Grafica News - May 2015
Understanding Exposure Lamp Variables Part 1
By Dawn M. Hohl-Nowlin, USA - Uncommon Enterprises LLC
Exposure is one of the most critical control points in screen printing. Incorrect or inconsistent exposure can create costly problems in production including stencil breakdown, excessive pinholes, sawtoothing, loss of detail, inaccurate image reproduction and reclaiming difficulty.
The objectives for stencil exposure include faithfully reproducing the original film/image and fully harden the stencil to stand up during the print run as well as allow easily screen reclaimability. A number of variables must be understood and controlled to achieve these characteristics. The exposure lamp's effectiveness is a function of the lamp configuration, type and power, lamp distance, and lamp age. Other exposure unit considerations include uniformity of light distribution, vacuum table glass and vacuum blanket type, condition and draw. In this multipart article we will take a closer look at each of these areas.
Lamp Configuration, Type and Power
There are two classes of exposure systems, the point light source and scatter, all things being equal.
Multi-point systems by definition use more than one lamp to create the needed light for exposure. A bank of special UV fluorescent lamps is the most common system in this category. These exposure units use a row of tubes at a relatively close distance to the glass and to each other to produce uniform distribution of light. Because of the number and length of the bulbs, light hits the emulsion in many different directions creating light scatter. Light scatter builds with time and will harden the stencil inside the image outline, closing up detail and reducing dot size. Exposure times with these systems tend to be longer as well due to the lower intensity of the lamps. These issues limit to the image accuracy and resolution that can be achieved with multi-tube exposure units.
Large format screens are often exposed with multiple lamp arrays, which creates a multi-point light source. These systems have inherent light scatter where the light overlaps, even when they are properly calibrated. But the benefits of shorter exposure times, and minimal space requirements offset these issues.
All UV light sources are not the same, and should be matched to the UV sensitivity range of photo emulsions. There are three main emulsion chemistries each with their own UV responses: diazo, SBQ (also known as one part photopolymer) and dual cure which is a combination of the two. Diazo emulsions are sensitive in the 370-420 nm range, SBQ emulsions have lower sensitivity between 320-370 nm, and the dual cures respond to both. Selecting an emulsion that responds to the spectral output strengths of your exposure lamp will keep exposure times lower and more efficient. While emulsions have preferred sensitivities, any UV lamp producing 320-420 nm waves will expose any of these emulsion types.
New on the block are multi-point UV LED lamp exposure systems which look to offer fast exposure times, efficient power usage, consistent light output over time and long lamp life. The few LED exposure units currently on the market are geared toward garment printers. The technology is worthy of research and observation as it develops.
Correct Lamp Distance For point light sources, the correct lamp distance to the exposure frame is important to ensure even distribution of light to the stencil. If the light source is too close Figure 2, the light intensity will be high in the center and quickly fall off at the edges producing severe light undercutting, gross underexposure and loss of image integrity.
For movable lamp systems Figure 3, industry rule of thumb describes the minimum lamp distance to be equal to the diagonal of the area to be exposed. If several screens are exposed at once, the diagonal of the total image area of all frames must be used. The distance is set from the glass cover of the exposure lamp to the glass of the vacuum frame. While this is the minimum lamp distance, a better recommend ation prescribes 1.5 times the diagonal of the emulsion area being exposed. The increase in distance produces a more uniform light across the surface area, but also increases exposure times. It is important to note any change in distance will require a new exposure time. On enclosed fixed distance systems Figure 4, the lamp distance is already preset to accommodate the vacuum area size.
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