Tips for Direct Projection
Learn about the intricacies of direct-projection exposure systems, how to keep your mesh from deteriorating, and the secrets of pad printing on inflatable objects.
Using a direct-projection exposure system can be a real benefit to large-format screen printers. Direct-projection systems eliminate the need for costly full-size film by using smaller positives that are projected onto emulsion-coated screens, enlarging the image to actual size and exposing the screen at the same time. By eliminating exposure as a separate step in the procedure and relying on smaller film positives, direct projection has great potential to reduce screenmaking time and material costs.
Those who print wide-format graphics can realize even greater productivity from direct-exposure by automating other aspects of screenmaking. For example, the projection system can be one stop on a complete screen processing line that features automatic emulsion coating, drying, projection exposure, developing, and delivery of screens to the production floor in the correct print order. In such environments, special conveyor systems are generally used to move screens through each stage of the process.
To realize the full benefits of projection exposure, equipment care and maintenance is crucial. One area of concern is the condition of the exposure lamp in the system. Replacement lamps for projection-exposure equipment are considerably more expensive than for conventional exposure units, tempting users to go past the recommended operating lives of the bulbs (typically rated at about 1000 hr) and rely on their system's light integrator to compensate for any falloff in output.
Sorry, but it doesn't work that way. Not only does lamp power diminish over time, but the spectral output changes as well, to the point where it no longer matches the requirements of the photoinitiators in the emulsion. This can spell disaster in the form of weak stencils and pinholing, eventually leading to lost production time that will be far more costly than replacing the lamp in the first place. So pay attention to lamp operation time and replace the bulb on schedule.
Another critical aspect of direct-projection is the alignment between the lens on the system and the screen to be exposed. One printer we worked with was having problems with poor dot reproduction on the right half of every screen. Our first thought was that this problem was caused by the projection system's optics. However, we found it to be an alignment problem. The distance from the lens to the plane of the mesh was supposed to be 5000 mm. It was correct on one side of the frame, but at the other end, the distance was 5005 mm. That 5-mm difference was enough to throw off the focus on the right-hand side of the image, leading to poor dot definition. Once we realigned the frame, the problem disappeared.
The need to remove haze and ghost images from the mesh causes an all-too-common problem that many screen printers don't recognize--reduced mesh life. Ghost images are usually caused by stencils that were underexposed, leaving the emulsion tacky and very difficult to remove from the mesh. Ink residue also can remain on the mesh after reclamation, leaving ghost images behind.
The problem is that the haze removers some printers use are very hard on mesh. Although environmentally friendly, non-caustic haze removers are now available that are very effective, many formulations still contain a fairly high concentration of caustic chemicals. These agents can very quickly degrade polyester mesh. Chances are you spend a lot more on your mesh than on screen chemicals, so be careful when using these products, keeping in mind that different formulas contain varying amounts of caustic chemicals.
One shop we visited was experiencing a problem with large-format screens that were ripping for no apparent reason. We traced the cause back to the haze-removing formula, which had an alkalinity that was higher than normal, resulting in a very aggressive mix that weakened the mesh. We switched to a different product with a lower concentration of caustic chemicals and found that it worked well without harming the mesh, thus eliminating the problem. We are not chemists and may not be using the correct jargon, but our point here is that you need to avoid using too aggressive a stencil cleaner on your mesh.
Advantages of specialization
Many of the advancements in screen-printing technology that we've seen over the years have been driven by the specific needs of niche markets. This is very true of the compact-disc field, which has been of real benefit to the screen-printing industry as a whole. Disc manufacturers presented the ideal set of circumstances for suppliers to develop their products: An easily identified, lucrative market where printing would be done in a generally controlled environment onto a reasonably consistent substrate.
The quality demanded by these clients has also driven the technology. Artists and recording companies are not interested in the limitations of screen printing. They just want the print on the CD or DVD to look like the litho-printed insert. The unavailability of a color-control strip to measure dot gain and density made it all the harder for CD printers to control the process in order to meet these quality expectations.
The print surface of the CD or DVD is coated with a UV lacquer, which in itself poses a problem because if the lacquer is overcured, UV screen-printing inks won't adhere to it. CD printers have generally used conventional UV inks to get the ink adhesion necessary, but have struggled with controlling the ink-deposit thickness to get the best results. Water-based UV inks can allow for a thinner ink deposit and have been used by graphics screen printers for some time. But these inks have been impractical in the CD industry because of adhesion problems and curing difficulties caused by the higher ambient temperatures in enclosed multicolor printing machines.
Recently, however, water-based UV inks have been released that are designed specifically for the needs of CD printers. These stable formulations offer more control of ink-deposit thickness and allow users to print tonal ranges of 10-90% at 150 lines/in. This is a big improvement over traditional ink systems, where 120 and 133 lines/in. are the norm.
Following closely on the heels of this development, new thinner gauge capillary films have been introduced that provide a genuine 2-micron stencil thickness with a very good Rz value. They are capable of controlling ink deposit to the point where you can get results with conventional UV inks that are comparable to those you get with water-based UV. Trials with such films have led to printed tonal ranges of 5-95% at 150 lines/in. Some tests even suggest that users can obtain 175 lines/in. with a similar tonal range.
Another, more established development in the CD industry is the use of precoated mesh. This has been an important step forward in a market where a high volume of screens are needed per day and stencils are seldom reclaimed--mesh is simply cut from the frame and discarded once the stencil is no longer needed. Precoated mesh offers a consistency in emulsion coating that can only be achieved by high-end coating equipment. It also can save money when all of the capital, labor, and space costs of setting up screen-coating facilities in house are taken into account. Although precoated mesh is a very useful product, its use will likely be limited to niche markets.
These recent advancements clearly show that screen printing is far from dead. In fact, competing analog and digital processes continue to spur new developments in screen-printing technology. The prophets of doom were somewhat premature.
Pad printing on inflatable balls
Visit gift and souvenir shops on your next vacation, and you'll likely see a wide assortment of inflated balls printed with multicolor cartoon characters and logos across their surfaces. Looking at these toys, you may think, "How do they do that?" The answer is pad printing.
If you look closely at the "equator" of such a ball, you will see that the print is actually made up of two individual images. Each hemisphere is printed separately, but on the same machine. It's done on a specialized multicolor pad-printing press that prints the balls while they are inflated. While there are many novel features to this press, the most important are the printing pads.
Normally, pads are made of solid, molded silicone rubber. The pads for this application also are made from silicone rubber, but they are hollow. Large air reservoirs mounted above the press pressurize the pad before ink pickup, then depressurize it during the print stroke. This variation in internal pressure enables the pad to pick up ink from the cliche, yet conform to the inflated ball surface during printing. Watching these presses in action is very impressive, as they can print up to 600 balls/hr with a four-color image covering 360° of surface. More print stations can also be added, allowing six to eight colors to be printed.