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Old 26-08-2008, 16:56   #46
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Glad someone finally mentioned the flame polishing, it's amazing on edges anyway. I'm not so sure about tempered glass windows myself. Having worked for a builder of high performance heavy weather craft, I've seen at least 2 glass windshields disintegrate from water impact. Glass is nice in the long run for a durable finish, but if your concerns are mainly impact... I'll take the poly.. (think about hitting a one foot square of 3/16 or 1/4 glass with a hammer, or the typical 1/2 poly with the hammer.... my money's on the poly!)
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Old 27-08-2008, 02:48   #47
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Quote:
Originally Posted by mdelannoy View Post
I work for an acrylic processor which also does ship windows.
... Crazing seems to occur at the areas that have been bend in the shipyard. The shipyard might not have used enough oven time. Therefore crazing has occured.
I figure that you can anneal (find annealing temperature here)
by putting the whole sheet in the oven. This might remove some if not all crazing...
I couldn't get your link to annealing temperatures to work.

Fabrication techniques such as thermoforming, cementing, machining, line bending, buffing, flame polishing, and screen printing can cause stress on parts made using acrylic sheet. Annealing, the strengthening of acrylic sheet through controlled heating and then cooling, minimizes the effects of fabrication stress.

Internal stresses in the sheet can cause crazing (the appearance of numerous tiny cracks in the material) when acrylic comes in contact with solvents such as glass cleaners or paints. Annealing helps to reduce crazing or large scale cracking by reducing internal stresses and increases the strength of cemented joints.

More: MINIMIZE IMPACT OF STRESS ON ACRYLIC SHEET THROUGH ANNEALING: Plastic Distributor & Fabricator

To anneal acrylic sheet, heat it to 180̊F (80̊C), just below the deflection temperature, and cool slowly. Heat one hour per millimeter of thickness – for thin sheet, at least two hours total.

More:
Annealing Acrylic
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Old 27-08-2008, 04:55   #48
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I saw a boat that was in Asia for 10 years, the good lady used to put baby oil on the exterior of the hatch material when she thought of it and it was still in pristine condition
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Old 05-09-2008, 19:35   #49
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Has anyone got an idea how to build an annealing oven?
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Old 05-09-2008, 19:43   #50
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From the annealing article cited above:

"It is possible to anneal with a restaurant-type oven obtained from a restaurant supply firm, but be sure the oven incorporates air circulation and accurate temperature control systems....To anneal acrylic sheet, such as CYRO Industries' ACRYLITE® FF, heat it to 180°F (80°C), "

For that matter, you oculd use any home oven, or any box built of sheet metal, as long as you added some srouce of heat and forced air, i.e. a convection oven or some heat guns with adjustable controls.

Or for small parts you could probably leave them on the dashboard of a car in Arizona.<G>
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Old 05-09-2008, 20:57   #51
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Lexan is a trade mark of GE. It is bulletproof. YES it will stop bullets! They do make Lexan that is coated on 1 side and very scratch resistant. They use it in NASCAR windshields. You can buy it from your local GE polymer distributor. I have made windshields from it before. Yes it is expensive but worth it.
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Old 05-09-2008, 21:20   #52
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"YES it will stop bullets!"
Which brings up an airline story that eventually made it to Mythbusters.

Someone at BOAC asked someone at the FAA how they tested aircraft windshields for resistance to bird strikes (hitting a bird, in flight at 500mph) and the nice folks explained how you got a frozen chicken form the supermarket, stuck it in an air cannon, and fired it at the windshield.

Brits called back, all befuddled because their windshields kept shattering. Finally, someone at the FAA says "Did you remember to DEFROST THE CHICKEN?"

Mythbusters confirmed, frozen chickens do a lot more damage. Even through Lexan. So much for bulletproof.[g]
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Old 06-09-2008, 08:36   #53
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Micro Mesh sandpaper

Someone said they weren't aware of sandpaper finer than 2000 grit, there is Micro Mesh. They sell kits to clear up airplane windows among other things. They go up to 12000 grit in the metal finishing section, not that you need that in this threads app.

https://www.micro-surface.com/default.cfm?page_id=1

John
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Old 10-01-2009, 23:47   #54
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Crazing....

That subject always cracks me up...

Listen guys, crazing in acrylic is caused by either thermal or mechanical stresses imparted during production, transport, manufacture or in use.

Acid rain,Ultra Violet degradation, Windex or the boat detailer's magic deck cleaner (to name a few) accelerate the process.

Once somthing is crazed it is significantly weaker than the same part in factory OEM condition.

How do you stop it? Make sunbrella covers (not vinyl backed sunbrella!!). Use cleaners specifically designed for Acrylic.

How do you repair it? You don't.

We have all seen the lotions and potions sold at boat shows to cure everything from warts to your crazed hatches. You still have the warts don't you?

