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Prepress & Screen Making



The diversity of screen sizes, mesh counts, and quantities of frames in the typical shop never ceases to amaze me. Yet, the screenmakers constantly complain that they lack enough frames or they're out of the mesh they need for the jobs on the schedule. It occurred to me this area has not been addressed, and badly needs attention.

To balance the capacity of the screen department, a number of variables need to be considered. These include standardization of mesh counts, selection of frame sizes, determination of minimum and maximum frame-inventory levels, and cycle requirement. Most shops are vaguely aware of these concepts, at least in the back of their minds, but no formal organization has been applied. This month we will have a primer on how these relationships work together so you can check your own situation an determine how efficient your shop is in this area.

Mesh-count selection

Let's start with selection of mesh counts. My experience has been that most printers are using far too many mesh counts. UV printers have it the easiest. They can settle on two or three mesh-count/thread-diameter combinations and be very satisfied. The textile printers will need four or five under most normal conditions.

The most rational approach, and the one that has worked for me, is based on the premise that the mesh must pass ink in a uniform manner and the stencil must be able to maintain a specified level of detail. My goal is to find the relatioship where detail is maintained and ink passes with the least amount of added squeegee pressure. To do this, I balance the percentage of open area and thread diameter.

When you study manufacturers' mesh charts, you'll notice most mesh counts are woven with two, three, or even four different thread diameters. The older designations were S (thinnest diameter) , T (most common diameter for that mesh count) , M (medium diameter for a specific mesh count), and HD (heavy duty, thickest diameter for that mesh count). The thread diameter changes for each designation as the mesh count changes. So a 34-micron thread may be an S designation at 255 threads/in. or a T at 305 threads/in. This is all very confusing and makes little sense from an engineering perspective.

A much better way to select the desired mesh is to consider the percentage of open area at each mesh-count range. For instance, you can select an open area of 36% ±2% for any given mesh count between 160-355 threads/in. By selecting the mesh count that is closest to the same open area, you establish a common open area for fine, medium, and coarse meshes (355, 305, and 230 threads/in., for instance). You've now established the range of meshes that will all pass ink at the same rate, regardless of how many threads per inch.

Frame size

Press requirements and preregistration systems determine frame size. Most presses support multiple frame sizes, but is it wise to go this route? The most common approach is to have multiple frame sizes in inventory in order to conserve mesh—in other words, use the smallest frame suitable for any designated job. But there are flaws in this way of thinking, because rarely does it conserve mesh. Actual yields will vary substantially, and you'll ultimately waste more mesh as selvage unless you choose the frame size that accommodates the width of mesh on the bolt. You'll need to piece together the length and width combinations in order to maximize the yield from the bolt.

Another consideration is the orientation of the mesh (warp vs. weft) on each frame. I am a big believer in consistency. Orienting the warp and weft in the same direction on the frame minimizes registration variations and makes tension more consistent over the life of the screen.

The labor component also plays a part. You can stretch mesh and prepare a small frame in roughly the same amount of time as you could a large frame; therefore, the economy is gone. If you use a static stretch-and-glue system, you'll spend a small amount of additional time and material to glue the larger frame. I'm not sure it's significant enough to worry about.

The goal with a standardized frame size is to reduce the amount of adjustment you have to make between sizes. Keeping the frame the same size dramatically minimizes the time needed to make frame-holder adjustments on press. The savings can be substantial, especially when the press isn't marked in some way to accommodate the various sizes or when you're using a multi-station press.

If you use a pre-alignment system, chances are very good you'll use a standard frame size. The object, whether you use a pin-registration system or a three-point lay system, is to load, lock, and go. Having multiple frame sizes can change this approach by adding frame-holder adjustment to the process. And while we are on the topic of holder adjustment, it's worth noting that any time you adjust a holder, you run the risk of not tightening down evenly or missing a lockdown altogether. This is especially true for carousel presses, some of which may hold as many as 18 screens. Of course, if you miss a lockdown, the chances of something moving during the run are much greater. Hence, in the interest of consistency, a predictable outcome, and repeatable future performance, the fewer things we change, the better off we are.

Consumption of mesh, emulsion, adhesive, and tape are short-term issues. The increased cost in these areas is far overshadowed by the much faster setups and increased earning potential of additional available press hours. Another component to consider is whether the press changeover will increase the running speed, thus lowering the cost per print.

Here are some guidelines for those who must use multiple frame sizes. At the outset, establish clear rules as to what size frames will be used for each situation. Once this is established, work through the variables surrounding changeover with different sizes. Systematically arrange and streamline the process to maintain as many common factors as possible.

Changes you make take time to settle. I always test changes by asking whether I'm adding value or adding cost to the final product. Value is defined as anything the customer would be willing to pay for if it were itemized on the invoice. Press changeovers certainly do not qualify unless we can significantly increase our prints per hour by making the change.

