Give it Your Best Dot
Are you printing the optimum halftone line count for your screens? Find out how to test the highest resolution that your screens and screenmaking procedures can deliver.
If you talk to your neighborhood printer and ask him what halftone he typically uses for high-quality printing, he’ll tell you what his favorite resolution is. But if you ask that same printer why he uses that resolution, he’ll stall a little. And if he’s honest, he may even tell you he’s not really sure. Most printers simply don’t have a clear idea of what their optimal halftone resolution is. They also don’t know how to determine it.
These problems are often compounded by the fact that no one wants to rock the boat and challenge what has already been established as a workable system. “If it ain’t broke, why fix it?” That’s the common attitude in many shops, yet they all want to create higher quality work on a more consistent level. A question that typically comes up is, “Why doesn’t everyone use the same resolution for the same type of work?” The simplest answer for this is that halftone printing comes with a lot of variables, and these variables combine with each other in an alarmingly large number of ways to create unexpected results. A thinking printer will do the homework and then stay fresh for the anticipated surprises in results.
There’s no substitute for knowing what you can handle in regards to halftone resolution and tonal range. You’ll find it difficult to move into deeper areas of more complex artwork and separation issues until you’ve determined the levels of resolution and range your shop can support. Fortunately, you can use a simple test to see what halftone resolution is best for your shop and quickly use the results to standardize your whole process. Before you execute this test, however, it’s a good idea to take a serious look at several of your film- and screenmaking procedures so that the test will deliver the best possible snapshot of what you can print. The initial review of the film and screen production will determine whether these areas need refinement, then the time will come to test your screens with traditional and index dot patterns to figure out what resolutions they’ll support.
Reviewing the films
Checking your films can be a really quick process and doesn’t need to be overly scientific. All you need is a loupe and, if possible, a light table. The things that you are looking for are the edge quality and the density of your dots on the film.
The edge quality of your films is simply how clean and sharp the edges of your dots and straight lines appear under magnification. Poor edge quality will cause a wide range of problems, but most noticeably will contribute to excessive dot gain and tonal compression. The effect of tonal compression is that the upper values in the 70-90% will all flow together and the 0-40% will appear really light (Figure 1). This happens because a dot that is not round or has a larger surface area tends to expand faster than a dot that is in a round shape. A round dot is better at holding shape because the surface tension of the ink—the force that keeps water droplets as round, dome-shaped drops on a flat surface—helps to maintain its shape. Ink flows more easily away from the center of a dot that takes an irregular shape. Remember that from computer to film to screen is already two reproductions and that images never get any better as they go throughmultiple copies. The goal is to keep them somewhat close to the original.
When the edge quality checks out, then it’s time to look at the film density. The Dmax, or density, of the film is a measurement of the amount of light that can penetrate the black areas of the positive. Some shops acquire a densitometer to read and record the opacity of their positives to make sure they’re achieving a high-quality barrier for proper screen exposure. A visual test on a light table will suffice for most of the printers out there. You can do this by taking your films to a light table, overlapping two films, and assessing the top film’s opacity. Can you read the bottom film through the top one’s blackest areas? Consider it a red flag if you can.
Poor Dmax on a film positive causes the smaller dots to disappear and the more open areas to be gritty and inconsistent in the final screen. A light film positive will produce dots in the screen that look ripped on the edges or oblong in shape, even if the screen looks fine to the plain eye. The emulsion has to work harder as a result of exposure, and it takes additional pressure and time to rinse out the screen.
Don’t be fooled by expensive equipment or new processes. Always check your positives with a loupe. In the case of a computer-to-screen machine, check the printed surface of the screen. I’ve seen several newer systems perform with worse quality than vellum upon close inspection, so don’t blindly trust that you’re getting the best results without seeing for yourself.
Checking the screens
Always use films that are in good shape to create the best screens possible. A great screen print comes from quality screens with well resolved halftone dots. Proper exposure is key. Undercutting and underexposure are serious issues you should work to avoid.
Undercutting is a problem that’s typically caused by poor vacuum seal during exposure, allowing the emulsion on the film to come away from the surface of the coated screen. The light then flows around the dots to some degree and causes poor exposures and bad edge quality on the final screens.
Underexposure is usually caused by the exposure unit’s bulb being too old or the improper calculation of actual exposure time. You can resolve these concerns by using a manufacturerprovided exposure calculator—available from most emulsion companies—and offsetting the age of the exposure unit’s bulb life in this calculation to account for declining effectiveness of the bulb over time. [More information about calculating exposure time can be found in Ross Balfour’s article, “In Pursuit of the Perfect Stencil,” beginning on page 82 of this issue.]
Apart from exposure issues, frequency (or moiré) presents the largest problem that printers face in creating screens. This issue can be a relatively simple one to fix, but it can still cause a lot of headaches. Frequency is a surprisingly common problem for even experienced printers to have on a recurring basis.
