Correct glue for Lexan windshield

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

rv7charlie

Well-Known Member
Joined
Nov 17, 2014
Messages
1,559
Location
Jackson
Back when the RV-x guys started using it, the only place we could find it was from marine products distributors, like Jamestown Distributors. My impression was that the product was created to bond plexi to other substrates in marine applications.

Do read their docs carefully before ordering, to be sure you're buying the right stuff; they make multiple products for different purposes, and even various primers for different substrates. Read them again before beginning the installation process. There are specific requirements on *minimum* thickness of the sealant bead, UV protection (at least on the products available when I did my canopy), cleaning, priming, etc. The product has a great record over a couple of decades in the RV community, but the processes do need to be followed. Biggest downside back when I did it was cost.
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
5,936
Are you sure that it is Lexan? I ask because I can't remember ever seeing Lexan crack like that. Plexiglas, yes, but never Lexan. Mind you, every window or windshield that I've ever worked with was Plexiglas.
It does crack. My first Jodel windshield cracked from the screw holes due to thermal stress. The cold weather caused it to shrink and pull and crack. The next one had oversized holes and I attached it with #6 brass screws, nuts, and washers. Didn't crack at the holes anymore.

The first one, besides cracking at the holes, also crazed. It's just a flat wrap, and that stress causes crazing after some years. Crazing is tiny cracks. One day, while fuelling it, I splashed some fuel on the windshield and it shattered. Literally. That forced me to replace it. The sudden chilling caused by the fuel hardened and shrank the plastic unevenly and it failed due to the crazing. The new one crazed much sooner, right after I made a new canopy cover out of nice, fresh marine-grade vinyl. The outgassing of solvents from the vinyl--the stuff you smell--attacked the Lexan. The old cover I'd made out of a waterproof, UV-resistant nylon that I couldn't find anywhere anymore.

Lexan is nice to work with, but it has its drawbacks.
 

Eugene

Well-Known Member
Joined
May 26, 2017
Messages
1,441
Location
Merrill, Wisconsin, USA
I received a little warning about Sikaflex 221. This glue is not designed to work in direct contact application like contact cement as example. It required some thickness to act as a middleman between two surfaces. If that's the case will not work for me against fiberglass. Original windshield was installed very tight. Very much like contact cement would look like.
 

rv7charlie

Well-Known Member
Joined
Nov 17, 2014
Messages
1,559
Location
Jackson
Tight, especially if the screw holes didn't have any clearance around the screws, is likely what caused it to crack. That's what caused the RV-x early adopters to seek out Sika; when a $1500 canopy cracks, it's really painful. Plexi & lexan expand (grow) a lot when heated; much more than even aluminum. That's the reason for the instruction calling for ~1/8" bead thickness Sika; to allow the plastic to expand/contract without shearing the bond. Sika is somewhat flexible after it cures. Depending on the design of the frame, you can sometimes achieve the thick bead by creating a big filet, instead of just applying it along the contact line.

edit: I'd suggest contacting Sika before purchasing. As I mentioned, they make a *lot* of different products. IIRC, many years ago I used a 29x series for my RV7. The current product for bonding plastic sheeting seems to be 295UV, with UV resistance. The UV resistant stuff wasn't available when I did my canopy, and we had to do some elaborate under/overcoating with their UV protectant paint.
 
Last edited:

Markproa

Well-Known Member
Joined
Feb 9, 2017
Messages
150
Location
Bellingen, NSW. Australia.
Check out some of the marine forums, there is a wealth of experience fitting windows. Sika make a system for fitting Lexan without fastenings. Do not use any of the modified silicones, it doesn't stick to Lexan. From my experience Lexan moves more than acrylic so use large holes if using bolts; I'm not keen on rivets as you can't control pressure.
 

Rataplan

Well-Known Member
Joined
Feb 14, 2021
Messages
120
By gliders the plexiglass canopy is glued on a glass fiber frame these frames are quite sturdy . in all cases the holes and all sides around should be smooth . If you glue it it is also important to know what kind of bond you want between the frame and 'glass' . A flexible one or a fixed one . that depends on what interaction is desired between the glass and frame . There is also a non glue no holes in the 'glass' option: Make a window of a strip with the thickness of the glass + 1~2 mm . which fits around the 'glass' with some mm space between . Make a second window just like the first one but of a wider strip so the inside of this second window frame will be smaller and will cover also the 'glass' . The first window you attach on the door frame , with adhesive or nails , as long the nails or bolts are flush with the surface of the window frame. The second window frame can be attached on the first with nails or bolts or screws . The glass itself can now be clamped to the door by attaching the second window frame. Because the first window frame is thicker than the glass there is space for an elastic kit or neoprene band. The space between the first windowframe and the glass also allows an elastic kit .
 

BBerson

Light Plane Philosopher
HBA Supporter
Joined
Dec 16, 2007
Messages
14,960
Location
Port Townsend WA
The second photo shows the crack doesn't intersect the hole. It looks like forced curvature from one rivet.
The forced curvature at the tube will cause a crack eventually.
 

Eugene

Well-Known Member
Joined
May 26, 2017
Messages
1,441
Location
Merrill, Wisconsin, USA
Here is picture of 2007 Skyboy. Different techniques were used and no rivets visible. In the corner you can see some micro cracks starting already. So, not very successful approach either

IMG_5833.JPGIMG_5834.JPGIMG_5824.JPGIMG_5832.JPG
 

rv7charlie

Well-Known Member
Joined
Nov 17, 2014
Messages
1,559
Location
Jackson
If it really is Lexan, fuel will make it craze.

The RV canopies that use Sika don't have any rivets at all, and the canopy actually has a lot of lifting force at 180+ MPH. The clecos in the pic are just clamps; no holes in the plexi.
canopy glue side.jpg
 

BBerson

Light Plane Philosopher
HBA Supporter
Joined
Dec 16, 2007
Messages
14,960
Location
Port Townsend WA
Need lots of rivets but none of them pushing down and bending the lexan. The Lexan should be loose fit in loose holes so it can expand or shrink without cracking. The frame is dissimilar expansion.
 

speedracer

Well-Known Member
Joined
Feb 4, 2020
Messages
113
When I installed the canopy bubble into the canopy frame in my Long EZ I did the following: I drilled approx. 50 1/4" holes along the bottom of the bubble then filled them with wet flox (flocked cotton + epoxy). Then I glassed the bubble into the frame before the flox cured. Ten years.... no cracks.
 

Yellowhammer

Well-Known Member
HBA Supporter
Joined
Feb 21, 2020
Messages
528
Location
Born In Alabama, reside: Louisiana (unfortunately)
John Croc has a very informative video on Youtube explaining the differences between Acrylic and Lexan. He also had a section in one of the previous months issue of Kit Planes magazine. If you search for in on Youtube you will find it and John does an excellent job explaining the pros and cons of the two types used in aircraft.

It taught me everything I needed to know. Hope it helps.

SIncerely,

Yellowhammer
 

Old Koreelah

Active Member
Joined
Oct 4, 2013
Messages
25
Location
Australia
A very useful discussion: one aspect of lexan (polycarbonate) not mentioned is that it’s hygroscopic- it absorbs moisture, so any efforts to heat-form it must be preceded by a thorough drying-out process. I discovered this the hard way when gently heating my screen; it suddenly turned milky and blotchy.

Heating overnight in an oven at 60 degrees C is recommended to drive out the moisture before any attempt to heat-form.
From memory, 160C is required to achieve a compound curve.
 
Top