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Screen Printing UV Special Effects - Thick Deposit Graphics and Textures
By Laura Maybaum, Graphics Market Segment Manager, Nazdar
For the digital and screen printing industry, the most common problems associated with static are dust attraction, printing defects and shocks to operators. Learn how to identify and mitigate these real-world challenges.
Special effects printing, such as overprint clears, metallic effects, functional inks and security inks has grown significantly in the last few years. Most specialty applications used by UV graphic printers such as mesh, stencil and printing techniques can be used without much consideration to the overall process. However, when printing thick deposit effects to simulate textures or add 3-D structure to the print surface, the process can vary from creation of the artwork to pre-press to printing and finishing.
This article provides a processing guide for addressing these considerations upfront. Doing your homework can make a big difference in effectively overcoming the obstacles to successfully (and profitably) printing these effects. Consider the recommendations as general guidelines to help you define where to start and to lead you to establish shop parameters specific to your location and each application.
While there are many thick printing inks and effects, I will start with UV heavy build-up clears. This type of product typically is used to overprint graphics or directly onto the substrate to provide a glossy, water-clear 3-D effect.
Applications may include:
Considerations when Making Stencils
Depending on the print conditions, a stencil build-up of more than 150 microns can leave too much ink deposit, resulting in under-curing or too great a height of ink which causes significant issues with transfer when printing. If the deposit thickness is not achieved with a single print pass, multiple print passes can be used. A higher stencil build-up also provides a stencil gasket to the substrate and produces a clean printed edge on the detail. Otherwise, the printed image may appear jagged.
Figure 1 is a side view of a stencil with extreme stencil build-up. The top of the image shows the stencil surface to be very smooth. When the stencil comes into contact with the substrate, the ink flows to the stencil edge but not beyond, producing a clean printed edge.
Stencil Build and Artwork Size
Figure 2 is an angled view of a printed image cut through the substrate and ink film. The left curved line is small enough in width for the ink surface to round up. As this detail becomes large in width, the surface begins to flatten out. The comma shaped artwork on the right shows a rim around the edges and a dip in the middle.
The artwork and stencil size ratio is a significant consideration when exposing a stencil; there is always some degree of under-cutting of the light resulting in an angle along the stencil's edge. Thicker stencils exaggerate this under-cutting and result in a change in size from the artwork to the stencil.
Figure 3 is a close-up view of a thick stencil using a clear emulsion, photo-film artwork and an 8k metal-halide lamp. The image shows clearly that the printed size of "rio" will be reduced. See how the detail in the dot in the "i" is more affected than that of the "o."
To achieve the desired printed effect, the balance of the artwork, stencil thickness and stencil exposure have to be qualified prior to moving onto full scale production. When starting to use a heavy build-up clear, it is best to establish the stencil build-up and run test images to determine the largest and smallest artwork size that prints cleanly and smoothly across the surface of the detail.
UV ink does not attack a stencil system like a water-based or solvent-based ink does, so a textile grade capillary film or emulsion can be an appropriate choice. However, the solvent used at press or in reclaiming can potentially "lock-in" the stencil. During reclaiming, the stencil may come off in chunks or sheets. The screen department may need to take special actions to filter out "lock-in" or non-dissolved stencil material. If this is an issue, consider moving to a solvent-resistant SBQ/pure photopolymer product. The other option to reclaiming a "locked-in" stencil is simply to cut the mesh out and re-stretch the screen.
Choosing a stencil system comes down to a choice between time and money. Capillary film provides fast processing at a higher cost; direct emulsion is less expensive and is more time consuming.
The quickest means to providing a high stencil-build is to use a thick capillary film. Typically, a capillary film sinks into the mesh about one-quarter to one-third of its thickness, so choose the capillary film product appropriately. Also consider applying direct emulsion on the squeegee side of the screen to help secure the capillary film to the mesh. Capillary films also have the added advantage of being applied and dried quickly. If speed is required in the process of making a screen, capillary film is the way to go.
Because most graphic printers are familiar with direct emulsion, it is easier to establish stencil making procedures that are not significantly different from current stencil processing. Start with an emulsion with high solids content, but note that there are new products on the market that provide extremely high solid content that are ideal for this application. Higher solids content results in thicker emulsion coating with less dripping and faster drying.
Begin by coating the substrate side of the screen using the round edge of the coater. Coat as many times as it takes to produce a glossy, smooth coating on the squeegee side of the screen. Then coat two to five times on the squeegee side of the screen. More coats may be applied, provided the emulsion does not drip or sag on the mesh while drying. Figure 4 shows a screen being coated from the squeegee side of the screen.
It is critical to dry the screen as much as possible after this initial coat to drive as much water out of the coating as possible; water retained in the stencil will affect exposure and may result in severe under exposure. This is true for emulsions and capillary films.
Once dry, coat the screen using the round edge of the coater on the substrate side of the screen with multiple wet-on-wet face coats. Apply light, even pressure when coating wet-on-wet to help deposit as much emulsion to the surface as possible without causing dripping. Dry the screen and repeat as many times as it takes to achieve the stencil build-up required. Again, be sure to dry the screen completely.
Step-test coating of the initial coating and the face coating should be conducted to determine the procedure that works for each application.
