What type of clearance to give to a channel on an Aluminium extrusion designed to frame glass?

[swiftarrow]

Hi friends! This question might seem very simple, please bear with.

I'm a mechanical engineer just joining the workforce, and need to re-design the product of my company. As part of this, I need to re-design the aluminium frame for a large glass sheet.

The previous design used a 3M tape to stick the glass to one side of an aluminium frame. Now we want to change it, so that the glass slots into a channel in the aluminium frame, and sticks there due to a bead of silicone in the slot (similar to how solar panels are built).

My question: the glass sheet is about 4mm thick... how much clearance should I give in the slot that it must fit into? The Silicone needs to create a weather-tight seal.

Thank you for your replies!

 

[racookpe1978]

Well, until "you" can more precisely define that "About 4 mm thick" dimension of the glass your company is going to be inserting into the aluminum trim around that "about 4 mm thick" glass, I have to recommend that the new aluminum trim be "just a little bit thicker than 4 mm" so it will always fit around glass that is "about 4 mm thick"

8<)

Measure 200-400 pieces of your glass.
Get the mean and standard deviation of the glass thickness.
Decide how many of your alumimun trim pieces you can aford to have a non-fit or sloppy fit with the silicon inside the trim edge (how many rejects you can afford to waste time forcing into a too-tight trim piece)
size your Al trim accordingly
or
Get a new glass supplier who can meet your tolerances better.

 

[brut3]

From the description you gave above it sounds like you're describing the silicone sealant being applied in a non-structural application. Below is a link to Dow Corning's technical manual and recommendations for silicone applications in conditions similar to what you are describing.

http://www.dowcorning.com/content/publishedlit/62-1444A-01.pdf

From the description that you gave above though, it sounds like you may be better served to silicone the glass panel into place once it has been set in the grooved channels of the aluminum extrusions.

 

[racookpe1978]

The glass is heavy: It will attempt to displace any silicon bead trapped between the Al trim on the lower side of the window, and - at the same time - be "pulling down" away from the silicon bead in the upper side of the window. Left and right sides will let the window slide down, or if one edge is tight and the other loose, then the glass can tip slightly.

the Al trim needs to be tight enough to trap the lower half silicon bead so it doesn't squish out around the glass, and tight enough so it doesn't let the top half pull out and slide the whole sheet of glass slide down. (The glass is "floating" on the lower silicon bead like a very viscous fluid, so if the lower half trim is too "loose" then it just slides down and the glass falls out of the surrounding trim over time.)

Too tight, as mentioned in nbr 1 reply, and you can't easily get the trim to fit when you are aasemblying the windows.
Too big a notch, and you use too much silicon filling in the edges around the glass.
Way too big, and windows get air leaks because the silicon doesn't "touch" the glass all around.

I do not recommend that the silicon be "filleted" around the edges of the trim, but rather be injected into the notch in the trim before the glass is inserted in the notch.

 

[dhengr]

Swiftarrow:
Does your phone work? You should talk this over with your/several extrusion suppliers and with several silicone suppliers to get a range of opinions and hone in on a method and materials. Consider the expansion and contraction of the three different materials. Also, discuss the thickness of the silicone caulk which will allow this kind of relative movement without debonding at the glass or the aluminum. The several mm thickness of silicone in the gap will be in shear to accommodate this movement, and this shearing action should not be high enough to cause a bond failure. Also, many times the glass panel is supported on a number of high durometer rubber blocks in the channel of the lower frame piece, then the silicone just fills the void and makes it water tight. How are the corners of the aluminum frame going to be joined? How do you assemble the frame around the glass panel, and what size is the glass panel? Do you ever disassemble these frame? Then some rubber gasket system btwn. the glass and the alum frame might be better.

 

[swiftarrow]

Thank you all for your answers. I think I got what I was hoping for, which was a better understanding of the variables involved.

To reply:
@racookpe1978, a comprehensive statistical evaluation is definitely the right way, but multiple pressures require less precise approaches.

@dhengr, my phone does work. Before talking with suppliers, I wanted to understand the scope of the problem better, which yours and other responses here has helped alot.

