Direct Thermal Labels
Direct thermal labels are manufactured with a heat-sensitive coating. When the label passes beneath the thermal printhead, tiny elements within the printhead heat specific points on the label. The heat causes the coating to chemically react, turning dark and creating the required image.
There is no ink ribbon, toner or other consumable involved. The label itself contains the material needed to produce the printed image.
This makes direct thermal printing a particularly simple and cost-effective solution. There is no ribbon to purchase, install or dispose of, and changing from one label to another is generally very straightforward.
How long do direct thermal labels last?
One of the main disadvantages of direct thermal printing is that the printed image is not as resistant to environmental conditions as an image produced by thermal transfer.
Direct thermal labels can be affected by:
- Sunlight and UV exposure
- Heat
- Prolonged exposure to light
- Moisture, depending on the label material
- Contact with chemicals, oils or solvents
- Friction and abrasion
It is therefore difficult to give a single maximum lifespan for every direct thermal label. A commonly quoted lifespan of around 12 months is a useful general guide for many standard applications, but it should not be treated as a universal limit. Specialised direct thermal materials can provide considerably longer performance under suitable conditions.
Storage conditions are particularly important. For example, a label stored in a cool, dark environment may remain perfectly usable for a considerable period, whereas exposure to strong sunlight or elevated temperatures can cause the image or background of the label to deteriorate much more quickly.
What happens to a direct thermal label in sunlight?
If a direct thermal label is exposed to strong sunlight or another source of heat or UV radiation, the heat-sensitive coating can begin to change colour. Depending on the material and exposure, the label may gradually turn grey and eventually become very dark.
This can be a significant problem where the printed information needs to remain readable for a long period.
Direct thermal labels and barcodes
Barcode applications deserve particular consideration.
A barcode scanner needs sufficient contrast between the dark bars and the lighter background. This is often discussed in terms of print contrast or Print Contrast Signal (PCS).
If the background of a direct thermal label begins to darken or the barcode itself deteriorates, the contrast between the bars and the background can be reduced. Eventually, a barcode that originally scanned perfectly may become difficult or impossible to read.
For this reason, direct thermal labels are generally best suited to short- or medium-life applications where long-term barcode readability is not required, unless a specialised direct thermal material has been selected and tested for the intended environment.
Typical applications for direct thermal labels
Direct thermal is particularly well suited to applications where the label only needs to remain readable for a relatively short period.
Typical examples include:
- Short-life food products
- Fresh food and produce
- Price-weigh labels
- Supermarket labels
- Shipping and logistics labels
- Courier labels
- Receipts
- Tickets
- Queue management
- Temporary identification labels
- Some healthcare and laboratory applications
The major advantage is simplicity: no ribbon is required, making direct thermal printing fast, convenient and economical for suitable applications.
Thermal Transfer Labels
Thermal transfer printing works in a different way.
Instead of heating the label coating directly, the thermal printhead heats a thermal transfer ribbon. The ribbon contains a coating of ink, often referred to as the “ink” or “resin” layer. When heated by the printhead, this material is transferred from the ribbon onto the surface of the label.
The result is a printed image that is physically transferred onto the label rather than chemically developed within the label itself.
The process can be thought of as:
Printhead → ribbon → label
rather than:
Printhead → heat-sensitive label
The use of a ribbon is the main difference between the two technologies.
Why choose thermal transfer?
The biggest advantage of thermal transfer printing is durability.
The resulting image can be considerably more resistant to environmental conditions, depending on the combination of ribbon and label material selected.
Thermal transfer labels can be manufactured to withstand conditions such as:
- Long-term storage
- Sunlight and UV exposure
- Moisture
- Refrigeration
- Freezing
- Abrasion
- Chemicals
- Oils and solvents
- Outdoor environments
- Industrial handling
This makes thermal transfer the preferred choice when the information printed on the label needs to remain readable for a long time.
It is particularly important for barcodes that need to remain scannable throughout the life of a product or asset.
Thermal transfer ribbons
An important consideration with thermal transfer printing is that the ribbon must be matched to the label material and application.
There are three broad categories of thermal transfer ribbon:
Wax
Wax ribbons are generally the most economical option and are well suited to printing on paper labels.
They are commonly used for general-purpose applications where extreme durability is not required.
Wax/resin
Wax/resin ribbons provide greater durability than standard wax ribbons and can be used for applications where the printed image needs better resistance to handling, moisture or abrasion.
Resin
Resin ribbons are designed for demanding applications where maximum durability is required.
They are often used with synthetic label materials and can provide excellent resistance to chemicals, abrasion, heat and environmental exposure.
The choice of ribbon therefore becomes an important part of specifying a thermal transfer label.