05 Jun Why Use Cross Hair Registration Marks?
Why Use Cross Hair Registration Marks?
Let’s begin with a question. Why are register marks used to align the stencils on a multicolor job?
by Bill Hood, ASDPT Fellow and SPTRC Lead Researcher
Most screenprinters use registration marks to align the separate screens when printing with more than one color. Since each ink color used in screenprinting requires a different stencil, the belief is that registration marks are crucial for accurate alignment. But, are they?
A press operator will often spend more than 10 minutes per screen aligning the crosshairs on the press. After a press check, the micro-adjustment requires another 10 minutes to align the job correctly. The task repeatedly continues with each screen on the press until the registration is “close enough.” Why not more quickly align the image as closely as possible and then make the micro-adjustments based on the printing position? This practice reduces the possible registration time by 50 percent.
There is a common belief that registration marks have always been a part of the printing process. The invention of screenprinting predates the use of crosshair registration marks. Benjamin Walker’s 1884 patent, which was the first to use mesh mounted on a frame. At the time, press operators brought images into registration, not crosshair marks.
The first use of crosshair registration marks was in the making of animated cartoons. Winsor McCay, the famous cartoonist, first used them in the creation of his animated film Gertie the Dinosaur in 1914. There is no record of when screenprinters began using crosshair register marks, but we did not use them in the 1940s when I first started in the screenprinting process.
The first patent to mention the use of two intersecting lines forming a cross is the 1956 patent issued to M. Bregman (US002760273), so perhaps screenprinters got along for at least 72-years without the marks. Similarly, those in the textile segment were just fine without the flash gel unit for at least the first 80 years. Considering that neither the crosshair registration marks nor the flash gel unit add anything to the process and waste time and energy, it is safe to say that they are not as worthy as thought.
Note: Suzuki Harunobu (c. 1725 – 8 July 1770) of Japan, invented the three-point method of aligning paper onto a woodblock in 1765. His three-point alignment method made multicolor printing possible in more colors, accelerating woodcut printing techniques with revolutionary speed. His approach used notches and wedges carved into the woodblock that assisted in aligning the paper onto the block in position.
Fixed Process versus Screenprinting
While the crosshair registration marks work well in other fixed plate processes, the screenprinting process is a variable process. The printing plate (the mesh) is in an off-contact position and flexible. When attempting to align the stencil with the substrate in the off-contact position, the mesh and image shift as they move downward. Multiple images are no longer in alignment. Any variance in mesh tension, off-contact, and the parallelism of the printing triad will affect the registration. Also, the squeegee blade’s hardness, pressure, angle, and especially friction will move the mesh of each stencil differently and create even more problems.
When screenprinting with low-tensioned mesh, the image can be stretched out of shape. While all registration marks may print in alignment, the print can be grossly deformed and not be dimensionally accurate. A circle becomes an elliptical shape when stretched. The rosette pattern of the 4-color process is distorted as well and alters the color of the print. Test the amount of distortion by laying the transparent film positive down over the printed image will show any deviation from the original.
Thus, the screenprinted image cannot be dimensionally the same as the original art without altering the film positive to counter all of the existent variables.
Dependency on the registration mark alone is never a good idea in the screenprinting process. The commonly held belief is not a reality of actual registration if no matter how closely the marks get us, we still fall short of a successful outcome.
A Thought: Would rather have the crosshair marks or the print in registration?
The Impossibility of Dimensional Accuracy
No matter which camp you belong to, dimensionally accurate screenprinting is, for the moment, impossible. The use of crosshairs will never achieve this state of accuracy, nor will registration of the image. In fact, many practitioners, in an attempt to define accuracy, will resort to degrees of dimensional accuracy. They use terms such as, loose (±.030-inch,) commercial (±.010-inch,) tight (below .005-inch,) and critical (below .002-inch) Obviously, none of these definitions, no matter how close, are not capable of or successful in reaching the intended target of accuracy.
Dimensional accuracy is the ability to print an image that matches the original image in physical size and with each color in register with the other colors. To attain absolute dimensional accuracy in screenprinting is impossible, as screenprinting, unlike other printing methods, is not a stable process. There is no one root cause, but a combination of factors that ultimately create the problem.
Dimensional accuracy of reproduction continues to be problematic for those in screenprinting for a variety of reasons including mechanical, force, friction, and temperature. There is also the generation of the film positive in relation to the original art, the instability of the mesh due to tension, the light used in the exposure and how it is received by the layer of emulsion, the shrinkage caused drying of the emulsion, the necessity of off-contact in screenprinting, the flexibility of the squeegee, and so much more.
I am in the midst of writing a book on the subject of, “The Impossibility of Dimensional Accuracy in Screenprinting,” which hopefully will be published in the next few months. It has taken an extreme amount of time, as one variable leads to others in the quest. Look for it soon at ScreenprintBooks.com!
Until then, “May all your impressions be great!“