Injection Molded Keycaps

The keycap presses onto the switch through a cross-shaped stem socket molded under the top. The fit is a small interference: the socket walls flex slightly as the switch stem enters and grip it. Too much interference and the socket splits, often days after assembly rather than at once. Too little and the cap comes off with a finger. The socket is also where knit lines tend to form, because the melt splits around the cross-shaped core pin and meets again on the far side of it.

The stem hangs from the underside of the top. Where it joins, the plastic is locally thicker, and that spot cools last. As it shrinks it pulls the top surface in, leaving a faint dimple right in the middle of the key, where light and fingertips find it. Keeping that join thin and hollow, with thin webs tying the stem to the skirt, keeps the top flat.

The resin changes the feel and the difficulty. ABS molds easily, holds tight fits, and gives a smooth surface, but tops wear shiny with use. PBT resists shine and many chemicals and has a dry, slightly grainy feel, but it is semi-crystalline and shrinks more, so long keys warp more easily and fits are harder to hold. Pigments can change how PBT crystallizes, so the socket fit is checked in every color of the set, not just the first one molded.

Legends are applied after molding. Pad printing is flexible and quick to change, but heavily used keys wear through the ink over time. Laser marking changes the surface itself and suits dark caps well. Long keys such as the spacebar add stabilizer stems at each end and need to stay straight, so they get their own gating and cooling attention.