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Old 12-12-2007, 17:53   #1
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Cure for Plexiglass / Lexan Crazing?

An experienced and respectable cruising sailor recommends carefully using a heat gun to "erase" crazing from plexiglass & lexan hatches.

Has anyone tried this method with positive results?

Just curious,

Kirk
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Old 12-12-2007, 18:03   #2
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A heat gun would do no more then cause expansion and contraction in small spots, which could end up cracking it even worse.

Crazing is caused by UV's and extreme weather. I would imagine the only way to bring back the surface would be to heat it to almost a molten state in an oven and bake it for a while.

But with that much effort, a replacement maybe EZ'r.
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Old 12-12-2007, 18:14   #3
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I made snap-on covers for all of my Lexan. I had 2- 3/4" Lexan Hatches and my dog-house had 2 pieces of 3/4" Lexan. I had covers for all.

My storm windows were also 3/4" Lexan. I only had them over my windows at sea. I didn't bother covering them.

I agree with delmarrey, heating them could cause more harm than good. However, I never had any crazing so I didn't have to deal with it. However, I did help a friend reseal his Lexan windows. They were crazed and when we took them off, they fell apart.

My understanding is, once Lexan has crazed, it is extremely degraded. Plexiglass is a little less because it isn't tempered and crazing is only surface and can usually be polished out. IMO, plexiglass is a better option than Lexan, if it is going to be exposed to the Sun over long periods of time. Understanding, of course, that Lexan is much stronger, when new.
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Old 12-12-2007, 18:32   #4
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I heated a piece of Lexan AR to bend it, it crazed like crazy!
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Old 12-12-2007, 22:00   #5
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I agree with the others, I wouldn't put a heat gun near Lexan or Acrylic.

Quote:
Originally Posted by Kanani View Post
Understanding, of course, that Lexan is much stronger, when new.
Actually a very common misunderstanding - Acrylic and Lexan (polycarbonate) have approximately the same ultimate tensile strength when new, around 65-70 MPa.

But Lexan has an elongation before failure in tension of around 100% compared to acrylic of around 5%.

I certainly agree that Lexan is the inferior material for windows and hatches for the above reason, its softness so prone to scratching (which can presipitate failure) and uncertain performance in UV (in fact I don't know of any quality hatch/window manufacturer that uses it).
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Old 12-12-2007, 22:26   #6
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Quote:
Originally Posted by MidLandOne View Post
....Lexan is the inferior material for windows and hatches....... (in fact I don't know of any quality hatch/window manufacturer that uses it).
Quote:
Features:
Tinted Lexan
Cast Aluminum Almag-35
Support bars under Lexan
Standard in black finish
Option of white finish
Double opening/Lazarette or Inside/Outside opening available on some models
Aluminum and stainless steel hardware
http://www.pompanette.com/pompweb.ns...b?OpenDocument

And; This Old Boat - Google Book Search

The Com-Pac 27/2 Sailboat from Com-Pac Yachts - Gulf Island Sails

Quote:
2 overhead Lexan hatches
Shall I go on?
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Old 12-12-2007, 22:49   #7
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Quote:
Originally Posted by Gallivanters View Post
An experienced and respectable cruising sailor recommends carefully using a heat gun to "erase" crazing from plexiglass & lexan hatches.

Has anyone tried this method with positive results?

Just curious,

Kirk
You would assume he is suggesting melting the surface of the plastic to "weld" the scratches closed. It might be possible, but it sounds very tricky. There could be all kinds of side -effects.
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Old 12-12-2007, 22:53   #8
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heaps of manurfacturers use polyc. As a sheet you can buy it as is ,with UV protection one side or UV both sides. Yes it is soft and does scratch but boy is it hard to break. A lot of plastics are prone to crazing from solvents including glues. We anneal large acrylic tube with a hot air gun on a kind of roller system before gluing flat basses on them. This releases the inhearent tension and stops them from cracking when the glue goes off. They remain clear. A sharp blow will shatter 6 mm acrylic, but is unlikely to have the same effect on 6mm poly.
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Old 12-12-2007, 23:04   #9
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I am not sure I would consider Polycarbonate (Lexon is a trade name) as inferior for use in windows. It is certainly very more resilliant to impact. But yes it has a surface that very easily scratches. However, so has PMMA or Acrylic sheet (Perspex and plexiglass are tradenames). Except Acrylic is usually coated with a special coating that makes it much more resiliant to scratching.
Both materials are thermoplastics. This means they both have a melting point. Acrylic is approx. 140DegC and Polycarbonate is aprox. 270DegC.
One big difference between the two is what light they filter. Polycarbonate is better at transmitting visual light. In other words, it seems more transparent. However, it blocks a great deal of the UV spectrum.
Acrylic is not so transparent, however, it transmits most of the UV spectrum. The result is that the Acrylic is actually better at resisting UV damage. PMMA's are more susceptable and can end up actually being less resistant to impacts than Acrylic in as little as 12months.
Here's an interesting fact. All the family of thermoplastics are made from oil. It takes about 2Kg of Oil to produce 1Kg of Acrylic. If you heat Acrylic enough, it will ignite at 460DegC. That's very hot. But as it burns, it produces nothing more than Co2 and water.
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Old 13-12-2007, 01:29   #10
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Have you asked yourself why Bomar puts support bars under the lenses in their Lexan fitted hatches - the clue is in my post? It is not self supporting for other than small areas and small loads due to its elongation when loaded. Apart from anything else that presents sealing problems. (Although at least one hatch manufacturer, can't think which one at the moment, gives the choice of thinner lensed acrylic hatches with support bars, or thicker acrylic lenses without supports)

You may also like to clarify what Bomar use for lenses in all their other hatches apart from the Cast 100 line linked to?

