Cutting acrylic for portlights

[iansan5653]

It's about time to replace the surface mounted acrylic portlights on my Freedom 21. They're crazed, cracked, leaking, and the screws keep falling out. I plan to forgo mechanical fasteners in favor of Dow Corning 795 sealant as described here.

I'm pretty sure I can handle this, but the only thing I'm stuck on is how to cut and prep the new pieces. They aren't simple shapes (see the attached image) and they are pretty long, so I think I have to just remove them and use the existing portlights as templates (as opposed to measuring them and sending dimensions to a fabricator).

If I pay someone to do this, I'll have to deliver them one of the current pieces for them to trace. That would take a couple days and there's always the risk of rain so I'd have to tarp over the opening. Then I have to deal with duct tape residue, the potential for theft, etc. Not really a good option.

So I think my only choice is to try cutting them out myself and get it all done in one weekend. I assume I can do it with a jigsaw (though I'll have to buy one) but I've never cut acrylic before and I'm nervous about ruining a large, expensive sheet of acrylic.

I don't really have a specific question here, but any general tips on purchasing, cutting, and finishing large acrylic pieces would be greatly appreciated!

[Jammer]

I've worked with smaller pieces of acrylic before. If you use a saber saw be sure you support and clamp/weight the work down. Use a good fine tooth blade and go slow. Acrylic is easy to sand with a die grinder, drum sander, or RO sander so you have those choices for cleaning up edges and curves.


I think if I were duplicating an existing piece I would use it as a template and cut with a router with a template/bearing bit. Again clamping and supporting the two pieces is critical. The router will cut a more exact duplicate of the original shape and so there will be less cleanup.


On my boat the acrylic windows are beveled at the edges. I don't know how I would do that with any sort of precision, without special tools.

[HankOnthewater]

Hi Iansan
Acrylic wil break went you do bend like that as in the pic. You cannot use acrylic (perspex or plexiglass), you have to use polycarbonate. That is far more bendable and stronger, but, scratches easier.
Although I would not be surprised if the ordinal windows were heated in an oven or something like that to enable to bend those sharp curves, particularly if the sheets is thicker than 5 mm.
If we were to do it yourself on a boat like in the pic, you may need 2 or 3 others to help you install.

Both acrylic and polycarbonate can be cut with a jigsaw, table saw, bandsaw or hacksaw. However one can buy special blades for this acrylic cutting. For jigsaw reduce speed to reduce friction and therefore heating. Blade needs to be sharp to minimise heating.

Then securing and sealing is another story, as normal silastic will not stick very well to the sheets. I have used a Sikaflex system with a primer. Can't recall the product numbers , have them on the boat.

Oops, When I am checking your pic more closely, I see the windows are only on the side, and do not wrap around the front. In that case, acrylic will work as well.

Other tips:
- predrill holes 2-3 mm oversize as the window will expand and contract (due to temperature changes and the the size of your window)
- edges that are cut will be pretty rough, and can be sanded to an acceptable finish as your edges will be visible
- further improvement on sanding is to use a blowtorch after the sanding, see this video

https://www.youtube.com/watch?v=QFoy8fyaqhA

- bed the window in generous thick gooh (ie 3 mm), again to allow movement
- when putting everything in place, do not tighten the screw(bolts) very much, otherwise all the gooh will be squeezed out, leaving no gooh for movement
- tighten more or less all screws/bolts evenly
- one way of achieving this to put some small washers or other things with an equal thickness in the gooh (but these items need to be completely surrounded by the gooh)

[Knotical]

I have cut acrylic sheets with a hole saw for various projects, never with a jigsaw.

