Avoid screen printing mistakes

By James Ortolani

Many years ago, one of my industry mentors told me that screen printing was an easy business to get into, but then I’d spend the next 15 years trying to figure out how to do it better. At the time I thought he was exaggerating, but more than 20 years later, I am still learning new tricks to the trade. Below is a list of a dozen problem areas you might have in your shop along with some solutions to these daily dilemmas.

1. KNOW YOUR WHITE INKS
Most ink companies offer a standard, multipurpose white plastisol that can be used as a mixing white (to lighten other colors) and to print as a standard white with decent opacity on dark shirts. In fact, many designs even call for white ink on white T-shirts as a spot color or as a highlight to the design. The reason for printing white ink on white garments becomes apparent after the garment is washed several times and begins to lose its bright, white look. The shirt will fade in brightness, but the white ink in the design will remain crisp and bright.

Athletic white is a high-gloss white ink with good elasticity and opacity, designed for printing on athletic wear and team jerseys. Some printers use athletic white as their primary white to print on everything, but there are drawbacks to this practice. For instance, athletic inks are thicker and more difficult to print through finer mesh, and the ink has a gloss finish that is not suitable for some designs.

It’s also not ideal when you want the whitest white print on dark garments. For this, use a fast-flashing white ink to print an underbase, then flash and overprint more white ink. These fast-flashing whites use a resin that gels at a lower temperature than conventional plastisol, cutting flash times by half.

Another specialty ink — low-bleed (LB) white — is ideal for bright, white prints on fabrics prone to bleeding — a problem referred to as dye migration. LB inks contain dye blockers that attack dyes from the garment as they bleed through the white ink deposit. Dye blockers are usually in the peroxide family, so use caution when stacking garments after they exit the dryer. Dye blockers work by bleaching the dyes in the garment and this bleaching action continues for a short time after the garment exits the dryer.

Thus, if you stack hot LB prints right out of the dryer, you will get “ghosting” on the backs of the garments as they stack up. Avoid this by allowing the T-shirts to cool completely before stacking.

2. FLASH CURE PROBLEMS
Occasionally a printer will call me explaining that he has a washout problem with screen printed designs on dark garments. In some cases, the underbase color doesn’t wash out, but the colors printed on top of the underbase do. Here’s why: The key to successful flashing is to simply gel the fast-flashing underbase ink at 190°F for three to six seconds. After other colors are printed on top, the finished print should reach 320°F in the dryer, at which point all the inks, including the gelled underbase, melt and bond together and to the garment.

The delamination problem starts when the underbase is flashed at too high a temperature. If you exceed 190°F and get to the 320°F range, you risk totally curing the underbase. When that happens, it becomes a solid sheet of vinyl (or plastic), and plastisol ink will not stick to vinyl. That’s why, after a few washes, the top printed colors will delaminate from the underbase and wash off.

Another solution to speed up flash times is to cover the shirt boards on your press with a thin sheet of 70-durometer, heat-resistant neoprene. The neoprene cover insulates the shirt board, directing infrared (IR) energy into the ink film instead of absorbing into the mass of the shirt board. Aluminum shirt boards are the worst for absorbing IR energy and slowing down flash times, so covering aluminum shirt boards with neoprene can cut flash times in half.

Over flashing also can cause the dye migration and bleeding problems mentioned above. Dye migration is a hot topic in most shops today because plastisol inks can appear to give good opacity as the printed garment exits the dryer, but hours (or even days) later the dye from the garment can bleed through the cured ink film. Dyes in the garment will bleed through the ink for several reasons — excessive flash temperatures, excessive dryer temperature or improper garment storage. Garment dyes heated up in the dryer will begin a “gassing” process that penetrates the ink film, tinting the ink to the color of the garment. Garments kept in hot, humid storage also can fall subject to dye migration.

You can fight dye migration by using an ink system that is opaque and contains dye blockers. Also, print 100% cotton T-shirts when you can. Natural fiber is more absorbent than blends and less likely to release dyes that bleed through plastisol.

