UV Myths and Magic
By Robert D. Radcliff
Watts, Hour Meters, Joules, & Magic
I get calls all the time from people who are already in trouble with their UV curing systems. Their product isn't curing and they figure that a UV radiometer
will tell them what's wrong. It won't. A UV radiometer will measure the conditions in your curing system today. It can't tell you what the conditions were when life was good and product was curing trouble-free.
A radiometer is a tool, a very effective tool, at measuring the parameters of UV irradiance and UV dosage. You, the user, must make the judgment as to what those readings mean to you and your process. To be able to make that judgment, you should do some homework up front. You need to measure the various parameters that affect your curing process-as many as you possibly can. Don't guess. Just because the curing system worked fine on that job you did the other day, it doesn't mean that the system is set up to run the job you want to do today. You don't know what will happen if you haven't done your homework. UV curing is not magic; it's science.
If you want control of your process, you must measure and document every variable that can be measured. You should establish baseline parameters, document them, then constantly monitor the process and record the results. Compare your measurements to your baseline data and compare the results. As your database grows, you will be able to predict your curing results and set your parameters accordingly. You will also be able to predict when maintenance should be performed and schedule it during planned downtime. This paper presents the very basics that must be measured to begin controlling your curing process.
You Need To Measure, Not Guess
If you are responsible for the UV curing processes in your company, you must measure every parameter possible to control it and to consider yourself a professional. Since there are so many things you can't control that will bite you, doesn't it make sense to take control of every aspect that you can? "I dunno" is not an adequate response when things go wrong, and they will go wrong. It can be very costly for your company and for you professionally if you do not measure your process parameters and you produce uncured product. You know the downside of producing poor product. I'll skip that and tell you about the upside of measurement. Repeatability is the reason you measure your process parameters. Measuring allows you to establish control limits and helps you determine what went wrong when something goes awry. Leave nothing to chance. You are a professional.
Establishing Baseline Parameters
What do I measure? Among other things, the following items are a must when measuring for UV process control:
- A digital tachometer accurately measures belt speed or line speed.
- An hour meter indicates the age of the lamp.
- An amp meter gauges the current being supplied to the lamp.
- A radiometer measures UV dosage and peak UV irradiance.
There are other components that affect UV lamp output, such as ballasts, power supplies, capacitors, magnetrons and line voltage, but this is not meant to be a complete service manual.
Speed
Measure the speed with a tachometer to accurately gauge the line speed. Do not rely on built-in speed indicators because often they are not particularly accurate. Also, check the linearity of the speed adjustment. A setting on the speed dial of 30 fpm is not necessarily twice the speed as a setting of 15 fpm.
Many conveyor manufacturers do not use tachometers in their systems to measure speed. They measure the voltage to the motor speed controller and use that as an indication of the speed. This is nothing more than a programmable volt meter measuring voltage between the controller and the gear motor. This method might not be accurate for any given setting and also might not be linear. The percentage error increases as you depart from the point of calibration. You can be off 10-15 fpm and never even know it. Check the speed with an inexpensive digital tachometer. You also could buy a conveyor that has a built in tachometer. The point is to be able to ascertain the conveyor speed to control the energy.
Until you get a tachometer, you can calculate the belt speed manually with a stopwatch. Measure the total length of the conveyor belt all the way around. This is the total length not just from one end of the conveyor to the other, but also including the length around the rollers. This is easier if you first remove the belt and lay it out on the floor. Then, apply the following formula:
60 s/min x belt length (feet)
belt speed
|
=
|
time(s) for belt to
make one revolution
|
For example:
60 s/min x 24.6 feet
67s
|
=
|
22 feet/min
|
This is a very accurate method for measuring belt speed. Measure it at varying speeds and draw a chart correlating the speed setting to the actual speed. You should check your belt speed weekly. Believe it or not, it can vary from day to day.
Hour Meters
Virtually all manufacturers have hour meters installed on their UV lamp systems. These indicate how long the lamp has been powered up and should provide an indication of when to replace the lamp. However, these meters do not tell you how many times that lamp has been started. Starting the lamp is the most stressful part of running it. The hour meter also doesn't tell you what the conditions are under which the lamp was operated - how hot the lamp ran and how well it was cooled, when it was last cleaned and if the lamp was rotated on a regular basis. It also doesn't tell you if the lamp was properly handled so that no contaminants were deposited on the quartz surface causing hot spots, which eventually become weak spots. The manufacturer will specify that the UV lamp will last a certain period of time, usually 1,000 to 3,000 hours. Lamps may not last that long, or if properly maintained, actually last much longer.
Use the hour meter as an indication of the relative quality of your lamps. Note the number of hours each lamp lasts before you have to replace it. Note the lamp model number, date or lot code, and manufacturer. Then, compare this data over time. If the lamp you just replaced lasted 1,200 hours, for instance, you'll be able to tell if this is good or bad for your system and its operating conditions. The hour meter will tell you the length of time the lamp has been operated, but by no means does it tell you if there is sufficient UV radiation being emitted to cure your product.