This glass is actually mounted horizontally, and currently is adhered to the aluminium frame with two-sided 3M tape (super strong stuff, mounted in tension on the upper side of the glass) and slightly compresses a rubber seal underneath it. Given the feedback about different expansion coefficients, maybe this is the best way to do it.

The problem is that we want to change the frame from four pieces of aluminium to use two pieces of aluminium and two pieces of transparent plastic. Along the way, changing the frame to hold the glass in a slot seemed to be advantageous from the plastic piece's point of view. Perhaps it's not... I need to re-visit the question with my colleagues tomorrow.

@tbruton3, great link! Thanks for that!

 

[swiftarrow]

Hi friends!

Well, after quite a few hours of drafting and testing, we have come to the conclusion that just for structural integrity, we will need to put the glass into a slot in the AL frame.

Therefore, I have to resurrect this thread, as I'm back to square 1. However, I do have more information to share... just a bit.

Attached is a file with a rough drafting of the scenario and the dimensions that I would like your recommendations for.

The length of this will be 2235mm. We'll use whatever silicone which is right for the job.

Thanks for your help!

 

[dhengr]

Swiftarrow:
You young guys are all supposed to be expert at the use of CAD programs, why the heck not show the whole glass/frame system, in its correct orientation and with some dimensions, correct proportions, loads, etc. This would take an extra 5 minutes, and much better represent the real world conditions. This properly proportioned picture of your problem, with sufficient design info., means a lot to an experienced engineer and his first impressions and opinions of the actual problem. I doubt that you have no other loads or stresses on this system (a glass partition, railing or some such, you must explain) because all of these conditions must finally be combined for a final design. This is not a college text book problem where proportions and other loadings have no impact on the basic concept which is being explained in that particular problem. At the moment, your sketch shows a plate of 4mm glass laying in a horiz. position, and not much else, it is cantilevered out of the alum. frame piece. Then it implies that the alum. frame is only about 1mm thick (unlikely) and about 9mm high, and the silicon caulk gap is only about 1.5mm thick (again, unlikely). You show no means of supporting the weight of the glass or adjusting for irregularities in the framing (rubber blokes). This glass plate is about 7.25' long, and how high? What framing on other three edges, and then what attachment of the frame to real structure? These will all influence the stresses on the glass, its framing, and the gasketing/sealing. You have to start out looking at the big picture and then hone in on the details. And, you have to work all of these things in some sort of parallel tracks, every step backing up a bit and asking how does this influence the next step and all the other elements of the design.

 

[swiftarrow]

Dear Dhengr,

Thanks for your reply! I was not sure if I should provide the full drawings or not, hence tried to "generalize" my question.

I'll try to make a good drawing of the current application at work on Monday. In the mean time, I can try to explain it:
The glass is horizontally mounted. It is 4mm thick, 2.4m long, and 1.2m wide, and framed on all four sides. It is a specially hardened glass for PV applications, and is supposed to withstand hail and other impacts to it's surface. (Interestingly, we have found that it does not withstand any impact to the edge, but is quite resiliant on surface impacts.)

Our current mounting system uses double-sided tape on the upper side of the glass to tape it to an aluminium frame (not in a channel). This frame is then put onto a PVT collector, mounted in such a way that the glass rests on three longitudinal rubber strips (one on either side and one down the middle), similar to the weatherstripping used on doors and windows. When the assembly is tightened, the frame pulls the glass down onto the rubber strips, creating a seal.

The aluminium frame is only 1mm thick. At some points it is up to 1.5mm thick, but not more (only due to a design on the extrusion). However, it is not cantilevered; there is a symmetric support on the opposite side of the glass.

We want to replace this double-sided tape with an Al channel and Silica glue. This will be akin to what is done with PV panels today.

Thanks again!
Link

 

[swiftarrow]

Update: I got some feedback from the PV Panel industry: they use 1.2mm clearance at the root of the channel, and 1.25mm clearance on both sides of the glass in the channel.

