Bonding vs screws for Acrylic and Polycarbonate

[svyolo]

I don't have a huge amount of experience fastening plastics. Below is a part of a write up of Acrylic and PC from a company that supplies both to the marine industry and the military (including aviation). The author prefers acrylic.

"Due to its ductility Polycarbonate it is more challenging to install. I have seen Sika Flex 295UV with primer and Dow 795 both mentioned. I use and recommend both. Dont go over 4 ft continuous length with a fixed portlight. Remember the coefficient of thermal expansion for Acrylic and Polycarbonate is in the neighborhood of .000039 per inch per degree F. That means an 8ft plastic port will expand and contract up to 1/2 of an inch from the coldest day in Feb to the hottest day in summer. WOW!! Compartmentalize the job. It will be easier to install and less prone to leaks.

Never ever bolt a plastic portlight in place. Screws are fine to hold a lens till the adhesive cures. Take them out asap and fill the holes with the aforementioned products. Both of these products are rated at 700 + percent elongation before tear. Strong flexible and UV resistant."

These are not my opinions. But I do plan on bonding the windows (acrylic) to the doors and front windows. The rear windows will be 1/16" PC, in the frames. My skylight (version 1.0) will be bolt in PC. I don't trust myself to bond in the skylight, at least at first.

[Bcone1381]

Does your kit have round or square tube window frames?

[marcusofcotton]

For our winter extreme cold to sitting in the sun on a hot day, it sounds like a minimum of a 1/8 in thick bead to fall within the seven hundred percent elongation for a two-foot-long window.

[svyolo]

My kit the front windows are square tube. Easy. The bottom doors have round tubing. I was going to try heating the glass to bend the edges around to give me a bit more surface area to adhere to. I already tried it a few months ago on a scrap piece.

if you had to replace the glass in the front doors/wiindows, it is easy enough to remove the door /window to do it. The skylight would be much harder, and working on a ladder once the aircraft is finished and on the gear. My first piece of skylight glass will be fastened mechanically. Heat would soften the adhesive.

Most external skins on car doors have been bonded for quite a while. To change a skin, you just heat it a bit and pull the old skin off.

I am not sure what 700% elongation means with respect to adhesive.

[zkelley2]

That the adhesive can stretch 700% it's size when bonded before it tears.

[svyolo]

700% of what? Bead thickness of the adhesive? I didn't, and don't understand what they are alluding too. So 1/16" bead thickness can stretch 7/16", perpendicular to the thickness of the bead?

[Nev]

One possible way to attach the skylight would be to bond it to inverted U-shape extrusion, and then bolt the extrusion in place over the stringers. Or, bond the skylight directly to the stringers, and have the stringers made to be removable in that section.

We’ve had some very large glass windows bonded in place on boats. Thermal expansion was definitely a major factor, however when done properly they were extremely secure - albeit with very slow speeds and no lifting force.

[svyolo]

There is a lot going on upstairs if you have a skylight. The wing root fairings get fastened through the glass. The plastic supplier I quoted also recommended 4'x4' max. A 4 place skylight is just slightly smaller. But that supplier was a marine/military supplier. In a boat or small ship, the glass frame is supported by, well, a boat or ship. A aircraft fuselage is much lighter. I think for a 4 place skylight I would rather let the glass move with fasteners and oversize holes. Maybe somewhere down the road I would feel more comfortable bonding it.

My first skylight attempt will be secured with fasteners. My first windows will be bonded.

[Archer39J]

Consider the possibility of a skylight zippering off in flight. A nuisance on a boat, potential for loss of the craft in large swells maybe, but in a plane? I sure don't want to be the one who finds out.

[S Lathrop]

Has anyone used nylon screws and nuts to fasten the the poly-carbonate and a sealer to the window frames?

I had molded windscreens on some of my race cars and I used nylon screws to hold the windscreens in place. Never had a cracking issue.