Amp Meter
The amp meter is a gauge to indicate the current being supplied to the lamp. As such, it is a gross measure of the amount of electrical power being applied to the lamp. Lamps are not 100 percent efficient at converting this power to UV. In fact, only a small percentage of the overall power put into the lamp is converted to total UV output and even a smaller portion of that is of the specific UV wavelengths that cause the formulation to cure. Therefore, an amp meter is a poor indication of UV output.
The amp meter measures the appetite of the ballast for consuming electrical current. It does not take into account the condition of the reflectors, which are responsible for the majority of the UV reaching the substrate surface. Write down the amp meter readings anyway for each power setting on the power supply: 100 W/in, 200 W/in., 300 W/in., 400 W/in., etc. Allow the system to stabilize between taking readings on each setting. These readings will assist you in troubleshooting the system later. Also, note the UV radiometer readings for each power setting. For example, if you see the amp meter twitch and a corresponding dip in lamp output along with it, it could be an indication that a capacitor is beginning to fail.
UV Light
Do your homework. Decide what parameters you are going to use before you commit to a job and then take responsibility for it. Do not leave the outcome to guesswork. If possible, get information from your ink, coating or adhesive supplier regarding what wavelengths of UV are important for the formulation you are curing. What intensity levels are required to activate the photoinitiator? Your formulator knows exactly what was put into the formulation when it was developed. Remember, it's science - not magic. Most of the chemistry suppliers have UV measurement devices, so they can tell you what they expect to happen under certain curing conditions.
Your UV equipment supplier has information on the spectral output, intensity level and temperatures involved in using the system you purchased. Get this information. It may be possible to purchase lamps that spectrally match the response profile of your formulation. For example, if your product requires UV that is very rich in the 365-nm region, you can get lamps that have been doped to enhance the output in this part of the spectrum. Work closely with your equipment and chemistry suppliers to optimize your curing system. Basically, you want everything possible working in your favor.
You also need to establish your "cure window" before you begin the job. You have to know what parameters bound your upper and lower limits. Knowing the limits beforehand gives you an edge in troubleshooting problems later.
Set up your curing system using your best guess, perhaps using the settings from a similar job you've run previously. Continually monitor the curing characteristics as you increase line speed. Yes, that means running some test samples through that will end up in the trash. Pass a radiometer through with the product each time. When the cure properties are just beginning to become adversely affected and you are undercuring, note the line speed, the number of lamps you are running, the lamp settings, and the UV dosage and peak intensity values. Once you've identified the threshold of failure, multiply the UV dosage reading by 1.2 to give yourself a 20 percent cushion. This will be the upper limit of your cure window.
To establish the lower cure window parameters, slow the line speed down while monitoring the cure properties. When the cure properties become adversely affected (overcuring), once again note the line speed, the number of lamps you are running, the lamp settings, and the UV dosage and peak intensity values. Multiply UV dosage by 0.8 to give yourself a 20 percent cushion on the down side. Typically, overcure is really overheat. You may want to monitor substrate temperature as well.
You'll need to run these tests for each combination of ink and substrate that you use. Over time, you will develop a history that will allow you to cure this combination of chemistry and substrate properly time after time. As your database grows, the need to run sacrifice work pieces through the process will diminish. You can run quality control on your new lamps when they come in by comparing them to your established baseline data. You can track and then predict when the lamp needs replacement before it fails.
Watts
Lamps are specified in watts per inch, e.g., 300 W/in. This is a measurement of the power applied to the lamp distributed over the length of that lamp. For instance, if you apply 4,000 W to a lamp that is 10 inches long, you have a 400 W/in. lamp. The same 4,000 W distributed over a 20-inch lamp would be a 200 W/in. lamp. Now, this is input power. Very little of this power is actually transmuted into UV of any use.
The wattage specification is only an indication of the power consumption of the lamp; it does not tell you the UV output. The settings for 100, 200, 300 or 400 W/in. on the power supply are nominal figures. Each manufacturer's system is different, and furthermore, the actual Watts vs. the setting value changes because the oil-filled capacitors feeding the lamps age, causing a change in the lamp output. Watts into the lamp are converted to UV, visible light and IR (heat). The wattage level of the lamp is a better indication of the IR energy from the lamp. The wattage level determines the heat output. A brand new lamp has a higher ratio of UV to IR than an older one. The ratio of UV output to IR output declines over the life of the lamp. As the lamp ages, the UV output drops off, though you are still powering that lamp at 300 W/in. Where does that power go? It's must go somewhere, and if it's not UV, then it's heat.
Some lamps output more UV for a given wattage level. Since there are so many different lamps on the market today, we need to measure the UV output, not guess. The power to the lamp is first converted to IR, then to visible light and finally to UV. UV lamps are a good source of heat and light (that's why they are used to light stadiums). The smallest component of the output spectrum of a UV lamp is UV. So, when you get a peak UV intensity reading on a radiometer between 0.5 and 5.0 W/cm2, don't crucify your lamp supplier-it's about right. You should not expect a reading of 300 watts.
