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Process Control Precedes Color Control

Joe Clarke has spent the past 47 years in the lab and in the engineering department, in pre-press and on-press, as an R&D / technical researcher and as a manager of screen print production. Clarke has held executive positions as President of M&R Printing Equipment and as Vice-President at Wilflex [Poly One]. He has been granted a growing number of print-related patents, including one for High-Shear printing with Smilin'Jack - he is a member of the ASDPT, is an Associate Editor for NBM and an SGIA Fellow.

Clarke has presented hundreds of technical papers, written a couple books and published over 600 technical / management articles for which he has been awarded five Swormstedts; the international standard for excellence in technical writing.

Currently Joe Clarke is the President of CPR, a Chicago-based corporation which manufactures Synergy Inks including NexGen; environmentally & financially responsible T-Shirt inks. For more information on CPR, visit http://www.cprknowsjack.com/.

In process color printing, most will invariably argue that the biggest key to success is color separations that will work every time. Truth be known, of all the garments printed with true process color, most aren’t accurate and look washed out. Most four-color process work tends to fade into the shirt because print shops overdose the colors on clear and use unnecessarily fine meshes to print them. But the art of printing hundreds of CMYK colors from the same four films is actually feasible, but only if you pay your dues up front. This paper is about those dues and their associated costs.

The most detailed, superior prints are a result of the print parameters, ink, garment and good separations. (Images courtesy Lon Winters, graphicelephants.com)

The four major elements of process-color printing in order of precedence are: the pump, the ink to be pumped, the garment which receives the ink and the color separations. To tweak or perfect them out of sequence and get good results would be a stroke of luck, but you’ll never have consistency or predictability.

The pump is composed of three main elements—the screen, the press and the blade. The screen includes the robustness of the frame, the mesh geometry and printing tension, the flatness (not thickness) of the stencil on both sides and the thickness of the stencil on the substrate side. The press implies tolerance with respect to planarity, parallelism and gap. The blade is the relationship of the flexion and compressibility (not the durometer) of the printing blade to its angle, pressure (downward force) and its stroke speed.  

Many under-expose stencils to hold detail, but properly exposed screens that are used in conjunction with all best print practices will achieve detail without sacrificing any other facets of quality. 

Rules of the screen

Frames should be as large possible and should not deflect under the load of tensioned mesh.

More mesh generally ruins the halftone print, therefore use a 305/31 yellow. Also keep in mind in regard to mesh:

• Thread diameter limits the most sensitive highlight end of the tonal range.

• Orientation or widths and stretch should not be changed with the gloss side on the substrate side.

• Tensioned mesh opening limits the less-sensitive shadow end of the tonal range.

• Tension the 305/31 to 25N/cm² in the center; 20N/cm² at the corners of the image.

• Printing tension needs to be stable to stretch in order to lower tension around the perimeter.

Stencils should have a minimum RzS1 (stencil thickness on side one) and RzS2 (stencil thickness on substrate side) and widest exposure latitude. Other notes on stencils:

• Don’t use EOM (emulsion over mesh) as a spec for CMYK printing, as it is a result of achieving proper flatness.

• Do not under-expose to hold detail; you’ll gain edge-acuity but will lose all else.

• If RzS1 is low enough, overexposure will be very unlikely.

The frame must be robust with the largest margin (area around the image) possible. The mesh should have the thinnest thread and the lowest count and must be dyed for sharp edge acuity. Always stretch screens with the glossy side of the mesh on the substrate side. Never change orientation or width of the fabric once you set standards. The key is not static tension but consistent printing tension. 

Stencil thickness should be zero but, sadly, in order to get a sufficiently flat coating, you will build some thickness, measure the Rz and take what you get with the EOM. If you under expose to hold dots, you’ll make a mess of the image. If you have a good RzS1 and RzS2 you won’t need to under expose the stencil.

The use of thin white lines against the background serves to both build contrast and use the stencil to boost the ink deposit.

Press parameters

The press MUST be accurately calibrated:

• All points of all platens on the press should be on the same plane ±0.010".

• All screens should be on the same plane, parallel and above the platens ±0.010".

• Off-contact should be at minimum for printing tension, maximum for dimensional accuracy 0.120".

• Shim screens that are low tension at 0.060" per 2N/cm² loss.

