What kind of glue to repair a coffee grinder catch cup?
January 23, 2021 10:34 AM   Subscribe

My coffee grinder's grounds catch cup has a crack that is threatening to turn into a full break. I'd like to repair it before that happens, and I'm trying to decide on the best adhesive -- and mostly whether I really need a food-grade glue for this.

I'm trying to choose between three adhesives: regular epoxy, specifically labeled food-grade epoxy, food-grade silicone.

Epoxy seems like the right choice, mechanically, so the core of my question is whether this application really demands a food-grade epoxy. I can keep nearly all of the epoxy on the outside of the cup, there will be no liquid in the container, and the contact time in practice will be very short; so, I'm inclined to just use regular hardware store epoxy, and let it cure a full 24-48 hours before using the cup again. Will I poison my coffee if I do this?

Cons of the other options: silicone, as I understand it, will make a flexible joint, which I think is likely to fail again in the same way; food-grade epoxy doesn't seem to be sold in small amounts (and I think is usually a thin, pourable resin, which would be hard to work with for this application). And I'd probably have to order either one online.
posted by egregious theorem to Home & Garden (15 answers total)
Best answer: I used regular epoxy years ago on a salad bowl repair, and I'm here to tell the tale. Salad bowls have wet, acidic dressing that conceivably could leach something from the hardened epoxy, but the ground coffee is basically dry so nothing like that would happen. And also, hardened epoxy is practically like glass. So any interaction between the ground coffee and hardened epoxy is going to be negligible. So I agree with your reasoning. Go with regular epoxy.
posted by beagle at 11:01 AM on January 23, 2021 [7 favorites]

Response by poster: Thanks!
posted by egregious theorem at 12:10 PM on January 23, 2021

I'd probably use some packing tape with filaments in it to secure it over the glue.
posted by theora55 at 12:16 PM on January 23, 2021

Agree that regular epoxy is fine using in the manner you've described (outside of cup, no liquid contact). I've fixed several coffee/spice grinder cups exactly this way. A further choice is that I don't regularly wash that coffee grinder cup; I wipe it out with a damp towel from time to time. That way I don't have liquid coming in contact with the hardened glue, nothing is warming the glued area or spreading anything to the food contact area. I do rinse the actual grinder bowl and wipe dry, making sure the blades are turning freely. I have a grinder I use just for heavy sea salt and it will seize up if it gets wet.
posted by winesong at 12:37 PM on January 23, 2021

Prying apart the crack to apply epoxy in there could make the crack worse. I might make do just with some glossy tape on the inside and the outside. Since the coffee grounds are getting hit with boiling water, I wouldn't be concerned with 'food-grade' for this repair (but I'd be looking around the thrift store for a substitute, alternate catch cup).
posted by Rash at 1:08 PM on January 23, 2021

A "liquid plastic" or "plastic welding" kit would probably do the trick, but might be the price of a new grinder. Of course, you can then go on to fix other things with it.
posted by snuffleupagus at 4:11 PM on January 23, 2021

I'd force the break to completion and then clean the bits well and just use some Cyanoacrylate glue to put them back together.

Same it's not going to really matter but worth a try before going to epoxy to avoid the mess of dealing with epoxy for something you could just super glue together and be done.

Though I also like the "plastic welding" angle but we don't actually know the cup is plastic nor what sort of plastic it is and the solvents used are just as wee/ooo noxious to use the same way that epoxy is.
posted by zengargoyle at 5:40 PM on January 23, 2021

I'd go with a cyanoacrylate glue. The low-viscosity versions of these are good for crack repairs because they wick right in. Just ease the crack open as much as you can without making it run further, get a little glue as far into it as you can using a hypodermic syringe with a needle on it, then tape it closed and wipe up the squeeze-out.

CA glue deep in a glass crack won't cure anywhere near as fast as you're used to because it only gets very restricted access to water vapour, so leave the repair undisturbed for several hours before putting it back into service.

Excess glue on the surface will cure quickly and make a white bloom. You can scrape that off with a sharp blade.
posted by flabdablet at 8:32 PM on January 23, 2021 [1 favorite]

I'll cosign cyanoacrylate for this type of crack. I wouldn't force the break, personally, just let a tiny bit of glue crawl in as far as you can get it to go by gently and repeatedly straining the crack. If you don't get the tip of the crack, and it propagates later, well then you've got your through-break to glue from the other side.

