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Thermoforming plexiglass lenses for wing tip lights

PaulvS

Well Known Member
This is a first attempt at forming some plexiglass wingtip lenses for the older W-415 type wingtip. Vans previously had a kit for the light coves and the lenses, but the coves are no longer available, although the lenses for those lights are currently available from Flyboy Accessories. I want to fabricate both the coves and the lenses to house some LED spotlight landing/wig-wag lights.

I experimented with heating up some plexi and then forming it over the actual wingtip while it cooled. It turned out well enough for me to experiment further. The next step will be to make some nice smooth template plugs to form the lenses over.

I'm wondering what is the best way to make the plugs; either I could make them out of wooden blocks (with a bit of trial and error filing, sanding and sealing); or alternatively make some female molds of the wing tips and then use casting resin to make some polished facsimiles of the wingtips.

Any ideas how well this would work, or other suggestions?

Thanks,
Paul
 

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Forming plexi

I ve made molds of balsa which were sanded smooth. By experimenting with oven temps the plexiglass would droop and self form over the mold. Yours looks great!
 
plastic fun

In an earlier life I was a mold maker for vacuum forming all sorts parts made out of plastic sheet.
To bad you don't have access to a vacuum forming machine to get an easy forming process.
In a real machine, the clear plastic is clamped in frame, rolled into a heated furnace, allowed to droop down. Then rolled back out of the furnace, then the mold box is dropped down into the drooped plastic. Quite often we used a lower box pressed up to the drooped plastic to get a good seal. Then vacuum applied to the back of the mold blocks. The plastic is sucked up to the mold and allowed to cool. That way, it is the same thickness through out the part. Resulting in some quite clear wave free parts.
Think a wall terrarium for plants in your house.
By the way when building your mold don't put any metal fasteners in the mold as they will telegraph through the plastic and create little eyes for you.
We learned that real hard lesson on a wavy mold that was 14' long and 4 feet wide. We dug out a lot of nails.
We used high density fiber board wood (MDF and HDF sheet stock) for most of our molds and covered them with a few coats of fiberglass resin, firsts a couple coats of "B" then a top coat of "A".
If we needed a mold to make thousands of parts we had them cast out of aluminum then milled them to final shape.
Life gives you a lot of experiences if you work at it.
Art
 
That looks really good for a first attempt, much better than mine did. Last year I made some lenses for a smaller wingtip on a tapered wing I built for my RV-4. I started out knowing nothing and learned several things.

First I had no success with the heat and drape method. It folded over in one direction but would not make much of a "compound curve". Those who had success with the heat and drape method know more than me and perhaps they can add some detail.

So I built a vacuum assembly for about $20 in an afternoon (I can email pics of this apparatus but cannot post them here). I first tried some acrylic sheet which formed well but was very prone to "running a crack" when I tried to trim them. Lexan did not work well as it "bubbled" as I was told it would. The material I had best success with was PET-G.

I really struggled to get lenses without distortion (but your first attempt looks pretty good). Finally I found that I could eliminate most distortion by making the mold (I used plaster of paris to make a male mold) VERY smooth. I put on several coats of polyester resin and wet sanded it with 1000 grit paper. When the mold was "perfect" the optics came out pretty good.

Incidentally I heated the sheet (which was in a 2' x 2' square frame) with a propane "weed burner" torch. This is a torch head that goes on a propane bottle and is about 3" diameter and puts out quite a dragon breath. I heated the sheet from below until it sagged about 5" at the center of the 2' x 2' and frame and then dropped it down over the mold to be sucked down over the mold.
 
Tips

Thanks for the tips, that's encouraging. The first "mold" I used was not super smooth, so there are small dust-like imperfections on the inner surface of the lense. Vacuum forming sounds like a good idea and I will send a PM for the photos.
I heated the plexi over a small bar heater until it became pliable though I didn't have any way of measuring the temperature.
 
