TSB
screen printing => Screen Making => Topic started by: rmonks on March 04, 2012, 08:12:16 AM
-
I have got to fine tune my screen exposure times, so I am getting ready to make a test and start reading the instructions, it says to double the time you expect to expose the screen in, WELL my 3140 does not have a timer it has a light integrator, and measure light in units , so how do I go about doing a text.
-
I have got to fine tune my screen exposure times, so I am getting ready to make a test and start reading the instructions, it says to double the time you expect to expose the screen in, WELL my 3140 does not have a timer it has a light integrator, and measure light in units , so how do I go about doing a text.
Time is not the proper way to measure the amount of uv your screen is receiving . Light units is what you want to know as the integrator will automatically adjust for less output as the bulb ages. A new bulb in a MSP3140 should probably be set for 100 light units and work your way up or down depending on the type of emulsion being exposed.
-
which emulsion are you using? thats where you start. A photopolymer like Ulano, is about 26 light units. A water resistant dual cure is about 175 light units. This is assuming your bulb is on the newer side.
-
This is assuming your bulb is on the newer side.
Shouldn't that not matter? The integrator automatically adjust the time. A light unit is a light unit. If it takes 100 light units then it takes 100 light units. If you have a 1k bulb it will take X time if you have a 5k bulb it will just take less time. And transversely it will just take longer if your bulb is older.
Unless the integrator doesn't really measure the proper waveform and your bulb is SUPER old you shouldn't have to adjust light units as it ages. Just for EOM and emulsion type.
-
This is assuming your bulb is on the newer side.
Shouldn't that not matter? The integrator automatically adjust the time. A light unit is a light unit. If it takes 100 light units then it takes 100 light units. If you have a 1k bulb it will take X time if you have a 5k bulb it will just take less time. And transversely it will just take longer if your bulb is older.
Unless the integrator doesn't really measure the proper waveform and your bulb is SUPER old you shouldn't have to adjust light units as it ages. Just for EOM and emulsion type.
in my experience 14yrs., I NEVER adjust due to an aged bulb. Although my supplier says I should. Just my experience.
-
You can do a simple step wedge test, like this one:
http://www.smrsoftware.com/vellumexposure.php (http://www.smrsoftware.com/vellumexposure.php)
You can print out most any large film that has a mixture of text, lines and patterns, just make sure it repeats across the screen so you can test the exposure and resolution across each exposure area.
Check the tech literature on your emulsion to see the suggested amount of units the emulsion needs. Use this as a starting point.
Let's say it's 100 units for this example. We'll use the suggested formula in the step wedge as above, which has 8 exposure steps.
100 x 1.5 = 150 / 8 = 18.75
You would expose each step in the test at 18.75 units.
The proper exposure for a given screen is the step where the emulsion is fully exposed all the way through, and any fine lines, text and dots are still open but do not wash out. So in the above example, in let's say the 5th step the exposure looks good, you would have a proper exposure at 93.75 units. (or round to 94)
Now, if you have a bigger screen, and more time to kill, you could do more exposure steps, and get a finer result. On a 23x31 screen I would do 10 steps or more.
100 x 1.5 = 150 / 10 = 15 units in each step.
To get a proper charting of exposures, you will need to do a full step wedge test for each mesh count you use.
The nice thing about an integrator is you do this just once per emulsion/mesh combo, not several times a year like with a timer.
NOW.....
Ulano and other manufacturers make a one-step calculator film. This is a two layer film test. One layer (the one that contacts the emulsion) has a repeating pattern of text and lines in various points. The second layer has graduating sections of neutral density filtering. Each section filters out more and more light.
With this, you do ONE exposure, and the graduated sections of filters reduce the amount of light energy hitting the screen in steps.
It's a step wedge test, with all the steps done at once. Handy.
(http://www.texsourceonline.com/images/Product/large/1769.jpg)
-
This is assuming your bulb is on the newer side.
Shouldn't that not matter? The integrator automatically adjust the time. A light unit is a light unit. If it takes 100 light units then it takes 100 light units. If you have a 1k bulb it will take X time if you have a 5k bulb it will just take less time. And transversely it will just take longer if your bulb is older.
