The short version: perler beads are made of low-density polyethylene, a plastic that softens around 130 degrees Celsius. When you press them with an iron, the beads melt just enough at the contact points to weld together, while the middle stays cool enough to keep the hole open.
Fuse beads work because of how their plastic reacts to heat. The beads are extruded as hollow cylinders, then cut to length. When heat is applied, the surface of each bead becomes soft and sticky, and the beads bond wherever they touch. This is why ironing is so forgiving: you do not need perfect technique, only even heat and pressure.
Why do the holes stay open?
An iron heats the top surface of the beads, but a bead is a tube, so heat has to travel around the wall to melt the bottom. In the seconds you iron, only the outer edges melt. The hole keeps its shape because the bead wall is thin and the heat is brief. That is why over-ironing makes holes close up and the piece look like a flat sheet of plastic.
How do you get a clean melt?
Three things matter: heat setting, time, and pressure. A medium-high iron with no steam, pressed in slow circles for 10 to 15 seconds, fuses most projects. Too little heat and the beads only stick on one side; too much and they collapse. Our ironing guide covers exact settings and timings. Whatever you make, start from a pattern you trust, like the printable charts in our free pattern library, or generate your own with the pattern maker.