Hard and soft UV ink are made for different materials.
Hard UV Ink
Hard ink has very little flexibility. If the printed material bends too much, the ink layer may crack.
It is commonly used on rigid materials such as acrylic.
Hard ink usually offers strong adhesion, good scratch resistance, and better wear resistance.
Many UV printers use 395 nm or 415 nm curing lamps. Lamp power can be adjusted if the ink cures too quickly.
Neutral ink can also work on some rigid materials.
Soft UV Ink
Soft ink is designed for flexible materials such as leather, soft film, vehicle graphics, and TPU.
It bends better without cracking.
Its weakness is wear resistance. Repeated scratching may gradually damage the printed layer.
A useful Printer tip is to match the ink type to how much the finished material needs to bend.