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Old 16-06-2014, 11:44   #1
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Cast Acrylic source in Maryland

Hello All,

I need to replace the windows on my catamaran and would like to find a source in the Baltimore / Annapolis area. Looking for about four sheets of 4x8 dark tint.


Kurt

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Old 16-06-2014, 12:18   #2
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Re: Cast Acrylic source in Maryland

Not from Baltimore but a Google Map search for "plastics suppliers" of the area reveals the presence of -

Laird Plastics

and

Piedmont Plastics | Home

Which should get you started. Both of these companies are national distributors with regional locations and I have dealt with them before. Fun field trip, big warehouses full of racks and racks of plastic stock. Typically these types of suppliers will also offer cut-to-size, as well as machining and fabrications services.

Alternately, you could do the same search and maybe find a smaller outfit closer to home, not sure where you live.

For what it's worth, I suggest you consider inquiring for products made in the USA or Mexico, as opposed to China. Good luck with your project!
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Old 16-06-2014, 20:52   #3
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Re: source to bend acrylic or polycarbonate

On a related note, ALL of my aft cabin windows are cracked and crazed. Haven't yet decided on acrylic vs polycarbonate but need a source to bend them to match the existing ones. Look like 3/16" thick and want as dark as I can get them.
Suggested source along the SE coast? NC ideal
Ed
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Old 16-06-2014, 21:22   #4
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Re: Cast Acrylic source in Maryland

Ed-
Cracked & crazed sometimes just means too much sunlight, but usually means an improper installation (which is common) or petroleum damage, from petrochemicals or ammonia in cleaners and polishes.
If that's news to you, take a good look at how the originals were installed, you may need to change that.
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Old 16-06-2014, 21:30   #5
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Re: Cast Acrylic source in Maryland

Well, for starters most (but not all) hatch manufacturers use cell-cast acrylic sheeting for their hatch lenses. NOT polycarbonate! (except like one, maybe two) (but I think you know that already Sailorman)

"Uh, but polycarbonate is unbreakable!!!" you say?

Well that is really not true. You can break it just fine, thank you. However it is very resilient, much more so than acrylic which is harder, stiffer, and more scratch resistant than polycarbonate. (all qualities that generally make it more suitable for hatches, that get walked on)

The key to understanding the strength of a typical hatch lens lies mainly in the thickness of the lens itself in proportion to the size of the unsupported panel. To streamline production, most will use a single size thickness of material based on their largest hatch size. Blah blah blah.

Where I am going with all of this is that polycarbonate IS ultimately stronger than acrylic and IF you can identify the original spec as acrylic, you might be able to get away with going down a size in the thickness with new polycarbonate, which would make it a lot easier to cold-form.

That would be ideal. If you could make that work, you would be making life a lot easier for yourself. If the original is polycarbonate, match existing thickness and bend away!

The real down side of poly over acrylic is less scratch resistance and shorter service life, but this could easily be offset by much cheaper up-front costs for not having to make a mold and by simply not using a scrub brush and Softscrub to clean your lenses.

I have had experience cold forming thin-ish polycarbonate with metal forming tools, it's kinda interesting actually. Can also be rolled successfully. Acrylic is too brittle and doesn't really lend itself to cold forming. Looks like you have either cylindrical our conic shapes so should be workable.

Might be a good experiment just to get a strip of material in the thickness you want to use and hold them up to the curve and to see how easy/hard it is to make them conform.

If you can't cold form it then you have to thermoform it which means making a mold which is expensive, blah, blah.

I am setting up to slump-form a cambered companionway hatch slide for my boat with a friend who does some really crazy architectural Corian thermoforming. I have the mold made and should post some pictures.

Best of luck with your project!
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Old 16-06-2014, 21:47   #6
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Re: Cast Acrylic source in Maryland

Quote:
Cracked & crazed sometimes just means too much sunlight, but usually means an improper installation (which is common) or petroleum damage, from petrochemicals or ammonia in cleaners and polishes.
If that's news to you, take a good look at how the originals were installed, you may need to change that.
Yes, very obvious that the last install was done poorly, cracks radiate from the screw holes, holes same size as screws, no gaskets on screws, tightened too much. I have stopped the serious leaks and am hoping to used 3M VHB tape, little to no screws during the replacement.

Delancy - I think the current thickness is Ok for the size of the cut-outs in the cabin top. I am also thinking to add some insulation to the sidewalls in the form of balsa as it is only 3/16" thick now and will transmit far too much heat in the tropics. Yes, acrylic is my first thought but still want to read up on recent advances to PC. I think the cabin top is a little thin and probably flexes more than I would like, maybe part of the cause of cracking but ALL of the glazing is crazed, some more than others. The much thicker (and larger) salon windows are on next year's list.

Pulled both saildrives out this WE. Replacing the saildrive seals is my last issue preventing me from going into the water.
Ed
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Old 17-06-2014, 10:09   #7
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Re: Cast Acrylic source in Maryland

So here's a shot of my mold for my companionway. I cut frames on the CNC and vacuum bagged the whole works with aliphatic wood glue. That green stuff is moisture resistant MDF, that I used just because it was lying around and the right thickness, and what is missing us a layer if Masonite I laminated over the top as a forming surface.

Masonite is good stuff for this purpose. It's actually heat-exploded wood fibers sprayed on a screen and then compacted. The wood fibers hold themselves together like felt so there is no adhesive.

You're going for as smooth a surface as possible to minimize what is known as "mark-off" to maintain the optical properties. Sometimes people line their molds with actual felt or sometimes cotton jersey but these can still show. I think Masonite is best.

