Printing at 400 degrees Celsius
Only available in German
published in: Elektronik, WEKA Verlag, issue 12/2009
This technical article explains in detail what everyone should be aware of when printing on thermal paper.
He explains the key factors of the thermal printing process and shows how their precise interplay can ensure the best printing results in applications using thermal printers.
When printing thermal paper, the print elements in the printer’s print head are heated to up to 400°C in less than 1 ms. The majority of the heat flows into the thermal paper and reacts with the thermosensitive layer. This heat dissipation is therefore so important because the print head is essentially cooled by this process. A good heat transfer also favors the lifespan of the print head.
If during the printing process the components in the thermosensitive layer melt, parts of them are mechanically removed and contaminate the print head. Good thermal papers support a kind of self-cleaning effect. If the heating times are correctly selected and the print head is mechanically properly adjusted, only a small portion of the material will settle just behind the print elements. If the settings are not correct, the deposits will gradually build up towards the print elements or burn directly onto the print elements. In this case, the print elements are thermally insulated and deteriorate extremely quickly.
A wrongly selected or of poor quality thermal paper can damage a printer within a short time. Only with specified papers can the stated lifespan be achieved with certainty.
What should be considered when choosing a product?
Is there a lot of printing in the application?
In general, only high-quality paper should be used. The more printed material there is, the more important it becomes.
1. The surface roughness of the paper surface: thermal paper gradually wears down the print head during operation. Good papers have a smooth surface and are therefore less mechanically abrasive. Topcoat papers, due to their very low surface roughness, generally produce even less wear on the print head than non-topcoat papers.
2. The ion content in the paper: Thermal papers contain free charge carriers due to the production process in the form of Na+, K+, and Cl+ ions. These ions have a very aggressive effect on the glass passivation of the printhead under high humidity and gradually destroy it. Good thermal papers have a very low ion content. The values must be below (Na<500ppm, K<100ppm, and Cl<300ppm).
What temperature constraints is the term subject to?
First, the paper must be selected to match the printing speed. A faster printer requires paper with higher dynamic sensitivity than a slower one (Figure 3). High-sensitive papers achieve full blackness with low energy transfer.
And the static temperature behavior, i.e. at what temperature the blackening of a thermal paper begins, must also be properly selected. Standard thermal papers start developing the image at about 70°C. There are also special papers, e.g. for the parking ticket area, where the blackening of the paper begins at 90°C or 120°C.
Should the expression be archived longer?
Leuco dyes have the tendency to revert to their original state, i.e., reflecting white light. Sunlight, plasticizers, oils, and water accelerate this process. The dyes have different strong resistance properties against the various influencing agents. Thermopapers are now manufactured with specified image stability (under proper storage) of 5, 25, and even „99“ years.
Should it be printed on pre-printed paper?
The thermal printing technology also allows printing on pre-printed paper. However, there are some things to consider to avoid damaging the sensitive printheads. When pre-printed paper is used, the effects described above become more pronounced. Since the printing ink of the printed paper is located between the printhead and the thermo-sensitive layer, it must withstand the high thermal and mechanical stresses. Unfortunately, even the best printing ink is not as durable as good thermal paper. As a result, during the printing process, more deposits accumulate on pre-printed paper than on pure thermal paper.
Therefore, some rules must be strictly adhered to: In the areas where printing is done, there should be as little ink application as possible, or even better, no ink at all. Only colors that meet the specified thermal printing requirements should be used; they must have a temperature resistance of at least 230°C.
Safety colors, metallic colors, or fluorescent colors must not be printed. Similarly, hologram foils should be placed outside the print image.
The heating power (darkening) must be reduced; a different „printer setting“ must be chosen.
Should the expression be made tamper-proof?
Generally, with the increasing value of a ticket, the risk of forgery increases. From the outset, you should therefore provide the ticket with a suitable security element. The following are available for selection:
„UV fibers“ = fibers that become visible under UV light „Anti-photocopying printing inks” = printing inks that are difficult to copy „Individualized holograms“ = holograms applied in addition „Micro-printing” = texts or digits so small that they are difficult to copy „Integrated RFID chips” = RFID tag embedded in the ticket or „IR-readable barcode“ = here the image produced by the thermal head absorbs in the infrared spectrum.
How important is our environment to you?
Thermo paper can and should be recycled! It is important for the use of thermo paper in connection with food that the thermal paper is approved by the ISEGA Research and Examination Society mbH, Aschaffenburg. This independent institute examines the papers with regard to contact with food and issues a certificate (ISEGA approval) if they are found to be safe. Some European manufacturers of thermo paper have also obtained the „FSC“ or „Swan“ environmental certificate. The award is based on compliance with standards for the total emission of pollutants, even during production.
Versatility of thermal papers:
Manufacturers of thermal paper offer a wide variety of different paper types in their product range, which have been optimized for a wide range of applications.
There are other ways to customize thermal paper for an application:
Water-resistant papers for refrigeration technology or for florists;
2-ply papers for creating through-holes in the POS area; colored papers, transparent thermal films, tear-resistant paper, etc. Even a 2-color print, e.g., red/black, is possible for better visualization. In any case, it is advisable to seek comprehensive advice before making the decision to use them. Because only a good choice will lead to a good result.
Author: Dipl.-Ing. (FH) Klaus Baldig is the Development Manager at GeBE