Anyone who has built a top hat separator, or just read the threads by people who have, know that the most challenging part of the construction is the side wall. Many different materials have been used, and each seems to have its own set of problems. And of course, the real challenge lies with securing and sealing the wall material.
I chose plexiglass for my sidewall, and like others who have used it, I cracked mine while securing it with screws, and that was in spite of taking extreme care. So, I started to think about how sidewalls could be secured without mechanical fasteners that project into the airstream, or dados that create a ledge at the drop slot, or messy glues that make an otherwise neat construction look sloppy.
At this point my proposed solution is merely conceptual and it needs to be developed and tested further, but it builds on the design used for my first separator and shared in the thread titled: " 5", Rectangular Inlet, Bellmouth outlet, Top Hat Thien Separator".
Here is a revised sketch of my original design showing the needed changes. First, the plexiglass retaining rings now show a 1/8?x3/16? dado that I added during construction of my separator. But, new to this design are 1/8? radially drilled holes that intersect with the dado. The drilled holes will be used to inject adhesive from the outside, and the spacing of the holes will be determined by the viscosity of the adhesive.
(https://www.dropbox.com/s/fz1wlnrjsg92gtw/Separator%20Concept.jpg?raw=1)
The following photo was taken during the construction of my separator. It shows the dado being cut with a slot cutter. In my build, this dado was filled with rope caulk prior to installing the plexiglass. However, in my new concept no rope caulk will be used. Instead the plexiglass will be installed over the open dado, but it must fit tightly or be held temporarily by clamps.
(https://www.dropbox.com/s/qu44asubhqw00nh/Separator-40.jpg?raw=1)
Once the plexiglass is securely in place, the adhesive is injected from the outside through the radial holes. Here is a video showing how that might work. Sorry about the video camera not being able to hold focus.
VIDEO
https://www.dropbox.com/s/ug17hdaqsyw4tkp/Separator%20Concept.MPG?dl=0
Retired, please do not take this the wrong way, but I do believe this is a case of unnecessary over engineering. The acrylic will bend but you have to treat this stuff like it is real glass. You cannot torque your screws in and you can have to soft clamp it one section at a time as you go. It took me 2 tries until I got it on the third that by pre-drilling and countersinking my holes before installation then using clamps in that manner, I used a vix bit and then sank the screws in by hand VERY slowly and careful not to apply pressure to the plastic, just seat the screw into the slot. By adding an adhesive, if a chunk of material comes flying through there and damages the acrylic, then you are kind of screwed on just opening it up and replacing it. I will try and get some photos up here this afternoon to show you.
Best of luck on this, you have really put in a lot of work on your build.
Don
i think if you were to use a popsicle stick (or whatever) to just lift the plastic from the frame a small bead of silicone could be applied. im gonna go with lexan since i think it will stay clearer longer than the plexi will since its in an abrasive environment.
i picked up a scrap piece (1/8 x 6 x 72) for $12 at a glass and mirror store
Quote from: Don_Z on September 04, 2011, 08:31:37 AM
Retired, please do not take this the wrong way, but I do believe this is a case of unnecessary over engineering. The acrylic will bend but you have to treat this stuff like it is real glass. You cannot torque your screws in and you can have to soft clamp it one section at a time as you go. It took me 2 tries until I got it on the third that by pre-drilling and countersinking my holes before installation then using clamps in that manner, I used a vix bit and then sank the screws in by hand VERY slowly and careful not to apply pressure to the plastic, just seat the screw into the slot. By adding an adhesive, if a chunk of material comes flying through there and damages the acrylic, then you are kind of screwed on just opening it up and replacing it. I will try and get some photos up here this afternoon to show you.
Best of luck on this, you have really put in a lot of work on your build.
Don
Don, I know you've been successful, but believe me I put my screws in very gingerly. I even drill slightly oversized holes, and I tested it in waste pieces several times. When I tried countersinking, the plexiglass broke before I even put in the screws. Something very strange happens to plexiglass once it's integrity is compromised. Are you sure you have acrylic, or could it be polycarbonate?
You did find one fault with this proposal, i.e. it might be a little difficult to replace a piece of broken acrylic after the fact. I would never wish any of us bad luck, but I would say the odds are high that the acrylic could break after the fact. No so much because of a hard impact, but because something causes it to flex, and the least amount of flexing could cause a drilled hold to send out a hairline crack.
Quote from: dabullseye on September 04, 2011, 08:40:32 AM
i think if you were to use a popsicle stick (or whatever) to just lift the plastic from the frame a small bead of silicone could be applied. im gonna go with lexan since i think it will stay clearer longer than the plexi will since its in an abrasive environment.
i picked up a scrap piece (1/8 x 6 x 72) for $12 at a glass and mirror store
I purchased my acrylic at a glass and mirror store also. I paid about $14 for a 1/8 x 6 x 72. His price for polycarbonate (don't know if it was the Lexan brand name) was about $25. I told him what I was doing, and he recommended the acrylic. Now I don't remember why. However, if I had it to do over again, I would go with polycarbonate for sure.
I got the Optix brand at the home depot .096" for $23.00. What kind of countersink are you using?
Okay, I just got a closer look at the way you screwed your plexi onto the ribcage and I believe I see the source of your cracking problem. Your screws are too far apart causing the material to be allowed to flex. I used 4 screws set 1 1/4" at the ends and from there I spaced them out at 2 1/4". I started by using a 1/16th bit for my initial holes with a spoilboard under the plexi for support and then followed with a #8 countersink with the tapered bit set back to where only 1/4" was above the countersink still using the spoilboard. I had used 1/2" MDF for my build so the holes were placed @ 1/4" from the edge of the plexi. From there it was just a matter of clamping top and bottom at the same time making sure that the plastic was snug against the rib cage before setting the pilots and screwing them in by hand. Also when starting at the leading end and when you get the the final end make sure you have plenty of clamps to get the plastic snug against the ribs until it begins to follow the form without stress. Pain in the butt? You bet, but the payoff is well worth it.
I think my countersink, which was not used in my final installation, is a 5 fluted 82 degree bit. It's a 40 year old Craftsman.
The spacing of the screws may or may not be an issue because my plexiglass was really jammed tightly against the retaining rings before I inserted the screws. There was no gap anywhere, in fact, I had to work like the devil to push the two captive ends into place. I think a bigger difference might be the thickness of the sheet.
Whatever the reasons, I would never use a material again that is this fragile or requires such care during construction. If I wasn't testing the bellmouth idea and wanting to see what is going on inside the separator, I probably would not have gone with a clear material at all. I think galvanized sheet metal is the way to go. It's slick and would have very little frictional losses.