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How to Avoid Color Changes in Screen Printing

(November 2002) posted on Mon Dec 09, 2002

It's true: Colors do change with screen printing. Find out why and what you can do about it.


By Carol Swift, Peter Kiddell

To help prevent the edge from deteriorating, keep squeegee pressure to a minimum. If you are printing a very long run, prepare several squeegees identically and have them ready and waiting in their holders when you start the job. Change the squeegees at regular intervals, before any edge wear becomes apparent. Don't even think of taking the squeegee off, sharpening it, it and putting it back on the press immediately.

Floodbar settings

Many printers do not realize that the floodbar can radically affect how much ink is printed. The floodbar's job is to coat the mesh with a consistent amount of ink. It doesn't just prevent ink from drying in the mesh--it plays a critical role in the consistency of the entire printing process. Altering the floodbar's pressure or angle can result in an excessive or insufficient amount of ink on the mesh. The speed of the floodbar is important as well: If it's too slow, the mesh will be overloaded with ink; too fast and the mesh will suffer from "ink starvation."

The floodbar's edge has an impact on its performance, but this edge won't wear as quickly as a squeegee's. You're much less likely to see color shifts resulting from changes in the floodbar edge than you are from making adjustments to the floodbar during the print run.

Machine settings

Process control is the key. Consistent setups mean consistent color. Variations in job settings will send color out of control. The classic problem occurs when printers change shifts and the incoming press operator alters the machine settings to suit his style. This causes the color to change. Modern presses that use computerized controls help reduce this possibility by making it easier to set up the press to exact specifications, maintain those settings throughout the print run, and recall them if the job is repeated later.

Substrates

An often-ignored aspect of screen printing is the consistency of the substrate. Paper, board, and plastics are generally produced in batches, and a good-quality supplier will maintain the surface finish of the substrate throughout the batch. But this is not always the case. Sometimes changes in manufacturing will alter the color of the substrate, as well as its gloss level and absorbency. When any of these things changes, it leads to a color shift even if nothing in the printing process has changed.

As a screen printer, you face real difficulties because of the range of different substrates onto which you might be asked to print a promotion. You may be asked to match a litho-printed image, but without careful process control, you have no chance. Your probability of success increases dramatically if you use such tools as a spectrophotometer for line art and a densitometer for process color. But if you rely on the traditional method--flying by the seat of your pants--the variables will be so great that the process will be completely out of control.

Viewing light

Nature has played an unkind trick upon us, making the same color appear different under different lighting conditions. The human eye is remarkably sensitive to these changes. You can minimize this variable by making sure the pigment and pigment level are precisely the same throughout the print run. Try to mix all the ink you'll need for the job in one batch. Don't change ink systems or brands during the middle of a run. Color perception is a very complex area. To gain the best possible control, be sure you have a closed loop of ink manufacturing, mixing, proofing, and color measurement.

Drying

We have all suffered color changes caused by incorrectly set dryers at one time or another. A common example when printing paper or board is having white image elements turn yellow due to excessive dryer temperatures. UV-based lacquers also have a tendency to yellow. The printers that suffer most from color changes during drying (or, in their case, firing) are glass and ceramics decorators. Their pigments can change completely from printing to firing. The final colors are affected not only by the firing temperature, but also by oxidizing or reducing qualities of the atmosphere within the firing zone.

True colors

The difficulty in reproducing colors accurately and consistently is one of the things that makes screen printing so interesting and challenging. Your odds in achieving correct color only improve when you take the variables of the printing process out of the picture. This discussion highlighted some of the key areas where you should focus your attention. Learn to control these elements of the screen-printing process, and you'll be able to deliver quality color time after time.
 


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