Willey explains how to maximize efficiency in your screen department.
By Jane Willey
You should also post and follow these standard parameters at all times: * minimum and maximum tensions for each mesh count * time that coated screens stay in hot drying * exposure times for each mesh count * setup measurements * coating procedures for each mesh count This list reflects only those items that are required for minimum control, so you may wish to include several others. The screen loop and the process checklist are the first set of constants necessary for screenroom efficiency. The second set of constants is the environmental conditions that you need to adequately support your screen loop. The environment The physical environment of your screenmaking department should be an enclosed area where new or reclaimed screens enter, cycle through the loop, and then exit into the job staging and production area. The entire enclosure should be temperature and humidity controlled; have sealed floors, walls, and furniture; have separation of wet and dry functions; be safelighted from coating through image development; have adequate ventilation; and be absolutely spotless! Maintain a temperature of 68-95°F in the general area, and 85-95°F in the hot (drying) rooms. The hot rooms must have circulating, filtered airflow. Maintain humidity in all dry-function areas at 50% RH or less. All surfaces should be smooth and washable, with no tendency to shed. For example, the area should have no raw concrete or unfinished wood surfaces (floors, walls, ceilings, racks, cabinets, etc). In my opinion, the only acceptable safelights are permanent sleeves that are manufactured for the requirements of the screen-printing industry. Yellow fluorescents ("bug lights") quickly lose their initial protection levels, and it is difficult to judge their effectiveness with the naked eye. Adequate ventilation is necessary for the entire area, but especially in those areas where VOC contamination is likely (screen reclaiming, coating, static-frame gluing, etc.). Fighting dust and dirt is a constant battle, but in this case, cleanliness equals quality, and quality equals efficiency. The screen loop and the screen department environment are process constants--their steps and conditions should never change. What does change is the amount of space and equipment you need to produce the correct number of screens to efficiently balance your production requirements. The larger your print-production floor and the more shifts you run, the larger your screen loop and screen department will need to be. The scale of operation The variables that determine the size of your screen department and screen loop are as follows: * number of machines * number of colors per machine (primarily garment printers) * number of frames per size (by outer dimension or "OD") * number of shifts * number of setups per machine per shift * number of mesh counts used Ideally, keeping six months' worth of records will give you a perfect profile of your plant's screen requirements. This information should give you a clear picture of a typical day (or shift). It will describe the number of jobs run, number of colors in each job, sizes (ODs) of the frames used, and mesh count mix. In lieu of these records, use the following formula to estimate your requirements: screen-loop size (SL) =
number of colors (C)
x number of setups per shift (SU)
x loop factor (LF)
x number of machines (M)
x number of shifts (S) The loop factor is actually a combination of three factors: * the number of production shifts supported by the number of screenmaking shifts * the variety of screen outside diameters (ODs) * the variety of mesh counts It is critical that your screen loop be large enough to keep the presses running. Yet, you want to be efficient enough to have the fewest number of screens possible to accomplish 100% print-production efficiency. Depending on your individual needs, the loop factor can vary from 1.75 to greater than 10. The ratio of these variables based on historical records will provide an accurate loop factor for any particular situation. If you do not have historical records, you have to estimate. I suggest using the following formula: loop factor (LF) =
shift ratio factor (SRF) x screen outside dimension factor (ODF)
x mesh count factor (MCF) You can find values for shift ratio, number of ODs, and number of mesh counts in Table 1. To see how the formula works, assume a shop runs one production shift and one screenmaking shift. This plant uses two frame sizes, but each press in the shop is dedicated to a specific OD frame. You know exactly how many of each frame size needed per shift, so it's as if you were only running one OD. This works any time the frame sizes employed during a shift remain the same. If each press wasn't dedicated to a single frame size and the frame sizes changed each day, then you would use the factor for two frame sizes from Table 1 instead. The plant uses three mesh counts.
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