16 Jul Achieving the Correct Exposure
If you are among the many who have problems exposing stencils, this article will explain how to put an end to the stress that you are going through. Become a fly on the wall as we walk a printer through the correct exposure time before making the exposure, every time, without fail.
A month ago, I received a call from John, a novice textile screenprinter at his wit’s end. He stated that he was using an ultra-fast exposing, pure polymer direct emulsion that was recommended by his distributor. The emulsion was peeling from the mesh during development.
John assumed that since the emulsion was not cross-hardening, he needed more than the 20-seconds exposure time he used in his first attempt, which was recommended by the distributor. However, after increasing the time in increments of 1 minute, he was now up eight minutes with no success. At this point, he had spent nine days coating each of the screens and allowing them to dry. His question was just, “How long should I be exposing this emulsion?”
John had prepaid my fee to ask his question, and I was obligated to provide the answer. As is often the case, I would have to explain at some point that the answer will probably lie in the inability to accept the responsibility of thinking the problem through and following directions. If John had done this nine days earlier, the resolution would have been quick and easy. And, the enormous amount of time failing time and time again could have been saved. John gave a nervous laugh and a statement that he was ready to do whatever it takes to correct his errors.
I explained that to resolve any problem we would, start at the root of the issue and proceed in an orderly fashion.
I asked what sort of emulsion he was using and found that he had purchased Ulano Orange.
For those unfamiliar with this emulsion, Ulano Orange is ready-to-use one part emulsion. With its high solids content of 44 – 46% provides good mesh bridging on a coarse mesh, and especially good edge definition. Its high viscosity (or thickness) improves control when coating screens by hand and makes Orange an excellent choice. Ulano Orange is a fast-drying emulsion when a conventional emulsion over mesh ratio is utilized, which reduces the time to prepare a stencil.
Stencils made with Orange have an excellent exposure latitude and are incredibly durable when applied and correctly exposed. Ulano Orange is a fast-exposing SBQ-photopolymer direct emulsion formulated for imprinted sportswear printing. The fast exposure speed is of benefit to low volume printers with weak light sources.
I pulled up the Technical Data Sheet from my morgue of resources kept on my computer and asked him to do the same. Unfortunately, not knowing what a technical data sheet was, he had not known to ask for one when he ordered the emulsion from his distributor, nor had the distributor accepted the responsibility to send one along with the order.
Next, I asked John what sort of lamp he had in his exposure system, which the reply was that it was unit with fluorescent tubes. There are unfiltered black light tubes and super diazo blue tubes, which offer the fastest exposure times for fluorescent tubes, and then there are plant-light, filtered black light, and daylight fluorescent tubes which are all slower and can double the exposure time. He was using black light tubes, but they were unmarked as filtered or unfiltered.
From the photos he sent, I could tell that his coating was extremely thick as the emulsion created bumps as the emulsion flowed away from the mesh. With a 45% solids content, i.e., 55% water, if the emulsion flows this rapidly when placed in a horizontal position to dry, it is too thick. If stencils require a high emulsion over mesh ratio, it will prove more beneficial to begin with a thinner base coat and build up the thickness with face coats on the substrate side after allowing the base coat to dry.
The exposure time for Ulano Orange using unfiltered black light tubes can average between 1 minute up to 4.5 minutes depending on the emulsion thickness and between two and nine minutes for filtered, blacklight tubes. And, since his coating was thicker than the coating method suggested by Ulano, we knew the exposure time would probably be at least nine minutes and due to his thicker stencils could be as high as 20 minutes or more.
Determining Proper Stencil Exposure
Exposure times are based on the thickness of the emulsion. The only way to correctly determine the thickness of the dry emulsion is to use an electronic thickness gauge, which John did not have.
And, luckily, John had a friend who owned an automotive paint and body shop near his shop, and he was able to borrow an electronic thickness gauge for a few hours.
John called around and found a screenprinter who had a Stouffer 21-step Transmission Guide that he could borrow to use for our testing purposes.
I had him choose five screens of the same mesh count, thread diameter, and tension level. Then I instructed him to prepare the stencils in the following manner.
Step 1: Apply Magic Mesh Prep Degreaser (which he had) with a bristle brush for two minutes on each side of the tensioned mesh. Rinse with a pressure sprayer until all degreaser has been washed away. Be sure to remove any residue on the frame or where the mesh meets the frame.
Step 2: Coat the screen on the substrate side until the squeegee side is glossy with wet emulsion, followed by a single coating on the squeegee side. Place the coated screen in a drying cabinet at 72˚F (22˚C) at a relative humidity of below 50%.
Dry until the moisture level in the emulsion is between 4 to 6%. The moisture level should be measured with a pin probe (non-destructive) moisture meter. A moisture meter measures the internal moisture level of your emulsion with a touch of the Read button. The benefits of a moisture meter are:
- Stronger Exposures: When the entire emulsion film is dry it will expose much better.
