UV light output and exposure times
I thought I’d post some findings after testing my new platemaker unit from China. Spoiler alert, get a 21 step Stouffer gauge if you don’t want to waste tons of polymer while tuning.
I have a little experience with screen printing so photopolymer was not alien to start with. I originally had a tabletop exposure unit from Amazon with LEDs (only 16 LEDs in the array). My first plate I exposed for 3 minutes and the entire plate washed out. The Stouffer gauge got me to 16 minutes and plate was OK (still using table top unit), but gauge still was under the 16-18 Jet-USA recommended (by 2 stops). I decided to stop being subjective and I bought a UV light meter, it measures 3 bands, UV-A, B and C with A being what we need for photopolymer. To start, I measured daylight, 1PM in Florida, clear sky but Sun is still south 22 degrees and I’m 28 North. About 4500 uW/cm2 UVA (that’s micro watts i.e. 1millionth). UVB and C were negligible at 13uW. On a related note as soon as I stepped indoors UVA was unmeasurably low even with windows all around, so plates should be safe while handling. My table top unit was 5800-6200 uW/cm2 at the glass depending if between LEDs or over one. Interestingly, the higher reading was NOT directly over an LED but between 2.
Then on to my new platemaker. A Chinese Alibaba purchase, and the tubes say GERMANY PL-XL 36W which is Phillips. I set the sensor head on the vacuum table and turned on the lamps. Initially the reading was 12600 uW but the sensor is 1 inch tall and the plates would not be that close to the tubes. I removed the drawer and placed the sensor at plate height and got 8000 uW. Stouffer gauge has me at 12min for an exposure on the platemaker, so that slightly higher reading is reasonable.
I have a UV flashlight for spot curing 3d resin prints with a high power LED. At 2 inches above the sensor the reading was 40,000 uW! What a difference a light source can make. Makes me want to buy a roll of LEDs and change out from the tubes. Oh, and all artificial UV sources had UVB and C at zero. For a lark, I put one of my films over the sensor and UVA dropped to zero. My Epson eco tank with all-black-ink does well, it seems. Also, Kreene (or the Chinese version of it anyway) blocked 16% of the UVA.
End conclusion? No-one can advise you what your exposure should be, the range of light output is huge depending on source. For 11 bucks, get a Stouffer gauge, it’s indispensable!
If you do further research be careful of math units, some specs are given in W or mW and meters instead of cm. Also you will see mJ/cm2 which is Joules. That’s just time in seconds added to the mix. 1W/cm2 for 1 second = 1 Joule/cm2. 1W/cm2 for 2 second = 2 Joule/cm2. 2 Joule/cm2 could also be 2W/cm2 for 1 second.
Here endeth the lesson…
Jason
Good for you Jason. I think your scientific approach is definitely the way to go. People need to realize that the more they know about the theory of how things work, the better they will be able to solve problems that come up.
The rather primitive way we used to determine if a film negative was dense enough, in the absence of a UV meter or a transmission densitometer, was to hold the negative up to a bare fluorescent light tube on the shop ceiling. If we could see the tube at all through the negative, even the barest outline of it, it wasn’t dense enough.
Visually, my films show a slight pass of visible light, very red, indicating it is the longer wavelengths being passed (over 600nm based on color), but we are going back to subjective without a spectrum analyzer. But while mine would fail the “no outline of light” test, they’re great at UV. I hold good detail but all unexposed areas wash to bare backing plastic.
When we held the negative up to the light, it looked white or bluish if I recall correctly.
With plates having a clear plastic backing, which ours did, we made an exposure through the back of the plate first. This cured the photopolymer enough to prevent the washout process from washing down to the base. Then when we exposed the top of the plate with the negative, we would cure the image down to the layer we already created by the back exposure. The back exposure did two things. It created a polymer layer in the non-image areas which helped to support the image areas, and it made the as yet unexposed upper layer of polymer more light sensitive. By varying the back exposure, we could adjust the relief (height difference between the image and non-image areas).
Modern flexo plates have a black carbon layer on the top. They are not exposed with a negative. A laser burns the carbon layer off in select areas to create the image. It makes a “negative” which is 100% attached to the plate and no kreen or vacuum is needed to hold it in contact. After exposure, the “negative” is removed during the washout process.
Anyway, this is how flexo works. Flexo is a relief process like letterpress, except that the plate material is much softer, the inks are more fluid, etc. By explaining a little about flexo plates, perhaps it can give you some ideas that you can experiment with.