3. SCREEN PREPARATION
New screen printers commonly ask about screen tension (measured in Newtons/centimeter). The key here is to be consistent. Whatever tension level you get your screens up to, it is important that all screens on press for a job are stretched as closely as possible to the same N/cm reading. If tension readings vary, screens will shift or slip differently under the pressure of the squeegee, causing registration problems. Most experts in our industry agree that 25 to 30 N/cm is a good place to be with your screen tension for standard, everyday printing.

The benefits of printing with high-tension screens are well-documented, so investing in retensionable aluminum frames should be high on the list of improvements needed in most shops. High-tensioned screens provide better opacity, lower ink consumption and lay down a thinner ink deposit that is easier to cure and produces a print with a softer, more desirable hand.

4. CHECKING THE CURE
The best way to check for a full cure on your prints is to conduct random wash tests in a standard washing machine. But in a high-paced production setting, you cannot wash every garment, nor would you want to. Instead, closely monitor all the variables that may affect ink curing.

Some companies use temperature strips to check the accuracy of their dryer temperature. I prefer using the pistol-type surface thermometer, or ray gun. Simply point the temperature gun at a plastisol print as soon as the garment begins to exit the oven. The digital readout should show at least 320°F to ensure a full cure. If the reading is less than 320°F, make the necessary adjustments to the dryer. Either slow down the dryer’s belt or turn up the temperature.

5. COLOR MATCHING
Printing for today’s corporate customers often involves matching plastisol colors with Pantone PMS colors. You will not match those colors by mixing standard, off-the-shelf plastisols, so purchase a PMS color matching system.

Also buy a digital scale and a “recipe” book showing all PMS color formulations. Since plastisol inks have a matte finish (low gloss), compare the plastisol color with both the Pantone “U” (uncoated) and “C” (coated) color chips to see which best matches the color your customer desires. Usually, you will find plastisol more closely matches the U color chip. By the way, always do color matching in natural light; artificial lighting reduces the brightness of a color.

6. PRINT COLOR ORDER
The common rule of thumb in setting up the screen/color order for printing on white T-shirts is to print light to dark — yellow, orange, red, blue, green and then black, for example, would be typical. However, use the exact opposite color order when printing on dark garments to reduce the amount of ink pickup on the backs of screens. This technique makes the last colors printed appear brighter and more opaque.

The exception to this rule is if you are printing a large block area of color. In this instance, move this screen to the end of the color order to avoid ink pickup on the backs of other screens. Conversely, a screen with a small design element should be moved to the front of the order.

Finally, make sure your color separations (artwork) are butt to butt (a.k.a., butt registration). This allows you to print colors in any order since the inks don’t overlap, though it’s still a good idea to follow the general rules above.

7. USING DIRECT EMULSION
There are many techniques for coating screens with direct emulsion — some screen printers prefer a one-on-one coating technique (one coat on each side), and some prefer two coats on each side. I find that the more passes I make with the scoop coater, the more emulsion I end up scraping off from prior coats. A good technique is to start by coating the contact side of the screen with one coat, flip the screen over, and coat one time on the squeegee side of the screen.

Be sure to use the correct emulsion for the mesh count you are coating. There are some direct emulsions that work on a wide range of screen meshes, but it is important to read the technical data sheet for recommendations. Screen emulsions are manufactured with different solids contents to allow for better “bridging” (coverage) on various mesh counts. For example, you will need an emulsion with a higher solids content to fill the holes in coarser meshes such as 60 and 86. Fewer solids are necessary in emulsions designed to coat 230 and 305 mesh.

Learn to properly expose screens by determining optimum exposure time for your shop’s conditions. Your combination of exposure unit, emulsion, mesh selection and screen coating techniques are unique. An exposure calculator can help you determine the ideal exposure time for the emulsion and screen mesh you use for each job.

