If you’re either very new or very old in this business, you’ll have come across four-color process (4CP) printing. For those who are very experienced (20 years or more), 4CP or CMYK printing was the only real way of achieving photo-realistic reproduction. For more than 150 years, this was the way all full-color printing was done for all printing processes.
Right from the beginning, I’d like to clarify a major misconception about process color. It may be referred to as full-color printing but, in reality, it is far from full color. We can reproduce a wide range of colors, but nowhere near what we see with our eye.
A healthy human will see somewhere between six and 10 million colors and tones depending on age and gender. The very best printing process will reproduce about 750,000 colors. At best, we will reproduce about 1,000 colors and tones when printing on fabric. Sorry, that’s the best we can do and it won’t get better unless we add more colors to the sequence. This is why simulated process became so popular.
With the advent of digital in the early ‘90s, process-color printing began to fall out of favor. This was primarily because it was so difficult to control. In fact, Joe Clarke’s 1986 book “Control Without Confusion” has been considered the definitive bible for this type of printing and went a long way toward introducing the variables and how to control them. I was fortunate enough to have provided the subject art and the actual project printing for that work.
For those new to the industry, CMYK or process color reproduction is experiencing somewhat of a renaissance. For one thing, it’s easier to separate true process color than it is for simulated process. A second reason why the interest in 4CP is returning is the rise of direct-to-substrate/garment printing (DTS or D2).
All digital printing is CMYK or true process color reproduction. It provides the opportunity to deliver proofs and short runs of a process color image and then have the main, high-volume run done with screen. This dual reproduction of the image by two different printing methods opens the door for the return of true process color. The challenge is to make sure the color is consistent between the two methods.
It should also be noted that while we refer to this method as four-color process, be aware that process color is almost never printed with just four colors. It’s common to add a highlight white to the sequence, even when printing on white shirts. It’s also quite common to add an additional red, blue, green, or purple if the image has strong variations of one of these colors in it. So, the most common approach to process color is really either five color process (CMYK plus white) or six color process (CMYK plus white plus color.)
For inkjet or digital reproduction, it’s very common to have even more. Some of the more common variations are CMYK plus four whites and CMYK, Lc,Lm,Ly, Lbk where L = a lighter version of the primary CMYK colors. This helps control the very light grays, pastels and tertiary colors.
|Four-color process (4CP) delivers a very wide range of color with a very limited number of printed colors. These examples are good choices for 4CP because the colors in the art do not have to be precise, just vibrant. (All images courtesy Andy Anderson, Anderson Studio, Inc.)|
Controlling the process
The goal of any type of halftone printing is to recreate various colors and tones through the use of printed halftones. Here, we will look at the guidelines to achieve the best possible reproduction on press.
You can hand (or manually) print process color, but the results will be inconsistent. This is because the human body is simply not precise enough to get the job done day-in, day-out without variation. We hold the squeegee differently or at a different angle; the pressures and the speeds change as we get tired, for example. It is much better to have an automatic press for consistent results.
In the beginning, choose artwork where the color reproduction isn’t critical. Examples would be landscapes, sunsets, water scenes and similar subjects that do not reference colors we recognize or know what they should look like. Critical color is also known as memory or reference color and is very difficult to work with. The starting goal should be to get a wide range of secondary and tertiary colors in the image without printing more than five or six screens.
Pick images that have a wide range of color and tone (light to dark). Images with lots of detail are easier to print than those with soft, gentle gradients or large areas of a single color.
The primary control points are:
1. Mesh specification,
2. Mesh tension,
3. Squeegee factors,
4. Squeegee pressure,
5. Squeegee angle,
6. Print order.
Mesh is designated by thread count, thread diameter, percent open area and weave type. The 300-305 yellow polyester mesh will provide the most control. Dyed mesh is important because it helps to capture the sharpest possible stencil. The thread diameter should be 34 micron and the weave type should be plain. The mesh is designated typically as 305.34 PW. This combination of thread diameter will have an open area of about 29 to 31 percent.
In addition to pure, saturated color, 4CP can deliver a wide range of grays, browns, and neutral colors such as tan, sand and beiges.
Mesh tension is very important for several different reasons. The biggest is that it is the primary method by which ink is metered to the print surface and how the ink is transferred. The higher the tension, the less pressure is needed to get the ink to shear (cut.) But it goes beyond just this. The screens need to be at the same tension screen-to-screen.
