One common use is cross-linking polymers. If you've ever used cable with XLPE insulation, it was initially extruded onto the wire as normal polyethylene, then s

One common use is cross-linking polymers. If you've ever used cable with XLPE insulation, it was initially extruded onto the wire as normal polyethylene, then spooled back and forth between drums passing under an e-beam system like this, to perform the cross-linking. It's phenomenally tough stuff after that; a lot of cars use XLPE (in its thinwall TXL spec) wiring.