Screen Print Methodology: Winning White

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/.

White ink is a research project. We ask ourselves which part—art, film, mesh, stencil, ink or blade—will save the day. While some in the industry lead us to believe there is a single smoking gun, a singular solution underpinning your puzzlement, it is the random input of raw materials that makes the print unpredictable and the wide range of images and garments that makes the task formidable. Let’s look at those variables.

Art

Artwork can be used to fool the eye of the beholder. The best artwork for a winning white on a dark shirt is an image that has a high spatial content, high spatial frequency and high luminous and chromatic contrast. High spatial content means one with lots of white—the polar bear on a snowscape, but only if the image is very busy (high spatial frequency), and if those elements have a high luminous contrast (black and white) and chromatic contrast (pastel yellow and violet). This type of print engineering (that which fools the viewer) is very conducive for day-to-day printing.

Screen mesh

The primary constraints to mesh selection are twofold: one’s ability to produce a quality stencil on said mesh and the level of calibration on the press. For those talented with stencils with a well-calibrated press, select a lower mesh count with a thinner thread for plastisol and a lower mesh count with a thicker thread for discharge.

A low count/thin thread mesh will allow the plastisol white to transfer with minimal pressure and produce a very smooth and bright white. However, your coating method will include face coats, and this mesh will not be as bulletproof as your low count/thick thread screen.

A low count/thick thread mesh keeps discharge from drying in the screen and can pump a lot of “bleach” into the knitted fabric. However, when using this mesh, the coating on the inside of the screen needs to be built up, which means lengthy exposure times and using an extrusion-type squeegee blade. 

Stencil

If artwork is composed of a lot of tiny elements, the stencil will have a major effect on the whiteness of those areas and contrast can be further increased with a thicker stencil (higher emulsion over mesh). But, if there are small elements and larger areas of white, the stencil will only affect the brightness of the small parts and leave large white areas that don’t look as bright when compared to the smaller print elements. Stencils per se are all about application—all of them work and nearly all work well if applied as intended.

If using a low-count, thin thread mesh, bridge the mesh with face coats. The goal is to have a very high-solids coating with a higher-than-average viscosity. If it is low count, thick thread mesh, build up the stencil on both sides. The same high solids, high-viscosity emulsions will help save passes, but be sure it is transparent enough to see through during image alignment on press. Dual cure emulsions that are intended for high edge acuity and high resolution will serve your purpose. If the image does not have many fine details, a pure photopolymer will suffice. But if the detail is really high, go with the dual cure.

Ink

First let’s look at opacity. White ink has a tendency to block the background color of the garment in a relatively thin deposit. Here’s the rub: about the only ingredient that helps white achieve opacity is TiO² (titanium dioxide, white pigment). However, if the white is overloaded with TiO², it becomes far more difficult to print. The printed surface will be rough and it will not matte down well at all. 

So, the plastisol to look for is one with a high absolute viscosity, low yield stress, low-to-medium plastic viscosity, with low surface tension and fast recovery. It starts out thick with a false body, thins readily with the least amount of shearing energy (that is, squeegeeing), loves cotton and polyester but regains its body immediately when the shearing force is removed. This one will matte down the surface with minimum pressure and apply a smooth layer without sacrificing detail. 

White that has a very slight bluish cast can cover the yellow/gray cast intrinsic in white pigments. Such a white has been seen to be more responsive to squeegee pressure versus print speed, so the result is best when this white is transferred with an extrusion model blade. The same recipe will work for an underbase as well as an overprint white, but all underbase inks (white included) should have a low surface tension, which will make them easy to overprint. These will be flat (versus glossy) and will look and feel dry. 

Blades

To get edge definition in a print, seek a blade that makes a tight seal between the stencil and the substrate immediately after the ink is transferred. This means the squeegee must run at or near vertical. Soft blades buckle and hard blades need much angling. In both extremes, the edge of the blade is nowhere near the downward force. And, if run fast, they hydroplane, resulting in a rough finish. 

If the image is small enough to fit in the sweet-spot of the screen, a single-axis blade may do the job. Even so, putting a bevel on the print side of the blade will allow you to print near vertical and produce a smooth finish. 

If printing an extrudable ink, the mesh can be pre-loaded without ruining the print. A metal flood-bar risks ripping the mesh, so consider a polymeric flood-bar which allows a screen to be preloaded without damaging the image, the stencil or the mesh. This type of blade allows users to alter the matte down of the image without shifting its registration.

The combination of the right squeegee and flood-bar will run as fast a print-stroke as the press will allow. High speed puts ink into an optimal shear response and causes it to flow to matte down all those conspicuous fibers… even with white ink.

Press settings

No matter how hard you work on the preceding facets of the process, if the press is not calibrated you’ll have precious little luck. Sadly, industry estimates say that 20 out of every 100 presses warrant the label of “calibrated” and of those 20, about five are well calibrated. It is beyond the scope of this article to detail calibration but, suffice it to say that if every platen doesn’t print the same, if the images are darker or lighter from front to rear, if one or more press heads give you reason to pause or if you’ve got a favorite head, you’re out of calibration. If and when you need or decide to calibrate, be prepared to invest 30 to 60 minutes per color and, if you’re the sort who can do it faster, consider that this may be at the core of the problem. Once you are in cal, check the press every week to be sure nothing has changed.

Award-winning white prints are not about “a single smoking gun.” Stellar prints are recognized as such due to great artwork printed by those who know how to orchestrate all of the variables. Make sure the given art is “cool” and designed to fool the observer, the stencil must accommodate the restrictions placed upon it by the mesh, the ink should be to spec, the blade should extrude with a flood which pre-loads, and the press calibration can make or break the whole deal.