Replacing acrylic windows

[Neeves]

We need to replace 3 acrylic windows. The windows are large, the largest is 1,000mm x 500mm and the other 2 are not much smaller. The acrylic is 10mm. The aperture, the actual window is actually quite small - the overlap is aesthetics and has a huge area of sealant.

The problem is the acrylic windows are not flat.

There is a slight bend, 10mm over the 500mm dimension to 20mm over the 500mm dimension. The curve is in one plane only.

I understand I could have the windows cut to size, flat, make up a former and address with a heat gun - but do not know how well this works with 10mm and upto 1,000mm long acrylic. I'm assuming a normal decorators heat gun, applied patiently.

It has been suggested that a 20mm curve in 500mm can be made in situ - but I cannot think how to bend the acrylic in situ and keep the bend as the sealant sets off - there is nothing I can think of to maintain the curve - and the acrylic will obviously be under tension trying to return to a flat format. In a boatyard I could rig something up to keep the curve in place - not so easy on a swing mooring.

I read, somewhere, that if you can induce the curve and attach then the tension disappears when installed as the acrylic bends (creeps?) to its new shape. If this is the case I could simply have the windows cut, clamp up and leave for a week, a month until they take up their new shape. If this is all correct - how long does it take to bend acrylic cold. I could have the windows cut, bend, leave for a month, 3 months - if its a year - that's a long time! And to speed things up I could clamp up a leave in the sun, of which we have plenty (though not today - its teeming down.

Any advice greatly appreciated.

Jonathan

 

[NormanS]

Presumably the acrylic is bolted through the (ply?) structure?
One way of doing this is to put a small washer on each bolt between the acrylic and the ply, so that all the sealant doesn't get squished out. Start in the middle, and work towards the ends.

 

[Neeves]

No - no bolts. The acrylic overlaps about 100mm all round and is bonded with a sealant adhesive (I believe a Dow product used to hold the glass panels on high rise buildings (at the time Petronas Tower was quoted). If there were bolts or some form of positive attachment it would be so much easier.

And no ply - all glass and foam.

Jonathan

 

[waynes world]

Hi Jonathan, how about a couple of ratchet straps and blocks of wood either end of the new screen to push and hold the new screen into place while it sets. If there is a point for the straps to hook onto this could work,

 

[Neeves]

Hi Jonathan, how about a couple of ratchet straps and blocks of wood either end of the new screen to push and hold the new screen into place while it sets. If there is a point for the straps to hook onto this could work,

Something I had not thought of! Maybe possible but definitely fiddly.

Has anyone cold bent 10mm acrylic?

The obvious answer is to heat set - but never having done it before (and acrylic is not cheap) I was looking (at other options) and for a lot of inspiration.

Jonathan

 

[macd]

Jonathan: if you have a hunt on-line you'll find all the info you need for allowed bend radii on acrylic. Manufacturers would be a good place to start.
I'd caution against taking a heat-gun to the pieces: the stuff can be heat-bent, of course, but really needs a staged cool-down if internal stresses are to be avoided. Commercially, of course, it's done in big ovens. I doubt that cold embrittlement is a problem where you are :encouragement:

 

[rogerthebodger]

You could use some vacuum glass lifting handles to pull the perspex into shape until the sealant sets.

http://www.ebay.co.uk/bhp/glass-vacuum-lifter

I have one large perspex window that is curved by about 20mm but mine are held in with bolts holding an inner frame.

You can see how my big windows are fixes in just next to my inside steering station.

 

[Mister E]

Some sort of heat box using steam may be they way for you to go. Then hold the Acrylic down to a former but allow an extra 10% in the bend for spring back.
I have bend 10mm Acrylic using a hot air paint stripper but it was a much tighter radius and showed some signs of overheating. That did not matter for what was needed at that position as it was going around a pole.

 

[Neeves]

Everything I read suggests that cold bending is possible for a radius greater than 200 time the acrylic thickness - but most then suggest the section needs to be held in a frame. I have assumed that if the sealant/adhesive is good for the application it would replace a frame (but that's just gut feel). I had read that cold formed acrylic would 'set' (and I could encourage that by putting in the sun) but cannot actually find any confirmation that it works. I could then simply clamp up, sit in the sun, let 'set' - say a few weeks, then instal before it flattened itself (in case it has a memory).

I did wonder about glass lifting suckers, we use them when we clean the hull from the dinghy. Again they are good for a short time - I've never tried them for the length of time a sealant takes to set off!

Jonathan

 

[Neeves]

Some sort of heat box using steam may be they way for you to go. Then hold the Acrylic down to a former but allow an extra 10% in the bend for spring back.
I have bend 10mm Acrylic using a hot air paint stripper but it was a much tighter radius and showed some signs of overheating. That did not matter for what was needed at that position as it was going around a pole.

I did think of a conventional domestic oven - but its not big enough!

Commercially this is all possible but simply making the former would cost more than the acrylic (and acrylic is not cheap) - hence my parsimony.

Jonathan

 

[PaulRainbow]

I did wonder about glass lifting suckers, we use them when we clean the hull from the dinghy. Again they are good for a short time - I've never tried them for the length of time a sealant takes to set off!

Jonathan

The sealant used for bonding car windscreens should be up to this job Jonathan, it sets fairly quickly.

