Ensuring Uniform Vacuum Draw in Screen Exposure
Discover the consequences of poor vacuum drawdown, and learn how to test the completeness and consistency of vacuum on your exposure unit.
Last month, I spent some time addressing the issue of clean glass on exposure equipment. Here, I continue my look at screen exposure by focusing on the vacuum blanket and the vacuum drawdown process.
Few screen-printing topics seem more mundane than vacuum blankets. Yet, when you really look closely at this component of the screenmaking process and consider all of the implications it has on stencil and print quality, you begin to appreciate why this subject deserves to be addressed. Most notably, you need to be aware of vacuum drawdown because it directly affects image definition, acutance (edge sharpness), tone reproduction, moiré formation, and registration.
Function of the vacuum blanket
The fundamental purpose of the vacuum blanket is to hold the emulsion of the film positive in intimate contact with the emulsion of the coated screen. Anything less than intimate contact will cause undesirable light scatter (halation) that will result in undercutting and image degradation. The question is, how do you know that the vacuum is uniform and complete?
This is not an easy question to answer. Readings on gauges can be deceptive. I am often told by printers that they are achieving 26 in. of vacuum or some similarly high value. Is this good or bad? How do you know what is really happening? The answers to these questions are really quite relative.
In the first case, the pressure indicated on the gauge may or may not be reflective of the actual pressure on the screen where it and the film positive make contact. Most likely, the reading reflects the vacuum as measured at the vacuum inlet where it meets the glass. It is entirely too common for the inlet to seal at this location prematurely, resulting in localized high vacuum and no vacuum in the screen area. This obviously creates some big problems. Chiefly, the contact between the positive and the emulsion is incomplete, undercutting results, and the image is compromised.
To minimize the risk of this situation, always run a bleeder cord from the vacuum inlet to the center of the frame. This cord facilitates the complete drawdown of the blanket and prevents localized high-vacuum areas. One of the easiest, cheapest, and most effective materials to use for the bleeder cord is the common plastic cord used to seal window screens into their frame. It has very fine ribs that grip the window screen when it is forced into the groove of the window frame. These ribs make excellent channels for the air to escape from under the vacuum blanket of your exposure unit.
If your vacuum frame does not have a bleeder cord, you can simply add one yourself. Fashion a loop at the end of the cord approximately 1/2-in. in diameter. You can do this by taping the cord back on itself. Position this looped end directly under the vacuum inlet on your frame. This will keep the inlet from sealing to the glass while at the same time providing a channel for the air to escape. If you have more than one inlet, make one cord for each.
The cord should be at least long enough to reach the center of the frame. I usually make mine long enough to encircle the inside of each frame that I am exposing. It is important to keep the cord out of the image area, as this would leave a mark in the image upon washout.
Checking for consistent vacuum pressure
With the bleeder cord(s) installed, the next question is whether you are achieving a high enough vacuum for a good exposure. This is a bit more difficult to determine. The easiest test I have found is to look for Newton's Rings in the area of the film positive. This effect looks very similar to the rainbow-like shimmer you see when oil floats on water. An example would be a puddle in the street after a rainstorm. The Newton's Rings are very tight and reticulated. They almost look granular. The object is to have them be uniform and tight. If they are uneven or nonexistent, you have an insufficient vacuum.
To view the rings, put your face next to the vacuum glass, about 2-3 in. away from the glass. Adjust your vision so that you are viewing the area where the inside surface of the glass comes in contact with the film positive. Your viewing angle should be about 15°; relative to the glass. The best conditions for viewing Newton's Rings are under subdued indirect lighting, such as you might find in a typical screen room. If you do not easily see the rings, you may have a low vacuum or localized vacuum situation.
You may not have a vacuum frame that allows you to view the exposure side of the glass. This would be the case if you are using a fully enclosed exposure unit. With such units, the easiest way to make sure that you have good vacuum and screen/film contact is to create a test exposure with a positive that you know is in perfect contact with the emulsion.
The test image should be a halftone of 55 or 65 line/in. The halftone should contain 1% to 10% steps, in 1% increments. The idea is to determine what the finest dot is that you can expose. Your test exposure will be done on 305-, 355-, or 390-thread/in. mesh. If you do not use fine mesh counts, change the line count and use a coarser mesh (e.g., 35 line/in. on a 230 thread/in. mesh).
