Outsider Needs Opinions

[ronbo613]

Hello everyone-I wandered in here while looking for information about cast acrylic, maybe some of you can give me some of your opinions.
While you guys try to keep water in, I try to keep water out. Here's my website, I've attached a small photo below as well. We make these housings for filmakers and photographers, you may have seen some these guy's work on TV, or in surfing or other watersports movies and magazines.
Anyway, I used to make these housings out of Lucite L, but it's almost impossible to find it now. If I can find it, a full sheet costs about $250.USD. Now I've got to decide what I'm going to use instead of the Lucite. Most commonly available are Plexiglas G and CyroGP(AcryliteGP). After checking the technical specs, they look pretty similar. I use 1/4" for most of the work, including routing the edge of the plastic, drilling, tapping and cutting(mostly with a sabre saw). In front of the lens is 1/8" cast acrylic, tubing is cast acrylic. Glue using Weld-On 4.
Since many of you have used these plastic sheets, which of these two brands would you prefer? The machining is very precise so it must be easily machined. Out of the two, which one has the better optical quality? After all, it's going to be made into camera gear. Any other cast acrylic sheet you think I should consider?
Any advice or opinions would be greatly appreciated. Thanks in advance-Ron B

 

[Anonymous]

In an acrylic course that I took that taught how to build things for aquariums, the teacher stated that the following 3 brands were the best for our application (which is basically the same as yours). In order of preference: 1. Spartech Polycast. 2. Acrylite GP. 3. Plexi-glass G.

Lucite L was 4th on his list and he said it was made for the bath and spa industry and he advised against using it, so it sounds like you'll be making a better product with either of the two that you have available.

Oh yeah, :welcome:

Louey

 

[ronbo613]

Thanks for the input Louey-I use 3/8" Plexi for some parts so I'm hoping the 1/4" will be at least as good. I think I may have used some Acrylite in the past, it may have been the extruded stuff.
First hand info is the best. The technical specs can only tell you so much. I like to hear from people who actually work with stuff.
I may have originally picked Lucite because it is very easy to machine and cut, the Plexi G and Acrylite may be harder and not scratch as easily. I want to do the best job I can, so I've got to make a good decision here.
Thanks for the welcome as well.

 

[Anonymous]

I don't remember coming across Spartech, but I never have problem with Acrylite GP. There is very minimium spectral difference between any of the acrylic brand, and I don't think you will have any issue with them. You may have some non-uniformity problem with extruded acrylic, but for cast material, they are flat unless you put stress on it.

As far as machineability, they are pretty much identical.

 

[Acrylics]

Hi Ron,

Any of the 3 major brands of cell cast acrylic will machine a little better than the Lucite L but won't glue *as* easily, the higher mole weight of cell cast acrylic is responsible on both counts. The cell cast material's higher mole wt means it won't melt as bad but also means it's less soluble so doesn't glue *as* easily.

Because of the continuous casting process by which Lucite L sheet is made, it will be more consistent (in thickness) than cell cast acrylics. Of the cell cast acrylics, the Polycast has been more consistent in my experience than the other two, followed by Plex G, and then Cyro GP. Thickness variation in my opinion is about the only downside to using Cyro GP (which tends to run on the thin side) but if a few thousanths of an inch doesn't bother you, it is good material. Like Lucite L, they are all sold in metric thicknesses except Polycast.

For our underwater camera gear, I use Polycast exclusively and would recommend it above the Plex G and Cyro GP materials though it's rare in 1/8" thickness (but still available).

As far as optics go, they are all very similar.

Same with hardness, they are all within a point or on the Rockwell scale so there's no difference insofar as scratching goes.

You can still find Lucite L fairly easily but may have to have it shipped in. Contact Laird Plastics in your area to get it as they've always had it every time I've wanted it.

HTH?
James

 

[ronbo613]

Thanks again everyone for your help. I've been checking around, the place I normally get my plastic from carries Plexi G, so I'm going to give that a shot. It's less than $100/sheet, about half what I was paying for Lucite. For some reason the Polycast seems hard to find around here, but I'm going to look for some cutoffs of Polycast and Acrylite to check them out. I've got to keep working, hopefully the Plex will work until I can figure the very best material for the job.
You know James, as I was checking around, I did find suppliers for Lucite in Portland, but none here in the San Diego area. One would think since we got everything else(and more), there would be a couple sheets of Lucite around.
The 1/8" material is what the lens shoots through, I'll have to run some optical tests to see which plastic would be best. I've noticed the quality of the 1/8" Lucite L has gone down over the past few years, lots of aberrations and chunks of stuff in the plastic. We are experimenting with float glass for this purpose, optical quality is about the same as the 1/8" acrylic and it does not scratch easily.
Thanks again for your information, I'm off to the plastic warehouse.

 

[Anonymous]

I thought you are in South Dakota for some reaons.

Anyway, I am surprised by the impurity you find in the plastic, since I don't remember anything that is not perfectly clear... What spectrum are you running your optical tests on? I assume you are doing regular Vis. There are various type of gloat glass out there, some are more clear than the others, such as iron content (green). In general, acrylic is better than glass in the visible region, but for thin material the optical advantage may not be significant compare to other issues such as scratch resistance, physical distortion due to load/pressure, etc.

