Direct & Indirect Thermal Labels

Direct Thermal

Direct thermal printing uses virtually the same process as thermal transfer printing - with one exception. Direct thermal printers do not use ribbon. In direct thermal printing, the thermal process occurs on the paper substrate itself - requiring a specially coated paper for optimal performance. The heat generated from the print head causes a chemical reaction with the top layer of the coated paper that essentially burns the transmitted image onto the paper.

Advantages
Images produced by direct thermal printing are clear and defined; and direct thermal printers are cheap, simple to operate and easy to maintain. Direct thermal printing for bar code labeling is certainly an improvement over ink jet and laser technology, especially when it comes to speed and resolution.

Disadvantages
Direct thermal images are far less durable than thermal transfer printed images; and direct thermal technology is also constrained by substrate restrictions. The coated paper required by direct thermal printing is very sensitive to light, heat, and abrasion - limiting the range of applications for direct thermal printing to short-term projects. Direct thermal images have no chemical resistance, and very short life spans. Primary uses for direct thermal printing are "Point-A-to-Point-B" applications, such as baggage checking in airports, or parcel delivery. Bar coding projects requiring durable images and substrate flexibility are not well served by direct thermal printing technology.

Thermal Transfer

In thermal transfer printers, the thermal transfer process occurs not on the substrate, but on the ribbon. As the ribbon and substrate come into contact with the print head, heat from the print head melts the ink in the ribbon and releases (transfers) the ink directly onto the substrate. Ink used in the thermal transfer process is generally composed of wax, resin, or a wax/resin compound, according to the requirements of the project. Wax ribbons are less expensive and less durable, and are more suitable for short-term projects; resin ribbons are premium quality transfer agents, and are optimal for long-term, highly durable printing needs.

Advantages
Thermal transfer printing produces dense, high resolution images - color or black-and-white - and can be used on a wide variety of substrates. Since thermal transfer printing uses ribbon as the transfer agent, images produced by the process are highly resistant to chemicals and extremely durable. Thermal transfer printers work at high speeds and require little maintenance.

Disadvantages
The primary disadvantage of thermal transfer printing is that each ribbon only has a one-time use. However, thermal transfer ribbons can be easily disposed and are comparatively environmentally safer to dispose of than the other printing technologies discussed. Thermal transfer printing is the industry's most suitable solution for bar code printing applications. Extremely versatile, thermal transfer technology allows users to print crisp, resilient images on a virtually unlimited universe of substrates - a primary requirement of most bar coding projects. For absolute assurance of quality and reliability when it counts, there's no other choice. Thermal transfer printing is the industry's best solution for bar code printing.

Thermal Transfer vs Direct Thermal

You may know that the two types of thermal labels are thermal transfer and direct thermal. But what are their differences?

Thermal Transfer

Thermal transfer labels require a ribbon when printing. Both the labels and the ribbon feed through the printer, and the printer applies heat to the ribbon to transfer the wax and resin onto the label (hence the term thermal transfer). The printer uses the ribbon similarly to the way an inkjet printer would use ink to print on paper, but with heated wax and resin replacing the ink. Ribbons are available in colors other than black, so you can have multicolored printing with thermal transfer. The resulting label is fairly scratch resistant, can be detailed, and can last a long time. View our thermal transfer labels selection.

Direct Thermal

Direct thermal labels do not require a ribbon to print. The labels feed through the printer and the printer applies heat directly to the label (hence the term direct thermal). As the print head runs over the label, it heats the label and a chemical reaction with the face stock causes the heated portions to darken. Think of a receipt printer when you think direct thermal. Since direct thermal printers do not require a ribbon, they can be smaller and take up less space. It is also cheaper to buy direct thermal labels since you do not have to purchase ribbon. The downside to direct thermal labels is that they are not scratch resistant (premium direct thermal is somewhat scratch resistant, but not completely heat resistant) and they do not last as long especially when stored in warm places or in direct sunlight. View our direct thermal labels selection.

How to Tell Them Apart

Ever wondered how to tell the difference between a thermal transfer label and a direct thermal label? There is one simple test that you can use to tell if a label is direct thermal. Take the label and scratch it quickly with your fingernail as if you were lighting a match. It may take a couple of hard strikes. If a dark mark appears on the label, it is direct thermal. If no dark marks are left on the label, it is thermal transfer. Some coated labels may take a quicker, harder strike to leave a mark.

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 standardized 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.