Our glass is thicker and the span is longer, so I'm not convinced that these numbers will work in my application. But it's good to have something to go by. Drawings tomorrow

 

[MikeHalloran]

Before you get all lathered up about RTV, take some time to mock up your future production line in the lab, assemble some real glass panels with some real aluminum channels, and develop answers to the questions that will arise, e.g.:

- How do you control location of the channel to the glass?
The auto industry used to use short rubber blocks, sometimes L-shaped in section, to guide windshield glass onto a bead of RTV. Now, I think they use robots to apply urethane adhesive to the glass, and then to slap it in place. ... but their adhesive sets fast.

- How do you maintain dimensions over the RTV until the RTV sets? I hate VHB tape, but it does have the virtue of instant adhesion.

- How does the channel shape impact assembly time?
The U shape implies radial assembly of the frame around the glass, which has to be slower than just placing something atop a flat sheet of glass.

- If you can't handle the RTV assemblies right away, how much time elapses until you can, and where do you store all that immobile work in process?

- Can you, personally, assemble the proposed assembly, without making a mess? If you can't do it in your conference room without fouling the rug, etc., then your factory will be a sh_thole in short order.


Your work on this "simple" assembly is only just beginning, and you can't do all of it at a CAD station.

Be careful about what you do. I've executed a lot of product changes because somebody "wanted" the change, only to find that the change didn't solve the problem at hand, or caused a whole lot of new problems.

Be careful how you talk about what you do.
Don't characterize the 'old' design as 'bad', or the 'new' design as 'superior'. Either way, you gore somebody's ox, and you never know who owned the ox until the damage is done.

Mike Halloran
Pembroke Pines, FL, USA

 

[swiftarrow]

Mike Halloran,

Thanks a lot for your kind guidance. All good points, and all taken.

Regarding the assembly issues, we are in the solar module industry, and make a PVT module. I've visited a plant which produces flat plate PV panels and seen how they do it: it is a groove as we are proposing to do, and it is radially assembled in a framing machine. I don't exactly know what RTV means, I'm guessing it's silicone glue or something similar?

I'm actually not the driver behind many of the changes that we are making. Our product has a large number of design flaws and assembly problems which I've been hired to solve. I am the only Mech Engg here, and am "in charge" of the whole thing, but I'm taking input from everyone in the company: the production guys, the assembly floor guys, etc. They have more experience with the actual product than I do. Everyone has seen the problems, and have formulated ways to make it better. My role is more to evaluate the proposed solutions from a feasibility standpoint, and implement them if it looks good. I believe the oxen are safely out to pasture, and might even be happy with what is going on. But thank you for your concern!

 

[AELLC]

I want to know who the oxen are and why are they out to pasture.

Oxen = ordinary bulls who have been castrated to eliminate their aggression and make them trainable for work as draft animals.

 

[dhengr]

Swiftarrow:
So then, the light alum. channel is really only glass edge protection, not really a structural support system/element. And this glass rests in an almost horiz. position (or the slope of a roof, but not in a vert. plane) to protect a photovoltaic panel from the elements? Then the glass and alum. frame pieces, as a unit, are clamped down to the rubber strips and PV panel? How are the corners of the 4 alum. frame pieces attached together, around the glass? Do they have to be water resistant (water tight), air tight? You might actually be better off with a rubber gasketing system which is “U” shaped and pinches btwn. the alum. frame pieces and the glass, but allows some differential movement and expansion and contraction. The gasket is (2)(2.4m + 1.2m) = 7.2m+ long, wrapped around the glass, butt spliced on one of the long edges; then the alum. frame pieces are pressed over the gasket mat’l. Reread my first post on this issue, is the 1mm of silicon btwn. the glass and alum. framing thick enough to shear sufficiently to prevent debonding btwn. the glass/alum and silicon? How will you finish and tool the squeezed out silicon? Does silicon creep with temp. or time, so that the bottom 1mm gap will eventually squeeze out?

I’m not trying to pick on you for your sketch and first descriptions. The sketch does show the detail at the glass and alum. frame, and apparently to reasonable scale too, but it completely misrepresents (or misleads on) the rest of your problem. Why do that, if you want a complete and meaningful discussion of your problem? I thought this might be a handrail in-fill panel or some such, btwn. the floor and the handrail; or part of a glass wall. Listen well to those guys out in the shop, they do have some good ideas; they have their hands and heads in it every day. They don’t always express them very well, you sometimes have to tease the real problem out of them with your questions. Sometimes the same question from three slightly different directions. They don’t have all the details worked out, but that’s where you come in, in finessing those final couple steps.