To assure perfect contact, we will lightly spray the positive with adhesive and apply it to the screen (I suggest using 3M's Scotch 77 Spray Cement). Hold the can 18 in. above the positive and lightly mist the surface. By holding the can high above, you allow the aerosol adhesive to slightly dry before it hits the emulsion surface of the positive. When you are done, the positive should be lightly tacky. You do not want to permanently glue it to the screen--this is a temporary situation. Make and spray adhesive on a few sample test patterns, then apply them on old screens to get an idea of how tacky you want the positive to be.
Once you have a film that adheres correctly, make your exposure based on previous run exposure-time tests that you've run. Wash out the screen and look at the halftone areas under a 10x or higher magnification loop. I like to use a 50x microscope. You should see cavities and craters where the very fine 1% and 2% dots washed out, but the mesh thread will be blocking the openings. Carefully determine at what line count all of the dots are visible and printable. This will usually be in the 3-5% range, depending on the mesh count and thread diameter. You now have a benchmark to which you can compare your vacuum-exposure results.
Attach the same positive (after you have cleaned off the adhesive) to a coated screen, place it on the exposure unit, and draw vacuum on the blanket. Make your exposure and wash the screen out as before. Now compare this screen with the test screen you exposed earlier. If the screen you just developed looks the same as the one on which the film was applied with spray adhesive, chances are very good that your vacuum is adequate. If you have missing dots or the dots resolve at a different tonal range, you are experiencing undercutting and halation.
Other problem areas
Inconsistent vacuum pressure can also result from vacuum leaks. These can occur due to broken or damaged gasket strips, which surround the vacuum blanket and are often attached with some kind of rubber cement. The gasket can lose adhesion, and consequently, compromise the seal. Examine the entire length of the gasket to make sure that there are no nicks or other problems that could result in an incomplete seal.
While you are considering the gasket seal, do not overlook the uniformity of the frame-to-frame seal. This means that the top frame of the exposure unit is parallel and even to the bottom frame. If the metal tube of the frame is too thin, it can warp over time. This results in partial-seal or no-seal conditions. One way to test for this is to use a smoke stick (available from companies like Granger or a laboratory supply house.) These sticks emit a nontoxic smoke plume that is used to test for air-flow direction and for leaks in seals. They are perfect for this use. Simply move the smoking stick along the gasket edge while the frame is under vacuum. Anywhere there are leaks in the gasket, the smoke will be drawn into the frame.
The blanket material itself can be a source of vacuum problems. I have seen numerous vacuum frames that use wetsuit material as the vacuum blanket. This material can work well, but it can also cause a lot of headaches. Wetsuit material comes in two types, cloth-backed open-cell neoprene and sealed-surface open-celled neoprene. The concern with both materials is the open-cell neoprene. This is a microscopic network of air bubbles that form the neoprene rubber layer. If the surface of the wetsuit material is compromised in any way, air will leak through the open cell portions resulting in an incomplete vacuum.
The most common causes for surface disruption on these materials are abrasion caused by repeated contact with frames during exposure and surface degradation caused by exposure to high levels of UV energy and ozone formed by exposure lamps. Both situations happen naturally over time. Also note that if you have a very large frame, the joint where the blanket is seamed is also highly susceptible to failure.
Instead of open-celled neoprene foam for a vacuum blanket, I suggest you use solid rubber sheeting. Many different types of sheet rubber are available that provide a variety of different properties. I do not want to go into all of the factors here, but the ideal material should be very flexible and conforming, tough, and capable of exposure to high levels of UV over a long period of time. Rubbers are rated for these kinds of situations.
Evenness of vacuum drawdown
I would like to conclude this discussion by touching on the evenness of vacuum draw. This is an increasing important aspect of the vacuum frame as more and more printers move toward pin registration systems. Ideally the frame should draw from the center out, in the form of an "X" (exposure-equipment patents have even been issued on this principle). The idea is that a uniform and progressive evacuation will guarantee registration accuracy by preventing film from creeping across the screen surface.
Because the forces exerted on the frame during evacuation can be substantial, it is very easy for the mesh and positive to creep unevenly as total vacuum is approached. The result is a distortion of the image on the stencil. You can see this distortion by comparing the image on the positive directly with the washed out image on the screen. The images should match-up perfectly. If they do not, revisit the bleeder cords to make sure you are drawing evenly.