- Better Press Performance: Once screens are dry and exposed well, presses stop less for pinholes or stencil issues.
- Controlling Screen Room Environment: High levels of humidity due to poor ventilation, wet weather, wet air due to nearby reclaiming will prevent screens from reaching the ‘green’ area of the meter. This indicates excess humidity is preventing screens from drying completely. Adding dehumidifiers, HVAC or moving reclaiming/developing processes will show better moisture levels in the emulsion when measured with a moisture meter.
Simply press the metal sensors against the dry emulsion and press the READ button to determine if it is ready to expose. This measures the internal moisture levels, not what may feel dry to the touch.
As John did not have a moisture meter, I gave him instructions on how to guesstimate stencil drying until he could order a moisture meter. In a pinch, you can lay the screen on a flat hard surface and then press down with powerful force on the emulsion with your forefinger and twist. If the emulsion does not have a slight twirling pattern, it should be ready for exposure.
Step 3: Measure the thickness of the coated mesh with the electronic thickness meter. In this instance, the five coated screens varied from 104.86-µm to 137.60-µm in thickness. The difference was due to a human not being able to control the speed of the coating stroke, which applied more emulsion when slow and less emulsion with more speed. It was obvious that whatever exposure was correct for the thinnest stencil, the thickest stencil would need more exposure to become fully cross-hardened throughout the thickness. I will explain more about that in a moment.
John’s 137-threads-per-inch mesh with a 64-µm thread diameter, tensioned to 25N/cm2 provided a mesh thickness of 98-µm thickness on all five screens as measured with the thickness gauge. However, after coating the mesh, his emulsion over mesh ratio varied due to not being able to coat the screens evenly. We started with the thinnest stencil, as determined by the thickness gauge and made a step test using the Ulano Exposure Test that he had purchased to determine what the proper exposure was for this particular stencil.
Step 4: Developing the stencil. We wet both sides of the stencil with cold water and waited for 30 seconds to allow the water to soak into the unexposed emulsion and soften it. Then we applied a forceful spray on the substrate side of the stencil until the image cleared in the stencil. We located the area with the best resolution and tested the squeegee side to assure that the emulsion was hard and not slimy.
The coated stencil measured 104.86-µm. After developing this stencil, we discovered that the proper exposure time for this stencil was 13 minutes.
The next stencil to be exposed was measured with the electronic thickness gauge, and it was 108.78-µm in thickness. Since we had determined that our base time for 104.86-µm was 13 minutes, we knew that for each µm of stencil thickness the exposure time as 0.124 minutes, we only had to multiply 137.60-µm by 0.124 to understand that the correct exposure for the thickest stencil was 17.06 minutes.
For this stencil, we registered and taped a film positive for a job to be printed onto the dry stencil and made the exposure for 17.06 minutes, and it proved to be a perfect exposure. The fine lines developed perfectly, as did a speck of dust on the exposure glass, which had to be blocked out as a pinhole.
To reiterate, the final stencil thickness for the five screens ranged from 104.86-µm to 137.60-µm in thickness, and the exposures ranged from 13.00 minutes to 17.06 minutes.
All five stencils developed just fine, and John was happy to have his problem solved.
I explained that John would need to test each combination in use at his shop as to mesh count, thread diameter, mesh color and coating technique to create a chart of the different base thicknesses and necessary exposure time based on final stencil thickness. Each of the combinations will likely have a different base number for exposure per micron calculation.
And, finally, I told John that he needed to purchase an electronic thickness gauge for USD$149. to start measuring his stencils to find the correct exposure time. And he would need to purchase a Stouffer 21-step Transmission Guide for USD$10 that he should place on his exposure unit in a position that would expose onto every stencil going forward. The Stouffer Scale will prove to be beneficial to ensure that the exposure was correct for each stencil. And, that if he notices the exposures progressively becoming more extended, it means that the UV power of the aging lamps is growing weaker and the bulbs need replacing.
I sent John the directions for altering his exposure when using the Stouffer 21-step Transmission Guide when his exposure lamps age or the weather changes. If the small Stouffer Guide is placed on every stencil made he can quickly determine if the exposure is moving away from the correct exposure needed to properly crosslink the emulsion and small steps can be made going forward.
John emailed three weeks later to let me know that he has since purchased an electronic thickness gauge, a Stouffer Scale and a slightly used moisture meter for USD$300. He is now measuring each of his stencils before exposure. He is more efficient and effective in his exposures and has not had any more failures. And, John stated that by measuring his stencils, he has been able to become more precise in his coating technique and the numbers are closer than they were when he did not know the thickness differences. He tracked his time savings and cost and quickly realized that his return on the USD$459 in precision process tools had a return on investment in less than 2-weeks for his small shop, and it was the best investment he had ever made!