I have heard many people confuse the terms Watts and Joules. I'll explain the difference so you can impress your supplier and your boss. UV dosage is measured in Joules/cm2. MilliWatts are units of measure for lamp intensity, and milliJoules are a measure of the accumulation of the MilliWatts over time of exposure. The formula for calculating Joules/cm2 is: 1 W/cm2 x 1 s = 1 J/cm2. The calculation for MilliJoules is the same, only the UV level is 1,000 times less:1 mW/cm2 x 1 s = 1 mJ/cm2.
A good analogy to explain the difference between mW/cm2 and mJ/cm2 is one that most people new to UV seem to understand. Let's say that it is snowing outside. The rate at which the snow is falling, or how intensely it is falling, is analogous to MilliWatts. The total amount of snow that has accumulated in a given square area is similar to MilliJoules. Milliwatts are intensity; MilliJoules are dosage.
Reflectors
I need to insert just a few words regarding the reflector inside your UV irradiator. The reflector supplies as much as 70 percent of the UV that reaches the curing surface. The other 30 percent is emitted directly from the bulb itself; yet more time is spent discussing when to replace the bulb than reflector maintenance. Gauges such as hour meters and amp meters give you information about the power supplied to the lamp and how long that power has been applied, but they tell you very little about the UV energy that performs the bulk of the curing, which is reflected energy.
UV irradiators attract dust and fumes that are given off as the curing product passes through the intense UV and thermal environment. These impurities gather on the reflector, causing it to become less efficient. The UV portion of the light spectrum is much more difficult to reflect to begin with than visible light. As the reflector becomes dull, the UV wavelengths tend to be lost, even while the visible light looks as bright as ever. Generally speaking, the shorter the wavelength, the sooner its intensity drops off. If you have a means of measuring the various UV wavelengths separately, you will be aware of deterioration in the shorter wavelengths first. As the short-wave UV drops off in relation to the longer wavelengths, you will foretell the need to clean the system earlier than if you just monitor the longer wavelengths.
Document Your Measurements
Measuring does no good if it is not documented. I suggest you develop a form for entering data for each job run. The measurements only have meaning if you have something by which to gauge them. You need to be able to compare these measurements to the last time you ran a similar job or under similar circumstances. You will want to analyze trends and yields, and perform quality control on incoming supplies, such as lamps, reflectors, power supplies, capacity, inks, coatings, formulations, etc.
If you document the measurements properly, you will have a basis for comparison; otherwise, they are just random measurements. To illustrate, if you are presently driving your car 60 mph, that bit of data has a certain relevance. But it might have more meaning if you notice that you are in a school zone. It becomes an urgent call for action if you also remember that the last time you drove that speed you got a $150 ticket and the six points have not yet dropped off your record. Documentation may seem like a waste of time, but I guarantee you that if you do it faithfully, you will reap the rewards.
What should you do if you get readings outside the control limits? After you've established the upper and lower limits of your cure window, what do you do when the process reaches the edge of that window according to the readings on your radiometer?
Don't panic. Remember, you added a 20 percent margin of error to your threshold value. You can afford to wait until the current job is complete before performing system maintenance.
Maintenance of bulbs is relatively simple. Turn the equipment off and allow it to cool before handling it. The temperatures inside curing equipment can approach 500°F. Before removing the lamp, note the rotational orientation of the lamp. Clean all the lamps and the reflectors with isopropyl alcohol and a lint-free cloth. Be careful not to scratch the reflectors. Remember, over two thirds of the energy reaching the curing surface is from the reflector. Also, do not touch the quartz lamp or the reflector with your bare fingers. Oil from your skin deposits itself on the quartz and causes a hot spot that weakens the bulb in this area during operation. Fingerprints on the reflector can permanently mar the reflector material.
Clean moving mechanical parts such as the shutter mechanism. Vacuum debris such as paper dust out of the lamp housing. When you reinstall the lamp after cleaning it, rotate it 90 to 180 degrees from its previous orientation. This counteracts any bowing effect that may have occurred as the lamp was used at high temperatures for an extended length of time. It is possible, especially for long lamps, to actually bow out of focus.
Restart the curing system and allow it to stabilize.
Take your radiometer measurements again; it is likely that the readings have improved and no further action is required. If the readings still indicate substandard operating parameters, then change the lamp, reflectors or both.
Measure and Document on a Routine Basis
If you continue to log your measurements on a regular schedule, you will minimize the number of problems you have with your UV curing system. You will have data readily at hand to answer your supplier's questions when a problem arises. As you gain experience, the number of times you have to call out the cavalry will decrease.
One of the chief benefits you will derive from measuring and documenting your curing system is the improvement in the quality of the product reaching your customer. You will be able to avoid producing inferior product. Never expect your customer to do your quality control work for you, since you are a professional who has control of your process. Produce top quality products and you will sleep well at night. The results of doing all you can to control the curing process: your customers and your boss will think you are a UV magician!