What are the "chunks" in the plastic? How big is it? I suppose there are some impurity in there during the process of polymerization, but I always assume they are minimal. I am very interested to know.

For a underwater project that I vaguely involved in, we use acrylic dome to cover the camera. I don't think we every run any optical test on the dome, and you make me feel that we may be a bit careless. I think they are talking about using a type ceramic to replace the acylic, but that will cost a lot more.

 

[ronbo613]

Anyway, I am surprised by the impurity you find in the plastic, since I don't remember anything that is not perfectly clear

Generally, cast acrylic transmits about 95% of the visible spectrum, the 5% not transmitted is on the "blue" end, nearly into the ultraviolet(UV) range. If you are familiar with photography, a filter commonly used to protect a lens is a UV filter, so that works out pretty good. This is especially true for the kind of work we do on the oceans of the world. Blue sky, blue or green water, losing 5% of the blue is no big deal, if anything, it might provide a little "warming" effect. Blues and greens tend to have a "cold" look.
If you look really close, it's possible to see small areas where the plastic may have cooled too quickly or too slowly, leaving a visual aberration, like a clear "swirl". There are often small nicks, probably from mishandling and sometimes there will be a speck of sand or dust that is encased in the plastic, dirt in the mixing/heating apparatus or the mold, or in the case of Lucite, on the rollers of the continuous casting machine. I worked my way through college in an injection molding factory, so I learned a great deal about casting and molding. Guess it paid off(the college did, too).
Most of the time you can see optical imperfections by holding up the plastic to the sun next to a neutral colored wall and look how the light shines through the plastic onto the wall. You'll be able to see "swirls" and imperfections because they will cast shadows and prismatic effects. If you look at a piece of extruded plastic(which is "pulled" through rollers) compared to cast acrylic, you should be able to see the difference.
We test for optics by shooting photos through the material under controlled conditions, then compare them side by side. I have two or three experienced photographer check them carefully in blind tests and see which one is the winner. You could check them with a densitometer, but I like the experienced eye better.
There are many types of good optical glass. Some of it is pretty expensive. We get our float glass from a company that makes glass for telescopes and other optical equipment. Cast acrylic may indeed have a slight optical advantage over even the best glass, but as the plastic becomes scratched by sand suspended in water, while the glass does not scratch, it probably winds up being very close in real world conditions. All really good glass is iron-free(you can tell glass that contains iron if you look at the edge, it is green).
As far as the domes go, the super high end ones are glass, we're talking thousands of dollars. We use domes that are cast with acrylic, then polished like a lens. A couple hundred bucks. They are exposed and tend to scratch and become hazy after a period of time. You can't really use vacu-formed domes for optical purposes.

 

[Anonymous]

Since it seem that you don't need to cement the acrylic, have you ever consider using silicated (or whatever process they call) acrylic? If I remember correctly, the Cyro SR is one such type. The SR stands for sratch resistance. I made an instrument using SR acrylic for a lab a few years ago, but I did not use it to see if the SR process make any difference. I feel that it is no match for the abrasion due to sand and other particles in the real world, but would like to see if you have any experience.

I will talk to the engineer that used the acrylic dome and see if they do any checking on the optical quality. They went a long way on uniformity correction, and other issue with camera, but if they missed the distortion due to the acrylic dome, that will be something they need to be aware of.

 

[ronbo613]

Most of the scratch resistant plastics have a little less optical quality than the standard versions. Also, unless they can sell truckloads of the stuff, local plastic stores won't carry it.
Cast and polished acrylic domes have pretty good quality. If there is any distortion, it will probably be near the edges of the dome. For the best quality, use some kind of black coating inside the housing where the dome is connected to the housing. Roughly 5% of the light reflects inside the plastic, this makes images appear soft.

 

[Anonymous]

I think I order the SR plastic from www.smallparts.com. It is 4X the price of uncoated acrylic, but it was not my money...

Here is one of the camera that my company use with a dome

 

[ronbo613]

It's hard to tell the exact size of that dome, but it looks like a compass dome we use for smaller fisheye lenses, like the Nikkor 16mm. I'm guessing there is a "lip" or flange at the bottom that holds the dome to the housing?
For the best quality, you'd want a dome that fits very close to the lens. The very best situation would be a dome that matches the "roundness" or arc of the lens itself. If you need to use the dome that is in the photo, you can probably get a little better quality by painting the bottom edge of the dome black. This will stop some of the light from bouncing around inside the plastic an off of your reflective housing body.
I'm working on a dome port housing today. I'll take a few photos to show you what we are using.

 

[Anonymous]

It is not very big. About 12 inch or so, since cutting a hole on an existing sub is never a comfortable thing to do, so we keep the damage to minimium.

For whatever reason, it is a hemisphere, not something that matches the lens at all. I guess it is just something they have as a COTS. I will keep your suggestions in mind when we do it the second time.