Precision Process Tools for Screen Printers - Part Two
In Part II of this article, we explore the design and artwork stage, the film production stage, and the inspection stage for Screen Printers.
By Ray Greenwood, Technical Services Associate, SGIA
Why You Can't Afford Not to Buy Precision Process Tools in Today's Print Environment
Design/Artwork Stage: Measurable Item
Color includes the measurement of client supplied colors, specification of selected colors or conversion of selected colors to usable mixing formulas.
The Pantone Matching System (PMS) color chip books are probably the most universal source for communicating color. Surprisingly, many small plants do not use this resource. Matching a custom or created color to a similar or identical color in the PMS guidebook under a standardized light source is still the most effective way to identify a color to all parties and assure the same color we mix ink for and print with is the chosen color. Too many printers and designers do not replace worn or aged Pantone color chip books. Figure 1 shows the color differences in books ranging from old to new. (See Figure 1)
|Figure 1: Pantone color matching guide. Old, faded book (right); new book (left).
Densitometer and Spectrophotometer:
|Figure 2: A reading of the hue variations with a reflectance densitometer.
Densitometers are precision measuring devices that utilize internal light sources, of exact intensity and spectrum that are calibrated to a permanent traceable standard to measure colors and print stock materials and quantify their color numerically. (See Figure 2)
The color-related capabilities of most general purpose densitometers are:
- Color density - how densely a given ink or coating may be applied based upon how much of the visible color it reflects (versus how much of the base stock can be seen).
- Hue - exactly which shade of a particular color we are dealing with. A densitometer can give only numerical coordinates based in RGB or CMYK for hue information.
- Stock gloss subtraction - the true reflected color without interference from light reflected by the materials' gloss.
A spectrophotometer has all functions of a densitometer plus the ability to express colors by coordinates in L.A.B. or C.I.E. color space or in several other color space systems. Both of these tools have numerous dot and resolution functions. (See Figure 3)
|Figure 3: Densitometer screens show dot function mode for measuring dot loss and gain, as well as half-tone percentage (top) and color-density mode (bottom).
Ink Drawdown Equipment for Color-Sampling:
As a screen printer, you already have most of what you need for basic color drawdown work. The most accurate use of this method is to have small-scale screens of exact mesh count, thread diameter, tension, open area and stencil thickness as those used in production. After the color is specified in the design phase, ink samples are mixed and printed on job stock using identical squeegee angle, hardness and speed settings. These printed samples are checked for correct ink deposit thickness, dried carefully, dated and presented to customer service for written customer approval. (See Figure 4)
|Figure 4: An adjustable-drawdown bar for ink film and color density testing.
What these Tools Can Do for Your Process
- Correctly quantify and identify the color that a client has chosen.
- Repeatedly mix that color in small or large quantities at different time periods.
- Modify the color to print on differently colored stock, objects and garments.
- Monitor color during the printing process to determine if the actual ink color or method with which it was printed has changed (change in density or dot quality).
- Warrant your colors against possible client dissatisfaction by having a verifiable reference.
Film Production Stage
|Figure 5: A bench-top transmission densitometer.
It is important to know how many lines per inch (lpi) of half-tone dot you can produce and what your d-min and d-max (the smallest and largest printable dots the film can resolve accurately) are, as well as the edge resolution of lines and dots in the films as dictated by the dpi or scan rate of your output device. In other words, you may want to know how well your film is rendering shades of gray or if it is accurately producing the specified tonal gradations.
Line Count and Angle Variation:
These vary from a simple, film-based tool, showing interference on all but the correct range of line count and angle, to a methodology-based tool that uses protractors and measuring loupes to positively verify line count and angle. The film-based tool is usually accurate within 2 - 4 percent, but takes one minute. In contrast, the protractor method is exact but takes 10 minutes.
For most printing plate exposure processes, a minimum film density (opacity) in dark areas of 3.25 to 3.75 is acceptable. Since screen printers may use exceptionally large formats (giving greater lamp distances and longer exposure times) and high-intensity UV-based exposure systems, a more practical film density number is a minimum of 4. In all shops, regardless of the specified film density, having consistent day-to-day density is important.
