Thermal Transfer Printing is a unique process that stands unparalleled by other variable imaging technologies for bar code labeling solutions. Unlike any of its counterparts, thermal transfer printing provides fail-safe durability, versatility and high-resolution output.
One of the first printing methods developed, dot matrix printing uses tiny round hammers to pound ink from a coated fabric ribbon onto a substrate – typically paper, which must be pin fed into the printer. Normally dot matrix printers use one or two column dot hammers – the more dot hammers on the print head, the higher the resolution.
Dot matrix printers are readily accessible and very inexpensive – most typically used to type forms, checks, and other documents that require carbon copies. Since the pressure applied by the print head to transfer the ink ribbon to the paper helps create a carbon copy, the dot matrix process is extremely useful for record keeping. Due to multi-pass, ribbons for dot matrix printers are relatively inexpensive.
Primary drawbacks include low print quality, limited graphic print capability, very slow print speeds, noisy operation, and a lack of resistance to chemicals. For bar code printing, especially, a defined image is crucial. The edge definition of dot matrix images is rarely clean or linear – greatly compromising the integrity of the bar code. Dot matrix printing’s poor image quality, combined with its low printing speed and inability to resist chemical solvents, severely inhibit its performance – serious concerns which can greatly increase costs to manufacturers.
There are basically two types of ink jet applications for bar code printing – direct print applications and label applications. Direct ink jet printing is very effective for printing bar codes and expiration dates directly on canned foods, liquid bottles, etc. For these uses, a special direct ink jet printer is necessary – these machines are very expensive, and are usually incorporated directly into the assembly line. Ink jet technology can also print bar codes on labels using a standard office printer.
The serial printing technology of ink jet printing is conceptually very similar to dot matrix – ink applied to a substrate in an up-and-down or side-by-side droplet form by pressure – however, the actual delivery method of ink jet printing differs from dot matrix substantially. Ink jet printers do not use direct pressure for the transfer process, nor do they rely on ink ribbons – instead, they use a combination of liquid ink, ultrasonic pressure and an electrostatic field. Ink jet printers create images by propelling ink into tiny, highly controlled jet streams and onto the substrate. The quality of ink jet images is determined by the size of the ink droplets – small droplets produce cleaner lines, large droplets tend to blur.
Ink jet printing technology is virtually the only solution for direct printing on a wide variety of substrates – from cartons and cans to plastics, glass, and paper. Assembly line ink jet printing can print at extremely high speeds and code large quantities in seconds. It is the most efficient method for alpha-numerical coding of consumer goods.
While ink jet printers do allow users to have more flexibility, their low image resolution, limited durability, and high unit price make them a less attractive option for bar coding purposes. Although assembly line ink jet printing services a large audience, it is a very niche application, and the printers tend to be extremely expensive. Ink jet printing is also less solvent resistant, and specialty inks are required for high speed assembly line printing – a factor which entails costly maintenance and frequent routine attention. The ability of ink jet printers to print directly on many substrates makes it a very useful printing technology; however, for bar coding projects that require quality high resolution and good durability, thermal transfer printing is by far the better choice.
Laser printing, much like photocopying, uses a light source to generate an exact duplicate of the image it receives. The light source – a laser beam – creates an electrostatic image that is then charged onto a photoreceptor; the electrostatic charges from the photoreceptor attract the toner to produce the printed image.
The laser printing process produces beautiful, dense, high resolution images for text and graphics.
For bar coding purposes, the most serious constraint of laser printing technology is its substrate limitation. Laser printers can only print on certain types and sizes of labels – and certainly not directly on any non-paper surface, such as plastic or metal. Laser printing is also not the fastest option, and laser images are far less durable than thermal printed images. Toner cartridges and drum kits used in the laser printing process are extremely expensive, and hazardous if not disposed of properly. While the images produced by laser printers are much cleaner than those of ink jet printers, the lack of versatility, durability, and environmental safety – combined with high unit and maintenance fees – typical of laser printers certainly makes laser printing a less efficient alternative for bar coding.
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.
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.
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.