How flexible is plexiglass ?

[Sven]

I got the teak for the forward hatch ... 8' by 4 3/4" by 1 3/8" ... beautiful, but it should be at $12 per foot !

I have a fairly good idea how I'm going to make the frame, but I'm less sure about the plexiglass bending. I'm assuming a 1/2" is thick enough and I'm planning on smokey gray clear.

The curvature of the hatch needs to be about 3/4" per foot (if you can imagine what I mean). IOW, 0 " goes up to 3/4" over a foot and then back down to 0" overt the next foor. Do I need to heat bend that or can I just bend it cold ?

Any insights ?

Thanks,



-Sven

 

[wurzner]

You would certainly benefit from attemting a poor man's approach to thermo forming it. I woud get 2 decent commercial heat guns (hair drying style) and make a curved surface (2 yard sticks or old flat battens), put weight on the ends, and SLOWLY heat the plastic and let it sag. This is how we thermo form, except a sheet is put into a large oven until it sags, and them either vacuum or pressure fit onto the mold. For what you are trying to accomplish, this will allow the material to deform a little so it doesn't spend the rest of its life trying to pull the screws out of the frame.

If I'm not mistaken, acrylic may be a better choice...make sure to check the properties expecially for UV. Acrylic is much cheaper, but also more prone to pitting. I've been out of the plastics business for 10 years and I don't recall all the material characteristics.

Good luck and let me know how it turns out.

shaun

 

[footrope]

Plexiglass is acrylic?

I have bent thin acrylic to as much as a 90 deg. angle with a heat gun quite successfully. But, based on what I've read on this board, including Shaun's reply, the thick stuff will be much more difficult. You might do a search for bending or heating acrylic and see if there is any other advice or warnings.

 

[Sven]

some links

I did a search for bending before asking here. I did find some useful information:

Sheets http://www.tapplastics.com/shop/product.php?pid=334&

Bending http://tapplastics.com/shop/product.php?pid=169&PHPSESSID=1221157dbd9f268d8de8eada80ad1f2e

Bending tips http://www.austinpcmods.com/Plex Bending Tutorial.htm

Bending pdf http://downloads.kruitzkraft.com/windshield.pdf

Bending http://www.wizdforums.co.uk/showthread.php?t=4119

And the discussion here for selection of material http://www.ericsonyachts.org/infoexchange/showthread.php?t=3468&highlight=lexan

You are right, I think it was acrylic that was the best choice given the UV exposure. The bullet-proofness of lexan doesn't seem to be that much of a selling point.

Thanks for the suggestions.


-Sven

 

[NateHanson]

I've thermoformed 1/2" acrylic with a propane torch, but only a much smaller piece. It's tricky because you have to heat the whole area uniformly for it to bend smoothly. Id be very suprised if you could accomplish this with such a large piece. Plus the risk of scorching or bubbling it is very real, and even acrylic that thickness is not cheap at all! I think I'd ask a plastic shop to do it for me. Perhaps see if you can build the form for them, to save money.

I've also heard of people doing this in their oven at home (acrylic forms at 350 degrees). But there's a slight chance of blowing the door off of your oven, and temporarily removing your eyebrows, because acrylic gives off a volatile/flammable gas as it reaches forming temp.

 

[Glyn Judson]

Protecting your new hatch from crazing.

Sven, Might I suggest that you consider protecting your new acrylic from the heat of the sun once it's installed? The good folks at Hastings Plastics in Santa Monica told me this trick years ago when cutting the larger hatch for me at the same time I dropped in a small Lewmar hatch on our boat. There are two examples right in MDR today of identical 10" Lewmar hatches I installed in a couple of Ericsons. They're on the front slope of the cabin of my old E25+ and on an E27, and so get a lot of sun exposure. In the case of the Ericson 25+ I added a 1/4" thickness of closed cell foam to the canvas cover I had made for the hatch when installing it in 1989 or so. I used to own that boat which now belongs to Harlen Holmes on the 1000 dock of SMYC so you can see it for yourself today. The other is on a friends E27 who chose not to add the foam as a part of his cover over an identical Lewmar hatch I installed a few months later. Within a few years the acrylic on the E27 began to craze and as of last checking, Harlan's hatch is as craze-free as the day it was put in. Glyn

 

[ted_reshetiloff]

I would like to hear how this project comes out. I would guess you may want to buy extra material in case you don't get it right the first time. There is a marine plastics shop in Annapolis that I have used for years for windows and stuff. They could probably replicate/duplicate your hatch if you sent them the old one or gave detailed specs, talk to Alex or Keith. I am not connected financially with these guys other than I have spent a bit of money with them...

Maritime Plastics
110 Severn Ave
Annapolis, MD 21403

(410) 263-4424

 

[Sven]

Thanks for all the helpful advice !

Having watched some of the videos and gotten an engineering feel for the problems I now see it as an interesting challenge !

I might go for clear rather than gray colored glass since it is half the cost ! If I didn't think I'd mess up the first sheet I might go for the color, but I see at least one failure in my future

My plan is now to get the frame put together, then bend a 1/4" sheet of plywood over it as a form, clamped in place or held down with some temporary screws.

I'll lay the acrylic across the top and let gravity do the bending as I'd very slowly heat along a line starting across the top and moving that line down the curvature of the hatch as the acrylic bends, first down one side then the other. Heating to 350 degrees, to a 1/2" depth, without bubbling the top surface will be the real challenge. I can already imagine my arms sore from moving the heat source back and forth ! America's funniest home videos special coming up ?

Wide tipped torch or electric heat gun ?

I'll have time to figure this out on paper before I actually start so I'll keep accepting ideas for a while



-Sven

 

[Sven]

I added a 1/4" thickness of closed cell foam to the canvas cover ... <snip> ...as of last checking, Harlan's hatch is as craze-free as the day it was put in. Glyn

Excellent idea ! I've been looking at hatches all over the place and they all look like they have crystal spiderwebs in them.

Thanks,



-Sven

 

[NateHanson]

I'd use a wide-tipped torch. That's what I've used, and I expect it's hotter than a heat gun. It also has the advantage of igniting the gasses that start coming off when the plastic hits the right temperature. It let's you know when it's getting close, and it also keeps you from inhaling them and then blowing up.

I think if I were doing it I'd start by heating one half of the sheet for 5 minutes or so to pre-heat, and then flip it over onto the form to do the bending as you heat the other face.