Unless the integrator doesn't really measure the proper waveform and your bulb is SUPER old you shouldn't have to adjust light units as it ages. Just for EOM and emulsion type.
Rich will know better, by my guess is that emulsions are sensitive to a spectrum of wavelength and the integrator only measures one specific value. The ratio of the measured and used (correct wavelength for the particular emulsion) light can change over time as the bulb gets older. The differences are minute and for all practical purposes probably not even worth worrying about, but when the high definition is important it does play a role.
Another factor could be the ability of the light to penetrate the emulsion. The stronger the bulb, better it will penetrate all the way through. Weaker bulb will produce the same amount of light (over longer time), but the opposite side might not be linked as well as it used to be when the light was stronger.
I have noticed the improvement in the stencils (ever so slightly) after replacing the bulb. So it does make a difference somehow . . .
pierre
-
This is assuming your bulb is on the newer side.
Shouldn't that not matter? The integrator automatically adjust the time. A light unit is a light unit. If it takes 100 light units then it takes 100 light units. If you have a 1k bulb it will take X time if you have a 5k bulb it will just take less time. And transversely it will just take longer if your bulb is older.
Unless the integrator doesn't really measure the proper waveform and your bulb is SUPER old you shouldn't have to adjust light units as it ages. Just for EOM and emulsion type.
Rich will know better, by my guess is that emulsions are sensitive to a spectrum of wavelength and the integrator only measures one specific value. The ratio of the measured and used (correct wavelength for the particular emulsion) light can change over time as the bulb gets older. The differences are minute and for all practical purposes probably not even worth worrying about, but when the high definition is important it does play a role.
Another factor could be the ability of the light to penetrate the emulsion. The stronger the bulb, better it will penetrate all the way through. Weaker bulb will produce the same amount of light (over longer time), but the opposite side might not be linked as well as it used to be when the light was stronger.
I have noticed the improvement in the stencils (ever so slightly) after replacing the bulb. So it does make a difference somehow . . .
pierre
You are correct Pierre in the exposure unit does not know the type of emulsion nor the thickness.the wedge test Evo suggest will get you exactly where you want to be as nothing will do better than an actual exposure test. I have seen customers using 100 percent photo polymer emulsion exposé screens in 7 seconds and 9 seconds was too much but also have seen where the emulsion has taken 5 minutes. The wedge test is the answer here for this person but 100 light units will be a good starting point. The person here at NuArc that can really give a lesson on all the variable is Ron Hopkins who monitors these forums quite often. Anyone wanting specifics should call out to him.
-
I have noticed the improvement in the stencils (ever so slightly) after replacing the bulb. So it does make a difference somehow . . .
pierre
The difference is the longer the exposure time, the more time there is for light to creep through the mesh threads and scatter and refract, causing loss of sharpness at the edges of the stencil image areas. This is what dyed mesh is for, it reduces the light creep. (think of each mesh thread as a fiber optic strand)
You always want the exposure to take as little time as possible.
An integrator will help to ensure proper exposure, but a newer lamp will ensure a faster exposure and a sharper stencil.
-
Evo, but wouldn't a brighter light just mean that the "scatter" would just be brighter as well? Meaning that "undercutting" would be just as significant with a brighter light as it would a dull light.
I think Pierre's assessment of light penetration all the way through would seem to make more sense to me.
We would obviously need some ridiculous amount of expensive research to really know what is all happening here and it's unlikely we will see this as we are talking about such a small difference and that difference is mainly noticed by the pros like Pierre. ;)
-
Evo, but wouldn't a brighter light just mean that the "scatter" would just be brighter as well? Meaning that "undercutting" would be just as significant with a brighter light as it would a dull light.
I've been wondering the same thing--wouldn't using dyed mesh and emulsion, as well as blacking out any non-reflector surfaces inside your exposure unit affect this to a much greater degree than the wattage, or amount of UV? It certainly does in thin stencils, from what I've seen. Not sure if the actual cross-linking of the emulsion is better, or strengthened somehow by a faster UV reaction. Of course, in thicker stencils, the higher wattage is essential, as Pierre mentioned--but I have heard of people having to dial down their wattage--that supposedly, too fast is just as bad as too slow?
