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Of all the different printing methods currently used, screen printing generally has the largest number of critical variables as there are more parameters to measure and keep constant. It's ironic then that as a whole the screen printing industry measures less and owns fewer tools than any other print industry segment. When trying to solve a plant's process issues, you must first measure the existing screen, ink, artwork and press set-up specifications before any changes or improvements can be made.

Figure 1: Pantone color matching guide. Old, faded book (right); new book (left).

In shops that use precision process tools, troubleshooting begins by asking what specifications are in use and verifying that the tools used to measure them are still in calibration and being used properly. Troubleshooting is much simpler after you're sure the materials and press settings have not strayed from normal shop specs. In plants that are not well equipped to measure variables, verification of troubleshooting problems becomes trial and error or, in some cases, guesswork.

For most screen printers, the lack of interest in precision tooling often comes down to a combination of two or more viewpoints:

• The tools are too expensive
• We don't need to be that precise
• We don't have time to measure things like that
• We are too small to need tools like that.
• Cost is the major factor in not purchasing these tools.

Figure 2: A reading of the hue variations with a reflectance densitometer.

The Cost of Precision Tooling
Overall, precision tool costs have dropped in recent years. The tools are more affordable for three reasons. First, new sensor types for electronic tooling are available. This makes some tools cheaper to build, and makes some cheaper type tools repeatable enough to be worth their investment. Secondly, automated manufacturing methods and overseas work forces have revolutionized the affordable precision tool industry. Finally, rising fuel costs for transport created the need to streamline local production through process improvement and standardization.

If the printing of any product becomes more repeatable, higher quality, faster, or improves process control because of these precision tools, you can save money in several areas: Overhead costs, raw material costs, process and print time consumption, defects and waste. Tool cost, as opposed to utility, is the main reason why printers do not purchase precision tools. This reasoning is faulty in most cases because all but the most rigidly controlled engineering-based facilities rarely take the time to properly research their true and total bottom-line costs. Some costs, such as ink, stock, labor and shipping, are easily calculated, assignable to every item printed and can be passed on to the customer.

Other costs, such as "true" overhead and frictional production losses, are harder to calculate because they are small and cumulative over time. These costs can best be calculated quarterly or seasonally because data on loss per printed unit can be very small. True overhead costs include items such as seasonal peak-load pricing for electricity and gas, which can make summer production costs higher and winter costs lower (or vice-versa) in some regions.

Frictional production losses include long-term material losses of emulsion, adhesives, tape, mesh, block-out materials, chemicals, lamp life, screen making labor or on-press failure time through inefficient, error-prone or trial-and-error screen making. This loss rate includes discarded squeegees that could be resurfaced or sharpened, and mesh with poor tension, which loses its usefulness too quickly. In short, unseen overhead and material costs generally are more than enough to justify the cost of precision process tools.

All five major factors of process streamlining are interrelated with cost as follows:

Overhead costs
This is the least understood cost factor for most print shops. Higher process or print-time consumption (through slower or problematic print runs) consumes more lights, heat, electrical power, gas and process water. In many cases, labor also is attached to this figure. What should also be included is the cost-per-square-foot usage of floor space of any given machine in the process (in dollars per hour divided by the monthly building rent).

This also should reflect the amortized loan cost of the machines themselves. These turn out to be small numbers (possibly one half of 1 percent per printed piece). But over a year, and thousands or millions of impressions, they become significant. When certain jobs print poorly or require troubleshooting, this overhead cost is higher per printed piece and true costs can be excessive.

Raw material cost
When printing is problematic (e.g., screen issues, make-ready problems and curing problems), it consumes more raw materials than were originally quoted. Many plants quote a "buffer" of raw materials into each job, so true losses are rarely recalculated on a per job basis. In many cases, buffer limits are exceeded and this fact is never known.

