The press operator (Øpêrãtǿré þréssąrĭűs) is a very particular entity. Their high level of eye-hand coordination and willingness to take a shop-cart of junk and out of it, build a racecar makes them an invaluable piece of the screen printing loop. They can understandably be territorial and defensive, but part of the reason is they rarely know the identity of the components of the print job. Specifically the films, mesh, stencil, blade and ink arrive at press with no specs.
When the image is easy, it won’t cost too much delay. But when the image is complex there is no telling how much can be lost. Any operator’s ability is unequivocally based upon his or her understanding of the variables that comprise each job. If they don’t have specs on the core components, how can they be expected to diagnose the results? For example, if you asked them why this one looks so good and asked for specifics, it’s likely they won’t know because they don’t have core specs. By addressing these four top issues—stretched images, screen blow-outs, slow print speed and ink build-up—it will become possible for all departments to speak the same language.
In this scenario, let’s assume the image is a relatively simple three-color line art image. Right off the bat, the first strike off doesn’t line up. So, the press operator micro adjusts, twists, tweaks and prays and finally gets to “as good as it’s going to get.” Why won’t it line up? From the press perspective, the operator would say it’s either the register board or the artwork is out… and it’s not possible to ‘un-stretch’ an image.
Here we have the symptom that the image cannot be aligned; the conclusion that register boards don’t register or the art must be out; and action of ‘prove to the boss the image can’t be aligned.’
Plan B: Unless the film positive alignment has been verified with the artist’s initials, the screen department should never assume they’ll fit on press. If they use high vacuum levels and/or if the screens are not robust, they should periodically align film to mesh and inspect their fit with a parallax-correcting magnifier. Only now can we be reasonably sure we’ve gone to press with screens that fit and permit registration.
Now look at the errant image. If it is off in the stroke direction, the likelihood is the low off-contact distance is too low, there is excessive squeegee pressure, or both. If the color is stretched side to side, it may be due to low screen tension and will require excessive off-contact distance. If the image is out between colors, it is likely the differences in press settings and blade pressures vary with the colors in question. If the image is shifting platen to platen, call the field tech—the press is not registering consistently.
The press operator is excited to finally have a long enough run to get something done other than set-up and changeover “sample” sized runs. Then, 37 shirts into a six-color, two flash press run, there’s a sound more harsh to the ears of a screen printer than breaking glass—the screen blows out right on press. Been here? Three shirts are now blown, and ink is dripping on the floor, the platen and your shoe and you swear the next person who asks “what happened” will die. But what did happen? There are no parts on the screen or in the ink, no burrs on the flood-bar. It must be weak mesh (the boss probably got a deal on some medical gauze or something), the press operator decides.
This time the symptom, screen has blown-out, leads to the conclusion that it was a foreign object, flood-bar, blade ends or maybe it just wore out, which inspires the action shoot another… and another.
Plan B: Number every screen and consider tensioning them for “printing tension” and not for “static tension.” The former is sensitive to the greater increase in resistance when the blade is nearest the inside of the frame, the latter does not. If your flood-bar is made of aluminum, keep it off the mesh. If your idea of squeegee pressure is gauged in metric tones, you might investigate to see what is causing the issue. It is likely to be presses that are not, from the parallel-top down—carriage, blades, mesh and platen.
Mesh with “durable” threads filter the ink and force the operator to use excess pressure and ink. Turns out, this requires an extra hundred degrees to gel, which causes the mesh to flow and cool, and so on. This combination is deadly to screens. The good news is nearly all of it is within your control.
Slow print speed
Scenario 3: The blade on the white ink is moving so slowly you can hardly tell a job is in process. This operator is using the logic that white ink is so thick and, since a lot of it is needed to be put down, it has to run slow. There’s also a second and third white on press. Here, we have the symptom of a blade running at snail’s pace, concluding that it must be the blade, leading to the action of trying a shorter blade, harder blade, softer blade, triple blade and finally, print slower.
Plan B: Let’s pick the color which is most likely to print slowly—white. The reason is some whites contain a lot of fillers, especially as is the case with less expensive inks. White inks with a lot of fillers tend to have more opacity, but they also make the ink very thick and tacky to where they don’t flow well on their own. And likely, the press operator is trying to jam it through a bulletproof screen mesh.
When you run boatloads of pressure and the blade is folded onto the mesh, the shear-stress (pressure) is actually a lot lower because the area of blade-to-mesh contact is greater. This makes the ink fight back even more and you are forced to reduce the shear stress (speed) to compensate for low pressure. Catch-22?
If the blade is far enough from the frame, run a higher durometer blade at a near-vertical angle and crank up the speed. Keep the flood-bar off the mesh and run it at a comparable speed. If the flooded ink film contains craters or circular voids in the ink film, you have exceeded the shear rate of the ink—slow down a bit until the craters disappear.
In our final scenario, our press operator is considering what causes so-called ink build-up. They’ll say it’s a lot factors—mesh, color, type of shirt, how the art is done… and 28 minutes later the operator finished regaling us with tales of build-up. The solution is to flash as many times as possible, press operators say. But this is not the natural progression of plastisol. This symptom, dry ink accumulation on the undersides of subsequent screens, finds the conclusion ‘maybe it’s the ink, maybe not’ and the process of flash-cool, flash-cool, flash-cool or revolve is put into action.
Plan B: Ink build-up has three discrete forms; hydraulic, absorptive and thermal. Each of the three cause the ink’s two phases—solid and fluid—to separate. The result is a sticky, dry-looking ink which accumulates to the underside of subsequent screens. Hydraulic build-up begins at the perimeter of the image and is due to excessive shearing forces. Reduce the off-contact distance and blade pressure as well as the blade speed.
Absorptive build-up starts in the central area of the image, not on a flashed under base. It is due to the ink sacrificing its fluid into the absorbent garment (and therefore is worse on 100 percent cotton). Increase the blade speed until the image won’t print then increase pressure until it does.
Thermal build up can operate independently or in conjunction with the other two. It will often show on the side of the screen image closest to the flash unit. It drops the fluid viscosity and therefore causes blurring and spider-legs when printed on an under base. Do whatever you can to reduce the heat. Reduce flash-to-platen distance, add flash additive, reduce lamp or panel temperature. If that fails to improve the situation, reduce the ink deposit and begin again.
Warn the press operator
If a capable press operator knows the identity and consistency of the tools he or she receives from the other departments, they will be oh-so-much better at diagnosing and troubleshooting the job.
Allow each department to label their own work and initial it prior to sending it to the next department. Empower the subsequent department to refuse the work without labels and a sign-off. By the time the identified tools get to press the operator, they will be basking in the sunshine of success… and you’ll make a lot more money. Diagnosis takes time, mis-diagnosis takes way more time. Troubleshooting only exists because your practitioners can’t build a prognosis based on the tools you’ve given them before they get to press.