These are the tips I follow-
-get a new, sharp fine tooth blade
-be slow and consistent
-clamp/support it well, don’t use your other hand to hold it down
-practice a lot on scrap acrylic first - you will chip and melt it at first but will get the hang of it after a few tries
-cut with the protective plastic on
-cut bigger than you need if you are unsure, you can always sand it down

Good luck

[chris mac]

We just replaced our windows. I had them formed by a shop since ours needed to be heat molded and installed them myself , but it got me curious about acrylic. So I've been building shelves and things for the boat.
I find most electric cutting tools either chip or melt the acrylic as you are cutting. With the jigsaw it seemed to melt, then reform behind the blade. It was a PIA. a grinder seemed to work best for me.
Don't polish the cuts too much. Anywhere the Dow 795 is against needs to be roughed up as well. Sand paper works well. The Dow doesn't stick well to smooth product.
The most important part is prep. Do your homework, understand the process and prepare everything properly. Then the windows go in easy, and stay in.
Good luck

[JPA Cate]

Hello, Iansan,

The two pieces may not be mirror images. You'll need to make really sure. Otherwise, plan on using the other side for its own template. When you take them off, notice the size of the holes, vs. the screw's diameter. That clearance has worked for those screws, even with the smoky acrylic. It is critical to make sure you have at least that much clearance with the new glazing.

Jim uses a saber saw to cut acrylic, using kerosene as a lubricant and cooler. You may want to sand the edges anyway, don't want 'em too sharp.

We have rented a commercial pizza oven to heat bend acrylic (facilitated by eating there often enough). You need a really smooth form to bend them on. However, if that is only 6 mm (1/4"), I'd think you could bend it in place, working from forward, astern, using the old holes. I did have to increase the picture size to be able to see how much there is paint and how much acrylic.

When we replaced our old hard dodger "windows," we found out what they use for gluing them in on skyscrapers, and that is what we used, according to the instructions. Once the glue is set it is quite strong, but we left the screws in and didn't disturb it once installed.

You CAN do this. It may help to have someone lend a hand when it comes to trial fitting the new acrylic, before the final fit with the glazing compound.

Ann

[Bowdrie]

If you go the route of using a router with the original part as a template, first cut the new part oversize a small amount with your jigsaw.
Router bits DO NOT like to cut a "surround cut", they give best results when used to trim a small amount, and avoid "climb cuts" if at all possible.
Sika #295UV is the standard sealant for most applications.
Any fasteners must use generously oversize holes, if any metal tries to constrain the movement caused by temp changes the material WILL crack, this includes fastener heads that fit in a countersink.
Best practice is no fastener holes, sealant only.

[Jammer]

Oh, on mounting these, when I resealed my acrylic windows, I used VHB tape instead of fasteners. I had someone help hold it in place since you only get one chance to get it right, but it worked out well. The VHB tape was chosen to be the same thickness as desired for the silicone, then I put the tape on the inside of the bonding area and the silicone on the outside, and pushed the window into place. Worked great.


On bending, for significant curves you do have to use heat, if you don't then even if it looks OK at first it will leave latent stress in the acrylic that will lead to premature crazing over time and with temperature cycling.

[garyfdl]

Not an expert by any stretch, but I have worked with it a few times.

First, I suggest making paper patterns from the old work.

It can be scribed and snapped when doing straight lines, if the length isn't too long (10-12"). If doing longer cuts you can make perpendicular cuts to the scribe and snap it in sections. They make an inexpensive tool, kind'a like a glass cutter, to make the scribing easier. Get one. It's not a bad idea to scribe the line 2,3 or even 4 times to make sure it's deep. Be careful to stay in the original line when doing it multiple times (especiallly with thicker material). When you snap it, snap away, not toward the scribed line.

The other 2 cents is, if the protective film comes off, you can use masking tape to protect the surface from scratching.

You might check with a local hardware store or a glaser to have it done. They work it regularly and I would think they would know 'tricks of the trade' for doing curves bends etc.

Good luck!

[Anders]

Get a jigsaw blade designed to cut plastic and the job is easy but you will need to polish the edges if they are visible.
Buy cast acrylic, not extruded acrylic and not polycarbonate.

