Tips For Screenprinters

An ongoing series of tips and tricks for screenprinters. They are in no particular order. We are continuously adding to this feature, so check back often to see what’s new.

UNDER EXPOSURE
Emulsions are composed of polyvinyl acetate surrounded by polyvinyl alcohol. During exposure, the sensitizer cross-links the polyvinyl alcohol, sealing the polyvinyl acetate inside, rendering it insolvable. If under-exposed, the polyvinyl alcohol will dissolve, allowing the sticky polyvinyl acetate to adhere to the mesh. The sodium-metaperiodate stencil remover has no effect on polyvinyl acetate and thus the stencil will be difficult to remove.

MEASURING FOR EXPOSURE
The thickness of the stencil determines the proper exposure necessary to completely harden the emulsion from top to bottom. By measuring the stencil thickness with an electronic thickness gauge, you can determine the necessary exposure time prior to exposure. By keeping a log of the correct stencil thickness to achieve opacity on a particular mesh count/thread diameter, you can reduce the need for double strokes and increase production.

SQUEEGEE DUROMETER
The hardness of squeegee material is measured with a durometer and the results are given in a specific durometer. Differing durometers will produce a difference in ink deposit, with lower durometer (softer) blades generally producing more deposit. The durometer of the squeegee should be noted on the Print Data Sheet for each job, as you will want to assure that you use the same durometer on repeat orders. The manufacturers stated durometer of a urethane squeegee can be off by as much as ±5° of the stated hardness. Measure before using!

SOCIAL MEDIA DESTRUCTION
Be careful what you say on social media sites. The same rules that are used in social settings should guide you on social media as well. Never talk about the three biggies; politics, religion, and any local “hot button” topics that can cause hard feelings and divide you from others. Your biggest client may not be of the same persuasion in these areas, but the money they spend with you, allows you to stay in business. You don’t have to agree with others, but you do not want to alienate your opportunities to earn a living for your family and employees.

EMULSION RE-WETTING
If you place wet screens in the same area as dry screens, the moisture will be drawn into the the dry ones. The solution is to place the wet screens into a drying cabinet, then when they are completely dry, move the screens into a dry cabinet. This goes for screens that have just been degreased, as well as the ones coated with emulsion. Use a dehumidifier to speed up the drying time, never a fan, which does nothing to remove moisture and blows pinhole-producing contaminates onto the screens.

PURPOSE OF COATING
We want to completely fill the mesh openings with emulsion. This means that we need to apply enough emulsion to the bottom of the screen until the inside of the screen is shiny with wet emulsion. In this way, we will know that we have filled the mesh openings. This will vary with the volume of the mesh opening, which is controlled by mesh count, thread diameter and tension. Thus, it will not suffice to coat all screens by a count, such as 1 over 1 or 2 over 1.

LIGHT SOURCE
The choice between multiple lamps and a single lamp is easy. Single point light sources control the light better and it is recommended to be used for a more detailed film exposure. Multiple lamps scatter light, attacking the films from multiple angles creating “undercutting” that reduce the image resolution.

PRINTING HIGHER RESOLUTION
There are three rules that come into play here. The first rule is that at least three of the largest particles in an ink must fit into the mesh opening. The second rule is the detail should be no smaller than the width of 2 threads and 1 mesh opening. The third rule is to use a low EOMR, as the thicker the stencil, the harder it is for the ink to be drawn from the mesh, due to the lower surface energy of the small detail. Once you meet these three rules and you still cannot produce the desired detail, you will need to switch to finer threads.

BIGGEST MISTAKE IN START-UPS
The biggest mistakes start-up businesses make is the failure to plan and prepare for the challenges. Planning forces you to make discoveries that will benefit your business. The most important thing when creating a business plan is to not get intimidated by everything you think needs to be in it. In its simplest form, it states what you are going to do and how you’re going to do it. You then project your revenues and expenses, and come up with a marketing plan.

SCOOP COATER WIDTH
The scoop coater should be of a width that is only slightly wider than the image to be printed. When coating a stretch and glue frame, if you come to close to the frame, you risk delaminating the mesh from the frame, especially if the mesh tension is low and more pressure is needed to get a consistent coat.

INK JET MEDIUM
Avoid using films with a “clear” appearance, as the emulsion is needed speed the drying process and keep edge resolution high reducing dot gain. Always use a “water proof” film. The slightly cloudy emulsion of the film does not affect the exposure process and allows ink to adher the film during output.

DYED MESH
Clear mesh (often referred to as white due to its milky color scatters light during the exposure and degrades resolution. Dyed mesh reflects UV light which helps the stencil hold more detail when printing small details, fine lines and halftones.

Scoop Coater Edge

The dull edge is used for coating and the sharp edge is used for applying face coats.

The reasoning is simple – you want to make as few strokes as possible in order to fill the mesh openings with emulsion. The larger radius of the dull edge applies more emulsion per stroke than the smaller radius of the sharper side. The mesh count does not come into play in this situation – only the volume of the mesh opening is affected as emulsion flows by gravity.