Also cosigning that cyanoacrylate inside a crack cures more slowly that you might count on and you can squeeze some out and glue your finger on an hour later. Ask me how I know.
posted by away for regrooving at 11:31 PM on January 23, 2021 [2 favorites]

(Optional adhesives geeekery.

My understanding is that food-safe epoxy resin still is made of BPA (an oligomer mix with some monomer), or related bisphenols you wouldn't want to eat. The food-safeness property of the epoxy is that it doesn't have other bad news The critical part in using it correctly is to hit the resin/hardener ratio precisely, and get them well-mixed, so they react completely with no leftovers. The cured polymer is harmless.

Cyanoacrylate adhesives are not rated food-safe, so take CA recommendations with that awareness, but I personally have no qualms with the cured material for this kind of application.)
posted by away for regrooving at 11:44 PM on January 23, 2021 [1 favorite]

The quantity of cyanoacrylate residue you're actually likely to ingest from dry coffee grounds rubbing against a tidily glued crack will almost certainly give your body less toxicity to deal with than the alkaloids in the coffee.

Food-safe ratings for polymers matter much more when (a) there's a substantial surface area of polymer exposed to the food and (b) the food contains water and/or oil in liquid form into which stray monomer could potentially leach.
posted by flabdablet at 1:11 AM on January 24, 2021

I'm on team superglue! If the container is transparent, it's very probably acrylic, which will bond *really* well with cyanoacrylate. Also, if you apply it from the outside and let it leech into the crack, I can't really see how it would end up having any contact with the coffee grounds.
posted by kleinsteradikaleminderheit at 4:45 AM on January 24, 2021

Though I also like the "plastic welding" angle but we don't actually know the cup is plastic nor what sort of plastic it is and the solvents used are just as wee/ooo noxious to use the same way that epoxy is.

There are no solvents, at least according to the amazon listing. It's some kind of filler that's applied as a gel or liquid and then hardened with UV.

posted by snuffleupagus at 12:00 PM on January 24, 2021

I'd still force the crack open and clean it well because at least the coffee I grind is oily and if that has seeped into the crack it will cause the bond to fail. The same way gluing your fingers together doen't last that long, skin oils start breaking the bond.

Plastic welding.... ok, I'm thinking of plastic model building and there is a gel like glue that is solvent with melted plastic in it that fills gaps and melts into each side a bit before evaporating and being just plastic welded to plastic. In the more advanced model building you just use the solvent directly and let it melt the edges of the pieces a bit before pushing them together and the same evaporation of the solvent yields the same plastic welded to plastic with no extra filler material. Probably just different uses of the term "plastic welding". When welding is done right, there are no longer two different pieces with some foreign layer of material in between them, they are now one piece just as if they were formed that way in the first place.

You can also get CA accelerant to aid in the quickness of full bond forming. Baking soda also works but IMHO leaves more crusty like residue. CA also just might not work with the sort of plastic (if it is that), it's hard to tell without trying. I've watched too many Adam Savage building videos using CA to not be highly opinionated. :)

I like Loctite ca/super glue because you can get it in an eye-dropper sized bottle and it has enough solvent and a tight enough cap that you can through it in a drawer and it's still good a couple of years later vs your random tube of super glue that dries out to unusable months after you open it. (that's why they sell such small tubes, the never last long)
posted by zengargoyle at 7:33 PM on January 24, 2021 [1 favorite]

Response by poster: Update in case the adhesive geeks are still here: I ultimately went with the epoxy. I have no doubt cyanoacrylate would have worked well too and probably would have been a bit easier to apply, so thanks for the suggestions -- but I had the epoxy on hand and not the CA, and I didn't want to run out for another glue. So far it seems to be holding up fine (I did clean the cup and the crack thoroughly before gluing, thanks zengargoyle).

And sorry, I definitely should have specified that the catch cup is ABS plastic, not glass or acrylic. Not sure how compatible that is with the various "plastic welding" solvents -- I think I've only used them in the context of PVC myself.
posted by egregious theorem at 12:37 PM on February 3, 2021 [1 favorite]

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