Thermal Printing - The Cliff Notes
- Thermal printing is an orchestra of many moving parts. The main conductor is the printhead. The printhead is the conductor of heat and composed of little square metal pins or pixels.
- Each metal plate consists of pins and are categorized in dots per inch (DPI). 203 dpi, 300 dpi, & 600 dpi are common options. 203 dpi is used in your typical shipping label whereas the 600 dpi might be used for a small barcode in which the lines need to be clean and crisp in such a small label space, such as tiny circuit board label or a label on your cell phone.
- Each pin can be heated up individually so depending on the image or barcode to be printed, the computer program tells each pin when to heat up and cool down to create the image needed.
- The pins heat and cool as the media (label or tag) move through the printer under the printhead.
- The pins in thermal transfer are square so as each pin heats it leaves a tiny square marking the size of a pinhead with straight edges that allow each marking to line up straight and touch, creating a clean line.
- Other printing technologies, such as laser, use rounded pins and lay down tiny dots.
- This is another reason why thermal printing is the best for printing barcodes. Clean and crisp lines are needed for optimum scanning/reading of the barcodes. Laser printed barcodes might look good to the eye, but under magnification, you can see the blurry edges from the laser printer laying down dots, whereas the thermal square pins allow the edges of the image to touch in a straight line.
2 types of Thermal printing:
Thermal Transfer – (“heat transfer”) in which the thermal (heated) printhead transfers the image onto the media substrate (i.e. label or tag) using a ribbon.
- This thermal transfer ribbon is coated with an ink that melts under the heat from the printhead pins and transfers the ink onto the label or tag substrate.
- The media substrate (label/tag) has a coating that allows it to receive and lock in the image.
- For the best image, make sure the ribbon formula (wax, wax-resin, resin) is matched properly with the appropriate label or tag material. For example, a wax ribbon would be used with a paper label and a resin ribbon would be matched with a polyester label.
- There are 3 main types of ribbon to choose from wax, wax-resin, resin as described in previous blog. Many media material substrates can be thermal coated. Paper, polypropylene, polyester, vinyl, etc.
- I often refer to the wax ribbon as a crayon that would take less heat to “melt” onto the paper substrate, where the resin ink is more like a permanent marker that would take more heat to transfer onto the slick, often glossy, synthetic material and create a lasting bond. The decision on which media to pair is driven by the needs of the end-user’s application. Thermal transfer is more durable and lasts longer than direct thermal.
Direct Thermal – (“direct heat”) in which there is no ribbon used and the pins heat up and there is a special coating on the substrate that reacts to heat and turns black, creating the desired image.
- Because this label is coated to react to heat and turn black, knowing the lifecycle of the label needed and how long the image (barcode) may need to last is key in determining IF direct thermal could work for your application. Direct thermal is a good solution if you have a shorter term need for the label and image and it is not exposed to constant direct heat, such as sunlight. If you take a direct thermal coated label and put in on your car’s dashboard, it will turn gray or black since the label is reactive to heat. Direct thermal media only fits if your application can take a paper-based label or tag and is not a harsh environment. A common benefit to using direct thermal is there is no ribbon to load. The label is the only media to load, which is generally quick and easy to load into a printer. Price is not often a driving factor since the price of the direct thermal labels is higher than the thermal transfer label, however, since a ribbon is not needed with direct thermal, the cost savings makes these solutions similar in cost.
- Insider Tip: For a quick self-test example to see how direct thermal reacts to heat, get a lighter and hold the direct thermal label over the flame (label side facing the flame) you will see it immediately turn black. For a thermal transfer label, it does not react the same to the heat from a flame.