It's one of the most important control factors in the day-to-day film consistency and standardization of the screen-exposure process. We are well into the age of thermally imaged film, advancing rapidly through the inkjet-imaged film age for certain applications and moving toward the age of direct imaging on screen without films. In all instances, the shape and edge definition of imaged dots must be inspected and measured to be controlled.
Transmission densitometers come in two basic types: Bench top and handheld. The bench top model is more repeatable in its readings than handheld units. These devices tell you the density of both the film base itself and the black areas that block light. (See Figures 5 and 6)
|Figure 6: A handheld transmission densitometer.
They also can read the dot area (spaces between dots relating to whether dots have grown or shrunk) and dot percentage (halftone gradation percentage). Unlike their cousin the reflectance densitometer, used to measure printed color on a solid surface, these models transmit a high-intensity collimated light through the film base and dot/line features and measure what is lost or blocked.
Low-powered, uncorrected, wide field of view (typically 15mm) linen testers and loupes of 10 times to 15 times (10X to 15X) magnification should be standard issue for anyone in a screen printing plant who must look at a print for any reason. Loupes of this power are only really useful for inspecting edge definition in line work and verifying registration marks while stripping films for pre-registration systems (pin registration) or lining up register marks between print layers. (See Figure 7)
|Figure 7: From the left, an 8 times magnification (8X) folding linen tester, 45X stand microscope, 12X lighted hand loupe, 20X lighted color-correcting loupe.
Higher-powered loupes (20X to 40X maximum), which are generally kept in the production office, are ideal for inspecting dot and line-edge quality and structure when troubleshooting color shifts, poor layer-to-layer printability in four color process and film or screen defects. Loupes (greater than 35X to 40X with magnification) are frequently classified as "microscopes."
The critical differences between loupes and microscopes are generally field of vision (how big an image area can be seen at one time), ability to be focused without rigid mounting (loupes are generally more portable) and the ability to operate with local light sources instead of illumination systems. Knowing when to use a particular magnification of loupe is almost as important as having one in the first place.
If you print four-color process on a regular basis, you should have one microscope in the plant to troubleshoot your film and printed product. The microscope type you need depends on what you print and how much you need to magnify or if you will need to photograph through the eyepiece. In a critical situation where the presses are stopped, a portable microscope pays for itself saving troubleshooting time. (See Figures 8 and 9)
|Figures 8 and 9: Binocular microscope for measuring and high-magnification inspection (more than 100X) and portable stand microscope for 35X to 100X magnification.
For printed circuit, membrane switch, solar and industrial screen print applications, bench top microscopes with measuring stages, reticules, precision light sources, photographic filters and software are almost a requirement. More than just image shape and resolution can be analyzed when you are viewing at higher magnifications than 60 to 200X.
Inspection Light Sources - Backlit:
A quality backlit light table for inspecting films and screens will save time, prevent errors and allow you to develop a proper QC program for inspecting films and screens. Film stock defects (thermal, silver-based and inkjet) are common. In inkjet film-positive production, ink supplies and machine defaults are routinely responsible for wide variations in dot density.
It is far cheaper to catch this problem on the light table than to wait until screens are burned and product blanks are ruined. This goes for all types of film and screen.
Inspection Lights - Non-Backlit:
With the volume of high-intensity LEDs and cheap electronics on the market, it pays to have a few bright, portable inspection lights on hand. Higher-power loupes are almost useless on press without a nearby clean light source. Unlike high-intensity, filament-style bulbs, LEDs give cleaner, brighter light with fewer hot spots, longer life and better ability to illuminate at odd angles.
Precision Measuring Reticules:
A reticule is an insert for a microscope or loupe that has engraved scales for measuring small widths and lengths of the items you view. This is ideal for checking film line counts, measuring a film's actual dot diameter versus the same images exposed to screens to calculate dot or line loss and shrinkage (frequently referred to as generational loss). (See Figure 11)
|Figure 11: A precision-measuring reticule as seen through a loupe or microscope eyepiece.