All the work you put into the screen is for nothing if the press is not accurately calibrated. A 10-color press will take four to eight hours with two people for original calibration. This will not be the time to look for a shortcut. Single point off-contact adjustment will never work for CMYK unless every screen is at precisely the same tension at all five points.

*Editor’s note: For a step-by-step on manual press calibration, see Printwear January page 62 or search the article archives on www.printwearmag.com using “press calibration” as the search term.

Blade guidelines

Blades (squeegees) should be biaxial and permit zero-degree angling (be at 90º or vertical). Additionally:

• Keep the length to the size of the image plus 1" total.

• Blade edges must be less than the mesh frequency 0.003".

• Minimize pressure to bring the mesh into contact and to clear the openings.

• On a calibrated press, a 305/31 mesh should be tensioned to 25 N/cm² at 0.120" off contact; pressure will be 28 PSI.

• Blades are bi-axial 85 Shore-A to adapt to both stroke and perpendicular directions.

• Pressure on the platen during the print stroke should be at or near zero.

• Print at maximum stroke speed for maximum contrast, finish and hand.

Always minimize the blade to shoot for the (impossible) sweet spot of the T-shirt screen. The edge of the blade should be less than one mesh period so it can run fast to properly transfer and clear the ink after every pass. Keep the angle at or near zero if your blade is biaxial and the pressure as low as possible. With respect to platen deflection, follow the prescription herein and the pressure on the platens will be near the ideal zero so they won’t deflect. Always print at maximum speed for the best looking and most consistent image. The conditions herein will permit maximum speed.

A flat, smooth, bright, high-fabric mass garment will give screen prints maximum value.

Ink considerations

Use a pseudo-plastic ink with a minimum yield when stressed, a maximum absolute viscosity, minimum plastic viscosity and minimum surface tension. Pseudo-plastic inks are short and more stable during the press run. Low-yield stress means the ink will flow with minimum fluid pressure absent moiré. Maximum absolute viscosity causes the ink to print a sharper dot while minimum plastic viscosity allows the ink to print with a soft hand. A minimum surface tension allows the ink to print brilliant images at top speed.

Most process-color inks have similar chemistry to the line colors with an omission or two and one or two additives… this is why they don’t work very well. The recipe above will allow you to compare spec to spec and to select an ink that will meet your needs.

Concerning color, no one makes or can make an RFU (ready for use) CMYK ink that will work without supplied specifications. Other points to consider here:

• Don’t mask off the color bars.

• Print YCM then K in order to create accurate RGB and near neutrals.

• Observe for optimal red (combine Y with M), blue (C with M) and green (Y with C).

• Observe for optimal near-neutral gray (Y with C and M) throughout the tonal range.

Unless inks are custom made to fit a consistent spec within tight tolerances for your pump, sequence, press, blade and shirt, nobody makes a ready-for-use balanced set of colors. You may need to add concentrate, toner or clear to adjust your inks to print the proper secondary colors. Never assume an ink will print the same way twice in a row.

The canvas

A flat, smooth, bright, high-fabric mass garment will give your handiwork maximum value. Select a ring spun shirt with the highest gram weight. Print a single color magenta halftone and observe to test the flatness of the fabric. 

Finally… the seps

Separations cannot be established until every other parameter is in place. Once that is so . . .

• Remap the tonal range to a color proof, through curves or, preferably, a CMS system.

• Become skilled and totally predictable at 45 lpi before increasing the line-count.

• Lower line-counts facilitate color balance and permit a longer tonal range.

• Film dot density must be sufficient for exposure.

If you are matching a commercial proof, most files have been built for sheet-fed offset printing and bear no resemblance to the tonal shifts inherent to screen printing. Whether through curves or with a color-management system, you will want to remap your tones to suit a proof. 

It’s important to stick with a 45 lpi until every single product, person and step in your process accommodates this line-count. Only then should you consider higher line-counts. A good 45 will blow the doors off of a mediocre 85.

If you will remember to buy the best consumables you can find, the preceding recipe will allow you to run virtually any original with a quality separation software or plugin. You’ll achieve color immediately and maintain it throughout the press run. The printed images will scream off the shirt, you’ll have a most efficient and effective process and, most importantly, a most satisfied customer.