Discover the consequences of poor vacuum drawdown, and learn how to test the completeness and consistency of vacuum on your exposure unit.
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.
Did you enjoy this article? Click here to subscribe to the magazine.