Most frequency issues with halftone dots in screen printing are caused by an interaction between the dot pattern on the film and overlapping areas of the screen mesh (typically called the knuckles), where warp and weft threads overlap each other. One of the primary ways to resolve this issue quickly is in mesh selection. Always have a ratio of at least three threads per dot. For example, 32-line/in. halftone on a film needs to have at least a 110-thread/in. mesh (32 x 3 = 96). Where this typically causes printers trouble is in the screen for the underbase white. You want to get the brightest white underbase possible, so you may opt for a 150- to 160-thread/ in. mesh with a common halftone line count for simulated process, such as 55 lines/in. Consider that 55 x 3 = 165 and you’ll discover that your choices will create some bad frequency patterns because there are not enough threads per dot and the halftone resolution is very close to the ratio of threads. The dots get lined up with the knuckles of the mesh and create that ugly feathering effect that wreaks havoc with a nice gradient (Figure 2).
So what’s the solution? Multiple factors are at work in this case, but I have found that lowering the mesh count is not always the right choice. The main reason that a 156-thread/in. mesh puts down more ink on a shirt as an underbase than a 180-thread/in. mesh is that the threads in the 156-thread/in. mesh are much greater in diameter and therefore trap more emulsion in a layer. Screen-printing geeks know this as EOM, or emulsion over mesh.
A common misconception is that larger mesh openings will always lead to a thicker, more opaque ink layer. Ratio of the size of the opening to the size of the mesh thread dictates ink flow, but does not necessarily lead to a thicker, opaque underbase. A better solution is to use a higher mesh count that has the proper ratio of threads per dot and attempt to improve the EOM by thickening the layer of emulsion. An extra face coat of emulsion gives a significant increase in underbase brightness without the frequency problems that will result from using a lower mesh count.
I’ve encountered quite a lot of resistance to this concept from experienced printers due to the added labor of putting an extra coat on some of the screens for underbases, but they can’t stand the moiré, either. So I contend that the extra time spent at the front end is well worth the time saved in trying to battle moiré on press.
One significant note to add at this point is that many CTS systems seem to produce far fewer frequency issues than film because they can get such an absolute seal directly on the screen’s surface. I still believe that the same ratio of dots to thread count should be observed using these systems so that the floating dots in the higher percentages will have enough threads to hold their shapes properly.
Testing the dots
The films are black and the screens are prepped, so it’s time to test a variety of halftone resolutions to determine which will cause the fewest headaches. First you will need an appropriate test file. This file is best executed in Adobe Photoshop, where you can create a grayscale document at 1200 dpi that you can then paste in varying resolution gradients of different line counts (Figure 3). Another option could be to create a CorelDRAW file and then import separate bitmaps of the different frequencies.
The idea with this test file is to create steps of different resolutions that will clearly show a preference when printed on a variety of screen meshes. The final prints will illustrate whether a specific resolution works best on a given mesh and how high the films and screen process can go with resolution before tonal compression and excessive dot gain make it less appealing.
I typically don’t mess with the angle of the dots for this test. I have tried all of the angles over the years, and through trial and error I now always print everything through 22.5° with no recurring issues. Expose and print your test film on all appropriate mesh counts and then evaluate which line count is best for the meshes.
Through this test I learned that 255-thread/in. mesh is not nearly as friendly a mesh to halftones as a 230- thread/in. mesh and that 47- and 55-line/in. halftones work very well on all the necessary meshes for the shop in which I tested them. The thing to remember is that this may not apply to every shop because the variables are different from facility to facility. Your shop has different mesh, films, screen tension, exposure times, emulsion thickness, and press variables. All of these elements can expose the need for a different halftone resolution that will create the best dots just for you.
The printers out there who have experienced the fantastic results that only index separations can bring will appreciate the index testing meth-od for determining the ideal resolution of the dots they can hold. For this test, it’s better to import the gradient strips into CorelDRAW or Adobe Illustrator so that the different resolutions will keep the square dots nice and sharp (Figure 4). Go through your favorite meshes with the same file and you’ll quickly see which resolutions you can hold with this method. You’ll more than likely be surprised at how high a resolution you can maintain. Several printers that I tested could easily hold index frequencies above 200 lines/in. and get some great prints that blend so well the naked eye can’t see any dots at all. This simple test may make you rethink the index method. If you can hold a finer dot for better detail than a traditional halftone and you have at least an eightcolor press, why not? The key is in conducting the test and seeing what really works in a production setting and then pushing it a little to get the best quality possible.
Become a dot detective
The best dot for your shop is only a simple test or two away. Once you’ve finished the test, you can really get a handle on standardizing mesh-to-resolution rates, eliminate moiré, achieve better detail, and improve overall quality control. Testing this way with the index method is an amazing process that can completely change the mindset of experienced printers when they see what kind of detail is possible without the edge-quality loss shown by conventional halftones. Use both tests to quickly find the best dot for all of your work now!
Thomas Trimingham is an award-winning art director, illustrator, and separator who has more than 16 years of experience in the screen-printing industry. He can be reached at firstname.lastname@example.org.