How to glue this diffuser?

[Mad ID]

Hi.
I have developed a small LED signalization unit with high brightness LEDs.
For better visibility, I made a laser cut diffuser which fits tightly inside a lid of my case. Picture of the lid with diffuser fitted is in the attachment.

The case will be exposed to direct sunlight and strong vibration. Can you please recommend a method how to glue the diffuser (plexiglass) to the lid (PC)?

Temperature range is -40 to 85C with and need to have some UV stability. The weight of the diffuser is 20 grams.

Super glue is not an option because of its viscosity. It easily goes between the lid and the diffuser which impairs esthetics.

Thanks.

Thank you.

[hlokk]

Is the diffuser clear with a frosted surface or does it have "diffusion throughout"? Usually for plastics, acetone or MEK is used via capillary action. I forget if it works between PC and PMMA (acrylic).

Depending exactly how robust you need it, you could probably use gel type superglue around the outside. Otherwise sikaflex should have an adhesive that will stick to both (although it needs to bond to the PC more)

[Balaur]

Epoxy all the way. I'm reasonably sure that you can find some two-part epoxy in your local hardware store that fits your application.
Else, there's always 3M and their very extensive range of solution and datasheets.

No cyanoacrylate glues of course because of blooming.

[salbayeng]

The preference for this sort of thing is a silicone rubber adhesive.
If you just want a little bead around the edge , then the standard acetic cure silastic like (silastic 732RTV ), e.g. window / aquarium adhesive is best.  It's kind of translucent but looks clear in thin sections. The silicone is undamaged by sunlight and wont harm the plastic.
If you want a full area transparent join then use the two part silicon rubber that the DIY PV panel builders use. (Sylgard 184  or PV 6010 )

I would avoid any kind of solvent cure adhesive. It is possible to get some really good joints using specialty solvent adhesives, (e.g. acrifix) but can end in disaster unless you know exactly what you are doing.

Also avoid any anerobic adhesives (Loctite) , the vapors from these will cause crazing of the plastic.

Also be careful with cleaning solvents , only use water + trace of detergent or alcohol. Many of the hydrocarbon solvent cleaners will cause the plastic to craze.


Have you considered sandblasting the inside of the lid? It's difficult to get uniform, but saves the hassle of diffusers and gluing.

[wraper]

Don't know how well will work in such case, but maybe LOCA. Apply, clean residues and then expose to UV.

[hlokk]

Epoxy all the way. I'm reasonably sure that you can find some two-part epoxy in your local hardware store that fits your application.

Loctite make a "mix in the nozzle" epoxy that I used a few days ago for ABS and worked well. I think they have a specific formula for plastic too? Wont work on Polyethylene or Teflon though (maybe not polypropylene) but should work for those two. Rough up the polycarbonate with a little bit of sandpaper first.

Also, pretty much all 5 min epoxies will yellow eventually (not sure if it has to be in UV or just age) but some 24hr epoxies wont. Though yellowing after a year on the inside and not in the main light path may not matter.

[Precipice]

Not a glue - but can you just heat stake / weld it?
Kinda shuffle a bead of the lid down so it clamps the diffuser, in, say, 10 places?

How sure are you there'll be no condensation in the gap? That could get to look quite crappy after a while, which would be a shame after you've gone to this effort!

[SirNick]

Are you building a 100x scale SMD PCB?  Is this a PLCC package?  That would be cool.  (Sorry, I have nothing actually helpful to say.)

[Mad ID]

The preference for this sort of thing is a silicone rubber adhesive.
If you just want a little bead around the edge , then the standard acetic cure silastic like (silastic 732RTV ), e.g. window / aquarium adhesive is best.  It's kind of translucent but looks clear in thin sections. The silicone is undamaged by sunlight and wont harm the plastic.

Have you considered sandblasting the inside of the lid? It's difficult to get uniform, but saves the hassle of diffusers and gluing.

Hi. You have been very helpful. I ordered this adhesive and will try in a couple of days when it arrives.
http://uk.farnell.com/dow-corning/732-clear-90ml/rtv-silicone-732-clear-90ml/dp/2289628

Sandblasting is a good idea but laser cut PMMA with some adhesive is very cheap if I pull it off nicely.

How sure are you there'll be no condensation in the gap? That could get to look quite crappy after a while, which would be a shame after you've gone to this effort!

I'm not, I have a nice IP65 case with from Bopla and hope there won't be any condensation plus the LEDs are high power and there will be some temperature rise inside. 

A lot of thanks to all of you, I will try all the options until one is good enough Cheers

[Jeroen3]

Have you considered sandblasting the inside of the lid? It's difficult to get uniform, but saves the hassle of diffusers and gluing.

Sand is too rough, use glass pearls, this makes it a smooth and even matte spread.

[Fraser]

Where mechanical stability is important I would use either a narrow plastic retainer frame that holds the diffuser in place and avoids contamination of the diffuser when glued to the enclosure with RTV. As an alternative to glue or RTV I would also consider the use of formed piano wire retainer that clips into small slots cut into the sides of the case. Such a clip requires no adhesive and is mechanically stable.

Aurora

[dannyf]

What about hot glue?

[wraper]

What about hot glue?

Are you serious or joking? It will fall off after a few months and almost instantly at low temperatures. And that without vibration.

[Towger]

Otherwise sikaflex should have an adhesive that will stick to both (although it needs to bond to the PC more)

Sikaflex is my vote, it is often used for just gluing windows in place on yachts and cars etc.