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Special Focus

Why Do You Need a Radiometer for UV Coating?

by Staff

August-September, 2005
While UV curing offers many advantages over other drying methods, it has its own requirements for maintaining process control. In particular, UV lamps and irradiators deteriorate over time, causing less energy to reach the cure surface. But just looking into a curing system to see if it is still working will not tell you if the system is actually producing any UV. In fact, as some lamps age, they actually produce more visible light, causing them to look brighter. Depending on several variables, lamps may last less than 250 hours or over 3000 hours. UV output must be measured in order to replace lamps only when necessary. The only way to accurately monitor UV output is by measuring with a radiometer. Using a UV radiometer can save time and money in the following areas:

  • Reduced set-up time
  • Reduced cost of spoiled materials
  • Lamps replaced only when necessary
  • Reflectors replaced only when necessary
  • Costly downtime avoided
  • Production of unsatisfactory product leading to unhappy/lost customers avoided

How a Radiometer is Used
A radiometer is used to quantify the dosage and intensity necessary to result in a proper cure, both in the lab before going to the production floor and while in a production environment.

Take a radiometer measurement after a test product is determined to be at optimal cure. This becomes the benchmark value for “good” product. Then continually monitor the curing characteristics as you increase line speed. When you are just beginning to undercure, note the line speed, the number of lamps you are running, the lamp settings, the peak intensity values, and/or UV dosage. Once you’ve identified the threshold of failure, multiply the UV dosage reading by 1.2 to give yourself a 20 percent cushion. Then slow the line speed down while monitoring the cure properties to measure for overcuring. Once overcuring is observed, multiply UV dosage by 0.8 to give yourself a 20 percent cushion on the down side. You now have your benchmark value, your minimum, and your maximum “cure window” value.

You’ll need to run these tests for each combination of ink and substrate that you use. Radiometers are typically used to take readings ranging from each day to even each hour to assure that the UV curing system is operating within your established guidelines.

What your UV Radiometer Measures
Radiometer functions range from measuring simple intensity and dosage to sophisticated mapping and graphing devices. Most UV radiometers on the market measure in the UVA spectrum (320nm-390nm), because the majority of the chemistry responds to this band of energy.

Most radiometry typically involves measuring UV Dose (millijoules/cm²). More sophisticated measurements involve measuring peak UV Intensity (milliwatts/cm²) in addition to UV Dose. The following is a good analogy to explain the difference between mW/cm² and mJ/cm²: imagine a faucet is dripping into a cup at the bottom of a sink. The rate at which the drips fall is analogous to mW/cm². The total amount of water that has accumulated in the cup is similar to mJ/cm²; mW/cm² is Intensity; mJ/cm² is Dosage. With a standard radiometer that measures only dosage, the detailed condition of the UV source cannot be determined. Particularly in a system with multiple lamps, you cannot tell if all lamps degraded equally or if there is a problem with just one lamp. The only way to get such information is to “map” the curing system with a more full-functioned radiometer that measures such information.

It is possible to fully analyze the entire curing system using a mapping device. It measures and stores the UV intensity and temperature data it encounters. The data is then captured and displayed on a computer.

Types of Radiometers
Pass-Through Radiometers A pass-through radiometer can be sent on a conveyor through your UV curing system. It accurately reflects how much UV energy your material to be cured is actually receiving. Specialty Pass-Through Radiometers Sometimes a pass-through radiometer cannot physically fit into and travel through the curing machine. For these circumstances, specially designed radiometers apply.

There are miniature sensors that can travel through nearly any type of unit. Generally coin sized, after retrieval these sensors are connected to a base unit that downloads the readings.

A new style of pass-through radiometer is the ultra-thin unit. With an ultra-thin profile of only 1/4”, the whole unit (not just a small sensor) can pass through the narrowest of curing environments.

When even the smallest sensor can’t fit, there are flexible, pass-through options that can safely travel through rollers. One option has 5 thermally-stable color changing zones on a flexible test strip. The other option also uses a flexible test strip. After exposing the test strip, it is measured by a dosimeter that produces a numerical value reflecting UV energy received. Online Monitoring Online monitoring is used when continuous monitoring is desired (not just spot-checking). While the overall cost of using online monitors is generally more than using one portable radiometer, if continuous measurements are required or if using a portable radiometer is NOT an option, online monitoring is desirable, effective, and reliable. Online monitors read relative intensity only (percentage of output compared to when the bulb was new). They are used to determine the ongoing efficiency of any given machine over a period of time. A measurement between different machines or different factories is not the aim of online monitoring.

Multiple Curing Stations
When you need to monitor multiple UV curing machines, a portable radiometer is less expensive than online monitoring. Radiometers allow machine-to-machine comparison. Readings also can be compared between R&D and production or to other plants. Probe-Style Radiometers A probe-style radiometer is best suited for UV environments that can be accessed only through a small opening.

Rigid Probes
Rigid Probes are handheld devices featuring a long, thin, tube-like sensor that can extend into UV curing areas while they are operating. Only a small access point is required to measure UV intensity.

Flexible Probes
A major benefit of some flexible probe radiometers is the ability to match removable probes to the specific UV wavelengths you need to measure.

You Have the Radiometer – Now What?
A radiometer will only measure the current conditions in your curing system. It can’t tell you what the conditions were when your product last cured properly. If you want control of your process, you must measure and document every variable you can. You should constantly monitor the process and record the results. As your database grows, you will be able to predict your curing results and set your parameters accordingly. The principle reason you measure your process parameters is repeatability.

Measure and Document on a Routine Basis
If you continue to log your measurements on a regular schedule, you will accomplish the following:

  • Develop a history that will allow you to consistently cure your combinations of chemistry and substrate properly.
  • Diminish the need to run spoiled workpieces through the process.
  • Perform quality control on your new lamps when they come in by comparing them to your established baseline data.
  • Predict when your UV lamps will need replacement before they fail.
  • Have data readily at hand to answer your supplier’s questions when a problem arises.
  • Call for assistance less frequently.
  • See improvement in the quality of the product reaching your customer.
  • Be able to avoid producing inferior product.

Any absolute measurement is only as good as the instrument’s calibration. Radiometers should be calibrated to a NIST (National Institute of Standards and Technology) traceable standard to provide accurate readings and ongoing NIST traceability. Typical recalibration is performed every 12 months. n

UV Process Supply serves the production needs of radiation curing professionals throughout the world. Using the brand names SAF-T-CURE and CON-TROL-CURE, all products are selected and tested to be the best for use in UV applications. For further details on Radiometers and other products offered by UV Process Supply, call (800) 621-1296 or visit www.uvps.com.