A Tale of Two Thermals

Direct thermal and thermal transfer both use heat for imaging, but only one is approved for UL 969 labels. Do you know the difference?

Admittedly because they sound similar, they can be easily confused. But each process requires very different materials–there is no one material fits all approach. In addition, permanency of the image varies wildly between the two. As such, it is important to understand each process. Here’s a breakdown:

di·rect ther·mal [dih-rekt | thur-muhl]
Printing method utilizing heat impinged upon a specially coated substrate so that the heat turns the surface black. No ribbon is required.

So what does that mean? Well let’s start with the components. Direct thermal is in many cases all about the material. It is coated with a special heat-activated layer. Apply enough heat, and the material will turn black. How much heat depends on the specific coating. Sometimes the friction from a swipe of the fingernail is enough, while other times you might need the heat of a lighter. The direct thermal printer applies this same principal through the use of a special a heated (thermal…get it?) printhead. Heat is precisely applied to the material to form text and images. Nothing else besides the heat and the specially coated material is used.

Perhaps the most common example of this would be a receipt from a cash register or label from a deli counter. These are quick variable information situations that need to last only a short while. That’s important why? Because direct thermal has a short lifespan. Ever left a receipt in the car and it turned black? Or found one in a forgotten pocket and it had faded into nothing? That’s the drawback of direct thermal. The heat reactive coating can be effected by outside factors and be altered. Because of this, it is not a suitable printing for UL 969 applications.

ther·mal trans·fer [thur-muhl | trans-fur]
Similar to a thermal print system, except a one-time ribbon and smooth surfaced materials are used to eliminate the problems of fading or changing color inherent in the direct thermal print process.

Starting with the components, we again have thermal transfer coated materials. There is no heat activation properties, rather it’s an ink receptivity coating. Next comes the ribbon. This is the big difference between the two. Thermal transfer requires a special ink ribbon. The printhead then uses heat to transfer ink from the ribbon on the material.

Because there is an actual layer of ink, instead of heat turned coating, thermal transfer is very durable. As a result it is often seen in industrial and manufacturing applications. The label on your freezer or on the back of your washing machine most likely have some if not all the printing performed with a thermal transfer process. It’s also because of this durability that thermal transfer can be an approved printing method for some UL969 applications (usually under the PGJI2, PGJI8 categories).

If you have trouble remembering the difference, think of it this way: direct thermal prints directly on the substrate with heat, while thermal transfer uses heat to transfer ink from the ribbon onto the substrate.