Thermoforming A Curved Acrylic Hatch

[St. Elsewhere]

The OP's equipment looks like something our company used except it also included a hood that was the size of the table full of heat lamps that had temperature sensors that controlled the heat closely. Compound curved parts were formed with molds under a silicon rubber blanket in a frame as shown in the OP's picture, and regulated vacuum pressure, with closely regulated heat. It was a very expensive piece of equipment that required some learning curve to get it right. Not at all well suited to one off parts, but reasonably effective for doing them in the low thousands.

[Sailorman Ed]

Delancy - I need 4 curved ( cast acrylic preferred) windows, 3/8" thick about a 10" R. Too big to fit in a kitchen oven. Prior experience is that if you heat too quickly, you get internal bubbles ( moisture). I could use the old windows as a mold. Possible to do? HOw to heat? All the others are flat.
Ed

[SV Siren]

Originally Posted by Sailorman Ed

Delancy - I need 4 curved ( cast acrylic preferred) windows, 3/8" thick about a 10" R. Too big to fit in a kitchen oven. Prior experience is that if you heat too quickly, you get internal bubbles ( moisture). I could use the old windows as a mold. Possible to do? HOw to heat? All the others are flat.
Ed

How big are we talking? Window panel size specifically?? I did thermoforming for 10 years, producing literally millions of parts...also R&D, prototyping and tool design...can help with input. We heated ABS, Polycarb, PET, you name it, we would heat polycarb up to 360 in as little time as 2 minutes (using IR, and cal rods, top and bottom heating) but that is the top end of the range, as it begins to really sag if only supported on the outside edges, less so for thicker sheets. The sheets once heated and cooled over a mold will shrink considerably, so make them oversize, I would suggest on the longer straight edge, and not the curve. I could go on but it's late..hit me up if you need and more suggestions...

[Delancey]

My thoughts are 3/8"? Is that the original thickness or are you trying to beef it up? If trying to beef it up I suggest you reconsider and match existing instead. It lasted this long right?

Next is are you sure you can't cold form the curve? Doesn't look compound and cold forming will get you better optical qualities than messing around with thermoforming.

Lastly where? Not sure where you are located by suggest you try to find someone with the equipment required since those are big pieces to try a junkyard oven. Try sign fabricators or people who do Corian fabrications. Most of those guys would be interested in your project just because it's off the wall and might be willing to you out just for fun if you bring them beer.

[JPA Cate]

If you build a plywood (door skin and some braces) form, for the panels, we hired a local pizza parlor's pizza oven after hours, once, to form dodger panels for our hard dodger, but we had the forms from the original ones. They were able to control the temperature, and we did not get many bubbles in the acrylic. As it flows, gravity will affect it a bit, so finished, some parts will be invisibly thicker than others.

Ann

[a64pilot]

We make aircraft Windows. We do have a form but it's covered with pool table felt. We hang the sheets in an oven until they soften then drape them over the felt, seeing as how you have the old windows, maybe you could cover them in felt and use them to mold with.


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[JPA Cate]

I didn't know about the felt--that's a great idea. Thanks.

Ann

[Delancey]

Originally Posted by Ann T. Cate

If you build a plywood (door skin and some braces) form, for the panels, we hired a local pizza parlor's pizza oven after hours, once, to form dodger panels for our hard dodger, but we had the forms from the original ones. They were able to control the temperature, and we did not get many bubbles in the acrylic. As it flows, gravity will affect it a bit, so finished, some parts will be invisibly thicker than others.

Ann

Please please do not follow Ann's advice and use a pizzeria oven or the oven at your home. Thermal degradation of acrylic can produce methyl methacrylate, ethyl acrylate, and acrolien. These are POISONS!!!

When I say junkyard oven I do not mean one you would eat from. I thought that was obvious.

[jamhass]

Originally Posted by Delancey

Please please do not follow Ann's advice and use a pizzeria oven or the oven at your home. Thermal degradation of acrylic can produce methyl methacrylate, ethyl acrylate, and acrolien. These are POISONS!!!

When I say junkyard oven I do not mean one you would eat from. I thought that was obvious.

methinks the comment about controlling the temperature takes care of the concern.

[Delancey]

You pays your money and you takes your choice. - Mark Twain

[Jim Holland]

Originally Posted by captainwireman

I plan on a no fastener method. Sorry I do not have the 3M specific product recommendations handy but I will use 2 side tape (vhb?) with adhesive sealant. The plexiglass manufacturer recommended no holes drilled or use of screws if possible. Sounds like you feel there will be more than enough flex. I have a sample of the tape and it is very strong. Do you agree?

I know that this thread has been inactive for four years so apologies to all if posting now is considered not good form. However, I am really curious how this project turned out, and if it was possible to cold bend the acrylic using vhb tape, how it was done, and whether the vhb tape held it ok.

I've just rebuilt an old teak hatch, and having spent a fair amount of time removing the huge gaps from the mortised mitre joints, don't want to screw it up now. A few years ago I rebuilt a similar hatch and fastened 3/8" acrylic with screws and finishing washers, and while the acrylic bent ok, and it looks fine, I vaguely recall a fair amount of force needed to be exerted torquing the fasteners to get it to bend.