You can follow the Mil.spec procedures from the USAF/USN and sand the lens until you are below the crazed line, sand again in successive grits and buff with 4 or 5 different compounds until clear again. Remember annealing? Yea.... you need to get the mechanical and thermal stress you just put in with the sanding and polishing out! Well at 170 degrees F for 12 hours (1/2 inch acrylic) and cool down at the same rate, oh and now you've found the seal puddled in the bottom of your wife's oven and the parakeet is dead from the MMA vapor being outgassed during the process. New seal, new oven, new wife?

By the time you have compleated this repair you could have purchased two hatches, thrown one overboard and had the most expensive yard in the States instal the second with the gold plated rag-surcharge and be ahead of the game.

I repair thousands of hatches every year. I know replacing the lens is an option but repairing the crazing with potions or flames is not realistic.

Let me know if you want some pictures of really crazed hatches. I haveseen every make and model you could imagine.

I would like to help. But even the Hatchmaster cant make crazing dissapear. (yet)

TD
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Old 11-01-2009, 09:09   #55
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WOW!!! I am amazed at the level of expertise on this forum. However I think I'm at information overload because now I dont know what product I should use in my wheelhouse side windows.
For the front facing windows I'm going to use tempered glass. The last motosailer I owned had removable windsheilds with tempered glass and I know of three times they were dropped from about 5 ft. and they never broke.
I had planned to use polycarbonate for the side windows and dutch doors, mainly to keep the weight down, ( the top half of my wheelhouse is removable for trucking purposes and possibly french canals ) , but now I'm not sure what I should use. I already have two sheets of 1/4" polycarbonate that was given to me so perhaps I should just use what I have for now with the understanding I'll be replacing it in a few years.

I sure wish that I had found this forum a few years ago when I started this project.
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Old 11-01-2009, 10:37   #56
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Originally Posted by quidam View Post
WOW!!! I am amazed at the level of expertise on this forum. However I think I'm at information overload because now I dont know what product I should use in my wheelhouse side windows.
For the front facing windows I'm going to use tempered glass. The last motosailer I owned had removable windsheilds with tempered glass and I know of three times they were dropped from about 5 ft. and they never broke.
I had planned to use polycarbonate for the side windows and dutch doors, mainly to keep the weight down, ( the top half of my wheelhouse is removable for trucking purposes and possibly french canals ) , but now I'm not sure what I should use. I already have two sheets of 1/4" polycarbonate that was given to me so perhaps I should just use what I have for now with the understanding I'll be replacing it in a few years.

I sure wish that I had found this forum a few years ago when I started this project.
Any of them can work just fine. I would not worry too much about it, especially if you already have the polycarbonate and if you are not planning a trip to the North Sea or something There have been very successful applications of polycarbonate, acrylic and tempered glass.
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Old 11-01-2009, 16:16   #57
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Lexan Vs Plexi

Both Acrylic and Polycarbonate have specific uses and installation requirements.

Cast Acrylic (of a specific thickness) is in accordance with CE and ABYC guidelines, and installed on virtually all of the big blue water sail boats produced on both sides of the pond. Polycarbonate is commonly used as a replacement due to its ease of fabrication and incredible initial strength. The USCG and USN require Polycarbonate on their vessels but they also have a PM cycle of 36 to 42 months for change out. My Tax dollars at work...

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 coefficiant of thermal expansion for Acrylic and Polycarbonate is in the neiborhood 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 afformentioned products. Both of these products are rated at 700 + percent elongation before tear. Strong flexible and UV resistant. Kinda like me!

Been to a boat show lately? Seen any screws? Glass is good so long as your boat does not twist or torque. Show me a fiberglass boat that does not twist and I will show you a coctail barge tied to the dock.

Listen... so long as your not running the Bermuda or Whitbread install the free stuff and call it a day. You will be OK...... Use the 795, its the best. Make sure its a current batch and do not apply it below 50 degrees F.

Tony (Hatchmaster)
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Old 19-06-2012, 12:34   #58
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Re: Cure for Plexiglass / Lexan Crazing?

So... now that my brain is a blur and my eyes glazed/crazed over does anyone know what the minimum thickness should be for cast acrylic on fixed portholes approx 60" in length?

Thanks,
Mike
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Old 19-06-2012, 12:59   #59
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Re: Cure for Plexiglass / Lexan Crazing?

Mike, if you mean "the minimum thickness to prevent breaking" that will depend on the length AND the width, and the impact or force you want to protect them against. Among other things.

Then you'd also have to pick a specific acrylic or other material, since strengths vary widely with the materials.
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Old 19-06-2012, 18:19   #60
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Re: Cure for Plexiglass / Lexan Crazing?

"Remember the coefficiant of thermal expansion for Acrylic and Polycarbonate is in the neiborhood 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."
I'm only getting .0234" change per 10" length.....? (30 degrees to 90 dgrees)
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