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I'm not sure if this is the right forum but seems more relevant to my question.

I'm building a rear-projected screen in a form of a large table (3m x 2m) with frosted/diffused acrylic or polycarbonate sheets as a tabletop. The ultra-short throw projector below the tabletop will project an image on the sheet. The table will be holding max 20-30kg equally distributed weight.

What would be the best options to secure the tabletop sheet and prevent it from bending? Any special kind of mounting tools or screws?

I can't have solid structures supporting the sheet from below as it would cast shadows from the projector. Is getting a very thick sheet the only option?

  • can you have a frame at the edge of the table? – Jasen Nov 30 '20 at 10:50
  • @Jasen, I could - every edge is 10 cm wide. But would 10cm from both sides enough to hold 300cm width? – Arturs Vancans Nov 30 '20 at 11:35
  • Some idea of your plans and dimensions seem important here. Your question is quite vague as it is. – isherwood Nov 30 '20 at 13:15
  • it would be much better than just bolting legs to the corners of a slab would., – Jasen Nov 30 '20 at 18:41
  • Does your table/screen need to be flat? I would think that some kind convex installation could be much more rigid, although your projector may not focus on such surface properly. – Arvo Dec 1 '20 at 15:43
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Consider tempered glass instead.

While using glass brings a real fear of breakage, thick glass actually doesn't break so easily, and it is quite strong. 2m x 3m is on the large size. Typical glass table tops that I remember seeing are more in the (at most) 1m x 2m size. I did a quick search and found a local (to me) vendor - local is important because picking it up yourself or getting a local truck delivery will be a lot cheaper than sending long distance - where this size is in the $1,000 range. From the same vendor, a table top 1/4 the size is ~ 1/8 the cost and 1/4" thick instead of 3/8" thick. So clearly this is a viable option, but whether it is practical or not will depend on your budget and other factors.

Glass also tends to not get scratched up as easily as most plastic, which may be a big advantage for a display medium.

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    I was considering glass, however, due to it being brittle, it would be safer to subdivide the area in maybe 3-4 tiles. Easier and cheaper to replace the broken tiles. But then it means I would need some kind of support mechanism from bellow. – Arturs Vancans Dec 1 '20 at 5:50
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I would recommend putting a solid wood edging around your table top. (Of course, you could use metal if you have the fabrication skills or willingness to outsource.) You want a dado for it to slip into to provide not just support from below, but also support on top to prevent the edges from bowing up from the weight.

Additionally, I'd suggest a thick piece. The thicker it is, the less it's going to bend. A thicker piece may impact light transmission and clarity, so you may have to make a compromise. You also have to consider that a piece of acrylic that is 2 x 3 meters (~ 6 x 9 feet for those on this side of the pond) will do some bending on its own if it's not thick enough! That's a heavy piece of plastic and will weigh a fair bit on its own.

Finally, contact the manufacturer to ask them for specs on the product you're looking to purchase. If anyone knows how much weight it can support, they will. They'll probably have something that will tell you how much weight per meter of span it can safely take before bending. They'll probably also have data on light transmission at each thickness so you can factor that in, as well.

One last thought for you - this may not be the best time to design something out of acrylic. With all the stupid fear about the pandemic, stores here in the US are wrapping all the cashier stations in it as a barrier so it's in short supply and prices have skyrocketed. As expensive as this piece is going to be, it'll be even more so now.

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    I suggest you replace "all the fear about this stupid virus"' with something a bit more neutral like "with the pandemic". As is, you kind of come across as a pandemic denier and that's likely to start some controversy that's not relevant. – Kat Nov 30 '20 at 23:29
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    @Kat I think FreeMan comes across more as a normal human being frustrated at how one tiny little - but very dangerous, even deadly, for many people - virus can disrupt the world to such a degree. It is "stupid" in the sense of "viruses don't have brains, not even neurons" and "stupid" in the sense of the effects on humanity in so many ways. That doesn't deny its existence. – manassehkatz-Moving 2 Codidact Nov 30 '20 at 23:50
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    @manassehkatz-Moving2Codidact it could be read either way IMO, which is why I said "kind of". It's not denying the existence of anything but could be read as denying the severity of it. It's more the "fear" than the "stupid" that suggests that to me. But maybe that's just me. No offense meant FreeMan if that wasn't your intent! – Kat Dec 1 '20 at 0:22
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One option would be air pressure under the sheet supporting its weight, air won't block the light from your projector, this would reqiuire enclosing some of the space under the table and providing a window for the projector to shine through. if there's any leaks the air will need replenishing.

