Understanding the simple physics behind a hot wire is the difference between clean cuts and a melted mess. There are only three things to balance: heat in, heat out, and speed.
Resistive heating
A wire has electrical resistance. Push current through it and it dissipates power as heat — that is Joule heating, P = I²R. A nichrome wire glows somewhere between roughly 200 °C and 400 °C for foam work (often not visibly glowing — that is fine). You control the heat with the voltage/current from the power supply, and sometimes with PWM (rapidly switching the power on and off to set an average).
The hot zone and the melt
Foam does not "burn" in a good cut — it sublimes/melts away in a thin zone just ahead of and around the wire. The wire never really touches solid foam; it rides in a self-made gap of vapour and softened material. The kerf (gap width) is set by how much foam that hot zone removes, which depends on temperature and dwell time.
The feedrate balance
This is the heart of it:
- Too slow → the wire lingers, the hot zone grows, the kerf gets wide, edges round over, and detail is lost.
- Too fast → the foam pushes back on the wire faster than it can melt, the wire bows backward, and the cut becomes inaccurate (and the wire can snap).
- Just right → the wire stays straight, melting foam exactly as fast as it advances, leaving a narrow, sealed, accurate cut.
Hotter wire lets you feed faster; cooler wire needs slower feed. Denser foam needs more heat or slower feed than light foam. There is no single magic number — it depends on your wire, your foam and your machine — but the simulator's material presets give you a sensible starting point.
Wire bow and why it matters
Because the foam resists the wire, the wire always lags slightly behind its endpoints — it bows. At a constant feed on a straight cut this is invisible. But at sharp corners and acute angles, the inside of the wire travels slower than the outside, the lag changes, and you get rounding or gouging. Slowing down into corners, or designing gentler corners, fixes this.
Why only ruled surfaces?
The wire is a straight line. As the machine moves its two ends, that straight line sweeps through space. The set of all surfaces a moving straight line can sweep are called ruled surfaces — flats, cylinders, cones, hyperboloids, and any shape where every point lies on a straight line connecting the two ends. You cannot cut a sphere or a bulging curve in one pass, because no straight wire passes through such a surface. This is the single most important constraint in hot-wire design, and it is why tapered wings (straight lines from root to tip) are easy but doubly-curved shapes need multiple rotated passes (see indexed rotation).
Left and right travel must match in time
On a 4-axis machine the two wire ends can trace very different profiles, but they must arrive at corresponding points at the same time. If one end has to travel much farther than the other in the same time, it moves faster — and may exceed the melt speed, scorching that side. cncfoam.com warns you when the two perimeters differ by more than about 1.5× for exactly this reason.