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  #1  
Old 07-18-2012
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Forming Plexiglas

The hatches on my boat are wood with Plexiglas. What has happened over time is the Plexiglas has pulled the screws loose from the wood frame. I've taken one hatch apart and discovered that the Plexiglas was forced to conform to the curve of the hatch. The hatch is curved but when the plexiglas is laid on the bench, it lays flat, no curve to it. So all this time the screws have had an upward force on them. I've found a manual on line about forming plexiglass. The manual suggest heating the sheet in the oven, pulling it out of the oven then clamping it to the frame, its called drape forming. My question is why not clamp the plexiglass to the form then stick it all in the oven heat it up and let it cool down?
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Old 07-18-2012
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Re: Forming Plexiglas

I think the first way relaxes the "memory" in the Plexi. not really sure though either way seems doable to me. How about a heat gun?
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Re: Forming Plexiglas

Heat gun seems spotty to me, wouldn't heat the sheet evenly. The cooling might be uneven which could lead to stress.
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Old 07-18-2012
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Re: Forming Plexiglas

Quote:
Originally Posted by Captainmeme View Post
The hatches on my boat are wood with Plexiglas. What has happened over time is the Plexiglas has pulled the screws loose from the wood frame. I've taken one hatch apart and discovered that the Plexiglas was forced to conform to the curve of the hatch. The hatch is curved but when the plexiglas is laid on the bench, it lays flat, no curve to it. So all this time the screws have had an upward force on them. I've found a manual on line about forming plexiglass. The manual suggest heating the sheet in the oven, pulling it out of the oven then clamping it to the frame, its called drape forming. My question is why not clamp the plexiglass to the form then stick it all in the oven heat it up and let it cool down?
I did a little research, and the melting point of Plexiglass is 160C. So heating it to 140C or 150C should soften it. Those temps are well below the burning point of wood.

However they are also above the melting point of alkyd resin, the ingredient in varnish. So if your wood is varnished I would expect that to fail before the Plexiglass softens.

So in answer to your question, why not, the answer is "because the varnish might be damaged".
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Old 07-19-2012
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Re: Forming Plexiglas

meme, have you asked a plexi shop (where they have the right heating tools and experience using them) if they could just take your hatches, and bend the plexi for you? Since it is just labor time, the job might not hurt too badly.
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Old 07-19-2012
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Re: Forming Plexiglas

I have heated Plexi in the oven many times for the purpose of forming it. It is not difficult.

I would put the form in with the plexi or make sure you can easily slide the flexible plexi onto your form once removed from the oven. It will be very flexible especially if left in too long. Be careful clamping it as it will be soft enough to indent. Spread the load out to prevent indentations.
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Re: Forming Plexiglas

"meme, have you asked a plexi shop (where they have the right heating tools and experience using them) if they could just take your hatches, and bend the plexi for you? Since it is just labor time, the job might not hurt too badly. "

I should change my online name to Captain-cheap-cheap. That is the first reason I haven't done that, the second reason is I live in the back hills of Tennessee. Plexi shops are far and few between. I think I'll just make a sacrificial mold to experiment with. As a previous poster pointed out the heat would/could affect the varnish, just between you and me, I'm more concerned about the adhesive holding the frame together. I'll post the results.
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Old 07-19-2012
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Re: Forming Plexiglas

If you are using plexi of significant thickness, it might get expensive to do much experimenting.

I'm a bit suprised the old plexiglass didn't conform more to the shape it was in for years.
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Old 07-19-2012
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Re: Forming Plexiglas

Quote:
Originally Posted by Captainmeme View Post
The hatches on my boat are wood with Plexiglas. What has happened over time is the Plexiglas has pulled the screws loose from the wood frame. I've taken one hatch apart and discovered that the Plexiglas was forced to conform to the curve of the hatch. The hatch is curved but when the plexiglas is laid on the bench, it lays flat, no curve to it. So all this time the screws have had an upward force on them. I've found a manual on line about forming plexiglass. The manual suggest heating the sheet in the oven, pulling it out of the oven then clamping it to the frame, its called drape forming. My question is why not clamp the plexiglass to the form then stick it all in the oven heat it up and let it cool down?
The Plexi will expand as its heated. Clamping it down before heating will likely cause it to shatter as it heats/expands. Note that the Plexi will have to be quite hot to "drape" well and the molding frame fairly sturdy.

FWIW...
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Re: Forming Plexiglas

Find a local plastics shop. You might be able to get some scraps to experiment with.
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