To do the forming you heat your sheet to about 280 or 300 degrees in a sheet oven and place it in your mold to slump. If you do a male mold you might have to clamp the edges which can cause mark-off, in which case people oversize then trim the edges.

For my slight curve I should be able to put it in the mold and just let it slump. You'll notice the mold is bigger than the work piece, this just makes life a little easier.

I'll post some photos of the oven and the finished product when completed but it will be a couple months down the road as I am off sailing soon.

I will mention that in addition to mold costs, it is very expensive to fire up the sheet oven (like sometimes a thousand bucks!) so if you are trying to do this on the cheap, it's good to have a friend who has an oven in use already.

Do NOT try to do this in your oven at home. The process releases a lot if nastiness, makes your cookies taste yucky!
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Old 17-06-2014, 10:10   #8
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Re: Cast Acrylic source in Maryland

Mold in the bag
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Old 17-06-2014, 10:41   #9
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Re: Cast Acrylic source in Maryland

Just a quick mention about screws- typically these are seen in older installations and their purpose is to hold the lenses in place until whatever adhesive caulk is used for bonding cures. No ever does it but if the joint is properly designed you can actually just back the screws out after the caulk has cured and fill the holes with more caulk if you want.

More modern installations use Very High-Bond double-sided foam tapes as temporary fasteners. I doubt though that you could rely on that method for a cold-formed curve on the Prout without additional external clamping but screw-less installations look cleaner and perform better. If you thermoform and have a good fit you should be okay without screws, it looks like you have plenty of overlay area for an effective adhesive bond.

Another thing about the cracks radiating from the screw holes. Basic beginner-level mistake to not oversize the screw holes, also do not use counter sunk screws. Basically the thing here is that both acrylic and poly are likely to expand and contract more than what you're mounting to. More so with acrylic than poly, same size screw holes will constrict the material and cause said cracks.

I did a screen-wall facade job once for that utilized thirty foot long extruded alveolar acrylic panels that expanded and contracted up to 2-1/2" over it's temperature range. The challenge involved designing a clamp to hold the panels securely for the wind load but still allowed the panels to move. I can't remember the exact reason but we selected a Buna rubber as the bearing surface.
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Old 17-06-2014, 10:50   #10
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Re: Cast Acrylic source in Maryland

Good stuff, Delancey.

Wish I had known all that back when I owned my old 45' leaky-teaky cutter with wood-framed acrylic hatches (on which I made most of the mistakes you've outlined when replacing the acrylic).

Thanks.
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Old 17-06-2014, 10:59   #11
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Re: Cast Acrylic source in Maryland

In addition to plastic fabricators and Corian fabricators, there's lots of sign companies out there that do thermoforming of sign faces. Sign guys are usually creative types who will get a kick out of your project.

If you are able to make the mold yourself and can find a good sign guy local you're golden. In the business they are known as "Pan Formed Faces" so that's what to ask for.

Do yourself a favor and don't expect to nail it the first try. Either do mock-ups or buy enough extra material that you can afford to make a mistake or two on the day. Good luck!


If you do find you have to thermoform here's a sign guy in Charleston. Shop Local!

http://www.thesignchef.com/plastic/r...nt-sign-faces/

Just for fun and to illustrate the principle, here's a video of a cute girl poisoning herself.

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Old 17-06-2014, 11:07   #12
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Re: Cast Acrylic source in Maryland

One last quick thing about the Prout. It looks like the two halves are mirrored and that there is draft in the form making them asymmetric on the vertical axis which means x2 the mold cost. So again, if you can cold-form that would be preferable.
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Old 17-06-2014, 12:05   #13
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Re: Cast Acrylic source in Maryland

Ed-
I'm thinking that with all that 3D curving to match up, they originally used screws because they had to force the pieces to mate up. It could require some skilled work and then still 2-3 trips to match and rematch the curves. VHB tape is not a gap-filler, it requires perfect mating surfaces.
So you might want to consider using bolts, but with proper allowance for movement and properly sized holes to prevent stress cracking.
Or, perhaps, doing some mold making so that you have actual casts of the hull to use in the oven, so that the glazing can physically be cast to an exact mold of the hull, the first time around. In the long run, plaster is cheap.
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Old 17-06-2014, 12:08   #14
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Re: Cast Acrylic source in Maryland

They are going to cost more, because they are "marine", but they do fantastic work and have experience doing exactly what you need. In Annapolis:

Maritime Plastics
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Old 17-06-2014, 12:29   #15
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Re: Cast Acrylic source in Maryland

I could be wrong but I don't know if I see compound curvature on the lenses of Ed's boat, as in a spherical form. Instead it looks like either a portion of a cylinder or possibly a cone which simplifies things. You could confirm this with a piece of paper, which you will want to do to make a pencil-rub pattern before you demo anyway.

You rarely see compound curvature in this kind of thing except for jet canopies where they are required, or something like motorcycle windscreens where the part is small.

The reason being compound curves require vacuum forming which adds to complexity and is harder to do with the kind of thickness we are talking about working with, unless it is very slight, in which case why bother? Most people wouldn't because of additional cost involved with the added complexity of the mold itself.

One thing I might mention about mold making for something like the Prout - designers and builders are working with the same tape measure you are and they are designing and building to geometry that can sometimes be intuitive.

For example on the Prout, presuming the metric system was used, you might field measure and find that the surface has an interior radius of 240mm and that the rake of the bend is 60 degrees from level. If this is the case you will see it immediately and can recognize it as such.

Designers are people just like everyone and else tend to favor round numbers which can be can be seen if you look for them and if you can find them your task will easy. If it kinda sorta looks like it is maybe 238mm or maybe instead it's 241mm, you can be confident the original intent was probably for 240mm and that is what you should work off of, at least in my experience anyway.
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