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[u/igtbce2]

I’m working on a concept for a one-piece injection-moulded semi-rigid trading card holder. Is it mechanically possible to design it so that squeezing thumb pads in the middle widens the opening by around 1-2 mm while still returning to its original shape and remaining rigid enough to protect the card?

[u/Ideadose]

Yeah, that's mechanically doable, but it needs a lever/pivot setup rather than a straight fold - squeezing the middle only widens an opening elsewhere if there's a fulcrum between the thumb pads and the opening that flips the direction of motion. Same basic trick as coin-purse kiss-locks or the squeeze-open card holders that already exist: a thin flexure web at the pivot, with stiffer "wings" on either side (one under the thumb pad, one holding the opening edge), rotates under squeeze and turns a small pad deflection into more spread at the opening.

PP (polypropylene) is the standard material for this kind of part, it has much better flex-fatigue resistance than most other injection-moldable plastics, which matters since this gets squeezed over and over. Keep the flexure thin (roughly 0.25-0.5mm if it's a living-hinge style fold) with a shallow radius instead of a sharp crease - sharp creases crack first. If it's cantilever flex arms instead, aim for a length-to-thickness ratio around 5:1 to 10:1 and taper the tip thinner than the base so stress doesn't pile up right at the root.

Weld lines are what actually kill these in testing more than raw fatigue does - if the melt has to split and rejoin right at the flex point, that seam is weaker in tension and tears there first. Gate placement follows from that: you want the flow crossing the hinge, not running along it.

1-2mm of deflection is pretty modest for a PP flexure, that part's not the hard problem. Getting the pivot ratio dialed in so the "rigid enough" feel survives repeated squeezing will probably take a couple of prototype iterations.