 

[IFRs]

Be sure to evaluate the change in size from thermal - in the dead of winter will the aluminum shrink enough to crack the glass and in the heat of summer will the glass drop out of the aluminum frame. This will help define the depth of the channel. Question - rather than use goop, would it be cleaner, faster and more consistent to use a custom rubber extrusion ( EPDM for instance )?

 

[swiftarrow]

IFRs, thanks for that input!

I wasn't able to get the drawings today at work (too many simultaneous things going on!). I should be able to finish some urgent stuff tomorrow though.

 

[swiftarrow]

Dhengr, Thanks again for your post. I'll talk over the rubber gasketing system with our production manager.

I've finally had a chance to draft the current system that I'm talking about. It's not as simple as I thought! (no surprise there though...)

It's attached as a PDF.

I hope that this explains what our current system looks like. Note a slight correction: this system may not always be mounted horizontally; in most situations it will be mounted at a tilt of about 60 degrees to the horizontal.

Why this has to change:
[ol 1]
[li]It doesn't withstand snow load: the snow manages to compress the rubber list further, stressing the two-sided tape and de-bonding it. While this doesn't directly compromise the sealed area, it is still a problem.[/li]
[li]We are changing the short side of the frame to be a very small aluminium extrusion, essentially less of a frame and more of an edging to the glass (only the short side - the long side shown in the drawings will remain as is). This is necessary for the performance of the collector. Using the 3M tape on the short side with the new frame won't really work well, as that frame spans across a flimsy plastic wall, not an aluminium extrusion, which it can't rest on. So the short frame needs to hold onto both the plastic wall and the glass, and the best way to do that would be with a channel.[/li]
[/ol]

While I would love it if someone was able to look at the draft and say "you need XXX dimensions" (and have the right dimensions!), I can't honestly say I expect that

But if you knowledgable people could tell me what to consider in the calculations, I would be very grateful!

Thanks!!!

 

[MikeHalloran]

Random responses:

RTV means 'room temperature vulcanizing'.
It describes the behavior of some forms of silicone rubber, supplied as a viscous liquid that cures in the presence of moisture, or with a catalyst. It does not generally require heat to cure to full strength.
The cured product is relatively resistant to heat.
When you heat it a lot, as in a kiln, it decomposes, and all that remains is fine white sand, which is silica.

My own experience with double sided foam tape is that it sticks like crazy on the production line, and through shipping, and in any sort of accelerated life test you can dream up, then it spontaneously debonds in service, sometimes after your product warranty expires, but always soon enough to besmirch your product's reputation.

I have used foam tape as a fixturing means for RTV, as in using a narrow stripe of tape instead of a wide one in your current product, with a controlled bead of RTV applied alongside. By the time the tape fails, the RTV has fully cured and can handle a load alone. There are also (polyurethane?) adhesives used on windshields that cure fast enough to use on a production line.

Given a little bonding surface on the short side bulkheads, windshield adhesive could replace the extrusion and the W-list and the tape entirely; just position the assembly face up, run a bead of adhesive down the middle and around the edges, drop the glass in place, and wait a bit. You might need a temporary fixture to keep the glass from sliding to the side, or perhaps to guide the edges. Oh. That leaves the glass edges exposed and subject to impact. Maybe you need an aluminum angle picture frame glued over the glass. That could be the alignment means if the vertical leg is long enough.








Mike Halloran
Pembroke Pines, FL, USA

 

[swiftarrow]

Hi Mike, thanks for your thoughts, and sharing your experience with the double-sided tape... we have unfortunately had a similar experience.

I see what you are saying for the manufacturing, and we have rough designs for a future version which will do it exactly as you have mentioned. I can't get those drawings done soon enough! I think your trick with tape + silicone RTV (thanks for explaining that!) is awesome, and I'll definitely do something similar.

Here, however, we can't glue the glass to the lower portion, as it needs to be servicable; the glass frame needs to lift off if necessary.

So I guess the only way is to radially assemble a frame around the glass with a groove to hold the glass.

Thanks everyone for your continued input. I look forward to more!