The thing I’m working on has some metal parts glued into a plastic case. The plastic is smooth (the mould is finished to the “Society of Plastics Engineers” 600 paper spec), and the metal is shiny chrome plated brass. I never considered either of those to be especially hard to glue, so I went for a pretty straightforward epoxy, Loctite 5 minute instant mix clear. It’s a decent enough go-to, much like any other random 5 minute clear epoxy. The short curing time is great for the impatient (me) and the clear finish is great because you can be a bit lax about the cleanup, which is especially important given the short curing time.
So, I splurged some on the metal part, shoved it in the hole and wiped off any egregious amount of excess with white spirit. A week later, I was fiddling with the part seeing how strong it was and was kind of surprised/distressed about how easily the joint broke. Like, really far too easily. And incidentally always the same way: the glue detaching from the plastic.
Then it got me thinking. I had a vague recollection that 5 minute epoxies kind of suck and come to think of it, so do clear ones. In a sort of ad-hoc way I think I’d come up with this:
Ed’s rule of glue
Every feature added to glue makes it worse.
Fast curing? worse. Non, tinted transparent? That’ll cost you. Conductive? [sucks air through teeth]. It kind of makes sense, you can’t optimize everything at once.
That’s not to say you should never use that glue, it’s still perfectly fine for a lot of stuff, but slower curing and/or those ones that cure a nasty brown colour will almost certainly perform better. So, anyway, I tried some polyurethane glue I had lying around and it seemed to be a bit better.
Time to experiment!
So, which one to use? So first, the selection. There’s roughly an infinite number of glues out there, give or take a few. Some have wild and wacky properties, but even if you stick to the middle ground of “generally gluing stuff to other stuff”, there’s still a large selection.
So, I picked some somewhat arbitrary criteria and selected some glues that generally fitted:
- Specced to glue metal to plastic, bonus points for having ABS in the list.
- Available from local shops or next day delivery from RS
- Reasonable curing time to handling strength.
- Auto mixer if possible where applicable (better for large volumes).
- Not wildly expensive.
- Decent brands and available in the US (I’d normally be happy with RS pro stuff, but I need to be able to get it abroad easily).
The brands are mostly big, well known ones. 3M is always good. Loctite likewise seems fine. The other one, Araldite, I have a particular soft spot for. For years in the home gamer space it was pretty much synonymous with epoxy. My old DT teacher was a huge fan (rightly so) because you could fix all manner of sins with it. Countless projects were rescued with Araldite at my school.
And so, I came up with the following list:
- Loctite 5 minute instant mix. That’s the baseline, and my gut feeling says that other clear, fast setting epoxies won’t differ wildly.
- Araldite Standard. Bog-standard DIY shop glue. Slow setting (pot time of an hour, high strength in 24), dries kind of opaque yellowish.
- Araldite 2011. One of about a billion varieties. Similar to 2, but dries clear yellow. Allegedly slightly stronger on ABS. Also a “pro” glue, so comes in pro packaging which means you need one of those applicator guns and a clip on mixing nozzle for the auto mixer.
- Loctite 330. It’s an acrylic glue which I didn’t even know was a thing before today. You put glue on one surface an spray the activator on the other surface and put them together. Datasheet specifies 7387 or 7386 activator, but Loctite actually sell it packaged with 7388 of course. Fast cure time (30 mins to half strength) , and almost indefinite pot time, which is handy, since you don’t mix it with the activator. Not much by the way of instructions though.
- Araldite 2028. OK, I lied about not bothering with other fast curing clear epoxies. This one claims excellent performance on ABS, not so much in the datasheet which is not anywhere official I could find, but in this handy selection guide, which has a mysteriously chosen subset of the glues on offer. Also pro packaging with an auto-mixer.
- 3M ScotchWeld DP8005. Now I’ve heard about acrylic adhesives, I’m on a roll. Also RS had a relatively small selection of 3M and I felt things wouldn’t be complete without one. Claims to work well on ABS. Has an annoyingly short pot life (3 minutes and they’re not messing around).
- Everbuild gator glue. Bog standard polyurethane glue (basically the same as Gorilla
I love the logo too!