 

[Neeves]

My source of information came from a reputable fount of all knowledge marine:

http://www.ybw.com/forums/showthread.php?398649-Bending-Acrylic-Sheet/page2

Post #15 - and the specific comment on acrylic taking a set came from my neck of the woods. I'll try a PM.

Jonathan

 

[Reptile Smile]

This place: https://www.trentplastics.co.uk/acrylic/acrylic-sheets will cut them to size and lists bending as one of their services

 

[peteK]

I replaced acrylic windows last year, 10mm thick 1300x300mm with a curve too follow the coachhouse sides,these were held on with self tappers.
This year I had to remove one to cure a leak and it had formed a curve which made it much easier to reseal as I wasnt having to force it in to follow the curvature of the coach roof.

 

[Neeves]

PeteK, you seem to suggest that acrylic does set with time. In the absence of further advice I'll have one cut to size, clamp up top and bottom edges with an appropriately sized bits of wood down the middle (all suitably protected from scratching) and leave in the Sydney sun and see if I can get it to set nearer the curve I want. I'll 'over' curve to start with as I guess there will be some spring back.

I'll report back on success or otherwise.

Thanks for the advise to one and all

Jonathan

 

[rogerthebodger]

PeteK, you seem to suggest that acrylic does set with time. In the absence of further advice I'll have one cut to size, clamp up top and bottom edges with an appropriately sized bits of wood down the middle (all suitably protected from scratching) and leave in the Sydney sun and see if I can get it to set nearer the curve I want. I'll 'over' curve to start with as I guess there will be some spring back.

I'll report back on success or otherwise.

Thanks for the advise to one and all

Jonathan

I would use a piece of plywood to make the curve with the perspex on top. This gives the perspex support over its whole area and reduces the possibility of uneven curvature. You could also suspend an electric bar radiant heater a metre or so above to introduce more heat to the process.

This is a simple vacuum forming machine. You don't need the vacuum bit just the heater bit at the top with the supporting plywood underneath.

 

[Neeves]

Roger,

Thanks, I had naively assumed that the acrylic would bend evenly but using plywood will certainly even out the curve. The windows are grey, they look black from the outside, and I am hoping the Australian sun will be sufficient (we certainly get enough of it). Too little heat means whatever I do will take longer - too much heat and you, apparently, damage the acrylic with bubble formation.

Jonathan

 

[maxi]

Yes, worry not about the bending, I believe that you will be surprised at how easily the acrylic will conform, no heat guns required.

Owning a boat for the last 20+ Years that appears to have a very similar window configuration as your own I am well aware of the problems, and the difficult one is; that the adhesive bond between acrylic
& the adhesive is unlikely to last much beyond 2 years, whilst the acrylic/hull joint will last almost eternally.

The solution is, I hope, that the application of the correct primer to the acrylic prior to bonding with a specifically designed UV sealant may be the way to go, using a good quality double sided 3mm thick sealing tape that will (a) poivde a suitable gap twixt boat & window to prevent the sealant being squeezed out by the bending of the window & (b) will hold the window in place during the cure period.

To assist in keeping the window in place I used a couple of suction handles (similar to those used for lifting glass sheets) on the inside of the window and tensioned with a Virgo suitable cabin fittings. They are really cheap on line& work well.

All easily do-able on a mooring from a suitably stake dinghy.

 

[Neeves]

I'm having the acrylic cut today and will have access to it at least sometime this week.

Never having played with acrylic like this its all a leap into the unknown.

That's 2 votes for window sucker devices, I'll buy a second one. Finding something on the interior from which to tension them will not be difficult.

Thanks to all

And in case I have nothing new to report

Merry Xmas

Jonathan

 

[Neeves]

An update

So - I had my acrylic cut.

Its about 1000mm x 500mm x 10mm. Its trapezoidal so the longest length is 1m along the bottom but only 500mm along the top. This may seem, it is, large but the overlap on the structure of the fibreglass is 100 - 150mm all round. The curve is not great about 20mm on each edge. This curve was enough to make me think that tension alone would not be sufficient to get the right curve as the sealant set off.

I took an old kitchen bench top which is the same size, roughly, as the acrylic, and flat and placed a 30mm thick block of wood, about 50mm diameter, in the centre. I had a sheet of hardboard between the acrylic and the centre block. I simply took softwood battens along each edge and clamped each end, or each corner so bending the acrylic over the central block.

I've left the window for 3 months, undisturbed, and on unclamping I have what looks like a decent curved set for the window. I've re-clamped it, again, for the next week and after Easter I'll check it out to see if I've got the curve about right. One edge might be a bit too straight and I might need to balance the curve - but cold setting seems one solution to producing a gentle curve.

I already have one set of window sucker devices and will buy another, as mentioned they are cheap. I can then use them on the inside to tension the new window against the fibreglass of the cabin.

I have yet to remove the original window but am thinking of piano wire, like cheese wire, and 'simply' slit the sealant and then scrape off.

I have 2 more windows, with a similar curve, to replace if this is successful.

The windows are 17 years old, are not crazed, at all, but have developed one crack in each of the windows, from one edge toward the centre. I think the cracks are sun related, all on the same side. as we do not recall they have been hit (and if they had I'd expect there to be a chip and the crack to be 'in the centre' - its an age/sun thing).

Jonathan