Exposure and Artwork Film
Once the exposure test screen is made with the varying detail, it is used to conduct test printing. The resulting size of the printed artwork can be directly compared to the film's artwork to alert the art department of the size needed to build into future files to print the desired finished size.
Be aware that thicker stencil build-up inherently results in a very smooth stencil surface. When exposing, this smooth surface can stick to the film and glass. Extra care may be needed to separate the stencil from the glass. If so, go slowly in separating the two and be sure to clean the glass if any residues are left. In my internal testing, the artwork film is created using an aqueous (water-based) digital printer, so any water residue in the stencil will soften the ink on the artwork and stick to the stencil surface. In this case, I expected the film to be used only once.
Otherwise, spray down the stencil on both sides of the screen and allow the water to soak into the stencil. Continue to spray the stencil off and on with low pressure and intermittent high pressure. This process may take several applications. Once the stencil starts to open up, continue to use the high pressure sprayer, focusing more attention on the substrate side of the screen. The stencil should be able to handle a high pressure sprayer from 10-12 inches away from the screen without blowing away from the mesh. If the stencil degrades under pressure, then it was underexposed.
Take care not to under-develop the screen. Any residue along the artwork's edge or on the mesh threads may be seen in the printed image and result in jagged edges or mesh marks in the image. Thoroughly dry the screen.
Squeegee Pressure and Angle
Squeegee pressure and angle must be considered when setting up the press and experimenting to achieve the desired result, but when attempting to overcome big issues, changing these settings has minimal influence.
Flood Bar Pressure
Ink Level and Usage
Due to the thickness of the ink deposit, a thin gauge substrate does not provide enough support to the ink - a thicker substrate is required for most applications. Not only will a thicker substrate support the finished print better, if the substrate shrinks or expands during printing or as a reaction to the ink, a thicker substrate will handle these changes better and result in a flatter printed piece.
Checking for Cure
Because it is difficult to cure a thick ink deposit, I do not recommend tinting heavy build-up inks. The added pigment of the ink blocks the UV light from effectively curing the ink. Adding five percent of a colored ink can result in total cure failure. To gain color in these areas, graphics should be printed first and heavy build-up clear should be printed last. Depending on color, ink deposit, substrate and UV output, you may be able to get away with slight tinting, but the responsibility for qualifying and testing the finished print is on the printer's shoulders and requires good curing equipment in excellent condition.
Some special effect powders or pastes in heavy build-up inks may be used provided that the particle is somewhat transparent, such as with a pearlescent powder. In this case, less than three percent should be used. The addition of any powder or paste can lead to curing issues and should be qualified prior to full-scale production.
Post Cure and Flexibility of the Ink
With multiple layers of ink, the difference in height from the substrate to the ink could be considerable. If prints are stacked and the substrate is soft, as with coated paper, the weight of the stack would press the back side of one print into the surface of the other. The prints may stick together or emboss each other. Do not assume that if one substrate passes all the stacking and finishing processing with the ink, the same will be true of any substrate.
With extremely thick stencils, clean all the ink off on press and in the wash out area. Soak the screen with water for a few minutes prior to applying the stencil remover. This initial water soaking helps open up the pores in the stencil and allows the reclaiming chemical to penetrate through the screen. Otherwise, the stencil will likely blow off the screen in chunks or peel off the surface. If the stencil does not dissolve, the screen area should have a way to catch the solid waste before it goes down the drain. Over time, this gummy residue could build up and clog drains.
When using the same stencil and stacking ink upon itself, the ink only transfers to the areas with which it comes into contact. The result is that the image shrinks slightly in size with each printed layer. For example, printing a circle upon itself results in a cone shape when viewed from the side. The bottom of the print or first layer is much wider in dimension than top point with the last layer.
Figure 10 is a top-down view showing a square printed onto a white substrate. As indicated with the black arrow, the first layer is significantly larger than the seventh layer (seen in blue). This reduction in size helps provide a more rounded appearance to the print.
Remember that when the artwork is too large and ridges form around its edges, they become exaggerated with additional layers of ink. Figures 11 and 12 are three, five and nine layers of a heavy deposit clear plus three percent gold pearlescent powder. The smaller circles in Figure 11 show how the layers of ink lead to a cone shape. Figure 12 is a slightly larger circle that results in an outer ridge line and a sunken middle that is exaggerated with more layers.
When layering ink, UV light will prematurely yellow any ink. To see a demonstration, print two sheets. Expose one print to UV light once and the other print to eight or more UV cure cycles. The color of the repeatedly-cured print will look more yellow. When printing multiple layers, expect a slight yellowing of the clear. The more layers you have, the more pronounced this effect will be.
Dialing in the Ink to Gain the Desired Effect
Laura Maybaum is the graphics market segment manager for Nazdar and is a member of the Academy of Screen Print Technology. She holds a bachelor's degree in Print Manufacturing and a master's degree in Corporate Training from Bowling Green State University in Ohio. Maybaum has been active in the screen printing market for more than 17 years with positions at Nazdar, KIWO and Sefar America. firstname.lastname@example.org
This article appeared in the SGIA Journal, January/February 2011 Issue and is reprinted with permission. Copyright 2011 Specialty Graphic Imaging Association (www.sgia.org). All Rights Reserved.
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