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1) Printing volume - industrial printer for high-volume and desktop printer for lower volume
2) Printing type - thermal transfer for durable labels or direct thermal for short-term use.
3) Desirable label size - make sure the label printer is capable of printing the size you need.
4) Connectivity & Quality - USB, Wifi, Bluetooth, Ethernet and DPI, print speed
5) Additional - Cutter, peeler, rewinder, etc
Read more about choosing label printers from here.