[luvdunhill]

So, I have a Serpac case that I'm putting a small measurement circuit inside. The front has two largish BNC connectors and a switch between them. It looks that the connectors are a bit too tall for me to cut the plastic front end and expect it not to split. Even if this succeeded, I'm not sure the plastic would hold the weight of the PCB.

So, I had this bright idea! I took two pan head screws and JBWeld'ed them to the bottom of the case so that they would come up through two of the four holes in the PCB. Now, the PCB will work fine if these two middle mounting holes are simply secured to the bottom of the Serpac and tightened as much as possible.

So, I mixed up some JBWeld and dabbed some on to the case, stuck the screws in head first and let it dry. I even put a bit going up the side of the screw. Next morning, I screwed the PCB in and it was wonderful........ for about 3 minutes, then the JBWeld broke off. It seems that it couldn't handle the angle that the force was being applied on the front panel. The spent JBWeld was a thin, plasticy consistency when it came of and then it peeled nicely off the head of the screws.

Any ideas? I guess I could just drill small holes in the back of the case, but it wouldn't look as cool ...

 

[ericj]

epoxy bonds to uneven surfaces best. There's nothing for it to stick to if you're just slathering it on a flat surface.

I'd suggest drilling the case, countersinking slightly, and installing flat-head screws from the outside with a nut on the inside.

 

[tangent]

Serpac's site doesn't say what material their cases are made of. Digi-Key claims they're ABS, which I believe unreservedly for the opaque ones, and provisionally for the clear ones. (I don't have a better idea; it's certainly not polycarbonate.)

J-B Weld's site doesn't have much to say about ABS, but they do warn against using it on polyethylene and polypropylene, which are relatives of ABS. The closest positive bit of advice I could find is that their Water Weld product is probably a better choice.

Another alternative is suggested by this page on Serpac's site. No doubt you don't want to special order 100 of these standoffs, but it does stimulate the little gray cells, no? Their "instant adhesive" is probably cyanoacrylate. I believe it should work as they propose: with plastics, it'll work fine if the surfaces are clean, and you use a very thin, broad layer. Cyanoacrylate is very brittle, so it doesn't lend itself to uses where you need it to act as a filler as well as an adhesive, as in your original screw idea.

Standard epoxies have much the same sort of weakness as cyanoacrylate. You can use them a bit thicker, but they shouldn't be made to flex very much.

If you weren't willing to abandon your idea of bonding screw heads to plastic, I'd suggest some sort of flexible adhesive, like Duco cement.

Another alternative to consider is just put a blob of Blu-Tak underneath the board. This won't stop it from moving, it'll just support it and severely damp its movements.

 

[tangent]

[size=xx-small]duplicate, ignore[/size]

 

[keyid]

Epoxy will work as long as the surface is sanded to make a griping surface. Also bondo will work you also need to sand the area to make a gripping surface. Gloria glue or liquid nails is another option.
I would use bondo or marine epoxy.

Make sure there are no chemical residues before glueing

 

[holland]

I'm not familiar with the Serpac but you can use spacers instead of trying to use and adhesive, like what tangent linked to. You can also use computer standoff spacers and let it float freely as long as you have other attachment points to secure back and forth movement.

Another thing you can do is take a chunk of PVC or ABS, glue that to the case so you can thread a screw into it.

If it definitely is ABS, you can go to the HW store and get ABS cement which will bond to ABS. You should be able to find stuff in the plumbing section. There are items which will even allow bonding of PVC to ABS. They are chemical solvents which "melt" the items together and dissipates entirely in 24 hours leaving just the bonded items. You may be able to use plastic computer standoffs with this. You can buy some ABS pipes and glue a plastic computer stand to it, before you apply it to the case, to see if it will hold.

ABS, PVC, and CPVC are commonly used for plumbing. Just be careful with this, as this becomes permanent (no separating) since the bond between plastics is the joining of the plastics itself.

 

[luvdunhill]

Thanks everyone for the replies. The main issue is that I will not be using the front plate (for now) so I will need something that will hold the PCB fairly well on its own. I think this rules out the BlueTac, computer spacers, and the like.

I'm still mulling over my options though ... this has been very helpful!

 

[amphead]

Devcon, high strength plastic epoxy cures in five minutes and is extremely good at securing parts. It stinks really bad, because of the chemistry neccessary in creating a high strength bond. I am willing to put up with the surf board building smell for those jobs where nothing else works!

 

[swt61]

Quote:

Originally Posted by luvdunhill /img/forum/go_quote.gif Thanks everyone for the replies. The main issue is that I will not be using the front plate (for now) so I will need something that will hold the PCB fairly well on its own. I think this rules out the BlueTac, computer spacers, and the like. I'm still mulling over my options though ... this has been very helpful!

NP-1 is your best bet. You can find it at most home centers. It comes in a caulk tube, and will require a caulking gun. It dries pretty sturdy, but remains somewhat flexible.

http://www.basf-cc.com.au/sealant/seal8.htm