Making plates isn't necessarily rocket science, but it is, indeed, a science! It is beneficial to track and quantify platemaking conditions so that changes in the equipment or finished plates are found before they cause costly problems on press.
Measuring the UVA light output of your plate exposure unit is a great place to start. Most plate exposure units use fluorescent bulbs to emit UVA light – the specific type of light that cures flexo plate material. With the help of a simple device called a UVA light meter, you can quantify the amount of light being emitted by the exposure equipment. Fluctuations in the intensity of the emitted UVA light can cause drastic changes in finished plate. These fluctuations usually happen gradually over time, but problems with bulbs or electronics can cause changes to happen more suddenly. Diagnosing the source of these issues correctly can save a lot of time and headaches.
Another advantage to tracking the UVA intensity of your exposure unit is that you can learn what dosage of UVA light works best for your plate material, yielding the best finished plate. We measure the total amount of light emitted over a span of time in millijoules.
mJ = mW x Exposure Time (in seconds)
Once the desired number of millijoules for a specific plate exposure is known, we can expect that it will generally stay consistent (unless there are other major changes such as polymer formulation, or large changes in ambient plateroom or storage temperatures). This can make coping with changes in exposure intensity much easier.
For example, you discover a bulb is out in your exposure unit. You order a new full set of bulbs to remedy the issue. The new bulbs are in and working properly, but where do you start with setting the new platemaking times? It's simple if you know your desired "dose" of UVA light for each plate's back and main exposure.
Here is an example:
Old UVA bulbs measured output = 11mW average
Satisfactory main exposure time = 12 minutes (720 seconds)
11mW x 720 sec = 7920 mJ
New UVA bulbs measured output = 18mW average
Known mJ dosage for main exposure = 7920 mJ
7920 mJ / 18mW = 440 sec (new exposure time - starting point)
As you can see, using this calculation to find the expected exposure time can help narrow down the steps needed to adapt to changes in the plateroom. It is always a good idea to fully process and measure a verification plate and make minor adjustments as needed.
For more information regarding this tech tip or platemaking in general, contact me at [email protected].