Screen printers like to stick with techniques that work well consistently. This is a clear reflection of the printing process itself, because you don’t want to change the way a shirt is being printed halfway through the production run. It might start to look different, or even worse, it might become unacceptable to the client. So it stands to reason that a printer will always look for the best method that produces the highest quality reproduction in the shortest period of time.

Screen printers like to stick with techniques that work well consistently. This is a clear reflection of the printing process itself, because you don’t want to change the way a shirt is being printed halfway through the production run. It might start to look different, or even worse, it might become unacceptable to the client. So it stands to reason that a printer will always look for the best method that produces the highest quality reproduction in the shortest period of time.

The more experienced printers that have been around awhile have occasionally tried different methods and styles of printing with a wide variety of results. Often these experiences quickly can create a mindset of what does and doesn’t work in separations and printing and sometimes this in turn creates a real hesitation to attempting new techniques.

This “don’t fix what isn’t broken” mentality often will flow backwards from production into the art department. There may be very little time for changes in processes or testing new methods in a busy art department, and even less time to attempt things that may not end up being used at all. When considering these challenges, it is easy to see why so many screen printers stick with only one style of halftone. It is common to hear something like, “We use a 42-dpi elliptical dot at 22° on everything” in a lot of art departments. The downside to having such a fixed process for halftone printing is that a lot of diverse art styles can benefit from using a variety of halftone types that work best when used with the right type of artwork. A halftone that works incredibly on a pencil drawing may not be ideal for a watercolor print, etc.

The first step in advancing the detail and quality of screen printing is always addressing the ability of production to recreate the cleanest possible halftone dots on the press with the least amount of unnecessary dot gain. To try to forge ahead into art-related areas without defining production abilities first is clearly putting the cart before the horse. No matter how good the films are coming out of the art department, they will never look great with 35% dot gain and a lot of tonal compression (this is the tendency of the 0-30% and 70-100% to all appear similar).

Moving up in printing quality requires clear, honest, self-assessment and willingness to address and standardize procedures in production prior to developing new art and separation formulas. Once the printing variables are in place and halftones can be reproduced consistently on press, the hard part of moving a company to a higher level of printing is already done.

Now that the production department can produce well defined screens that print good quality halftones with minimal dot gain, it is time to address the art department and see if some new styles of halftones would benefit and improve both separations and final prints.

First, it is a good idea to review the different styles of halftones so that each type of dot can be clearly identified. Then the halftone styles can be looked at for how each type is best suited for different styles of artwork. Finally, the specific limitations of the dot types can be discussed to demonstration what to look out for with possible problem areas that can occur. The common types of screen-printed halftones are traditional, index or stochastic, and alternative styles.

A quick review of common screen-printing halftones
One relevant note is that halftones that work certain ways on garments may have different results on flat stock. Printing on the uneven surface of a T-shirt has always been an extra challenge for reproducing good halftones, so it is considered to be more difficult to standardize due to the changing nature of the uneven surface. This breakdown of halftones is meant for garment graphics only, so there isn’t confusion about what might work best on other substrates.

Traditional halftones
Traditional halftones are typically round or elliptical (Figure 1). The hallmark of standard halftones is that the dots form a pattern that has both an angle and a density (dots per inch or dpi). The specific property of a traditional halftone is that the dots shrink or expand in size depending upon the value of the image being reproduced. In other words, you have the same number of dots in a square of halftone but the dots just become larger and touch, overlap, or even become a solid area when the value approaches 100%.

Art that works well with traditional halftones
The majority of screen printers who print halftones use traditional dots that have an angle and a density to them. They are created from common RIP software quickly and they tend to work well on most graphics with a few exceptions. The artwork that works the best with traditional halftones is artwork that has smooth gradients in it with blended colors that flow gently from one color to another. A soft, airbrushed image or a design with a lot of subtle effects works particularly well with this style of halftone because the shrinking dots can emulate the slow fading of one color into another color or the background of the garment (Figure 2).

Artwork that has a lot of realistic or photographic imagery is ideal for traditional halftones and these styles of dots can represent the delicate transitions needed for photographic reproduction the best out of the current halftone methods while using the least amount of screens and inks. The traditional dot’s flexibility and overall wide range of use make it a clear choice for the most popular halftone style in screen-printing shops. A close look at these dots shows how they merge together when then blend past 50% density (Figure 3).

