Sun, 11 Sep 2022
This comes up a lot, so I'm setting all my tips down for posterity.
Setting plastic snaps seems pretty simple, but there are a few tricks to it. The folks at KAMSnaps dot com have a troubleshooting page, with a video that shows how things are supposed to work: the little plastic point is supposed to mash down into a little mushroom-cap that holds it together, but it doesn't always come out right. Here are a few more reasons that they don't address.
Excess fabric up the stem. If you punch the sharp stem through the fabric (vinyl, etc.) instead of pre-punching a hole, sometimes the vinyl climbs up the stem a little. This can cause the washer to stop too far up the stem, leaving less to mushroom. Use a hole punch rather than an awl, or at least make sure the awl has punched a hole larger than the stem and that you flatten any lip on the hole before inserting the cap.
Too much fabric. A 20 Regular snap will not have much stem left after getting through, for instance, two layers of marine vinyl. Consider using thinner promo vinyl, Oly-Fun, or some other non-fraying fabric for the back side of your piece.
Not enough stem. If you can't reduce your fabric thickness, use longer caps. I always use 20 Longs - life's too short to mess with broken snaps.
Not enough pressure. The stem should spread into a neat little bullseye. If it doesn't spread out enough, the washer may compress it right back down and pop off.
Off-center pressure. If you have trouble getting your bullseye centered, try pressing lightly down to get it started, then rotating your piece and pressing down again to finish the job. It's best to get it right the first time, of course, so don't rush into stamping it before you have it positioned exactly vertical and centered, but sometimes an awkward piece just won't sit quite right.
Too much pressure. This was the most counter-intuitive thing I had to learn. My caps popped off, so I tried pressing harder. I was all but standing on the press, putting my not-inconsiderable weight on it, and they'd pop off even more easily. And then I realized: I was going beyond making a mushroom-cap and into making a little donut that easily detached from the stem. Hand presses (even the fancier flat-bottomed one that can stand on a tabletop) are apparently more prone to this than the more expensive table press, so the expensive fix is to get the good press. I haven't done this, so I just press gently and keep checking until I have a nice bullseye, and no more.
Ultimately, though, plastic snaps are just too temperamental for me. I love the colors, and I have a giant assortment, but more often than not I find myself reaching for rivets, or for metal snaps. If you really love the look of the colored snaps, especially the engraved and shaped ones that only come in Regular length, I would highly recommend you invest in a real tabletop press. (Yes, they should pay me a commission for saying that, but they don't. Alas.)