I used to be pretty fanatical about high mesh tension with the results to support this approach. But recent improvements in polyester chemistry have made this less critical than before. You’ll achieve superior results with tensions of 25 N/cm or higher. I rarely go over 30 N/cm anymore.
Tensions below 20 N/cm lead to uncontrollable print variations. The excess elastic capacity of the mesh absorbs the squeegee force. Instead of the ink being sheared, the mesh is stretched first and only a partial release of the ink occurs. To achieve full ink release, additional squeegee pressure is applied with negative effect on print quality.
Low tension is the single biggest cause of uncontrollable print variation resulting from excess, unnecessary pressures. I could spend several articles just on this one area, but keep it simple and stick to 25 N/cm and you won’t have any problems. The bare bones minimum is 22 N/cm.
New screens lose tension rather rapidly during the first couple of cycles. This is mostly due to relaxing the knuckle friction where the threads overlap. On a 305 mesh, there are 93,025 mesh knuckles per square inch. You can imagine how fast the tension will drop if there is even the slightest give at each knuckle.
The up and down stretching of the mesh during each print stroke has the same effect as constantly bending a knot in a rope. Eventually the cord loosens and the knot comes undone.
The same happens with mesh over time. This is why it is so important to use re-tensionable screens. Re-tensioning after each run for the first few times will provide very stable screens. The bottom line here is, new screens are the most unstable when it comes to maintaining mesh tension. Use seasoned screens for best results.
From my experience, a triple durometer 70/90/70 squeegee provides the most control. The softer durometers on the outer edges provide some “give” at the print surface while the stiffer 90-degree
center keeps the blade from bending over during the print stroke.
Single-durometer squeegees are much more prone to deflection (bending) during the print run. The combination of edge durometer and edge sharpness are critical in shearing the ink with the least amount of pressure.
To determine if the edge is sharp, lightly drag a finger tip over the squeegee edge. If you can feel the individual ridges of your fingerprints, the blade is sharp enough. If you can’t, it’s time to sharpen the blade.
On the note of sharpening, I should add that it is not enough to put brand new blades in the squeegee. Even though the blades may be perfectly sharp, it is almost impossible to mount the blades perfectly flat in the holders. This is one of the most commonly-overlooked mistakes I see.
After mounting the squeegee, look down the edge of the blade. You will see that it wavers or bends due to the clamping pressure of the holder. Sharpening/honing the blade after it is mounted will result in a much truer edge and a perfectly-even ink shear in the screen.
Pressure on press is very simple: use the least amount possible. Using 25 N/cm screens and a sharp edge as described requires very little pressure.
Back the pressures out until the ink is left behind in the screen. Add 1/4-turn of pressure on each side of the squeegee until the ink cuts and nothing is left behind. You should be able to wipe your finger across the inside of the screen and not have any ink on it. If you do, it means the ink is too cold or the squeegee is not sharp enough.
Squeegee angle should be 15 to 25 degrees off of vertical. In this range, the downward force on the squeegee is focused directly over the cutting edge of the blade. If the angle is any flatter than 25 degrees, the blade will bend. This is very bad and highly inefficient. Keep the force over the edge for most accurate reproduction.
Finally, print order is very important. The preferred order for printing straight 4CP is YMCK. (Yellow first because it has the most area and is the most easily contaminated.) Because 4CP is most commonly printed with halftone angles that form rosettes, there is a tendency for the subsequent ink colors to be contaminated as the ink on the shirt can work its way up into the screens over time. Printing lightest to darkest minimizes the contamination in the following colors.
If using a highlight white (highly recommended), it would become the first color down followed by yellow. Do not flash between colors or after the white.
When adding additional colors to the process sequence, place them immediately after the closest color in the sequence:
Red after magenta
Blue after cyan
Green after yellow or cyan
Purple after cyan
Until next time
While there are many sub points I could add, these minimum control points for obtaining consistent results will give you the biggest bang for any efforts. Next month, I will go into the separation process in-depth. You will need Photoshop to obtain the greatest degree of control. If you don’t have it, I will provide recommendations for best results based on some of the other most common graphics programs available. You do not need special separation software, although you will get superior results using a dedicated separation software program.
Print practitioners beginning to use four-color process often have trouble obtaining a full tone range—pure whites and full, deep, rich blacks and solid colors are diffi cult to achieve. The key is to have the correct color settings and basic controls in place.