The visual drawbacks to traditional halftones are fewer compared to some of the other types, but they are important to the screen printer because they directly relate to specific problems that can occur on press. Because these halftones shrink with size when they are reproducing a lower density, there is a fall-off point where the dots will simply not show up on the screen. This level depends upon several factors, but common things that dictate the fall-off point are the density and method of the film or image that exposes on the screen (CTS, computer-to-screen imaging machines are the best at controlling fall-off points) and what the actual screen mesh is. A lower screen mesh has bigger threads and will commonly block the reproduction of really small dots. A typical fall off density where dot exposure stops on the screen is around 8-12% for film-based exposure systems and 4-5% for CTS systems.

The biggest drawback to traditional halftone dots is that they tend to gain in size on the final screen print. The reasons for this can be a variety of art and production issues, but the most common are: loose screen mesh (requiring higher pressure to get the ink to release onto the shirt), dull squeegees, poor exposure or underexposure of the screens, high print pressure, improper separations or print order, and poor dot quality. When these issues combine in a production run, the halftone may quickly gain in density to where the original image becomes almost solid ink with a loss of detail and clarity. This is one of the most common issues facing printers who want to start printing four-color process or detailed simulated process screen prints on garments. First, the dot gain needs to be managed on press and then the separations can be created and fine-tuned. Doing it in any other way takes a lot longer to achieve higher quality with screen-printed reproduction.

Index halftones
The main difference between the index style of dot and the traditional is that the index style of dot using a frequency pattern to reproduce density of value (Figure 4). These styles of dots are all the same size and they vary in distance from one another to recreate the proper value in an image. One other noticeable difference in a stochastic dot is that it is a square shape that will fit together like a checker board when different colors blend together.

The best types of artwork for index dots
Many screen printers use only index dots to reproduce their halftones. A lot of reasons may motivate this choice but it is common in certain styles of artwork and printing because of its specific advantages. Artwork that works the best as an index dot has shorter gradients in it and may already have a texture or grain to the image.

Index dots often produce a noticeable texture to the print because all of the dots are the same size so when a piece of artwork has a texture like a pencil drawing, painting, or noisy digital image, it is perfect for the grainy look of an index print. Fine artwork that is created by traditional media, like chalk or charcoal, works particularly well (Figure 5). Another feature that makes art work well for index printing is if the image has very small details, such as little faces, logos, or tiny typography, that need to be clear in the final print. Due to the nature of the index dot, where the dots stack next to each other so there are no open holes as in traditional halftones, small details can be maintained better than in other halftone styles.

Limitations of the index dot are related to the process of creating them and can be the major reason that screen printers decide to not use this style. The primary drawback to index or square dots is that to reproduce a blended set of colors extra screens are typically required. It is harder to create a wide color gamut because the dots do not shrink with lower values. The blending has to rely on the solid colors that are represented. A four-color, simulated print with traditional halftones may require up to six or seven colors with an index dot. When considering this, many screen printers won’t even give the square dot a second look because of the added screens and cost of setup.

The unknown advantages of the index dot do dictate a second look, however, as the big benefit to screen printers is that the dots experience very little dot gain and once one dot can be exposed, all of them can be exposed, so it effectively eliminates dot gain. Both of these advantages can make this type of halftone very attractive to shops that have unstable production environments and longer printing runs because the prints will turn out very consistent with lower issues, even though there are a couple extra screens. In effect, the tradeoff can be worth it if the artwork works with the dot style.

Alternative halftones
New styles of halftones have become popular again and they can provide a refreshing twist to the older standards of traditional and index dots. There are different methods of getting some of the newer alternative halftone styles. Some of the more common ones are line, mezzotint, and pattern halftones (Figure 6). The difference to using an alternative method to produce a halftone is that it can behave strangely on press and occasionally produce unacceptable results.

Art that works well with alternative halftones
In a sense, using an alternative halftone actually creates a new piece of artwork out of a graphic so it is less about the original artwork and more about how the alternative halftone works after it is applied to the final graphic. The exciting thing about this type of halftone is the random discovery or happy accident that can occur when a different style is applied. An old piece of artwork can appear like an etching or vintage ad if a line halftone is used. Other options can be a mezzotint or pattern that is overlaid on top of a photo to produce a wide variety of effects.

It is important for the screen printer to find an effective method of reproducing values using halftone dots. The key to raising the quality of your printing is to stay open to a variety of functional methods while still being able to apply the right process that will work the best with a specific artwork type.

Thomas Trimingham has worked in the screen printing industry for more than 15 years as an artist, art director, industry consultant, and head of R&D for some of the nation’s largest screen printers. He is an award-winning illustrator, designer, and author of more than 45 articles on graphics for screen printing.
He can be reached at ttrimingham@yahoo.com.