[Tupaia]

Done this got the tea shirt. It is a major undertaking but excellent results can be achieved.

1. Use cold cast Acrylic - extruded Acrylic will crack and craze.

2. Use silicone adhesive Dow 795 or GE SG4000 both are used to stick glazing to skyscrapers.

3. Do NOT use mechanical fasteners.

4. The best (only) results will be achieved by heat bending the Acrylic. A terrifying experience the first time you do it. It requires a large blow lamp the type used to melt tar on roofs, 2ft flames. Continuously and evenly heat the material, constantly turning it over and over until it goes limp, like a piece of leather. A sizeable pane will take up to 20 mins to reach this stage. If you over cook it it will bubble or if you under cook it, it will not form evenly and result in optical distortions, so practice on a few pieces first.

5. Create a curved mould using your existing windows. You need to do this before heating the final pieces "obvioulsy". This is a whole other process and must take into account the thickness of the windows so you can't use the windows themselves as moulds only as a template to make moulds. 4mm Melamine faced MDF board over battens I found works best. Yours look quite simple.

6. When heated the "over" sized Acrylic should be laid over the mould and clamped using battens outside of the window area.

7. Create a paper template from the outside of the old window to use on the newly formed piece. Use blue masking tape on the new cut lines before marking with felt marker.

8. When fully cooled (up to 12 hours) the piece should be released and cut to shape using a diamond coated jigsaw blade at medium speed. Leave the masking tape in place to prevent any chipping. Remove the tape once cut. The jigsaw should have a clean plastic over plate.

9. Mask the inside of the piece so that the joining area can be abraded using 200 grit.

10. Make sure everything is spotlessly clean and dust free, methylated spirit will not damage Acrylic.

11. Mask the reveal of the frame allowing for a neat fillet.

12. Apply an even spread of silicone to both the frame and the abraded section of the window and fit. Do not apply too much force so sufficient silicone thickness remains to allow expansion movement. Clamp or use water bags or sand bags hung from halyards to maintain pressure over night. Make sure the pressure is evenly spread.

13. Skim the excess silicone on the inside using a polyethylene joint finisher. This should leave a nice fillet between the masking tape on the inside of the pane and the frame reveal.

14. Remove the masking tape. Clean up using DRY paper towel.

15. It takes up to 7 days for these silicone products to reach full cure.

16. Once cured fill any joins using foam bedding beads to ensure only 2 sided adhesion. Finish with joint finisher. Clean up using DRY paper towel.

[iansan5653]

Thanks everyone, there's some really great information here. I feel much more comfortable tackling this now. I'll try and share updates when I get it done.

[danstanford]

Don't have any stress introduced by things like square inside corners or cuts into the ende from the saw. All this needs to be scraped or routed away or stress and resultant cracks will occur as the material expands and contracts from temperature changes. Polishing the edges using a torch can add damaging impurities into the edge of the material so use oxy-hydrogen mix if available.
Finally, you home oven can be much easier and less dangerous than a torch if you can fit the part into it. Heat slowly till it flexes easily then pay it over a form so it will have the desired shape as it cools.

As noted above, cast is much better for this job.

[Artey]

3rded - use a router with a trim bit/bearing bit, trim excess ahead of time with a jig or circular saw (fine toothed), and make sure it very well supported and will not move from its template once you begin cutting or handling the piece during cutting to keep it supported. I used wood working clamps and saw horses with a sheet of play lay between them. Another note - Goggles and mask! The router makes a nasty dust and shavings out of the acrylic!

[Jim Cate]

Final hint: if you need to heat the plexi, don't use a torch of any sort. The chances of a terminal local hot spot ruining the piece are too great. A set of infra red heat lamps is a much safer means of heating evenly and slowly. For short radius bends electric heating tapes do a good job of localized heating. And as Ann said upthread, flooding the cutting area with kerosene will improve quality of the cut.

Good luck...

Jim