Roger L. Jennings

R Jennings Manufacturing Company, Inc.

You want to increase your income and reduce your labor required by screen printing. One way to achieve these objectives is to eliminate flash curing when printing white or light colors on dark garments like black.

If white ink is printed on a black shirt, flash cured, and then printed again before the shirt is removed from the press, a typical production rate is 30-40 shirts per hour. There are screen printers who say they do more or less per hour, but 30-40 is the number quoted by most.

When the same screen printers print a dark color ink on a light colored shirt, like blue ink on a white shirt, they typically quote a production rate of 100-120 shirts per hour. Again, there will be some who say they do more or less per hour. So printing without flashing results in production at a rate 300-400% higher than printing with a flash.

These higher production rates can be achieved with white ink on black shirts by following these procedures.

110 Mesh on a Retensionable Frame Soft mesh results in ink being driven down into the shirt, soaking the shirt with ink, and allowing the color of the shirt to show through the ink film. The dull print then needs to be flashed and printed again to achieve a bright, white image when white ink is printed on black shirts, for example.

Mesh stretched to its ultimate breaking strength relaxes 20-25% within a few hours. The loss of tension occurs more quickly with high mesh counts like 230 and 305 than more coarse mesh counts. With more time, the mesh will relax even more. The slack should be taken out of the mesh bringing the tension back to its former level, plus 2-4 more N/cm2. Then the mesh will relax again, but only 15%. The mesh should be retensioned again to recover the lost 15%, and the tension should be ratcheted another 2 N/cm2 higher. By the next day, the mesh will only lose about 5% of its tension. The screen printer with a tension meter will witness this experience.

Mesh deflects more in the center of the screen than near the frame. Very tight mesh is harder to deflect, holds registration better, and contributes to the ink being cut, or sheared, more than mesh that moves laterally with the force of the squeegee. Tight mesh deposits a more consistent thickness of ink.

When asked, all screen printers respond saying they prefer to print with tight screens rather than loose screens. The tightest screens are the best, and optimal tightness comes from retensioning.

Capillary Film With liquid emulsion, the mesh count is used to meter the amount of ink that passes through the image area of the screen onto the garment. That function is fundamentally changed with capillary film.

Capillary film is purchased as a liquid emulsion that has been coated to a polyester sheet at a precise thickness from 10 to 700 microns thick. That film can be adhered to mesh with plain water, and after drying, the polyester sheet is peeled off and discarded.

Now ink passes around the threads of the mesh and into the image area like a mold without the influence of the mesh on the edges of the image. More importantly, the thickness of the ink deposit is determined by the thickness of stencil. Thickness is a determinant of opacity as long as the mesh is very tight so that the ink is on the surface of the garment rather than driven into the garment.

Thicker stencils result in heavier ink deposits and more "hand" or feel to the print. Most consumers want to see the image, but not feel a lot of image. So the objective is to print the maximum thickness of ink that creates the acceptable level of opacity or brightness to the image. If you have never used capillary film, most manufacturers and distributors will provide a free sample with instructions, or you can contact R Jennings Manufacturing for a free sample with instructions.

When the optimal printing technique (squeegee angle, pressure, speed) are used with a tight screen, capillary film properly applied, and an opaque ink that shears easily, that capillary film thickness is 50. If a person is new to screen printing and has not developed their technique, then 70-80 microns might be more appropriate. A thicker stencil like 70-80 can be created by coating the screen with liquid emulsion first before laying the capillary film on the mesh. The liquid emulsion fills the mesh and prevents the film from being drawn into the mesh. Similarly, capillary film should never be squeegeed into the mesh.

Opaque White Ink that Shears Easily All inks should be thoroughly stirred in the manufacturer’s container. An electric drill with a bent rod or Allen wrench in lieu of a drill bit will do the job after about 3 minutes in a quart and 5 minutes in a gallon. Then pull the rod straight out of the ink. Observe. Does the ink drop off the rod immediately like pudding from a spoon, or stay attached when the rod is 6" above the container of ink?

Ink should drop off the rod like pudding for the ink to shear easily from the mesh when printing. All experienced printers print off-contact, i.e. with the mesh 1/32 – 1/16" above the T-shirt, and more for compressible materials like sweatshirts. When pressure is applied to the squeegee, the mesh is deflected so that the mesh makes momentary contact with the shirt. As the squeegee passes over the image, the squeegee leaving a portion of the image area releases the tight mesh which springs back (or peels off) to its original position.

If ink is adhesive, it will draw out and continue to connect the mesh and ink on the garment rather than releasing from the mesh. By contrast, ink that shears properly will appear to drop out of the image and be entirely on the garment. There will be no ink in the image area of the screen or, when using very tight screens, built up around the edges of the image.

If ink does not shear properly, the choices are to change to a brand that does shear like pudding or modify the ink to shear as described. Ink can be modified by mixing with the same color ink that shears more easily. Another option is to add curable reducer. There are risks with curable reducer.

The risks are: (1) Too much curable reducer can result in the ink never curing properly regardless of curing temperature or time. The ink will wash off the shirt. (2) Shirts with man-made fibers like polyester, e.g. 50-50’s, will show dye migration. White ink on maroon shirts will turn pink. (3) Ink will absorb into the garment rather than stand up on the surface. Ink that absorbs into the shirt will need to be flashed and printed a second time.

Curable reducer should never be freehand poured into ink. It must be measured in with a scale. The appropriate amount will be some increment of 2% with the maximum of 10% by weight. After each 2% is added, the ink should be whipped again with the electric drill, and then the rod pulled from the ink to see if the ink drops off like pudding.

Conclusion The mechanics of 110 mesh on a retensionable frame, 50 micron capillary film and opaque ink that shears easily are easy to achieve. This combination duplicates an ink film thickness equal to printing, flashing and printing, but without flashing.

The challenge for most people will be finding the optimal printing technique. For many, that means slowing the print stroke, using less squeegee pressure, and maintaining a constant squeegee angle of 45 degrees. Until that technique is perfected, pulling the squeegee twice without lifting the screen or flashing curing is a much faster way to print than using the old print-flash-print method.

The 3 photos on the left show the steps for applying capillary film.  Notice the water is being sprayed up the screen as the film is being rolled down in the water.  Water makes the film stick.  If you have insufficient water, then you will end up with white spots in the screen which are air trapped between the mesh and film, and which need to be dragged off the screen from the side of the white spot using as light pressure as possible.   Click on the photos to enlarge.

The photo on the right shows how the ink on the stick shears from the ink in the can immediately upon removing the stick from the can just like you would expect from pudding.  Nearly all inks on the market do not perform like pudding.  Click on the photo to enlarge.

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