Who's got an AL-100? ;)
-
Evo, but wouldn't a brighter light just mean that the "scatter" would just be brighter as well? Meaning that "undercutting" would be just as significant with a brighter light as it would a dull light.
I think Pierre's assessment of light penetration all the way through would seem to make more sense to me.
More powerful/brighter lamp means more UV light energy hitting the emulsion means faster exposure (all the way through the emulsion) means less time for the light to scatter means sharper images.
-
Here is the simplest and easiest way to measure exposure with Diazo based emulsion..
Exposure testing with the wedge step is accurate, if YOU produce films with the same film the test was on. It was made from an imagesetter with 2400dpi resolution on thin and clear film
If you use cloudy waterproof film with budget ink refills from your 1400... you will be underexposed after you run your wedge test.
Burn your screen, if you can see a color difference from where the film was to wasn't... you're underexposed.
Add more light until that color line goes away. then back off for detail of your dots.
Photopolymer users are on their own as the only true exposure test is detail. If your dots won't wash out, back off till they do.
-
I assume the gist of it is that a more powerful light will 'push' UV to the print side of the stencil, moreso than a weak light, which will harden the shirt side quickly, but then hit cross linking 'traps', before it reaches the print side, resulting in an unusually long burn time to fully expose the print side. That certainly makes sense.
Great tip on the exposure calcs--I actually use a piece of the positive material I'll be burning under half of an exposure calculator lengthwise--the unusually crappy ones will change a stouffer test by two or three steps.
-
I assume the gist of it is that a more powerful light will 'push' UV to the print side of the stencil, moreso than a weak light, which will harden the shirt side quickly, but then hit cross linking 'traps', before it reaches the print side, resulting in an unusually long burn time to fully expose the print side. That certainly makes sense.
Yeah, that makes sense.
-
Guess you caught me napping on this one. I will not try to go back over all that has been said, but will give a few comments on the integrator and light source.
Concerning the integrator, much like a measuring cup, it is looking for a quantity of light and when the specified amount is seen it shuts the light off. This allows the unit to adjust for the aging of the lamp and also for changes in line voltage coming into the unit. There is a photo cell that looks at the light and sends a small amount of voltage to the integrator. In order to get the most accurate exposures, the lamps are made to output a specific nanometer range to match the emulsion. We put different filters in front of the photo cell depending on what lamp is being used so that we are not just looking at the quantity of light, but the quantity of light within the range of light that the emulsion needs. We all use the term "units of light", and as most of you know that is more of a number than a quantity. Honestly, it does not matter if you are exposing for 20 units or 120 units as long as you are getting the correct exposure and it is consistent. When we set up a unit in the factory, we set up it up so that 30 units is approximately 30 seconds. This gives a good starting point to work from. If you are dealing with a unit that has been in use for a period of time, then it will slowly take longer to reach the same amount of light as when the lamp was new. Other things that can effect it are the voltage tap settings and the age of the lamp. If you are dealing with an older unit and not sure where to start with your exposure tests, try setting the integrator at 60 units and time it to get an idea of the units to time relationship. Then you can usually get a starting point from the emulsion manufacturer for testing.
I will agree with some of the comments about the effect of time and light. In addition to having a good vacuum, the most important thing is to use a lamp that outputs most of its energy within the proper nanometer range. That way most of the light energy is being used for exposing the emulsion. If the lamp is not a good match, then it takes much longer for the correct quantity of light to effect the emulsion and this is when you get unwanted effects by having the wrong wave length working on the emulsion for an extra long period of time. The wattage of the lamp also plays in here. Larger units with the ability to expose multiple or larger screens, require that the lamp to screen distance be increased to get even coverage. As this is done, a more powerful lamp is needed and the wattage is normally increased to compensate for the difference and the fact that these shops are probably making more screens and desire faster exposure times.
Ron Hopkins
NuArc Sales Mgr.
M&R Sales and Service