EIT Part Number: P/N IM-0110 Rev A Issued April 2019
Page 4
Establishing a Cure or Process Window/Process Parameters
A cure, or process window, is a set of parameters that defines the equipment/process/source conditions necessary to
ensure proper curing.
Ideally, to establish a process window, a series of tests are run for different combinations of coatings, substrates and UV
sources so that a history can be obtained. This history helps establish the baseline parameters for various combinations
and provides important information for process control. The operator can compare a current measurement to the
established process window and adjust the process to obtain the desired curing results. A cure window assists in
troubleshooting when deficiencies in a UV process are found.
By analogy, when baking a cake, two process (oven) variables or process parameters are utilized:
1. Oven temperature
2. Cooking time
Correspondingly, with a UV process, there are three UV source variables or process parameters utilized:
1. Wavelength(s) of the UV source(s). This is controlled by the type of bulb used
2. Power (Irradiance, sometimes called Intensity, measured in W/cm
2
3. Energy (Energy Density, (sometimes called Dose), measured in J/cm
2
Each of these parameters can affect the degree of cure at the surface, at the substrate, or throughout the material.
1. Wavelength: The wavelength produced by a UV source is based on the source type. UV Broadband Sources are
mercury-based and characterized by a “broad” (i.e. wide) output across the UV spectrum. Typical bulb types include
mercury (sometimes called ‘H’), mercury-iron (‘D’) or mercury-gallium (‘V’). The actual spectral output can vary
from supplier to supplier.
2. Power/Irradiance: Irradiance is the “brightness” of the light source. Irradiance is measured in Watts (W/cm
2
) or
milliWatts (mW/cm
2
) per square centimeter. Irradiance is sometimes also called intensity.
Irradiance is important for adhesion of the ink/coating the substrate. Irradiance generally decreases with the square
of distance as you move farther from the UV source and/or as the light source output diminishes (reduced power)
for any reason. If you move twice as far away from a typical broadband source, you would expect the irradiance to
decrease by the square (2
2
) of the distance change and be approximately ¼ (25%) of the original value. This
inverse square law does not always hold true for UV LEDs.
3. Energy/Energy Density: Energy density (sometimes referred to as “dose”) is a measure of how much power was
received over a period of time. Energy is measured in Joules (J/cm
2
) or milliJoules (mJ/cm
2
) per square centimeter.
The Energy Density value is important for the complete cure of the material. It is sometimes the only value shared
by a formulator.
Note: In a UV process, variables or process parameters related to the formulation (ink, adhesive, coating, etc.)
and substrate must also be tracked and controlled. Examples include can formulation thickness and substrate
dyne levels.
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