Grafica News - Nov 2015
Understanding Exposure Lamp Variables - Part 3
by Dawn M. Hohl-Nowlin
One issue affecting exposure not often discussed is the uniformity of UV light hitting the stencil area.
Are corners of the stencil receiving the same intensity and quantity of light as the middle of the screen? If the difference is large enough the stencil can have reduced durability and poor image reproduction due to exposure variation. Light distribution becomes more critical if emulsion speed is fast and/or emulsion thickness is thin.
Another light distribution concern is hot spots. These occur when some exposed areas receive much more light relative to other locations. Image detail can be subsequently overexposed, to the point of closing in. Common casualties from hot spots are halftone highlight dots and small type or details.
Light uniformity is a result of lamp type and shape, lamp distance and placement, reflector design and cleanliness, and lamp age. Because these variables are complex, a radiometer is needed to map the exposure area to find the true light distribution picture. Measuring actual light takes all variables into account and can identify any major problems.
To map light distribution use the following procedure.
1. Divide the effective exposure area into 9 sections (Figure 1) or 25 sections for large format over 300 cm. Area should represent the largest emulsion area exposed for the lamp distance being tested. *Run this test at each distance used in production.
2. Using double sided tape, attach the probe, either to the outside of the glass (facing the lamp) or inside the vacuum frame (draw a vacuum before measurement), in the center of the grid.
3. Read radiant energy density (dose) for a 60 second exposure, and record. Repeat 4 more times in the center to total 5 readings. If it is only possible to read irradiance, record only the peak value reached during exposure. Average these 5 readings to establish a baseline for comparing the other readings.
4. Take a reading in the remaining 8 grid sections at a 60 second exposure.
5. Divide each grid reading by the average value calculated in step 3, and multiply by 100 to get relative percentage.
6. Evaluate relative percentages to see if they are within 20-30% of each other. Adjust lamp distance or position to get values within needed tolerance.
The type of glass used in an exposure unit can make a difference in how much UV light transmits to the stencil. Standard float glass, while inexpensive can contain iron that blocks ultraviolet light transmission by as much as 38%. The best option, while more expensive, is optically clear glass made with a reduced iron content allowing much more UV light to hit the stencil. Another excellent alternative is tempered glass which is much safer. Tempering improves the strength of the glass by 3-4X and if broken, it crumbles into small fragments instead of sharp shards (Figure 2). Tempered glass can be made with either type of glass formula, so select a low iron optically clear tempered glass to maximize UV light transmission.
Exposure glass should be cleaned regularly to remove dirt, stencil and adhesive residue, which will reduce pinholes. Select a glass cleaner without UV blockers, one specifically designed for graphic arts applications, and use lint free cloths. Silicone dust rollers (Figure 3)are another useful tool to remove fine dust from the positive and exposure glass. Grounding or antistatic measures in the exposure area can reduce the static (and subsequent dust attraction) created from the vacuum.
Intimate contact between the stencil and film positive is critical for accurate image reproduction, and becomes essential for halftone, fine detail and high tolerance printing. Inadequate vacuum will allow light to leak under the positive to expose the emulsion in the image area. This can close up fine details and highlight dots, affect registration and produce poor stencil edge definition.
To determine if proper vacuum is achieved, look for Newton rings to form during vacuum draw down. Newton's rings are a series of alternating, rainbow colored, bright and dark ring shapes that form between the film positive and glass when a tight seal is formed (Figure 4). You can see them by viewing the vacuum table glass at a low angle under subdued indirect light. Newton rings should appear where the positive and glass contact. There absence may indicate low or localized vacuum problem.
To address vacuum problems, inspect the vacuum frame blanket for leaks/tears and replace as necessary. Check the gasket strip running around the vacuum blanket for nicks or loose areas preventing an incomplete seal. Use a bleeder cord from the vacuum inlet to the inside of the frame to ensure drawdown is complete (Figure 5). A bleeder cord can easily be added by taping a cord with a small loop under the vacuum inlet of the frame. Make the cord long enough to circle inside the frame and avoid placing it on the image area during exposure.
Dawn Hohl-Nowlin is a Technical Trainer and Consultant on screen printing. Her industry experience includes 18 years with SPTF/SGIA, USA, overseeing screen printing workshops, developing training resources and conducting process related research. She is a member of the Academy of Screen and Digital Printing Technology (ASDPT). She can be reached at firstname.lastname@example.org