A Thermal Transfer printer is a printer which prints on paper (or some other material) by melting a coating of ribbon so that it stays glued to the material on which the print is applied. It contrasts with Direct Thermal printing where no ribbon is present in the process. It was invented by SATO corporation around the late 1940s.
Usage of TT printers in industry includes:
- barcode labels (as labels printed with thermal printer tend not to last long), or for marking clothing labels (shirt size etc)
- Printing plastic labels for chemical containers (because the cheaper types of plastic would melt in a laser printer)
Barcode printers typically come in fixed sizes of 4 inches, 6 inches or 8 inches wide. Although a number of manufacturers have made differing sizes in the past, most have now standardised on these sizes. The main application for these printers is to produce barcode labels for product and shipping identification.
The printers use a fixed width thermal print head, pressing onto a paper or plastic label, over a driven rubber roller called a platen. Between the print head and the label is sandwiched a very thin thermal transfer ribbon (or sometimes called “foil”), which is a polyester film which has been coated on the label side with a wax, wax-resin or pure resin “ink”. The ribbon is spooled onto reels up to 625 meters (1965 feet) long and is driven through the printing mechanism in sync with
the labels, at speeds of up to 12 inches per second (although 6 inches per second is adequate for most applications).
As the label and ribbon are driven beneath the printhead together, tiny pixels across the width of the printhead are heated and cooled so as to melt the “ink” off the polyester film and onto the label. This process happens very quickly and accounts for the fast speed of the printers and is dry instantly. Thermal printheads are often 203 dots per inch (8 dots per mm) or 300 dpi (12 dots per mm). Though some manufacturers now make 600 dpi printers to produce very small barcodes for electronics industries (look inside the battery compartment of your mobile phone.)
Because of the high print speeds, the label printers have become very sophisticated, with powerful processors and large memory capacities, to allow them to produce the label images to be printed at the same speed as the print mechanism. To achieve this speed, almost all thermal label printers use special internal description languages to allow the label to be laid out inside the printers’ memory prior to printing.
Each manufacturer has their own language and some are very complex and difficult to work with. For example to print a barcode on a label, the controlling computer would send a series of codes to the printer, requesting a particular barcode type and specifying its size and location on the label, along with the data to be printed as a barcode. The printer will then use pre-defined algorithms to construct the barcode, keeping very strictly to the resolution allowed by the printhead, to create the best possible barcode on that particular type of printer. Barcodes have very strict rules for accurate printing, to ensure readability in a wide range of circumstances.