For hatches you will find that builders such as Freeman, Lewmar, Maxwell, Atkins & Hoyle, etc all use acrylic for hatches (or in the case of Freeman, choice of acrylic or tempered glass, but not polycarbonate, and in Bomar's case mostly acrylic).

As far as I know they all mostly use acrylic or if not then tempered glass for portlights (Freeman, who is probably the best manufacturer of the lot, does use Lexan for its cheapest "Yacht Premium portlights though, but glass in most (all?) others and Bomar uses Lexan in (all?) portlights).

I cannot imagine anyone using Lexan for other than very small fixed windows but seems amateurs do - but there again I have also known them to use plain annealed glass and laminated annealed glass too, so maybe Lexan not much of a surprise .

For storm shutters where Konani used 3/4" (think it was) I would have no difficulty with, distortion and sealing is not a real issue and shock loads are what one might want resistance to. It has obvious benefits compared to the alternative of plywood.

Of course, people can use whatever they like (but they will have a very limited choice if they want Lexan), but any new build I managed would never be allowed to be fitted with Lexan hatches or windows (in fact the windows are always tempered glass because nothing else is allowed in them ).
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Old 13-12-2007, 05:56   #11
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I will let you know the real world results...!! I have one set of 6 mm , 10 mm acrylic, and 6 mm poly windows all on the same boat all now 6 years old. The poly is double sided uv resistant. My second boat has 10 mm acrylic (the minimum i would except as storm thickness in acrylic) and 6 mm poly (double sided uv resitant) now 3 years old... Tempered glass is a great option for those people who have the ability or need to pay for some one else to do it for them. I am a home builder and proud of it. You would have a very hard time telling me that the majority of production boats have tempered glass. I dont deal in brought in "marine" products, i deal in the materials that i deal with from day to day. If acrylic is "better" why is the sheet of choice for the roofing industriy now polyc. ? The anwser is simple ...inhearent strength and flex....glass is very expensive , has wonderfull resistance to scratching , is very strong, but cant take point shock loads, and cant be cut or shaped by an average boat builder. Acrylic has very poor bending ability, is better at scratch reistance, is very suseptable to uv (there is some uv treated but expensive sheet) and can be shaped by the average boat builder. Poly has incredible bending and deformation abilitys (thats why it is used as bullet proof clears) has poor scratch ability and Double uv has excelant uv ability. And can be shaped by the average boat builder. I find a lot of conversations about boats two edged... from my point of view it is important to spell out whether we are talking about an expensive bought off the shelf ., go to your local retail boat supplier, or are we talking about somthing that is possible for the "practicle boat owner". Are you in the "business" midlandone ?
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Old 13-12-2007, 11:35   #12
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(Putting work hat on...)

What you are suggesting is called flame polishing.

It is a technique often used most often in plastics... on acrylic to de-burr the edges of a saw cut piece.

Basically you take a torch and hold it 4-6 inches away from the piece and wait until the finish changes, then "push" the shiny slick surface around. On professional rigs they use a hydrogen torch setup with tips every 6 inches or so... sort of like an industrial sized shrink wrap heater with a conveyor belt under it.

I haven't tried it on Lexan, but the I'll bring my torch the next time I'm down at the boat.

What it doesn't work on... anything with vinyl in it. The vinyl will go gray with not a whole lot of heat.

As far as other methods go... nothing stopping you from wet sanding the surface and following up with plastic polish. It won't be optical quality, but if a window that warps light is good enough, it'd work. Until you get to 2000 or 2500 grit it'll still looked scuffed. Once to that point an electric buffer would be nice, never stop moving it.

I don't have any experience with it, but polycarbonate is vapor polished for eye glasses, ground and polished chemistry that attacks the surface.

On my boat I'm planning to ditch the beautiful bronze deadlights and use as thick of lexan as deemed necessary backed up with an aluminum backing plate. Shooting for strong enough that it adds stiffness to the cabintop. Took keep it clear I'm thinking a piece of 1/4 inch plexi on the outside as a sacrificial layer. Lexan = bedded in, plexi just bolted up.
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Old 13-12-2007, 15:56   #13
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On my boat I'm planning to ditch the beautiful bronze deadlights and use as thick of lexan as deemed necessary backed up with an aluminum backing plate. Shooting for strong enough that it adds stiffness to the cabintop. Took keep it clear I'm thinking a piece of 1/4 inch plexi on the outside as a sacrificial layer. Lexan = bedded in, plexi just bolted up.

Search for the thread on "Sexy Windows". That is the method I am going to use next spring.
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Old 17-12-2007, 21:10   #14
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I've successfully rejuvunated and polished aircraft plexiglass windows. It is a painstaking process requiring lot's of time.

We started with one canopy that had real, tool mark type scratches in it. By the time we were done it was beautiful.

This company - I am sure there are others - sells a kit for it. Basically various grades of abrasive papers, different polishing compounds etc.

Glass Technics Aircraft Window Polishing
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Old 18-12-2007, 00:22   #15
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Back in the day when I was a toolmaker, We use to flame polish Acrylic with a hydrogen flame. It is a cooler flame than many gases. It was very tricky but removed all of the machine marks. I have never seen crazing in PolyC. Usually it fogs up. I am surprised someone would say that Acrylic is the same strength as PolyC. I use to make bullet resistant windshield for the police. The other claim I found odd was that the "scratches in Lexan(polyC) can presipitate failure". You can't be serious!
One suggestion I will share with everyone is it is difficult to find a sealant to use on PolyC. The only thing I researched tha works is a GE product called Silpruf.
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