Glue). Generally considered OK for metal and plastic. Reasonable pot life and cure time. Single part, so no mixing needed. Also expands as it cures. This is not (as is popularly thought) useful for filling gaps since it expands as foam when the pockets of glue get too large. However, it does slightly splurge out so if you miss small areas, they get filled from the nearby ones. If you’re not familiar with British knock off brands, Everbuild is basically an adhesive company which makes cheaper versions of the well known stuff. So Gator glue/Gorilla glue, One Strike Filler/Pollyfilla, Stixall/No More Nails, etc etc. They also sell the delightfully named ASBO brand anti climb paint and anti graffiti painting. That makes me chuckle every time.
- Polycaprolacetone. I remember that sticking to everything really hard when I tried to use it. Applying it as an acetone based slurry, then reflowing it thermally after it’s dried. This is basically me messing around.
White crud on the two snaps on the right hand side. No glue was applied there.
- Cyanoacrylate (generic brand). Another baseline, known to be strong and effective, but outgasses this white crud everywhere which is electrically insulating and has to be cleaned off.
Oh and the surface prep. Did I mention I forgot the surface prep before?
- Clean both surfaces with IPA (isopropyl alcohol).
- The full monty: clean with IPA, sand with 400 grit paper (3M as it happens), then clean again with IPA.
- As-is, with whatever crud and release agent is left on by the moulding processes.
And it’s arrived!
Check out the swag, including a spiffy (and necessary) applicator gun, because as you can see, the pro packages don’t have full syringes. And 3 hours of cleaning, sanding, mixing, gluing and wiping off the excess, I present:
That’s a lot of gluing and there’s more to come.
So now, I need some way to measure the force required to eject the metal bits from a case. Naturally, since I’m doing the whole thing at short notice and in rather a hurry, I had to cobble together something with whatever was lying around or could buy locally. My solution is to go to a local DIY shop and enter a kind of trance until a solution based on the weird selection on offer presents itself. The result was this:
After a few iterations, I settled on pushing down on a set of old kitchen scales held down with cable ties and string. The lever is 4x and the scales go to 5Kg, and the extra weights on the end are 2.5Kg at 5x. Maximum force it can apply is 32.5Kgf.
It’s basically a parallel action mechanism with a long lever. I balanced it to make it neutral and marked graduations on the lever so I can put a weight at a known position to apply a known force. You can’t see the M4 screw on the underside which is embedded into the pressing bit with some brass threaded inserts. I used some 5 minute epoxy (surprise!) to glue them in, but not a clear one and the horrendous brown colour made me think I should have tried it too. Though being nasty coloured and fast curing (hard to clean off) is not a great combo. The big M10 studding isn’t going into bare pine. The holes are lined with some sort of plastic tubing which felt a lot like HDPE. Naturally it was 15mm, and they only had 14 and 16mm drills, so a lot of sanding was involved (later I found just hacking off the outer layer with a Stanley knife was faster. Oh well). It’s also quite loose (inner diameter 11mm), which is good because I can’t drill straight apparently.
Oh, also, I glued on the main pillar upside-down first time (look: it’s not symmetric) at the other end with the Gator glue (scar is visible just by the pliers). I realised after about 20 minutes and that was enough that one of the end grain to long grain joints was already stronger than the wood. Even if I find doesn’t measure up here, I’ve become a fan of the speed of polyurethane glue and strength on end grain for woodworking
Oh and check out how straight the pillar is 8-). There’s nothing to keep it straight except for the square planed end. I’m excessively pleased about that.
So the spare DP8005 didn’t cure in a nice solid lump, at least in not in a few hours, and it’s meant to have a pot time of about 3 minutes. I think it was a mixing problem. I’m not 100% sure I got the right nozzles and since it’s a 10:1 mix, that really matters. So I mixed some by hand, too (strongly not recommended) and tried that. But since mixing is slow and it has a short pot time, I didn’t have a chance to glue up all variants. When it’s properly mixed, this stuff sets FAST.
The first results.
Thoughts so far during gluing:
- The Loctite 330 activator is this yellow gunk that has to be cleaned off. It’s pretty nasty and it’s more or less impossible to clean it off interestingly shaped surfaces.
- Scotchweld DP8005 is made by 3M and being 3M they’ve really put the effort into the packaging. It reseals really well by plugging up the holes (unlike the Araldite which has a simple cap), with a keyed cap, so you don’t mix the two sides. Also it’s gritty! That’s apparently to keep the surfaces apart by the minimum spacing. 3M think of everything.
- Cyanoacrylate is and always will be a pain in the neck.
- My lab now smells of glue and solvents. Niiiice.
