (on the order of .040 thick) cut to the the shape of the hole. Then drape the acrylic over it which should give you the step you're looking for. Since the acrylic won't wrap around the corner of the tape edge it might take some poking with a stick to get it to wrap around the corners.

Custom lenses - DONE!

My Lexan lenses are done, and they turned out pretty good. Here's the basic process I followed:

Using 3/16 Lexan (polycarbonate) sheet, I machined a .032" thick step-down around the perimeter of the lens. This was done in my CNC mill, and it was really no problem getting a clean chip-free cut in the plastic. The main problem in cutting this stuff was it had a tendency to "goo up" around the mill instead of cutting cleanly. The fix to that was slower surface speeds (100 ipm) and higher chip loads (around .004"). The resulting cut was perfect.

Here's how my blanks looked:


Then a tool was made to fit the leading edge shape. Using RZBill's advise, I just used particle board and .020 aluminum sheet. The sheet is wrapped tightly over the particle board form and held in place with screws on the bottom side. This pic shows how the tool holds the blank in place. I'm relying on gravity to do the forming work.


Polycarbonate does need to be dried out thoroughly before it can be heated to forming temperature. Failure to do this is said to contribute to bubbles forming while heat-bending the plastic. This requires baking the blanks at 250F for 24 hours. Forming temperature is from 350-400F (GE Lexan thermoforming manual).

I got an old oven for $40 (spent the money myself Bill !) and stuck an oven thermometer inside it to verify the temp. After the 24 hour bake, I raised the temp to 350 and prepared the tool. I decided to carefully clean only the mating surfaces of the tool and the blank. No felt.

350 wasn't hot enough, so I went to 400F. That was the trick. The lexan formed fully to the tool in 6 minutes. No bubbles, no problems. I took the rig out of the oven, and used a straight piece of wood to help hold the free edge down against the tool while cooling. Here's the results:




They look a little better in the pics here than they do in person. Optical quality is very good (which was my main concern) but the post-formed lexan did retain some uneven shrinkage along its bottom edge after cooling. Its not really significant; only about 1/16" of "bow" along that lower edge and this is an 11" wide lens. Good thing is it fits the airfoil on the wing like a glove!

Probably I could do better with more practice....

Edit: The 3/16 material is pretty heavy for a lens such as this. After forming, the lens is very stiff and if not made *exactly* to contour, would be very hard to fit inside the leading edge. Also, the 3/16 material took a fairly long time to reach forming temp internally (5-6 min at 400F). Knowing what I know now, I'd recommend using 1/8 material for the lens. After cutting in the .032 recess, that would still leave a .093" flange which would be plenty. It would form faster (and maybe with less chance for problems), and fit better. Just a few stray thoughts.... have fun!

Bill,

How about a new thread telling us a bit about the CNC mill? I've heard of these things but I don't know anything about them. I can only imagine the cockpit (or firewall forward) customization something like that would permit.

How difficult is the earning curve? What sort of software is used? How big are they?

All the basic questions....

Thanks,
Dave

The old Bingelis book - "The Sportplane Builder, Aircraft Construction Methods" - the "blue one" - has a section on forming plexiglas. He uses the making of a leading edge landing light cover as an example.

Just ask at your next EAA chapter meeting who has a copy...

You could always call Duckworks if you don't want to go through the whole process of making the mold and getting some plexi.

They will make custom sizes for any model RV for just a couple dollars more than a standard size lense.

Richard, Duckworks lenses don't fit flush with the outer airfoil surface, which is major feature on the ones I did.