As an example, let's establish press time at $150/hr and 450 prints per hour. This means our press time per sheet is 33 cents ($150/450 prints). If our changeover takes 15 minutes to make holder adjustments, we will only be able to print 338 images compared to being ready to run without an adjustment in the first hour. This adds 11 cents to our cost per sheet during the first hour ($150/338.) You can do the math on whatever run size you have. But be aware that as press-run lengths get smaller and smaller, adding any changeover time to the process can have a significant impact on your over-all costs. In fact, it may make the difference between a profitable job and one that costs you money.

Frame inventory

Frame inventory is one of my pet hot-button issues. Money tied up in anything that doesn't directly produce value is waste. Proponents of lean manufacturing know this all too well. It's their mantra. With this in mind, finding the balance for your frame inventory is one of the keys to keeping the production flow moving smoothly.

Obviously, if you do not have a frame with the correct mesh count when you need it, you will delay production, or you will have to get creative at the expense of additional press adjustments and associated work-arounds that add cost and reduce efficiency. Using percent open area is very helpful in reducing the number of mesh counts with which you work. That is step one. Step two is to begin charting and tracking your daily usage by mesh count and frame size. If you are fortunate enough to use a single frame size, the process is relatively painless. When you have multiple frame sizes, you must determine whether the image really needs to go on the frame chosen or whether perceived convenience was your motivation. Do this over a two-month period. Keep track of which sizes and meshes you use by the day of the week and by daily volume. You may find that some very interesting patterns emerge.

You'll have a slice of a typical week at the conclusion of your analysis. Keep in mind the run lengths of the work you do during the test period. Make sure they were typical, neither too many big jobs, nor too many small jobs. The longer you track this, the more accurate your daily forecast will be.

Cycle requirement

The cycle requirement is where we pull it all together. Highly efficient companies expose screens just before they need them, sometimes only minutes or hours before going to press. The goal is to be comfortably close to press time—just in time—but never miss a press schedule to wait on frames. I try to have screens ready no more than four hours ahead of press time.

In the beginning, you may want to expose screens up to one day ahead until you become confident that your system works. The times will drop as you get better and better and control the flow of information within the company.

For the cycle to work well, the screen department must know well in advance when the job is due on press and what size frames and mesh will be required for each color in the image. This can be daunting. These decisions need to be made upstream of the screen department. In other words, I don't want the screenmakers choosing mesh on the day the screens are due.

The frame size, image positioning, and mesh count should all come to the screenmaking department along with the scheduled on-press time. The screenmaker can use this information to quickly determine how many frames must be prepped and imaged for the schedule in hand. Ideally, the screen department should have the schedule one day ahead of the press time. In reality, this is often not the case.

Last-minute changes and late art delivery often impact the schedule, and the screen department ends up with the job just a few minutes or hours before it is due on press. This is where your screen-usage trends come into play. You can forecast the inventory levels needed on a daily basis as determined by the recorded daily and weekly trends you've been studying. The job becomes easier and easier as you work toward reducing mesh counts and standardizing frame sizes.

My goal is to have screens in inventory for no more than 24 hours. Over the years, I've found that I get the best results when the screens are imaged within 24 hours of being coated. This is especially true for halftone work, which is the majority of what I do. My inventory requirement is for three days of production: Day one is reclaim, retension, degrease, and coat; day two is image and press prep; day three is print and breakdown. If the shop images 50 screens per day, the minimum requirement is 150 screens—a very tight inventory that I cannot recommend to any shop that isn't very experienced and controlled.

A particular screen-related variable can challenge the practice of managing inventory this way. That variable is how many screens (as a percentage of inventory) will break on press, or be torn in handling. The finer meshes are more susceptible to damage, and in tracking mesh failure, we know they have a definite life cycle. But the cycle is different for every company. This variable will add one to two days to your anticipated inventory level. In the beginning, you will run with more inventory, heading toward the five-day level. With time, you will trend toward three days, or slightly longer. I have yet to visit a high-volume shop where the inventory runs fewer than three days.

Finally, there is forecasting for growth. This could be the subject of an entirely different column, but it is something you must consider now. Any material change to the work day—adding a half shift or a full shift, or adding machines—will definitely impact your inventory levels. Be aware that the change is not linear. In other words, adding a second or third press will not double or triple your inventory requirement. This is a rate-related problem. As your proficiency increases, the need for more days of inventory decreases. But adding capacity changes the proficiency level (new operators and screen makers), and you will find the inventory requirement moving up initially before dropping again.

How long does it take to get a good handle on your situation? That's difficult to say. The more turnover you have in the screenmaking and press departments, the longer it will be. With a stable group of workers and good reporting, you should have the cycle requirement well established within three to six months.


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