Having just found this thread and learning of the existence of vhb tape, I'm keen to try eliminating the fasteners. The dimensions for the acrylic are 19.25" x 19.25" x 3/8" thick. The bend is in one axis only and fairly slight, I've calculated the radius on the bend at approx 190" ([H/2+W sqrd/(8xH)] where H and W are height and width of the arc) which comfortably exceeds the minimum cold bend radius (330 x thickness = 123.75").

As the hatch is wood I'm planning on putting a layer of epoxy on the ledge and letting it cure before applying the vhb tape (the multiple existing holes in the shelf ledge have all been filled with epoxy). As far as applying the acrylic to the tape, I understand it has to be done right first time, and am wondering if I'll need to clamp the acrylic in place using either a form or possibly battens and multiple clamps, while the tape cures? Once it has, the plan would be to fill the perimeter gap with Dow Corning 795.

If anyone has experience / insight about the best way to cold bend acrylic when using vhb tape, or any other potential snafus that may have been overlooked, I'd appreciate hearing from you.

[er9]

Originally Posted by Jim Holland

I know that this thread has been inactive for four years so apologies to all if posting now is considered not good form. However, I am really curious how this project turned out, and if it was possible to cold bend the acrylic using vhb tape, how it was done, and whether the vhb tape held it ok.

I've just rebuilt an old teak hatch, and having spent a fair amount of time removing the huge gaps from the mortised mitre joints, don't want to screw it up now. A few years ago I rebuilt a similar hatch and fastened 3/8" acrylic with screws and finishing washers, and while the acrylic bent ok, and it looks fine, I vaguely recall a fair amount of force needed to be exerted torquing the fasteners to get it to bend.

Having just found this thread and learning of the existence of vhb tape, I'm keen to try eliminating the fasteners. The dimensions for the acrylic are 19.25" x 19.25" x 3/8" thick. The bend is in one axis only and fairly slight, I've calculated the radius on the bend at approx 190" ([H/2+W sqrd/(8xH)] where H and W are height and width of the arc) which comfortably exceeds the minimum cold bend radius (330 x thickness = 123.75").

As the hatch is wood I'm planning on putting a layer of epoxy on the ledge and letting it cure before applying the vhb tape (the multiple existing holes in the shelf ledge have all been filled with epoxy). As far as applying the acrylic to the tape, I understand it has to be done right first time, and am wondering if I'll need to clamp the acrylic in place using either a form or possibly battens and multiple clamps, while the tape cures? Once it has, the plan would be to fill the perimeter gap with Dow Corning 795.

If anyone has experience / insight about the best way to cold bend acrylic when using vhb tape, or any other potential snafus that may have been overlooked, I'd appreciate hearing from you.

i dont think it will hold at all. at that radius your acrylic at its ends will have to bend .37" from flat. 3/8" acrylic that small is really stiff. it would be a job even for mechanical fasterners to hold it to shape when cold formed. even with screws it would put a lot of torque on the wood frame. 1/4" might cold form to that radius with tape.

if you have plenty of time, get your acrylic, place it on a flat surface and put a couple spacers under each end maybe an inch or so high so the acrylic floats above the surface about an inch or so. put 20-30 lbs of weight on it (or whatever it takes) so it bends evenly in the shape you desire but get it to over-bend 2-3 times the actual radius you want. if you have a nice warm sunny place to put it that would help and speed up the process. leave it alone for a month or two and when you come back to it it will have cold set itself to the new shape roughly. take your time and set it up carefully so you mimic the shape you want just exaderated. check it every week or so. if you can get it to set close to your radius it would probably work pretty well.

we have to be careful when we store plexi sheets. if we are storing them upright and were not careful and they get a slight bend from leaning at an angle, the bend will set into the sheet permanately. iv'e ruined 1" thick sheets before by being careless when putting them away in a rack at an angle, unsupported. when it came time to cut them up to make furniture, we couldnt use them because they were no longer flat...oops. thats why they ship and store them flat in bulk.

[er9]

sorry calculation error. your ends will have to bend approx .25" from flat. thats still a lot of torque for .375" acrylic. i still dont think it will hold though without pre-bending it.

[SV Siren]

You might want to try and build a jig a little bigger than your acrylic with an ever so "slightly" exaggerated curve to it, and place your jig and glass in a commercial convection oven at 300F. Let it heat up, and then try and bend it down to the form using clamps in the corners using a 2x4 with a towel between the glass and the 2x4, across the ends and let it cool. That should work. Just find a mom and pop type restaurant, and tip them a few bucks.

[Jim Holland]

Many thanks ER and Siren for your advice, very helpful. I do have time (I've put a temporary plywood "hatch" on the boat) so am going to try cold bending. Does it matter if the backing paper paper stays on the acrylic during cold bending? Ideally I'd like to keep it on to reduce the chances of getting sealant smeared all over the acrylic during installation. Although I imagine it would need to be removed for bending with heat, so maybe remove also for cold bending? Thanks again.