8. SQUEEGEE SELECTION
The standard squeegee used by many manual textile printers is a straight edge, 70-durometer model. I prefer to print with a dual- or triple-durometer squeegee. A multi-durometer squeegee gives you the advantage of a soft edge blade combined with the support of a more rigid inner core or backing.

A simple rule to remember is that if you want to print a surface deposit of ink without driving the ink into the garment, choose a soft squeegee in the 55- to 60-durometer range.

Whatever squeegee hardness you choose, be sure the edge of the squeegee blade is sharp. And have your squeegees sharpened on a regular basis. A dull edge squeegee does not shear ink through a stencil properly.

9. MESH SELECTION
Approximately 40 mesh counts are commercially available to screen printers, yet most shops stock just three or four mesh counts. To be at the top of your game, have eight to 10 mesh counts on hand at all times.

“Mesh count” refers to how many threads per inch are woven in the screen fabric. A lower mesh count allows more open area in the mesh so more ink can flow through these openings. A higher mesh count has less open area and reduces the flow of ink onto the substrate.

The diameter of the individual strands of thread also affects the open area of the screen. A finer thread allows more open area than a thicker thread. Ask your mesh supplier for a mesh chart showing all the available meshes and thread diameters in microns.

Here’s a breakdown of common mesh counts used in a typical garment screen print shop:

• 30 to 40 mesh is used for printing glitter inks
• 60 to 86 mesh is common for printing flock transfers, metallic and puff inks
• 110 to 156 mesh is an all-purpose mesh for printing basic spot color jobs
• 195 to 230 mesh is used for jacket printing and fine detail work
• 255 to 305 is great for four-color process and simulated process printing

10. PRINTING TRANSFERS
Transfers are a great inventory control option, especially when you don’t know if a particular design will be a good seller. Why direct screen print hundreds of garments in advance and hope they will sell? Plus, you never know what sizes and garment styles will be popular for a given design. Transfers can take the guess work out of the equation.

All textile screen printers already have 90% of the equipment needed to print plastisol transfers — a one-color one-station press, vacuum platen, conveyor dryer and a heat press. Now you just have to decide which type of plastisol transfer you want to produce — hot split or cold peel. To make a cold-peel plastisol transfer, you can use the same plastisol that you currently use to direct print T-shirts, 110 to 156 mesh and a coated transfer paper stock.

To produce hot-split transfers, purchase hot-split ink, uncoated transfer paper and 156 to 195 mesh. Be sure to reverse the image when you make your screens, set the dryer at 190ºF, and set the dryer belt speed to retain the transfer in the oven for 30 to 45 seconds.

11. YOUTH-SIZED GARMENTS
When printing a job that calls for both adult and youth sizes, you must create a separate set of smaller positives to print the art on the youth-sized garments. Adult-sized artwork looks out of proportion on children’s apparel. You also will need to purchase a set of youth-sized shirt boards — 8″ x 10″ or 10″ x 10″ boards are ideal. If you contract print childrenswear and youth garments for large department stores or a major brand, the buyer may request flame-retardant inks.

12. FIGHTING FIBRILLATION
Fibrillation is a technical term used to describe the little hairs (or fibers) in the makeup of garment yarn. These fiber tips can protrude through plastisol, making a print appear washed out. Thicker plastisols are tacky and will stick to the back of the next screen that it comes in contact with. The result is an audible snap when the screen is lifted from the ink deposit. This is the sound of the ink ripping away from the screen and at the same time pulling the micro yarn fibers up through the wet ink deposit.

Plastisol that is thick and tacky will have a high sheer quality that greatly contributes to fibrillation problems. Plastisol manufacturers have perfected ink systems with very good mat down qualities that fight fibrillation. These inks possess a low shear quality and are very easy to print with.

Control these screen printing variables and you will be in better control of your shop’s destiny. It is impossible to make all of these changes overnight, but it is possible to gradually integrate these and other ideas into your daily production until you have a smooth running production operation.