Face coats are applied after the original base coat is made. They are made on the bottom side of the stencil to fill the mesh openings that were reduced after the moisture was evacuated during drying. By filling the reduced mesh opening on the bottom side of the stencil, you are assured of a better edge resolution and a full gasket effect.

WET INK THICKNESS
The wet ink deposit is formed by the volume of the mesh opening and the emulsion over mesh ratio. A thicker thread mesh will deposit more ink than a thin thread and thus give more opacity. This can be increased by adding face coats to the stencil after it is dried. By measuring the thickness of the stencil you can set standards, which will eliminate the need for multiple strokes.

THE HARD TRUTH
A lack of education makes everything harder. Improper advice by others makes it worse. Be very careful where and from whom you seek your education in the screenprinting technologies. There is a lot of bad advice out there!

PREVENTING HAZE
Allowing ink to dry in the screens after printing will cause ink to dry in the knuckles of the mesh. This is true of plastisol, solvent base, water base, discharge and HSA inks. Excessive pressure and flashing create heat on the mesh, which open the pores of polyester mesh threads, allowing the ink to seep into the thread. Once cooled, the mesh will require either heat or haze remover to open the pores and allow the mesh to be be properly cleaned.

INK JET FILM STICKING TO THE STENCIL
During periods of high heat and moisture, if you have any moisture remaining in emulsion when you expose it, the ink on the ink jet film will likely adhere to the moist emulsion. High heat can cause dry emulsion to draw any moisture from the surrounding air and create the same effect. Use a dehumidifier in your drying cabinet to speed up the drying time and another in the dry cabinet to keep them dry until ready for use.

CMYK PRINTING
The use of a Flemenco (single angle) pattern is preferred over using a Rosette (multiple angles) pattern. All four screens should be at 22.5 degrees. Reprint one of your Rosette pattern jobs as a Flemenco and you will see a vast improvement!

STATIC AND DYNAMIC TENSION
The tension meter reads an average of tension across the mesh producing an average static tension of the mesh. Dynamic tension is the tension of the mesh recorded when the squeegee pressure moves the mesh into contact with the printed surface. There is a radical difference in static tension and dynamic at the print surface based on off contact. On a large frame dynamic tension can be more than 40 N/cm2. If the static tension is near the breaking point of the mesh threads, the mesh can fail under dynamic tension.

RINSING DEGREASER
Proper rinsing is necessary to assure that screens are properly degreased. After degreasing, always rinse the screen with a high volume, low-pressure water rinse, similar to water running from a garden hose without a nozzle attached. Rinse until the screen runs free of bubbles and the water sheets from the screen. If water does not sheet from the mesh, the screen is not clean of contaminates. Degrease the screen again to remove printing residues and contaminates still on the screen.

HIGHER PROFITS
The largest and most successful screenprinting operations are contracters and pre-printers. However, both contract and pre-print work are far more complex than most imagine. Competition is aggressive, deadlines are tight, margins are low, and because of the larger orders you will be dealing with few clients – lose one and you suffer greatly. While contract and pre-print work may be the fastest path to higher profits, it require extensive planning to be successful.

DRYING COATED SCREENS
When drying a coated screen place the “print side” facing up and the “squeegee side” facing down. Exactly as it is when you print with it. Gravity forces the emulsion to produce the proper emulsion over mesh ratio on the print side. This improves edge definition and increases the ink well which leads to better ink coverage

STENCIL THICKNESS
The total stencil system (mesh + emulsion) thickness should be smaller or equal to the finest line being printed. If the stencil system thickness is equal to or larger than the width of the line, ink transfer will be more difficult. The proper stencil thickness is not a end-all solution for printing fine lines. There are other parameters that a printer must take into consideration, such as the line’s angle to the mesh, and the thread diameter of the mesh.

COATING IN ROOM LIGHT
Most sensitized wet emulsions (with few exceptions) are only slightly sensitive to room light, such as fluorescent lighting. It is the amount of UV in the light source that creates the chemical cross-linking of emulsion. So, yes you can coat in ordinary room light and then place the coated screen in a light safe place.

REGISTER MARKS
Registration marks are used to line up the art and screens on press. Eventually, as one gains experience they become an annoyance as they learn that it is the artwork that needs to be registered. as I The added bonuses in not using register marks are that they don’t have to placed on the art or film positives, nor do require remember to block them out of the stencil.

FINAL HALFTONE COUNT
You should be aware that if the halftone line count is 55 lines-per-inch (lpi) on the film positive, the final print will be less than 55 lpi. This is due to the mesh being stretched from its static tension to the dynamic tension as the squeegee moves the mesh down to the substrate. The higher the off-contact – the lower the lpi will be when printed. Thus, you may be able to increase the lpi on the film and not have a moiré problem in the final print. This negates the myth of placing the film on the substrate to be printed and observing moiré.