Getting the Best Results from Your LED Exposure System

Facts and figures illustrate optimal techniques.

Continued from How Suitable Is UV LED Exposure for Your Stencil System?

Once you understand the output characteristics of your exposure unit and the peak absorption of the stencil system you wish to use, you can better gauge its compatibility. Figure 9 shows the typical absorption curves for the three types of stencil systems (Red: Acrylate sensitizer – Combined with diazo in dual cure emulsions; Blue: Diazo – Alone in single cure emulsions; Green: SBQ – Photopolymer emulsions.) Provided that an exposure system generates sufficient energy at the wavelengths where the stencil systems are reactive, the crosslinking necessary to harden the stencil will take place.

To return to LED systems, most of the ones on the market now appear to have been tuned to provide peak outputs at either 385 or 395 nm. Some units have a secondary source at 405 nm. LEDs have a very narrow emission profile (typically +/- 10 nm), but all of the typical emulsions and capillary films in use today are sensitive in those wavelengths. The questions are how efficient the reactions will be and whether they will provide suitable stencils for printing.

First, you must have the correct procedures in place to produce optimal stencils. Making sure that coated screens are thoroughly dried prior to exposure is essential because water will detract from the crosslinking in the diazo reaction. Earlier, we observed that diazo reacts with the hydroxyl group of the polyvinyl alcohol. As you all know, the water molecule is H2O – or, written another way, H-OH. This shows that water also has a hydroxyl group that can be an unwanted part of the reaction. If the diazo reacts with water instead of the polyvinyl alcohol, crosslinking with another polyvinyl alcohol chain will not happen and a weakened stencil will result.

Also be certain that the exposure time is optimized. The common tool for determining this is an exposure calculator, which incorporates neutral density  filters to produce multiple levels of exposure (five, in the case of the exposure calculator shown in Figures 10 and 11) in a single operation. This tool is ideal for diazo and diazo/photopolymer stencil systems.

Because the diazo sensitizer is yellow and loses its color when it undergoes the chemical reaction, this “bleaching” effect should be used as a quick gauge of whether all of the diazo in the stencil has been exposed, indicating an optimal exposure. Resolution should not be used to determine optimal exposure with these systems.

SBQ products are different, since their color has nothing to do with a chemical change but more with stencil thickness and crosslink density. To tell if an SBQ product has been optimally exposed, check how soft the squeegee side of the stencil is (rub the back side of the screen with your finger to make sure you can’t run any soft, underexposed emulsion into the open area) and watch for a significant loss in resolution as a sign of overexposure.

For diazo-based systems, you can see this most easily during the washout stage. Figure 10 shows a wet stencil in the process of being washed out. The distinct color of the diazo can be seen in the three factors on the right side of the test. The one furthest to the right (0.25) permitted only 25 percent of the light to penetrate through; moving to the left, the next two panels allowed 33 percent and 50 percent of the light, respectively, to reach the stencil. There is a small color change between the 1.0 factor (blocking none of the UV light during the test) and the 0.7 factor, indicating that in this instance, the correct exposure time is likely very close to the time of the test. Ideally, you want to see no change between these factors, as in Figure 11.

Other variables to consider: Yellow mesh can certainly alter one’s perception of the test results, and dual-cure systems are trickier to determine as the photoinitiators also impart a yellowness change that can linger past what may be considered optimum for the diazo.

This is why color alone is not a sufficient measure. When the test stencil is dry, also compare the apparent visual robustness, stencil edge quality, resolution, and square edge areas to find the correct point with the highest quality at the longest exposure to aid on-press durability.

Traditional multipoint light sources will not provide the resolution or edge quality of a single-point metal halide light unit. In tests done on multipoint LED exposure units, I have also seen an impact on resolution and edge quality. (See the images from such a test below.)

   

(From left to right: Diazo emulsion 110 white mesh, LED cure; Diazo emulsion 110 white mesh, metal halide [diazo/photopolymer bulb]; Dual cure emulsion 110 white mesh, LED cure; Dual cure emulsion 110 white mesh, metal halide [diazo/photopolymer bulb].)

The difference may not be an issue for some companies, and indeed it isn’t perceived to be an issue by many users in the field today. It gets back to the resolution and edge definition that the application requires.

A secondary tool in testing screen exposure is a 21-tone step wedge. These are sometimes incorporated into exposure calculators, as shown in Figures 10 and 11. Step wedges aren’t ideal as they can make it difficult to judge suitable exposure adhesion, which can be exacerbated by the harshness of the water washout and also by the chemistry of the stencil itself. Products with very good wet stencil hardness may give false readings for the exposure. But step wedges are another tool to aid consistency.

The challenge of determining optimum exposure can be seen in Figure 13, showing a test screen made with an SBQ-sensitized emulsion where one half was exposed in an LED unit (left) while the other was exposed with a metal halide bulb (right). The optimal time on the LED unit, taking into account the 21-step grayscale, softness of the back side of the screen, and resolution drop-off, would be the 0.5 factor, or half of the test exposure time. Through cure of the LED unit’s wavelengths can be seen to be good. The test image exposed by the metal halide bulb does not show such good through cure, with an optimal exposure between 0.5 and 0.7 using the same evaluation parameters.

Whatever method you choose to test screen exposure, use it regularly and be sure you’re consistent in evaluating the results. Also be consistent in washing out the exposed stencils. Much of the industry uses pressure washers for this task, which is understandable as the screens can be quite large. I prefer to use the physics of water and the understanding that unexposed areas are water soluble, washing out the images with a high volume of water instead. Only then will I carefully use a pressure washer on a low setting with a wide fan to complete the washout. I have also found that this is the best way to handle the 21-step grayscale, taking extra care not to wash too aggressively.

I have been asked on numerous occasions about the value of post-exposing a stencil after washout for additional durability. I prefer to always expose a thoroughly dried stencil to the optimum level as determined by an exposure calculator. If the stencil is weak to start with, not much can be done to improve it. But the relative merits of post-exposure vary a bit on which stencil system you are using.

With diazo systems, there should not be much diazo left in the stencil to crosslink if the screen was dried and exposed correctly. A small amount of additional crosslinking can take place, but in my experience, the added resistance benefits are often minimal. Dual-cure emulsions, on the other hand, will have some unreacted acrylates and photoinitators even if the screen was processed correctly, so a second exposure can enhance the solvent resistance of the stencil.

SBQ photopolymer systems, although very fast-reacting, may also not crosslink completely during the initial exposure because of the way that the reactive elements in the chemistry link together. Further exposure, after the stencil is dry, can cause additional crosslinking, creating a more durable stencil with additional water and solvent resistance. Additional crosslinking can also be obtained using certain SBQ emulsions designed to incorporate a low dose of diazo. Although this reaction will slow the overall exposure of the stencil, additional solvent and water durability can be obtained. This is particularly useful if very aggressive inks are being used.

To conclude, are we better able to determine the effectiveness of UV LED exposure technology? We come back to my favorite answer: It depends. Theoretically, all direct stencil systems are sensitive in the energy regions these systems emit. The power and spectral bandwidth of the lamps are critically important. Ultimately, the overall durability, edge quality, and resolution you require will help you determine whether this technology has a place in your business.

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