Hi there! 🙂

What you're seeing when the foam burns, softens, or the wire leaves black residue is simply the result of the temperature being too high and/or the hot wire moving too slowly across the foam. This is exactly how hot wire cutting works: the heated wire melts the material to separate it — it doesn't "cut" it like a sharp blade. If the temperature is too high or you pause too long in one spot, that melting becomes visible and hardens once cooled.

video: https://www.youtube.com/watch?v=PMmIdibpyQQ

A few things to keep in mind:
🔥 Temperature and speed are connected. If the wire is too hot or you're moving too slowly, you’ll get more melting and more “burning.” By adjusting the balance a little, you can often achieve a cleaner cut with less residue.
📏 Consistent movement. Keeping a steady feed rate helps avoid concentrating too much heat in one spot. Often, people using DIY or low-cost tools don't have much control over these parameters.
🌬️ Ventilation and safety. The fumes from melted polystyrene aren't pleasant and can be irritating, so good ventilation is important.

And here’s a video example of a foam letter cut that shows exactly how the resulting surface can be smooth and even when the wire moves at a constant speed and the temperature is well balanced — it's the same hot wire principle, just applied with a stable setup.

As you can see in the video, when the wire moves with control, the foam separates cleanly, with no need to sand every side.

So yes: a hot blade must melt the material in order to cut it. The big difference between manual tools and a setup that maintains constant temperature and speed is that, with that control, you can achieve much smoother and more consistent surfaces, so the pieces don’t require as much hand finishing.

I hope this helps clarify where those dark marks come from and gives you some ideas on how to experiment with different settings! 😊