RFID Printing & Encoding

How an RFID printer works

An RFID printer is a thermal label printer with an RFID reader-encoder and a small antenna built into the media path. RFID labels - sometimes called smart labels - look like ordinary paper or synthetic labels, but each one carries an inlay (chip plus antenna) laminated between the face stock and the liner. As each label passes through the printer, three things happen:

1. Encode: the label stops, or slows, with its inlay over the encoder antenna, and the printer writes data such as the EPC into the chip memory.
2. Verify: the printer reads the tag back and compares what it reads with what it wrote. If the tag does not respond, or the data does not match, the label is treated as defective.
3. Print: the printhead prints the barcode, text and graphics on the face of the label using direct thermal or thermal transfer printing.

The order of encoding and printing varies between models and settings, but the principle is the same: one pass produces a label whose printed information and chip data match, and whose tag has been tested. This is what makes on-site printing reliable enough for shipping labels, asset tags and retail tickets.

The data written is usually an EPC in the EPC memory bank, sometimes with user memory, passwords and lock settings. If these terms are new, read EPC encoding and tag memory first.

Desktop vs industrial RFID printers

RFID printers come in the same broad classes as barcode printers. The right class depends on volume, duty cycle and environment more than on the RFID function itself.

There are also print-and-apply systems that encode, print and apply labels automatically on a packing line, and mobile printers with RFID encoding for field use. Whatever the class, check that the printer supports the inlay types, label sizes and chip types you plan to use. Indian RFID's range of RFID printers covers desktop and industrial models.

In the real world

Inlay position and calibration

For reliable encoding, the inlay must sit in the right place relative to the printer's encoder antenna at the moment of writing. Two settings control this:

• Media settings: label length, gap or black-mark sensing, and label width, so the printer knows exactly where each label starts.
• Encoding (program) position: how far the label is advanced, or backed up, before the chip is written, and at what encoding power.

Most RFID printers have an RFID calibration routine. The printer feeds labels, tries to encode at a range of positions and power levels, and selects the best combination for that media. Run calibration whenever you load a new label type, a different inlay or a new supplier's batch - and after any repair.

The inlay's location within the label is called its placement or offset, and the distance from one inlay to the next along the roll is the inlay pitch. Printer manufacturers publish which inlays, placements and minimum pitches each model supports. Ordering media that matches these recommendations avoids most problems before they start.

Void and defective tag handling

Not every inlay works. A small proportion can be dead from manufacture, and others are damaged by rough handling, static, bending or crushing. An RFID printer deals with this automatically:

1. The printer tries to write and verify the tag.
2. If the tag fails, it retries a set number of times, sometimes adjusting power.
3. If it still fails, the printer overstrikes the label - prints a VOID pattern or text across it - so it cannot be used by mistake.
4. It then repeats the same job on the next label, so the data sequence is not lost.
5. After a configurable number of consecutive failures, the printer stops and reports an error, because repeated failures usually point to a setting or media problem rather than bad luck.

Monitor the void rate - voided labels as a percentage of labels printed. A low, steady rate is normal; a sudden rise signals a calibration, media or hardware issue. Voided labels should be collected and destroyed, not peeled and reused.

Encoding data from ERP and label software

What goes into the chip should be decided by the data design, not by the person at the printer. In a well-run system:

• The ERP, WMS or asset system holds the master data: product code, batch, asset number or shipment details.
• Label design software or middleware takes that data, converts it to the EPC format chosen (for example an SGTIN or SSCC-based EPC, or a company-specific asset numbering scheme), and sends a print job containing both the visible label content and the RFID write command.
• Every EPC that must be unique - most item-level and asset schemes - gets a serial number from a controlled source, so no two tags carry the same code.
• After printing, the printer or software reports which EPCs were written successfully, and this encoding log is stored for reconciliation.

Printers are driven by their command languages, which include commands to set up RFID parameters, write data to a memory bank, and read data back. Most organisations do not write these commands by hand; they configure templates in label software or integrate through RFID middleware. For GS1-based schemes in India, GS1 India allocates the company prefix on which SGTIN and SSCC codes are built.

Locking and passwords

Encoding can also set an access password, a kill password and lock states for memory banks. Locking prevents the EPC from being changed later by someone with a reader. It is common for supply-chain labels and asset tags, but it should be a deliberate decision because a permanently locked tag cannot be rewritten if the data design changes.

Label design: barcode, human-readable and RFID

An RFID label should still work as a normal label. Good designs combine three layers of identification:

• Barcode (linear or 2D) carrying the same identifier, or a related one, as a fallback when RFID cannot be read.
• Human-readable text so people can identify the item without any device.
• RFID inlay carrying the EPC, plus an optional RFID emblem so staff know the label contains a tag.

Practical layout rules:

• Avoid printing a barcode directly over the chip. The chip forms a small bump, which can cause print voids and also puts extra printhead pressure on the chip.
• Leave the inlay area free of heavy, dense graphics where possible.
• Choose face stock and ribbon to suit the environment: synthetic face and resin ribbon for moisture, abrasion and chemicals; paper with wax or wax-resin for general use; direct thermal only for short-life labels.
• For metal items, use on-metal RFID labels designed for printing, not a standard inlay - see UHF RFID labels for the label types available.

Pre-encoded batches vs on-demand printing

Many projects use both: pre-encoded labels for high-volume, predictable items and an on-site printer for exceptions, replacements and variable data.

Common problems and fixes

Quality control

A simple quality-control routine keeps printing reliable:

• Incoming media check: on each new batch, confirm the inlay type matches the specification and test a sample with a handheld reader.
• Calibrate on change: run RFID calibration for every new media type or batch.
• Keep verification on: never disable read-after-write verification to "speed up" printing.
• Track void rate: record it per shift or per roll, and investigate trends.
• Audit sample labels: read finished labels with a handheld to confirm the EPC matches the barcode and the system record.
• Test in the application: periodically check that labels applied to products still read at the real read points, not just in the printer.
• Store media properly: in original packaging, away from heat, humidity and sharp bends, which can crack inlay antennas.
• Maintain the printer: clean the printhead and platen, and keep firmware at a version supported by your label software.

Before committing to a label and printer combination, make sure the tag itself has passed the tests in RFID site survey and tag testing. Printing a perfect label is of little value if the tag cannot be read on the product. Hands-on printer set-up, calibration and encoding are also part of the Certified RFID Specialist Program for those who want guided practice.