Try to heat the plastic sheet from both sides in a very wind free area as it will give the plastic the best stretching process. If the sheet is evenly heated and allowed to sag it is like magic. We had timers for all processes.
The vacuum holes are very small in the corners and plenty of them for fast or slow and even suction.
We played around with slow and fast suction a lot. Just put a hand valve on the suction line. Size of suction line matters a lot. Then fans to cool the parts before mold retraction.
Been there done that person.
Have fun Art
 
Part size

Question about using wing tip as a mold.
If you used the wingtip to get the shape, wouldn’t the new part be too large? The part would be bigger than the tip it is replacing by the amount of thickness of the material.
 
solution

No, you use the old wing tip lens to make an ?? exact shape mold. Then add to the size enough to make up for the plastic shrinkage Per manufacturers specs and the region for cut off, then add the mold to a vacuum base board.
Yes you need the old part to make the new mold, But, you should have that.
You can actually pour a duplicate mold out of casting resin, then add shrinkage allowance and WaLa you have the mold.
No, all this stuff isn't free, But what is life about? Except a bunch of learning failures and practices.
Then pretty soon you are grinning like a cat in a creamery cause you can make something new from the old.
Art
 
could you give a rough description of what the surface of the vaccuum box is like. I assume it is porous with holes to allow suction. But would 'big' holes like a Masonite peg-board work? Or are those holes too big? What size holes and what spacing would work well?

In an earlier life I was a mold maker for vacuum forming all sorts parts made out of plastic sheet.
To bad you don't have access to a vacuum forming machine to get an easy forming process.
In a real machine, the clear plastic is clamped in frame, rolled into a heated furnace, allowed to droop down. Then rolled back out of the furnace, then the mold box is dropped down into the drooped plastic. Quite often we used a lower box pressed up to the drooped plastic to get a good seal. Then vacuum applied to the back of the mold blocks. The plastic is sucked up to the mold and allowed to cool. That way, it is the same thickness through out the part. Resulting in some quite clear wave free parts.
Think a wall terrarium for plants in your house.
By the way when building your mold don't put any metal fasteners in the mold as they will telegraph through the plastic and create little eyes for you.
We learned that real hard lesson on a wavy mold that was 14' long and 4 feet wide. We dug out a lot of nails.
We used high density fiber board wood (MDF and HDF sheet stock) for most of our molds and covered them with a few coats of fiberglass resin, firsts a couple coats of "B" then a top coat of "A".
If we needed a mold to make thousands of parts we had them cast out of aluminum then milled them to final shape.
Life gives you a lot of experiences if you work at it.
Art
 
The OP said they used the actual wingtip, not the old lens. My assumption of that process is that the new part is the same size as the wingtip on the inside of the part. The outside measurement of the part is now larger by a factor of the thickness of the material minus the shrinkage factor mentioned. I looked around and saw the shrinkage factor was about 2%. Is that enough to get back to size?
 
The OP said they used the actual wingtip, not the old lens. My assumption of that process is that the new part is the same size as the wingtip on the inside of the part. The outside measurement of the part is now larger by a factor of the thickness of the material minus the shrinkage factor mentioned. I looked around and saw the shrinkage factor was about 2%. Is that enough to get back to size?

PMMA (Plexiglas) is an amorphous plastic. Its shrink rate is 0.5%. Think about a pile of jumbled cooked spaghetti noodles to represent the molecules in an amorphous plastic.

Shrink rates on the order of 2% are exhibited by semi-crystaline plastics. (PP, PE, PET, Nylon for instance) Think about a mixture of cooked spaghetti noodles interspersed with blocks of hard uncooked noodles stuck together in bricks. These "crystals" are tightly compacted so the phase change from 100% amorphous to the partial crystaline state causes more shrinkage of the overall part.
 
could you give a rough description of what the surface of the vaccuum box is like. I assume it is porous with holes to allow suction. But would 'big' holes like a Masonite peg-board work? Or are those holes too big? What size holes and what spacing would work well?