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    That's a creative idea! But also seems like a potential source of troubles. If a leak occurs and pressure drops fast beyond the critical point, the sheets bend and create a chain reaction of other issues. – Arturs Vancans Dec 1 '20 at 5:32
  • I like this - you don't have to seal the table, just control the rate at which air enters and exits so it has a positive pressure. This would help with cooling the projector too. Downside, any forced air will have noise too. I'm sure someone smarter than me can figure out how many cubic feet per minute would be needed. – Criggie Dec 1 '20 at 8:36
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You can use a layered option. Frosted acrylic or polycarbonate tends to be expensive, so you can get a thin sheet of that, maybe 1/4" or 1/8", then get a clear support piece the same size but thicker as your base. You don't need to glue them together, or anything, just lay the frosted piece over the clear. In fact, I've done a fair bit of gluing/cementing plastics together on their large faces. It's really difficult to avoid bubbles even with small faces of less than 6", especially when using a solvent based cement, since you only have a short time to remove the bubbles, so I'd suggest against gluing them together.

Side note, acrylic will scratch more easily than polycarb, so I'd suggest getting the top piece as polycarb, even if you use acrylic as the support material. Heck, you could use a piece of glass as support, but then you still risk breakage.

You would also want to make sure you are supporting the full perimeter of the frame. Plastics tend to be less rigid than glass, but that's also what helps prevent plastics from breaking.

With plastics, you can also drill holes through the sheets and use screws or bolts to try to put tension into the sheet to help prevent sag by attempting to stretch the sheet. That's not easy, and I haven't tried it, and doing it too much could cause the holes to crack. It could also cause stress interference in the sheet or cause the sheet to warp. It might not be worth this approach, now that I think about it. But having the sheets held down tightly and more than just sitting there will help prevent sag.

I'm from the US, so getting a piece of acrylic or polycarb larger than 4'x8' (1.2192m x 2.4384m) would be a special order. I'm assuming you already have a supplier that can go that large? If you haven't, I found at least one by searching "acrylic sheet sizes uk", or whatever country you are from.

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You might try gluing thick or vertical pieces of acrylic to the edges of your sheet to stiffen it, with solvent-based glue.

  • that will distort the image. but it might be mostly fixable in software. – Jasen Dec 2 '20 at 3:27
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Add a mesh/grid of thin tensioned horizontal wires below the sheet - your frame will need to be strong enough to resist the tension.

Something like guitar wire, or perhaps clear fishing line will have minimal impact on the image projected - though do a test before committing.

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    I'd have to test that but putting anything under the sheet might cast shadows from the projector. – Arturs Vancans Dec 1 '20 at 5:30
  • @ArtursVancans yes it might - depends on the resolution / quality of the image, and whether the projected image has big or small pixels. Also, you'd want a way to add tension somehow - everyting sags with age. – Criggie Dec 1 '20 at 8:35
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    Yes it will. Remember CRTs? They had two lines in them. People didn't care (well, mostly didn't). They weren't objectionable and got ignored after a few seconds once your brain worked out to ignore them. – J.Hirsch Dec 1 '20 at 14:28
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    @J.Hirsch Quality CRT Trinitron tubes are an excellent example - they had one (or more if you were rich!) stabilising wire that ran horizontally, and were easily visible if you looked on a plain colour. One didn't notice them in a moving scene, or in a complex still image. However those were stabilising a mesh of hundreds of vertical wires, that noone ever noticed because they were more numerous and "between" pixels. I think a test run / trial is in order. – Criggie Dec 1 '20 at 21:47
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You can use something thick and rigid enough to not noticeably sag (like glass or thick plastic). But you can also accomplish it with relatively thin material. The material will bend but needs to be stiff so as to not sag in random places. Giving it an intentional slight curve in one direction will prevent it from bending in the other direction.

Make the frame just slightly narrower than the narrow dimension of the plastic sheet (making it adjustable will allow experimentation to optimize it). Give the sheet an ever-so-slight upward arch across the short dimension. On a surface that size, the arch will barely be noticeable and won't distort what's projected onto it. The weight of the sheet will try to flatten the arch, but if the long edges are constrained so they have no place to go, it will maintain a close to imperceptible upward arch. You can reinforce the shape by supporting the sheet with slightly curved or stepped sides on the short ends.

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