Anyway, I left them curing for 36 hours in my lab. Then I spent I don’t know how long pressing snaps out of cases and writing down the results. And then some awk code to collate and prettyprint the results of course.
Surface prep|| IPA clean |clean, sand, clean | none
Glue || ave min | ave min | ave min
Loctite instant mix 5 minuted epoxy || 14.7 8.8 | 12.1 9.2 | 9.1 8.0
Araldite standard ||* 7.9 5.6 |* 11.2 10.0 |* 4.8 4.0
Araldite 2011 ||* 6.5 4.8 | 11.5 11.2 | 5.4 5.2
Loctite 330 ||* 12.3 7.2 |* 16.3 14.4 |* 13.6 10.4
Araldite 2028 || 7.1 5.6 | 6.9 6.4 | 5.9 5.2
Scotchweld DP8005 badly mixed ||* 4.4 3.6 |* 4.1 4.0 |* 4.5 4.4
Scotchweld DP8005 || 22.1 20.0 | | 17.5 13.6
Gator glue (no water) ||* 11.7 11.2 |* 7.5 4.8 |* 9.7 8.8
PCA || 3.2 2.8 | 5.1 3.6 | 3.0 2.0
Cyanoacrylate || 21.0 15.7 | 22.6 18.4 | 21.6 19.7
* = Clean off excess glue with IPA
It’s a bit mediocre.
Well OK. I’ve established that it’s really important to clean the parts before gluing. Sanding seems to help a bit, mostly, but to be honest I didn’t do a great job of it.
The 5 minute epoxy came out pretty strong comparatively and although it seems strong enough it’s not exactly decisive. The CA is strong (as expected), but has outgassing problems. The DP8005 is excellent if done properly, but I think it cures a bit fast to be useful for this application. The Loctite 330 is decent, but that spray is just too horrible for this application.
My ad-hoc experiments earlier had Gator Glue doing better relative to the Loctite epoxy. I think something might be wrong there.
And wow, Araldite, you kinda suck 😦 Is this the end of my fond childhood memories?
No, seriously, RTFM, n00b.
OK, so I forgot a bunch of stuff. I forgot to dampen one of the surfaces with the polyurethane glue (Gator Glue). That might explain the so-so results. Time to repeat the experiment. This time, I also switched to genuine Gorilla glue, not because I expect it to be better, but because the results are going to be used in the US and Gator glue isn’t available there.
Also, the lumps of left over araldite 2011 seemed kinda not fully cured even after 36 hours. I mean they’re sort of hard, but not really solid like I’d expect. The data sheet claims it cures at 10 degrees C (colder than the room I used) but rather sneakily, the strengths are not listed for low curing temperatures.
Double also, in the DP8005 datasheet, it mentions that you should discard the first bit out of the nozzle because it won’t be properly mixed. Not only did I not do that either, but it seems like a really good idea for the other auto mixers as well.
Round 2. Fight!
I’m only re-testing the ones which (a) didn’t seem to come out quite as expected (b) won’t cause massive wastage and (c) don’t have other issues. So, I’m doing Araldite 2011 with a bake at 80 degrees C for an hour (far more than enough) and Gorilla glue with a water spray. I’m also going to be a bit more generous with the Araldite and apply it to both surfaces. I chose 80 based on a kind of gut feeling compromise: the datasheet listed curing times up to 100, but 80 still cured fast and the plastic starts to soften slightly at 85.
For surface prep I’m only doing the IPA clean since it works well, but not the sanding since it’s variable, hard to do and possibly too awkward to be good in production.
The only thing I am varying consistently is whether or not to wipe off any excess with IPA.
Surface prep|| IPA clean
Glue || ave min
2011, both sides, bake ||* 20.7 16.8
2011, both sides, bake || 26.9 19.7
Gorilla Glue, wipe after ||* 19.9 17.6
Gorilla Glue || 17.2 15.6
Scotchweld DP8005 || 22.1 20.0
* = Clean off excess glue with IPA
Well, that’s a bit better. The Araldite 2011 is the clear winner in strength terms. One measurement actually hit the maximum 32.5Kg the default setup of the press could reach and tore a chunk of the chrome plating off the underlying brass of the snaps . And the Gorilla Glue puts in a very respectable showing. Not quite as strong, but the worst cases are pretty similar. I’m pretty confident now that either of these will do in the final application with the correct surface prep.