The part in question screams to be molded on a male mold upside down. With the plastic held in a picture frame, heat it until it is easily deformed by a poker (or finger :eek:). Lower the male mold into the sagging plastic and turn on the vacuum. This will give the most even wall thickness as the plastic will have sagged to the approximate shape already.

The best place for vacuum will be the interface between the mold and the mold baseplate. For the sheet thickness involved, a gap of around 0.030" should be fine. Feed vacuum with a big hose and big hole in the baseplate. Make large passages (tracks with a dremel ball burr) from there to just before the edge of the male cavity. Shim the cavity off the baseplate by 0.030"

No vacuum needed anywhere else. Its needed at the very last point that is formed. Slot vacuum described has way more flow area then drilled holes too. Thats good for fast forming before the resin cools. As a rule, drilled vacuum holes impinging on a clear finished surface will cause "fisheyes". Undesireable. A general rule about vacuum hole size is to compare it to the finished plastic thickness. A hole diameter near the plastic thickness will show as a dimple and may pull a hole. Of course that is somewhat dependant on the forming temperature and pressures applied. One half sheet thickness or less is best in hot high pressure forming. Vacuum only systems can get away with larger vacuum holes or slots if they are placed correctly. In many cases, the plastic will freeze before it reaches the vacuum slot.
 
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If you have the time, Somebody will sell you the plastic

Bill answered most of the questions posed.
Male mold, sag to form, etc.
The next concern is how big a sheet you start with to form parts out of, That will drive the mold "box" which is really a plate mounted on the back of the mold base with a gap between for flow. Holes drilled at the mold/plate juncture were 1/16" and NEVER on the part itself.
As stated it will ruin the part with fish eyes.
You need the plastic to sag as deep/tall as the mold is usually so the vacuum can do the forming.
We usually wasted a few pieces of cheap plastic to get the mold heated up first. With clear plastic we found that the mold has to be heated first or the plastic will have a tendency to "freeze" before it forms completely. We got a lot of wavy parts from not following a set method of production.
Them new guys are always learning, But some never remember.
All this sounds like a lot of messing around. You are correct it is.
Time consuming for sure.
I know how, But won't take the time to do it. I would need to build a machine to do it right.
My three cents worth again. Art
 
To answer a few of the comments above:

In regard to the vacuum base board and the holes in it: Yes I used the "masonite peg board" like you'd hand on the wall in your garage for peg board. I think the holes are 1/4". It worked for me but may not work in all applications or sizes of parts.

Yes if you use the wingtip as a mold the lens will be oversized as discussed, but it still may fit OK as it will pull down onto the wingtip where it attaches and the fact that it is "oversized" may not be noticed. However if you use the entire wingtip as a mold you will need a pretty big vacuum forming rig. I used the wingtip to cast a female mold from before I cut out the cove where the light lens will sit so my mold was the size of the INSIDE of the lens. If you are making a lens to replace a lens you could use the original lens to cast a mold from, but in my case I was making a much smaller lens for my smaller (due to tapered wing) wingtip and had no existing lens from which to start.

In regard to the size of sheet/vacuum base to start with: I used a 2' x 2' piece to start with. It was plenty, but not all that much extra. Given that I was making a smaller lens due to my smaller wingtip I'm not sure that it would be a big enough piece to form a "full sized" wingtip lens but I think it would still work, but there would not be much margin. 2' x 2' is a handy size because it usually cuts out evenly from the raw material sheets that are 4' x 4' or 4' x 8' or 2' x 6' etc.

Incidentally: You can get aerosol "mold release" spray from most plastic shops. I tried it with and without and it did not seem to make any difference. There was really no tendency for the plastic to stick to the mold or vacuum board (masonite peg board and plaster of paris mold covered in epoxy resin).
 
I am interested in your vacuum mold device. I cracked a wingtip lens (well prop and engine too) when I taxied into a culvert. Can't seem to find the old 'bat wing' lens anymore.
Marty [email protected] KSUT
 
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