If you are doing industrial, solar or circuit printing, it is worthwhile to invest in a bench-top glass scale reticule for measuring entire screens for shrinkage or distortion. It is cheaper and faster than a microscope with equal screen size capabilities.
Screen and Film Angle Finders:
These tools create visible moiré patterns at specific line or thread counts. They are used to quickly identify or grade the actual thread counts of unknown screens, or the true line count of unknown films. With this information, screens or films can be paired properly to avoid moiré and loss of resolution. The accuracy of these tools is directly proportional to their cost of production. The higher the line resolution of the output device, the more accurate the tool will be. (See Figure 12)
|Figure 12: Film type angle finders.
What these Tools Can Do for Your Process
- Control of line count and dot sizes at the film stage will prevent unplanned color shifts during half-tone and four-color process printing.
- By making films more consistent in resolution and density, screen exposure times will be more consistent and the screen lifespan will produce more predictable color transitions on press.
- Consistent film density and resolution will prevent wasted time for excessive exposure testing, improperly exposed screens and excess time for blocking out and pin-holing.
- More consistent films and screens will allow better consistency in color matching and ink mixing which leads to less on-press troubleshooting and less incorrectly mixed ink.
The same tools used to visually inspect films for dot size and resolution also are ideal for inspecting finished stencils to verify proper-edge definition, development and image blockage in areas of highlight dots and fine lines. With these same tools, the vast majority of final troubleshooting can be done before the screen leaves the washout sink. Any screen troubleshooting done beyond film stages or the screen department is always more expensive because of the lost press time.
Inspection Stage Measurable Items
Thread Diameter, Thread Count and Open Area:
Knowing what these three measurements are and how much they change from raw mesh off the bolt to a finished, tensioned frame is imperative for four-color process printers, industrial printers and color matching in all screen printing types. These three items are primary controls for ink deposit thickness.
The tension applied to screen mesh affects all three of the aforementioned measurements. The format size of the screen printing process influences the mesh selection process. The need to keep off-contact (or snap-off heights) low can influence the thread diameter selected. This will have an impact on open area and final thread count.
Mesh Thickness before Coating:
Prior to coating, the thickness measurement of the tensioned mesh is the starting point for calculating the required emulsion thickness.
Loupes and Microscopes:
A portable or bench microscope is necessary for counting threads and measuring thread diameters. For thread counts of 160 and lower, a loupe or microscope between 45X minimum to 60X average will work; thread counts higher than 160 should use a microscope of no less than 60X.
Measuring thread diameter as well as the length and width of the open mesh areas requires a 100X microscope or better, with a reticule featuring calibrations of at least two to three increments per thread width. (See Figure 14)
|Figure 14: Counting threads with 120x microscope and a 25.4m reticule
For instance, if you know you will be using threads of 34?, a reticule calibrated a .001" (25?) per increment will not give an accurate thread-diameter measurement. The alternate method, when correct reticule sizes cannot be found for the more portable microscopes, is to count threads and spaces over a set measuring distance and statistically divide five to 10 sets of readings. This method has some error, but can be a good indicator of thread diameter.
Common in printed circuit board plants, these devices are affordable, highly accurate and have a large measuring area. They are precision, micro-etched reticules with an attached sliding eyepiece or pair of eyepieces. They are ideally suited for measuring and counting threads, or inspecting films and checking line count.
This is described in the previous section, which addresses film production, but is shown here in both portable and stationary formats
What These Tools Can Do for Your Process
- Find variations or changes in the mesh before they create changes or errors on press.
- Match mesh count and thread diameter more closely to fine-line, half-tone and four-color process films for resolution, pigment particle size and ink deposit.
- Quickly discover if printing or ink-flow difficulties are caused by mesh selection, obstruction or damage.
- Troubleshoot broken screens and immediately find the root cause.
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. firstname.lastname@example.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.