The Kodak ThermoFlex device does not directly create a flexo plate. Digital flexo media for use with the ThermoFlex has a black carbon layer (the mask) attached to the face of the digital plate material. The ThermoFlex ablates (removes) the areas of the carbon mask where images exist. The carbon mask is considered to be the equivalent of a traditional negative film. This digital flexo platemaking technique is known as Laser Ablatable Mask System (LAMS).
After the mask is exposed, the plate material is placed in a UV exposure unit to activate, or polymerize, the plate. Where there are voids in the mask, UV energy passes through and creates bonds in the plate media. The unexposed plate material is a monomer, which has individual free-floating molecules. After exposure to UV energy, bonds form between the individual molecules, linking them into long chains called polymers.
Figure 1. Digital plate laser ablation using LAMS
The mask ablation and UV exposure process resembles the traditional process of exposing plate media with negative film. However, there is an important difference: where the image areas exist, there is a physical void in the mask material. This void allows oxygen to interact with UV energy as it exposes the monomer material, effectively inhibiting the formation of polymer bonds at the edges of the exposure. This oxygen inhibition has the effect of sharpening dots and images, allowing for much finer and cleaner dots than is possible with analog (film) exposures.
Figure 2. Film (left) vs. digital (right) dot exposures
ThermoFlex devices are configured by default to expose an accurate, linear mask. Linear output accurately reproduces the full range of tonal values existing in the original file. For example, a 25% tonal region in the original digital artwork would be imaged on the device at exactly 25%, a 50% region would be imaged at 50%, and so on. This one-to-one matching of input values to output values is shown as a straight, diagonal line on an X-Y graph, and for this reason is referred to as linear output.
Many operators familiar with digital film output are used to creating linearization curves to compensate for daily changes in the process of digital film exposing and developing. Linearization curves are not required to maintain ThermoFlex accuracy. ThermoFlex devices are linearized when installed by a Kodak service representative, and should remain linear.
If an imaging test shows that a device is not imaging accurately throughout the tonal range, you should not create an adjustment curve to linearize the device. The lack of linearity indicates a fundamental imaging problem, and you should contact a Kodak service representative. The service representative may make adjustments to power, exposure, drum speed, or debris removal to bring the device back into linearity.
Due to the stability of Kodak devices, it is not necessary to monitor the entire tonal range to confirm accuracy of imaging; it is only necessary to monitor a 50% tone swatch on an ongoing basis. ThermoFlex devices are designed to lock into a linear imaging mode when a 50% value is accurately imaging as 50%. If that midtone value on the black mask is accurate, the rest of the tonal range falls into linearity. On the ThermoFlex Wide device, a simple 8x8 pixel checkerboard is typically used.
Figure 3. Typical 50% target
On ThermoFlex Narrow/Mid devices, you can use a 50% tint block of any dot shape. You can use a traditional transmission densitometer for reading exposed black masks. Additionally, you can use CCD readers designed for reading flexo plates, like X-RiteDot, CCDot, Betaflex, or QEA to read the mask after exposing.
Figure 4. QEA CCD reader positioned on flexo sleeve to read a mask
Note: The black carbon mask is fragile and susceptible to scratching—use extreme caution when measuring exposed masks. It is a good idea to include separate test patches for measuring on an output flat. If test patches are not part of a live plate, it is not a problem if the test patches get scratched.
After the device has been configured, you should check a 50% patch for accuracy at least once a week, as part of a managed quality-control system. If the patch no longer measures 50%, you should arrange to have a Kodak service representative adjust the device.
After the device has been configured to accurately expose the carbon mask, UV exposure values can then be determined by working together with the media representative.