Start with the existing process
Document conveyor speed, product spacing, label location, available mounting space, sensor positions, control signals and current label-data source. A retrofit is easier when the existing sequence is understood before hardware is selected.
Mechanical and timing constraints
Application method, product variation and conveyor motion determine where the applicator can be mounted and how much time is available to print and apply each label. Product detection must be stable enough to maintain placement accuracy.
Controls and communications
The replacement system may need discrete I/O, industrial Ethernet, serial communication or an interface to a database or warehouse system. Existing PLC logic should be reviewed so obsolete handshakes are not simply copied into a new installation.
Plan verification and maintenance
A retrofit is a good time to add label verification, clearer diagnostics, easier consumable access and maintenance-friendly mounting. Those details influence long-term reliability more than the initial installation alone.