Thermal Transfer Overprinter

Overview

A thermal transfer overprinter, or TTO, prints variable codes such as dates, batch numbers, and barcodes onto flexible packaging film while it is being made into bags or wrappers. It mounts inside a flow wrapper, a vertical bagger, or a labeling machine, and prints directly onto the moving film web before the pack is formed and sealed. The print is made by a line of tiny heater dots that melt ink off a ribbon onto the film, so the result is a sharp, dry, durable mark with no ink to maintain. TTO is the standard way to code flexible film because it gives high resolution at high speed, prints crisp barcodes and small text, and changes message instantly between packs.

The printer presses a Thermal Print Head against the film over a backing in the Print Action, drawing ink from a ribbon metered by the Ribbon Drive and held in a Ribbon Cassette. The Controller rasterizes the code and times the print, the Mounting Bracket fits it into the host machine, and the Power and Interface section powers it and exchanges signals.

The thermal print head

The mark is made one column of dots at a time by the Thermal Print Head. Its Heater Element Line is a row of fine resistive heaters, commonly twelve to a millimetre, each able to heat a single pixel. The Head Driver IC switches each dot on for a controlled time, so a hot dot melts ink off the ribbon and transfers it to the film while a cold dot leaves the film blank. The heaters sit on an Ceramic Substrate of alumina ceramic that carries them and sheds their heat fast, a Head Thermistor reads head temperature so the energy per dot is corrected as the head warms, and a Head Heatsink pulls residual heat away between prints. A Flex Connector connects the head to the controller. Even dot energy across the whole line is what keeps barcode bars uniform, so the temperature correction matters as much as the heaters themselves.

Ribbon handling and ribbon save

The ink comes from a roll of coated film, not from a tank, and using it efficiently is a big part of the running cost. The Ribbon Drive runs the Supply Spindle and Rewind Spindle under servo control so it advances only the length of ribbon actually used by a print, then leaves a tiny gap before the next, which is the ribbon-save feature that can double or triple the prints per roll. The Supply Servo and Rewind Servo hold matched tension across the head, Ribbon Roller guides keep the ribbon flat, and a Ribbon Sensor warns before the roll runs out. The ribbon itself lives in a Ribbon Cassette that drops in on a Quick Latch so a fresh Thermal Ribbon Roll loads in seconds.

Intermittent and continuous printing

A TTO has to suit two kinds of packaging machine, and the Print Action supports both. In intermittent mode, used on baggers that stop the film to seal each pack, the head simply presses down onto the still film while the Print Platen backs it, prints, and lifts. In continuous mode, used on flow wrappers where the film never stops, the Carriage Servo sweeps the head along the moving web so the head and film match speed during the print. A Head Carriage on a Linear Guide Rail carries the head, and Pressure Spring springs set even pressure across the print width so the whole image transfers cleanly.

Control and message handling

The Controller turns a stored message into the pattern of dots and times the print to the pack. Its Main Control Board rasterizes the code, calculates the current date and counter values, and times each column, while the Head Power Stage stage delivers the pulsed current the heaters need. The operator builds and previews codes on the Touchscreen, and the Message Memory holds saved layouts so the right format is recalled per product. A Trigger Input takes the print signal from the packaging machine so each pack is coded in the same place.

Mounting and integration

A TTO is a module inside another machine rather than a standalone unit. The Mounting Bracket fits it over the film web with the Print Platen held square behind the film by a Platen Bracket. A Mount Plate bolts the printer into the wrapper or bagger frame, and Position Adjuster screws set the print position across and along the web. The Power and Interface section provides a Power Supply for the head, a Interface Board that handshakes print, ready, and fault with the host, and a Interlock Input interlock that stops printing if a guard opens.

Use and variants

Thermal transfer overprinters dominate flexible-film coding because they print sharp small text and scannable barcodes that a Date Coding Machine ink jet struggles to match on film, and they need no ink upkeep. They suit snack bags, medical pouches, sachets, and labels, anywhere a code must go onto a film web during forming. The trade-offs are that the print is contact, so the film must be backed by the platen, and the ribbon is a consumable that has to be reloaded, though ribbon-save keeps that cost low. Variants differ mainly in print width and in whether they run intermittent, continuous, or both, and wider heads cover multi-up webs that carry several lanes of packs side by side. For coding rigid containers and uneven surfaces, lines use an ink-jet Date Coding Machine instead, since a TTO needs a flat backed surface to press against.