This is particularly true when calculating ink consumption. Material calculations usually are exceeded on high-coverage print runs because the ink deposit thickness is unknown. High-ink consumption and uncontrolled deposit thickness commonly use more make-ready stock than what is quoted, leading to more waste. Many large facilities already pay to discard waste. This can be as simple as a dumpster service, or as complex as a chemically safe waste stream disposal incinerator or recovery service. This should be amortized into your per piece overhead cost.

Print defect
Depending on the company type, printing defects can be called waste, cast-off, spoilage or, in the worst cases, disguised as make-ready or shipped in place of an acceptable product. Print defects are the worst offenders for uncontrolled and hidden production cost. Defects consume overhead cost, labor and raw materials, creating costly waste. More print defects can be prevented by proper process methods, which are backed up and enforced by modern process tools, than by any other method.

Only by calculating your true production costs can you really see how some of these instruments will save you money. But what is meant by precision tools and where do you use them in your process? Diagram 1 is a basic process model to illustrate what can be monitored or measured. While no two plants' process flow models will look alike, most have the basic processes outlined in the flow chart. In the figure, each process input or sub-process with a "red" highlight ring has critical parameters we can measure and control.

For each main process step (in yellow), we'll cover four main ideas:

• What measurable items are in each process step
• What tools are available that can be used
• What tools can correct or improve the process
• Actual before-and-after print results to illustrate how the tool helps improve print results.

The Printed Images
All of the printed images shown use a common image. Before proceeding, let me explain what test film image was chosen and why. Understanding the precise nature of the chosen images will better illustrate the tools. Both line and dot images are from an industrial film set used in the circuit industry. They were chosen for their highly technical precision level of film production.

The images were produced at 16,800 dpi, with a film density of 5.0. Though this is far beyond the resolution needs of most industrial and circuit screen printers, it is also well above any resolution issues of most emulsions available. This high output rate ensures any edge-definition defects visible in photographs can be properly attributed to the effect being illustrated, and not to the film itself. The difficulty level for printing and imaging the features in this test film is high.

The "line-work" test image will easily show print defects in squeegee or flood pressure settings, off-contact settings, tension levels or squeegee angle or wear. Meanwhile, the "dot" image immediately will show any defect in stencil thickness, press settings or Rz. This image is sensitive enough that once an ink of a given viscosity is paired to a specific screen and print stock there is only one range of settings to create a defect-free print. A screen change dictates a viscosity change and vice-versa. The image is ideal for demonstrating the failure points of emulsion thickness, screen tension, viscosity and various press settings.

The line work image width on the film is 98.4 X 1/10th of a mil. This is 250 microns. The spacing between those lines is 157.4 X 1/10th of a mil. This is 400 microns. The "dot" images are 250 microns and are approximately equal to a 70-73% shadow dot in 65 line process printing. To make sure we all understand the scale, there are 2.54 microns per 1/1000th of an inch (1/1000th of an inch is one mil.). The use of a shadow dot format, as compared to a highlight dot was chosen because any "tipping" of the dot stencil due to excessive off contact, screen movement, sticking, pressure and speed setting, poor stencil thickness or incorrect Rz will show immediately during printing as a dot diameter and edge clarity loss.

To further prevent skewing print results with unexpected variables, such as stock thickness and press print bed variations, each printed image or photo of a dot or line is the exact same dot or line image from within the same film image, not just a similar dot or line from somewhere else.

In Part II, we explore the design and artwork stage, the film production stage, and the inspection stage.

Ray Greenwood is SGIA's Technical Services Associate. He is responsible for helping SGIA member companies in all types of imaging-related technical inquiries. Greenwood, who helps the SGIA Digital Lab and SPTF Lab conduct workshops and research projects, has spent the past 20 years of his printing career working with semiconductors and circuits, as well as textiles and large-format graphics on a variety of substrates. ray@sgia.org

This article appeared in the SGIA Journal, 4th Quarter 2008 Issue and is reprinted with permission. Copyright 2008 Specialty Graphic Imaging Association (www.sgia.org). All Rights Reserved.